Common Communication Tests
Hundreds, even thousands, of different tests are made on communication equipment. However, some common tests are widely used on all types of communication equipment. This section summarizes the most common tests and measurements made in the servicing of communication equipment.
Most of the tests described here focus on standard radio communication equipment. Tests for transmitters, receivers, and antennas will be described, as will special microwave tests, fiber-optic cable tests, and tests on data communication equipment. Keep in mind that the test procedures described are general. To make specific tests, follow the test setups recommended by the test equipment manufacturer. Always have the manuals for the test equipment and the equipment being serviced on hand for reference.
Four main tests are made on most transmitters: tests of frequency, modulation, and power, and tests for any undesired output signal component such as harmonics and parasitic radiations. These tests and measurements are made for several reasons. First, any equipment that radiates a radio signal is governed by Federal Communications Commission (FCC) rules and regulations. For a transmitter to meet its intended purpose, the FCC specifies frequency, power, and other measurements to which the equipment must comply.
Second, the tests are normally made when the equipment is first installed to be sure that everything is working correctly. Third, such tests may be performed to troubleshoot equipment. If the equipment is not working properly, these tests are some of the first that should be made to help identify the trouble.
Regardless of the method of carrier generation, the frequency of the transmitter is important. The transmitter must operate on the assigned frequency to comply with FCC regulations and to ensure that the signal can be picked up by a receiver tuned to that frequency.
The output of a transmitter is measured directly to determine its frequency. The transmitted signal is independently picked up and its frequency measured on a frequency counter. Fig. 22-14 shows several methods of picking up the signal. Many frequency counters designed for communication work come with an antenna that picks up the signal directly from the transmitter [see Fig. 22-14(a)]. The transmitter is keyed up (turned on) while it is connected to its regular antenna, and the antenna on the counter picks up the signal and translates it to one that can be measured by the counter circuitry. No modulation should be applied, especially if FM is used.
The transmitter output can be connected to a dummy load [see Fig. 22-14(b)]. This will ensure that no signal is radiated, but that there will be sufficient signal pickup to make a frequency measurement if the counter and its antenna are placed near the transmitter.
Another method of connecting the counter to the transmitter is to use a small coil, as shown in Fig. 22-14(c). A small pickup coil can be made of stiff copper wire. Enamel copper wire, size AWG 12 or 14 wire, is formed into a loop of two to four turns. The ends of the loop are connected to a coaxial cable with a BNC connector for attachment to the frequency counter. The loop can be placed near the transmitter circuits. This method of pickup is used when the transmitter has been opened and its circuits have been exposed. For most transmitters, the loop has been placed only in the general vicinity of the circuitry. Normally the loop picks up radiation from one of the inductors in the final stage of the transmitter. Maximum coupling is achieved when the axis of the turns of the loop is parallel to the axis of one of the inductors in the final output stage.
Once the signal from the transmitter is coupled to the counter, the counter sensitivity is adjusted, and the counter is set to the desired range for displaying the frequency. The greater the number of digits the counter can display, the more accurate the measurement. Normally this test is made without modulation. If only the carrier is transmitted, any modulation effects can be ignored. Modulation must not be applied to an FM transmitter, because the carrier frequency will be varied by the modulation, resulting in inaccurate frequency measurement.
The quality of crystals today is excellent; thus, the off-frequency operation is not common. If the transmitter is not within specifications, the crystal can be replaced. In some critical pieces of equipment, the crystal may be in an oven. If the oven temperature control circuits are not working correctly, the crystal may have drifted off frequency. This calls for the repair of the oven circuitry or replacement of the entire unit. If the signal source is a frequency synthesizer, the precision of the reference crystal can be checked. If it is within specifications, perhaps an off-frequency operation is being caused by a digital problem in the phase-locked loop (PLL). An incorrect frequency division ratio, faulty phase detector, or poorly tracking voltage-controlled oscillator (VCO) may be the problem.
If AM is being used in the transmitter, you should measure the percentage of modulation. It is best to keep the percentage of modulation as close to 100 as possible to ensure maximum output power and below 100 to prevent signal distortion and harmonic radiation. In FM or PM transmitters, you should measure the frequency deviation with modulation. The goal with FM is to keep the deviation within the specific range to prevent adjacent channel interference.
The best way to measure AM is to use an oscilloscope and display the AM signal directly. To do this, you must have an oscilloscope whose vertical amplifier bandwidth is sufficient to cover the transmitter frequency. Fig. 22-15(a) shows the basic test setup.
An audio signal generator is used to amplitude-modulate the transmitter. An audio signal of 400 to 1000 Hz is applied in place of the microphone signal.
The transmitter is then keyed up, and the oscilloscope is attached to the output load. It is best to perform this test with a dummy load to prevent radiation of the signal. The oscilloscope is then adjusted to display the AM signal. The display will appear as shown in Fig. 22-15(b).
Most transmitters have a tune-up procedure recommended by the manufacturer for adjusting each stage to produce maximum output power. In older transmitters, tuned circuits between stages have to be precisely adjusted in the correct sequence. In modern solid-state transmitters, there are fewer adjustments, but in most cases, there are some adjustments in the driver and frequency multiplier stages as well as tuning adjustments for resonance at the operating frequency to the final amplifier.
There may be impedance-matching adjustments in the final amplifier to ensure full coupling of the power to the antenna. The process is essentially that of adjusting the tuned circuits to resonance. These measurements are generally made while monitoring the output power of the transmitter.
The procedure for measuring the output power is to connect the transmitter output to an RF power meter and the dummy load, as shown in Fig. 22-16. The transmitter is keyed up without modulation, and adjustments are made on the transmitter circuit to tune for maximum power output. With the test arrangement shown, the power meter will display the output power reading.
Once the transmitter is properly tuned up, it can be connected to the antenna. The power into the antenna will then be indicated. If the antenna is properly matched to the transmission line, the amount of output power will be the same as that in the dummy load. If not, SWR measurements should be made. Most modern power meters measure both forward and reflected power, so SWR measurements are easier to make. It may be necessary to adjust the antenna or a matching circuit to ensure maximum output power with minimum SWR.
Harmonics and Spurious Output Measurements
A common problem in transmitters is the radiation of undesirable harmonics or spurious signals. Ideally, the output of the transmitter should be a pure signal at the carrier frequency with only those sideband components produced by the modulating signal. However, most transmitters will generate some harmonics and spurious signals. Transmitters that use class C, class D and class E amplifi ers generate a high harmonic content. If the tuned circuits in the transmitter are properly designed, the Q’s will be high enough to reduce the harmonic content level suffi ciently. However, this is not always the case.
Another problem is that other spurious signals can be generated by transmitters. In high-power transmitters particularly, parasitic oscillations can occur. These are caused by the excitation of small tuned circuits whose components are the stray inductances and capacitances in the circuits or the transistors of the tubes involved. Parasitic oscillations can reach high levels and cause radiation on undesired frequencies.
For most transmitters, the FCC specifies maximum levels of harmonic and spurious radiation. Intermodulation distortion in mixers and nonlinear circuits also produces unwanted signals. Normally these signals must be at least 30 or 40 dB down from the main carrier signal. The best way to measure harmonics and spurious signals is to use a spectrum analyzer. The transmitter output is modulated with an audio tone, and its output is monitored directly on the spectrum analyzer. It is usually best to feed the transmitter output into a dummy load for this measurement. The spectrum analyzer is then adjusted to display the normal carrier and sideband pattern. The search for high-level signals can begin for spurious outputs by tuning the spectrum analyzer above and below the operating frequency. The spectrum analyzer can be tuned to search for signals at the second, third, and higher harmonics of the carrier frequency. If signals are detected, they can be measured to ensure that they are sufficiently low in power to meet FCC regulations and/or the manufacturer’s specifications. The spectrum analyzer is then tuned over a broad range to ensure that no other spurious nonharmonic signals are present.
It may be possible to reduce the harmonic and spurious output content by making a minor transmitter tuning adjustment. If not, to meet specifications, it is often necessary to use filters to eliminate unwanted harmonics or other signals.
Antenna and Transmission Line Tests
If the transmitter is working correctly and the antenna has been properly designed, the only test that needs to be made on the transmission line and antenna is for standing waves. It will tell you whether any further adjustments are necessary. If the SWR is high, you can usually tune the antenna to reduce it.
You may also run into a transmission line problem. It may be open or shortcircuited, which will show up on an SWR test as infinite SWR. But there may be other problems such as a cable that has been cut, short-circuited, or crushed between the transmitter and receiver. These kinds of problems can be located with a time-domain refl ectometer test.
The test procedure for SWR is shown in Fig. 22-17. The SWR meter is connected between the transmission line and the antenna. Check with the manufacturer of the SWR meter to determine whether any specifi c connection location is required or whether other conditions must be met. Some of the lower-cost SWR meters must be connected directly at the antenna or a specifi c number of half wavelengths from the antenna back to the transmitter.
Once the meter is properly connected, key up the transmitter without modulation. The transmitter should have been previously tuned and adjusted for maximum output power. The SWR can be read directly from the instrument’s meter. In some cases, the meter will give measurements for the relative amount of incident or forward and reflected power or will read out in terms of the reflection coefficient, in which case you must calculate the SWR as described earlier. Other meters will read out directly in SWR. The maximum range is usually 3:1.
The ideal SWR is 1 or 1:1, which means that all the power generated by the transmitter is absorbed by the antenna load. Nevertheless, in even the best systems, perfect matching is rarely achieved. Any mismatch will produce reflected power and standing waves. If the SWR is less than 2:1, the amount of power that will be lost or refl ected will be minimal.
The primary procedure for reducing the SWR is to make antenna adjustments, usually in the form of modifying the element lengths to more closely tune the antenna to the frequency of operation. Using many antennas makes it possible to adjust their length over a narrow range to fi ne-tune the SWR. Other antennas permit adjustment to provide a better match of the transmission line to the driven element of the antenna. These adjustments can be made one at a time, and the SWR monitored.
Time-domain reflectometry (TDR) is a pulse for cables and transmission lines of all types. It is widely used in finding faults in cables used for digital data transmission, but it can be used for RF transmission lines. (Refer to the section Data Communication Tests later in this chapter for details.)
The primary tests for receivers involve sensitivity and noise level. The greater the sensitivity of the receiver, the higher it’s gain and the better job it does of receiving very small signals.
As part of the sensitivity testing, the signal-to-noise (S/N) ratio is also usually measured indirectly. The ability of a receiver to pick up weak signals is just as much a function of the receiver noise level as it is of overall receiver gain. The lower the noise level, the greater the ability of the receiver to detect weak signals.
As part of an overall sensitivity check, some receiver manufacturers specify an audio power output level. Since receiver sensitivity measurements are usually made by measuring the speaker output voltage, power output can also be checked if desired.
In this section, some of the common tests made on receivers are described. The information is generic, and the actual testing procedures often differ from one receiver manufacturer to another.
To make sensitivity and noise measurements, the following equipment is necessary:
- Dual-Trace Oscilloscope. The vertical frequency response is not too critical, for you will be viewing noise and audio frequency signals.
- RF Signal Generator. This generator provides an RF signal at the receiver operating frequency. It should have an output attenuator so that signals as low as 1 μV or less can be set. This may indicate the need for an external attenuator if the generator does not have built-in attenuators or output level adjustments. The generator must also have modulation capability, either AM or FM, depending upon the type of receiver to be tested.
- RF Voltmeter. The RF voltmeter is needed to measure the RF generator output voltage in some tests. Some higher-quality RF generators have an RF voltmeter built in to aid in setting the output attenuator and level controls.
- Frequency Counter. A frequency counter capable of measuring the RF generator output frequency is needed.
- Multimeter. A multimeter capable of measuring audio-frequency (AF) voltage levels is needed. Any analog or digital multimeter with ac measurement capability in the AF range can be used.
- Dummy Loads. A dummy load is needed for the receiver antenna input for the noise test. This can be either a 50- or a 75-ohms resistor attached to the appropriate coaxial input connector. A dummy load is needed for the speaker. Because most noise and sensitivity tests are made with maximum receiver gain including audio gain, it is not practical or desirable to leave the speaker connected. Most communication receivers have an audio output power capability of 2 to 10 W, which is sufficiently high to make the output signal level too high for comfort. A speaker dummy load of 4, 8, or 16 V, depending upon the speaker impedance, is needed. Be sure that the dummy load can withstand the maximum audio power output of the receiver. Do not use wire-wound resistors for this application, for they have too much inductance. Check the receiver’s specifications for both the impedance and the maximum power level specification.
Noise consists of random signal variations picked up by the receiver caused by thermal agitation and other conditions inside the receiver circuitry. External noise cannot be controlled or eliminated. However, noise contributed by the receiver can be controlled. Every effort is made during the design to minimize internally generated noise and thus to improve the ability of the receiver to pick up weak signals.
Most noise generated by the receiver occurs in the receiver’s front end, primarily the RF amplifier and the mixer. Careful attention is given to the design of both these circuits so that they contribute minimum noise.
Because noise is a totally random signal that is a composite of varying frequency and varying amplitude signals, it is somewhat difficult to measure. However, the following procedure has become a common and popular method that is easy to implement.
Refer to the test setup shown in Fig. 22-18. The antenna is removed from the receiver, and a dummy load of the correct impedance is used to replace it. A carbon composition resistor of 50 or 75 V can be used. The idea is to prevent the receiver from picking up any signals while maintaining the correct impedance.
At the output of the receiver, the speaker is replaced with a dummy load. Most speakers have an output impedance of 4 or 8 V. Check the receiver’s specifications, and connect an appropriate value of the resistor in place of the speaker. Be sure that the dummy load resistors can withstand the output power.
Finally, connect the dual-trace oscilloscope across the dummy speaker load. The same signal should be displayed on both channels of the oscilloscope. The displayed signal will be an amplifi ed version of the noise produced by the receiver and amplifi ed by all the stages between the antenna input and the speaker. Follow this step-by-step procedure:
- Turn on the receiver, and tune it to a channel where no signal will be received.
- Set the receiver volume control to maximum. If the receiver has any type of RF o IF gain control, it too should be set to its maximum setting.
- Set the oscilloscope input for the lower trace (channel B) to ground. Most oscilloscopes have a switch that allows the input to be set for ac measurements, dc measurements, or ground. Grounding the channel B input will prevent any signal from being displayed. At this time, you will see a straight horizontal line for the lower trace. Adjust that lower trace so that it lines up with one of the horizontal graticule lines near the bottom of the oscilloscope screen. This will provide a voltage measurement reference.
- Set the channel B input and the channel A input to alternating current. Adjust the vertical sensitivities of channels A and B so that they are on the same range. Make the adjustments so that the signal is displayed something like that shown in Fig. 22-19(a). You should see exactly the same noise pattern on both channels.
- Using the vertical position control on the upper or A channel, move the upper noise trace downward so that it begins to merge with the noise signal on channel B. The correct adjustment for the position of the channel A noise signal is such that the peaks of the upper and lower signals just barely merge. This will generally be indicated at the point where there is no blank space between upper and lower traces. The signal should look something like that shown in Fig. 22-19(b).
- Now set both oscilloscope inputs to ground, thereby preventing the noise signal from being displayed. You will see two straight horizontal lines. The distance between the two lines is a measure of the noise voltage. The value indicated is two times the root mean square (rms) noise voltage [see Fig. 22-19(c)].
Assume that the adjustments described above were made and the separation between the two horizontal traces is 1.2 vertical divisions. If the vertical gains of both channels are set to the 10 mV per division range, the noise reading is 1.2 x 10 mV =12 mV. The rms noise voltage is one-half of this figure or 12 mV/2 = 6 mV.
Power Output Tests
Sometimes it is necessary to measure the receiver’s total power output capability. This is a good general test of all the receiver circuits. If the receiver can supply the manufacturer’s specified maximum output power into the speaker with a given low RF signal level input, the receiver is operating correctly.
The test setup for the power output test is shown in Fig. 22-20. An RF signal generator for the correct frequency is used as the primary signal source. It must also be possible to modulate this generator with either AM or FM depending upon the type of receiver.
It is also desirable to connect a frequency counter to the signal generator output to provide an accurate measure of the receiver input frequency. Most communication receivers operate on specific frequency channels. For the test to be valid, the generator output frequency must be set to the center of the receiver frequency channel. This will usually be known from the receiver’s specifications. The signal generator is tuned, and the frequency is set by monitoring the digital readout on the frequency counter.
Be sure to replace the speaker with a resistive load of an impedance equal to that of the speaker, such as 8 ohms. The dummy speaker load should also be able to carry the maximum rated output power of the receiver. Finally, connect an ac voltmeter across the speaker dummy load.
Turn on the receiver, and set the volume control to the maximum. If the receiver has a variable RF or IF gain control, you must set it to the maximum setting also. At this time, any other receiver features should be disabled. For example, in an FM receiver, the squelch should be turned off or disabled. In an AM receiver, if a noise limiter is used, it too should be turned off.
To begin the test, follow this procedure:
- Set the RF generator output level to 1 mV. If the RF signal generator has a built-in RF voltmeter, use it to make this setting. Otherwise, an external RF voltmeter may be needed, as shown in Fig. 22-20.
- Set the signal generator for modulation of the appropriate type. I AM is used, set the percentage of modulation for 30. If FM is used, set the deviation for ;3 kHz. In most signal generators, the percentage of AM and the frequency deviation for FM are fixed. Refer to the signal generator specifications to find out what these values are.
- With everything appropriately adjusted, measure the ac voltage across the speaker dummy load. This will be an RMS reading.
- To determine the receiver power output, use the standard power formula P= V2/R.
Assume that you measure an RMS voltage of 4 V across an 8-V speaker. The power output will be
P = V2/R = 42/8 = 16/8 = 2 W
An optional test is to observe the signal across the speaker load with an oscilloscope. Most RF generators modulate the input signal with a sine wave of 400 Hz or 1 kHz. If an oscilloscope is placed across the dummy speaker load, the sine wave will be seen. This will indicate whether the receiver is distorting. The oscilloscope can also be used in place of the audio voltmeter to make the voltage measurement across the dummy speaker load. Remember that oscilloscope measurements are peak to peak. The peak-to-peak value must be converted to the root mean square to make the power output calculation.
20-dB Quieting Sensitivity Tests
In most cases, the sensitivity of a receiver is expressed in terms of the minimum RF voltage at the antenna terminals that will produce a specific audio output power level. Most measurements factor in the effect of noise. The method of sensitivity measurement is determined according to whether AM or FM is used. Because most modern radio communication equipment uses frequency modulation, measuring FM receiver sensitivity is illustrated. There are two basic methods, quieting and SINAD. The quieting method measures the amount of signal needed to reduce the output noise to 20 dB. As the signal level increases, the noise level decreases until the limiters in the IF section begin to start their clipping action. When this happens, the receiver output “quiets”; i.e., its output is silent and blanks out the noise.
The SINAD test is a measure of the input signal voltage that will produce at least a 12-dB signal-to-noise ratio. The noise value includes any harmonics that are produced by the receiver circuits because of distortion.
The test setup for receiver sensitivity measurements is shown in Fig. 22-21(a). It consists of an RF signal generator, an RF voltmeter, a frequency counter, the receiver to be tested, and a voltmeter to measure the output across the speaker dummy load.
It is often necessary to provide an impedance-matching network between the generator and the receiver antenna input terminals. Most RF generators have a 50-ohms output impedance. This may match the receiver input impedance exactly. However, some receiver input impedances may be different. For example, if a receiver has a 75-V ohms input impedance, some form of impedance matching will be required. This is usually handled by a resistive attenuator known as an impedance-matching pad, which is a resistive T network that provides the correct match between the receiver input and the generator. A typical impedance-matching pad is shown in Fig. 22-21(b). It matches the 50-ohms generator output to the 50-V receiver antenna input. Because resistors are used, the impedance-matching circuit is also an attenuator. With the values are given in Fig. 22-21(b), the signal attenuation is 10 dB. This must be factored into all signal generator measurements to obtain the correct sensitivity figure. Different values of resistors can be used to create a pad with the
correct impedance-matching qualities but with lower or higher values of attenuation.
Some manufacturers specify a special input network made up of resistors, inductors, and/or capacitors for this or other sensitivity tests. This network helps simulate the antenna accurately in equipment that uses special antennas.
Follow this procedure to make the 20-dB quieting measurement:
- Turn on the receiver, and set it to an unused channel.
- Leave the signal generator off so that no signal is applied.
- Set the receiver gain to the maximum with any RF or IF gain control, if available.
- Adjust the volume control of the receiver so that you read some convenient value of
noise voltage on the meter connected across the speaker. One volt RMS is a good value
if you can achieve it; but if not, any other convenient value will do.
- Turn on the signal generator, but set the output level to zero or some very low value.
Adjust the generator frequency to the center of the receiver’s channel setting. Turn off
the modulation so that the generator supplies carrier only.
- Increase the signal generator output signal level a little at a time, and observe the voltage across the speaker. The noise voltage level will decrease as the carrier signal gets
strong enough to overpower the noise. Increase the signal level until the noise voltage
drops to one-tenth of its previous value.
- Measure the generator output voltage on the generator meter or the external RF
- If an attenuator pad or other impedance-matching network was used, subtract the
loss it introduces. The resulting value is the voltage level that produces 20 dB of
quieting in the receiver.
Assume that you measured a generator output of 5 μV that produces the 20-dB noise decrease. This is applied across a 50-V load producing an input power of P2 =V2/R = (5 x 10-6)2/50= 0.5 pW. This is attenuated further by the 10-dB matching pad to a level of one-tenth, or 0.05 PW, which translates to a voltage level across 50ohms as
This is the receiver sensitivity. It takes 1.58 μV of a signal to produce 20 dB of quieting in the receiver.
For a good communication receiver, the 20-dB quieting value should be under 1 μV. A typical value is in the 0.2- to the 0.5-μV range. The lower the value, the better the sensitivity.
Blocking and Third-Order Intercept Tests
As the spectrum has become more crowded and as modulation advances have permitted higher speed per hertz of bandwidth, the potential for adjacent channel interference has significantly increased. To ensure minimum adjacent channel interference, receiver specifications have become tighter. This is especially true in cell phones. Whole suites of tests must be passed by the receiver in
order to meet the specifications of a specific cell phone standard such as GSM or CDMA.
An example is a receiver blocking test that makes measurements to ensure that signals from an adjacent channel do not block or desensitize the channel being used. A very strong signal near the receiving frequency has the effect of lowering the gain of the receiver. Any small signal being received will be decreased in amplitude or even blocked completely. Some specifications call for the receiver to be able to receive a weak signal when the adjacent channel signal is 60 to 70 dB greater in level. The ability to meet this test depends upon the filtering selectivity of the receiver.
But perhaps the most difficult test is the third-order intercept test, designated TOI or IP3. This test is a measure of the linearity of amplifiers, mixers, and other circuits.
When two signals are applied to a circuit, any nonlinearity in the circuit causes a mixing or modulation effect. The larger the input signals, the more likely the amplifier will be driven into a nonlinear region where mixing will occur. Sum and difference signals will be produced. Some of the resulting so-called intermodulation products are problematic because they occur at a frequency near or inside the receiver bandpass and interfere with the signal being received. Such signals, because they are so close to the desired signal, cannot be fi ltered out. These intermodulation signals must be reduced as much as possible in the design of the receiver by selecting more linear components or giving greater attention to biasing schemes and operating points.
Fig. 22-22 shows two signals f1 and f2 that appear at the input to an amplifier. The nonlinear action of the amplifier generates a wide range of sums and differences, including those with the second and third harmonics. Most of these undesired products will be filtered out by the receiver IF filters. The third-order products cause the most problems because they will most likely fall within the receiver IF bandpass. The third-order products
are 2f1 ± f2 and 2f2 ± f1, where the terms 2f1 – f2 and 2f2 – f1 cause the most problems. To measure the third-order problem, two equal power signals are applied to the amplifi er or another circuit to be tested. The frequency spacing is usually small and often made equal to the normal channel spacing used. The power of the input signals is gradually increased, and measurements are made on the amplifier output to determine the levels of the test signals and the third-order products. As the power levels of the input signals increase, the third-order signal power increases as the cube of the input power change. On a logarithmic scale, the rate of increase of third-order products is three times that of the original signals.
Refer to Fig. 22-23. If you plot the output signal amplitudes versus input signal power increases, at some point the power levels reach their limit and flatten out. If you extend the linear portions of the two curves, they will meet at a theoretical point where the initially lower third-order signals equal the main input signals in amplitude. This is the third-order intercept point. The input power at that point is used as a measure of the intermodulation. In Fig. 22-23, the IP3 point is at 15 dBm. Typical IP3 values are between 0 and 35 dBm. The higher the IP3 value, the better the circuit linearity and the lower the intermodulation products.
Microwave tests are generally similar to those performed on standard transmitters and receivers. Transmitter measurements include output power, deviation, harmonics, and spurious signals as well as modulation. The techniques are similar but require the use of only those test instruments whose frequency response is in the desired microwave region.
The same goes for receiver measurements and antenna transmission line tests. The procedures are generally the same, but the equipment is different. For example, with power measurements, a directional coupler is normally used as the transmitter output to reduce the signal to a proper level for measurement with the power meter.
Data Communication Tests
The tests for wireless data communication equipment are essentially the same as those for standard RF communication as described above. The only difference is the type of modulation used to apply the binary signal to the carrier. FSK and its many variants, as well as PSK and spread spectrum, are the most widely used. Special FSK and PSK deviation and modulation meters are available to make these measurements.
For data communication applications in which binary signals are baseband on coaxial and twisted-pair cables, such as in LANs, more conventional testing methods may be used. For example, binary test patterns may be initiated in the transmitting equipment, and the signal viewed on the oscilloscope at the receiving end. Tests of signal attenuation and wave shape can then be made.
A common method of analyzing the quality of binary data transmitted on a cable is to display what is known as the eye diagram on a common oscilloscope. The eye diagram, or pattern, is a display of the individual bits overlapped with one another. The resulting output looks like an open eye. The shape of the pattern and the degree that the eye is “open” can be used to determine many things about the quality of transmission.
Eye diagrams are used for testing because it is diffi cult to display long streams of random serial bits on an oscilloscope. The randomness of the data prevents good synchronization of the oscilloscope with the data, and thus the display jitters and changes continuously. Sending the same pattern of bits such as repeating the ASCII code for the letter U (alternating 1s and 0s) may help the synchronization process, but the display of one whole word on the screen usually does not provide suffi cient detail to determine the nature of the signal. The eye diagram solves these problems.
Fig. 22-24(a) shows a serial pulse train of alternating binary 0s and 1s that is applied to a transmission line. The transmission line, either coaxial or twisted pair, is a low-pass filter, and therefore it eliminates or at least greatly attenuates the higherfrequency components in the pulse train. It also delays the signal. The result is that the signal is rounded and distorted at the end of the cable and the input to the receiver [see Fig. 22-24(b)].
The longer the cable and/or the higher the bit rate, the greater the distortion. The pulses tend to blur into one another, causing what is called intersymbol interference (ISI).
ISI makes the voltage levels for binary 1 and 0 closer together with 1-bit smearing or overlapping into the other. This makes the receiver’s job of clearly distinguishing a binary 1 from a binary 0 more difficult. Furthermore, noise is usually picked up along the transmission path, making the received signal a poor representation of the original data signal.
Too much signal rounding or ISI introduces bit errors. Fig. 22-24(c) shows a severely distorted, noisy data signal.
The eye pattern provides a way to view a serial data signal and to make a determination about its quality. To display the eye diagram, you need an oscilloscope with bandwidth at least fi ve times the maximum bit rate. For example, if the bit rate is 10 Mbps, the oscilloscope bandwidth should be at least 10 x 5 MHz, or 50 MHz. The higher the better. The oscilloscope should have triggered sweep. Either a conventional analog or a digital oscilloscope can be used.
Apply the baseband binary signal at the end of the cable and the receiver input to the vertical input. Adjust the sweep rate of the oscilloscope so that a 1-bit interval takes up the entire horizontal width of the screen. Use the variable-sweep control to fi ne-tune the display and use the trigger controls to stabilize the display. The result is an eye diagram.
Several different eye patterns are shown in Fig. 22-25. The multiple lines represent the overlapping pulses occurring over time. Their amplitude and phase shift are slightly shifted from sweep to sweep, thereby giving the kind of pattern shown. If the signal has not been severely rounded, delayed, or distorted, it might appear as shown in Fig. 22-25(a). The eye is “wide open” and has a trapezoidal shape. This is a composite display of the rise and fall times of the pulses overlapping one another. The steeper the sides, the less the distortion. The eye pattern in Fig. 22-25(a) indicates
the wide bandwidth of the medium.
In Fig. 22-25(b), the pattern looks more like an open eye. The pulses are rounded, indicating that the bandwidth is limited. In fact, the pulses approach the shape of a sine wave. The pattern shown in Fig. 22-25(c) indicates more severe bandwidth limiting. This reduces the amplitude of the rounded pulses, resulting in a pattern that appears to be an eye that is closing. The more the eye closes, the narrower the bandwidth, the greater the distortion, and the greater the intersymbol interference. The difference between the binary 0 and 1 levels is less, and the chance is greater for the receiver to misinterpret the level and create a bit of error.
Note further in Fig. 22-25(c) that the amplitudes of some of the traces are different from others. This is caused by noise varying the amplitude of the signal. The noise can further confuse the receiver, thus producing bit errors. The amount of voltage between the lowest of the upper patterns and the highest of the lower patterns as shown is called the noise margin. The smaller this value, the greater the noise and the greater the bit error rate. Noise margin is sometimes expressed as a percentage based upon the ratio of the noise margin level in Fig. 22-25(c) to the maximum peak-to-peak value of eye A.
The eye diagram is not a precise measurement method. But it is an excellent way to get a quick qualitative check of the signal. The eye pattern tells at a glance the degree of bandwidth limitation, signal distortion, jitter, and noise margin.
A pattern generator is a device that produces fixed binary bit patterns in serial form to use as test signals in data communication systems. The pattern generator may generate a repeating ASCII code or any desired stream of 1s and 0s. Pattern generators are used to replace the actual source of data such as the computer. Their output patterns can be changed to standard codes or messages or maybe programmable to some desired sequence. A pattern generator may be implemented in software at the sending computer.
Bit Error Rate Tests
At the other end of the link, the pattern generator sequence is detected and compared to the known actual pattern or message sent. Any errors in the comparison indicate errors. The instrument that detects the pulse’s pattern and compares it is called a bit error rate (BER) analyzer. It compares on a bit-by-bit basis the transmitted and received data to point out every bit error made. It keeps track of the total number of bits sent and the number of errors that occur and then computes the BER by dividing the number of errors by the number of bits sent. The BER tester must of course know the exact pattern or message sent by the pattern generator.
TDR Tests with an Analyzer
Special data communication test instruments are also available. A popular instrument is the TDR tester, also known as a cable analyzer or LAN meter. This instrument, often handheld, is connected to coaxial or twisted-pair cable and is able to make tests and measurements, some of which are
- Tests for open or short circuits and impedance anomalies on coaxial or twisted-pair cables.
- Measurements of cable length, capacitance, and loop resistance.
- Measurements of cable attenuation.
- Tests for cable miswiring such as so-called split pairs.
A split pair is a wiring error often made when a cable contains multiple twisted-pair lines. One of the wires from one twisted pair is wrongly paired with one wire from another twisted pair.
Many tests are based upon what is called time-domain reflectometry (TDR). The TDR technique can be used on any cable or transmission line, such as antenna lines or LAN cables, to determine SWR, short and open circuits, and characteristic impedance mismatches between cable and load. It can even determine the distance to the short or open circuit or to any other glitch anywhere along the line. TDR testing is based upon the presence of standing waves on the line if impedances are not matched.
The basic TDR process is to apply a rectangular pulse to the cable input and to monitor the signal at the input. The test setup is shown in Fig. 22-26. If the load impedance is matched, the pulse will be absorbed by the load and no reflections will occur. However, if there is a short or open circuit or impedance mismatch, a pulse will be reflected from the point of the mismatch.
The most sophisticated data communications test equipment is the protocol analyzer. Its purpose is to capture and analyze the data transmitted in a particular system. Most data communication systems transmit data in frames or packets that include preamble information such as sync bits or frames, the start of header codes, addresses of source and destination, and a finite block of data, followed by error detection codes. A protocol analyzer can capture these frames, analyze them, and tell you whether the system is operating properly. The analyzer will determine the specific protocol being captured and then indicate whether the data transmitted is following the protocol or whether there are errors in transmission or in formatting the frame.
Protocol analyzers contain microcomputers programmed to recognize a wide range of data communication protocols such as Bisync, SDLC, HDLC, Ethernet, SONET, OTN, and other network protocols. These instruments normally cost tens of thousands of dollars and may, in fact, b primarily a computer containing the stored protocols and software that read and store and then compare and analyze the received data so that it can report any differences or errors. Most protocol analyzers have a video display.
Special Test Sets
As communications equipment, especially wireless, has become more complex, it has been necessary to develop special test systems for specific protocols. For example, special test sets have been developed for GSM, CDMA, and LTE cell phones, Wi-Fi, and Bluetooth. These test sets combine multiple instruments into a common enclosure along with a computer for control. The instruments include signal generators (synthesizers) operating on the desired frequency bands along with the proper modulation and protocol for a given standard. This part of the test set permits testing of the receiver. Another section will include a calibrated receiver set up to receive any transmitted signal. An internal computer is programmed to conduct a precise sequence of tests and then to record and analyze the data. Test sets automate the testing procedure and identify those units (cell phones, etc.) that pass or fail the tests. These automated systems usually include internal spectrum analyzers for the display of results.
Fiber-Optic Test Equipment and Measurements
A variety of special instruments are available for testing and measuring fiber-optic systems. The most widely used fiber-optic instruments are the automatic splicer and the optical time-domain reflectometer (OTDR).
Splicing fiber-optic cable is a common occurrence in installing and maintaining fiber-optic systems. This operation can be accomplished with hand tools especially made for cutting, polishing, and splicing the cable. However, as the cable thickness has gotten finer, hand splicing has become more difficult than ever. It is very difficult to align the two cable ends perfectly before the splice is made.
To overcome this problem, a special splicer has been developed by several manufacturers. It provides a way to automatically align the cable ends and splice them. The two cables to be spliced are stripped and cleaved by hand and then placed in the unit.
A special mechanism holds the two cable ends close together. Then an optical system with a light source, lenses, and light sensors detects the physical alignment of the two cables, and a servo feedback mechanism drives a motor so that the two cable ends are perfectly centered on each other. An optical viewing screen is provided so that the operator can view the alignment from two directions at 90° to each other.
Once the alignment is perfect, the splicer is activated. The splicer is a pair of probes centered over the junction of the two cable ends. Pressing the “splice” button causes the probes to generate an electric arc hot enough to fuse the two glass cable ends together. The automatic splicer is very expensive, but it must be used because it is not possible for human beings to make good splices visually and by hand. Handmade splices have high attenuation, whereas minimum attenuation is best achieved with the automatic splicer.
Optical Time-Domain Reflectometer
Another essential instrument for fiber-optic work is the optical TDR or OTDR. It is an oscilloscope-like device with an LCD display and a built-in microcomputer.
The OTDR works as a standard TDR in that it generates a pulse, in this case, a light pulse, and sends it down a cable to be tested. If there is a break or defect, there is a light reflection, just as there is a reflection on an electric transmission line. The reflection is detected. Internal circuitry measures the time between the transmitted and reflected pulses so that the location of the break or other fault can be calculated and displayed.
The OTDR also detects splices, connectors, and other anomalies such as dents in the cable. The attenuation of each of these irregularities can be determined and displayed.
Optical Signal Analyzer
A newer breed of versatile optical test instrument makes multiple measurements. In addition to providing the OTDR measurement, this unit is a sampling oscilloscope capable of displaying signals of more than 10 Gbps. It can also be used to show eye patterns, optical output power, and jitter.
Jitter is a type of noise that shows up as a time variation of the leading and trailing edges of a binary signal. See Fig. 22-27. It appears as a kind of phase or frequency shift in which the time period for 1 bit is lengthened or shortened at a rapid rate. Jitter shows up on an oscilloscope as a blurring of the 0-to-1 and 1-to-0 transitions of a binary signal.
It is not much of a problem at lower data rates, but as data rates go above 1 GHz, jitter becomes more prevalent. On fi ber-optic data systems, jitter is a major problem. Furthermore, jitter is diffi cult to measure. Most optical fi ber communication networks such as SONET have jitter specifi cations that must be met. Therefore, some accurate method of measurement is needed. Some of the newer optical signal analyzers have a jitter measurement capability. Jitter is usually expressed as a percentage of the unit interval (UI).
The UI is the bit time of the data signal. For example, a jitter measurement maybe
0.01 UI or 10 mUI.
Infrared Wireless | Ultra wideband Wireless | Multiband OFDM UWB ( Communication Tests | Fiber-Optic | Antenna and Transmission Line Tests )
Wireless LAN | IEEE 802.11b | PANs | Bluetooth | ZigBee | WiMAX ( Communication Tests | Fiber-Optic | Antenna and Transmission Line Tests )
LTE Cellular Systems | 4G | 5G | Data Rate | TD-LTE | Voice over LTE ( Communication Tests | Fiber-Optic | Antenna and Transmission Line Tests )
Cellular Telephone | Duplexing | 2G & 3G | EDGE | GPRS | WCDMA ( Communication Tests | Fiber-Optic | Antenna and Transmission Line Tests )
WDM ( Wavelength-Division Multiplexing ) | Passive Optical | Transport ( Communication Tests | Fiber-Optic | Antenna and Transmission Line Tests )