Communication Test Equipment | RF Voltmeters | Power Meters

Communication Test Equipment

This section gives a broad overview of the many different types of test instruments available for use with communication equipment. It is assumed that you already know basic test and measurement techniques used with conventional low-frequency test equipment, such as multimeters, signal generators, and oscilloscopes. In your communication work, you will continue to use standard oscilloscopes and multimeters for measuring voltages, currents, and resistance. Coverage of these basic test instruments will not be repeated here. We will, however, draw upon your knowledge of the principles of those instruments as they apply to the test equipment discussed in this section.

Voltage Measurements

The most common measurement obtained for most electronic equipment is voltage. This is particularly true for dc and low-frequency ac applications. In RF applications, voltage measurements may be important under some conditions, but power measurements are far
more common at higher frequencies, particularly microwaves. In testing and troubleshooting communication equipment, you will still use a dc voltmeter to check power supplies and other dc conditions. There are also occasions when the measurement of RF, that is, ca, voltage must be made.

There are two basic ways to make ac voltage measurements in electronic equipment. One is to use an ac voltmeter. Most conventional voltmeters can measure ac voltages from a few millivolts to several hundred volts. Typical bench or portable ac multimeters are restricted in their frequency range to a maximum of several thousand kilohertz.

Higher-frequency ac voltmeters are available for measuring audio voltages up to several hundred thousand kilohertz. For higher frequencies, special RF voltmeters must be used. The second method is to use an oscilloscope as described below.

RF Voltmeters

An RF voltmeter is a special piece of test equipment designed to measure the voltage of high-frequency signals. Typical units are available for making measurements up to 10 MHz. Special units capable of measuring voltages from microvolts to hundreds of volts at frequencies up to 1 to 2 GHz are also available.

RF voltmeters are made to measure sine wave voltages, with the readout given in root mean square (RMS). Most RF voltmeters are of the analog variety with a moving pointer on a background scale. Measurement accuracy is within the 1 to 5 percent range depending upon the specific instrument. Accuracy is usually quoted as a percentage of the reading or as a percentage of the full-scale value of the voltage range selected. RF voltmeters with digital readout probes are also available with somewhat improved measurement accuracy.

RF Probes

One way to measure RF voltage is to use an RF probe with a standard dc multimeter. RF probes are sometimes referred to as detector probes. An RF probe is basically a rectifier (germanium or hot carrier) with a filter capacitor that stores the peak value of the sine wave RF voltage. The external dc voltmeter reads the capacitor voltage. The result is a peak value that can easily be converted to root mean square by multiplying it by 0.707.

Most RF probes are good for RF voltage measurements to about 250 MHz. The accuracy is about 5 percent, but that is usually very good for RF measurements. Oscilloscopes. Two basic types of oscilloscopes are used in RF measurements: the analog oscilloscope and the digital storage oscilloscope (DSO).

Analog oscilloscopes amplify the signal to be measured and display it on the face of a CRT at a specifi c sweep rate. They are available for displaying and measuring RF voltages to about 500 MHz. As a rule, an analog oscilloscope should have a bandwidth of three to five or more times the highest-frequency component (a carrier, a harmonic, or a sideband) to be displayed.

Digital storage oscilloscopes, also known as digital, or sampling, oscilloscopes, are growing in popularity and rapidly replacing analog oscilloscopes. DSOs use high-speed sampling or A/D techniques to convert the signal to be measured to a series of digital words that are stored in internal memory. Sampling rates vary depending upon the oscilloscope but can range from approximately 20 million samples per second to more than 50 billion samples per second. Each measurement sample is usually converted to an 8- or 10-bit parallel binary number that is stored in internal memory. Oscilloscopes with approximately 1 Gbyte of memory or more are available depending upon the product. Today, more than 50 percent of oscilloscope sales are of the digital sampling type.

DSOs are very popular for high-frequency measurements because they provide the means to display signals with frequencies up to about 70 GHz. This means that complex modulated microwave signals can be readily viewed, measured, and analyzed.

Power Meters

As indicated earlier, it is far more common to measure RF power than it is to measure RF voltage or current. This is particularly true in testing and adjusting transmitters that typically develop significant output power. One of the most commonly used RF test instruments is the power meter.

Power meters come in a variety of sizes and configurations. One of the most popular is a small in-line power meter designed to be inserted into the coaxial cable between a transmitter and an antenna. The meter is used to measure the transmitter output power supply to the antenna. A short coaxial cable connects the transmitter output to the power meter, and the output of the power meter is connected to the antenna or dummy load. A more sophisticated power meter is the bench unit designed for laboratory or production line testing. The output of the transmitter or other device whose power is to be measured is connected by a short coaxial cable to the power meter input.

Power meters may have either an analog readout meter or a digital display. The dial or display is calibrated in milliwatts, watts, or kilowatts. The dial can also be calibrated in terms of dBm. This is the decibel power reference to 1 milliwatt (mW). In the smaller, handheld type of power meter, an SWR measurement capability is usually included.

The operation of a power meter is generally based on converting signal power to heat. Whenever current flows through a resistance, power is dissipated in the form of heat. If the heat can be accurately measured, it can usually be converted to an electric signal that can be displayed on a meter.

Example 22.1
An RF voltmeter with a detector probe is used to measure the voltage across a 75-V resistive load. The frequency is 137.5 MHz. The measured voltage is 8 V. What power is dissipated in the load?

Communication Test Equipment | RF Voltmeters |  Power Meters

Power can also be measured indirectly. If the load impedance is known and resistive, you can measure the voltage across the load and then calculate the power with the formula P = V2/R.

Power Measurement Circuits

Relatively simple circuits can be used to measure power in transmitters and RF power circuits. An example is the mono match circuit shown in Fig. 22-1. It uses a 50-V transmission line made with a microstrip on a small printed circuit board (PCB). The center conductor is the segment labeled 2 in the schematic. On each side of the center, the conductor is narrower pickup loops labeled 1 and 3. An RF voltage proportional to the forward and reverse (reflected) power is produced as the result of capacitive and inductive coupling with the center conductor. The voltage in segment 3 represents the forward power. It is rectified by diode D1 and filtered by C1 into a proportional dc voltage. This voltage is applied through multiplier resistors R3 and R5 to a meter whose scale is calibrated in watts of power. Note the 50-ohm resistors that terminate the pickup loops for impedance matching.

The voltage induced in pickup loop 1 is proportional to the reflected power. It is rectified by D2 and filtered into a proportional direct current by C2. A switch is used to select the display of either the forward or the reflected power. Resistor R5 is used to calibrate the meter circuit, using an accurate power meter as a standard.

Another popular power measurement circuit is the directional coupler shown in Fig. 22-2. A short piece of 50-V coaxial cable serves as the single-turn primary winding on a transformer made with a toroid core and a secondary winding of many turns of fi ne wire. When RF power is passed through the coaxial section, a stepped-up voltage is induced into the secondary winding. Equal-value resistors R1 and R2 divide the voltage equally between two diode rectifi er circuits made up of D1 and D2 and the related components.

Communication Test Equipment | RF Voltmeters |  Power Meters

A voltage divider made up of C1, C2, C3, and L1 samples the voltage at the output of the circuit. This voltage is applied to both diode rectifiers along with the voltages from the transformer secondary. When these voltages are combined, the rectified outputs are proportional to the forward and reflected voltages on the line. Low-pass filters R3-C6 and R4-C7 smooth the rectified signals into direct current. A meter arrangement like that in Fig. 22-1 is used to display either forward or reflected power.

Both circuits can be designed to handle power levels from a few milliwatts to man kilowatts. When low-level signals are used, the diodes must be of the germanium or hot carrier type with low bias threshold voltages (0.2 to 0.4 V) to provide sufficient accuracy of measurement. With careful design and adjustment, these circuits can give an accuracy of 90 percent or better. Because the circuits are so small, they are often built into the transmitter or other circuit along with the meter and switch.

Example 22.2
If the forward power and reflected power in a circuit are known, the SWR can be calculated. If the forward power is 380 W and the reflected power is 40 W, what is the SWR?

Dummy Loads

A dummy load is a resistor that is connected to the transmission line in place of the antenna to absorb the transmitter output power. When power is measured or other transmitter tests are done, it is usually desirable to disconnect the antenna so that the transmitter does not radiate and interfere with other stations on the same frequency. In addition, it is best that no radiation be released if the transmitter has a problem or does not meet frequency or emission standards. The dummy load meets this requirement. The dummy load may be connected directly to the transmitter coaxial output connector, or it may be connected to it by a short piece of coaxial cable.

The load is a resistor whose value is equal to the output impedance of the transmitter and that has a sufficient power rating. For example, a CB transmitter has an output impedance of 50 V and a power rating of about 4 W. The resistor dummy load must be capable of dissipating that amount of power or more. For example, you could use three 150-ohms, 2-W resistors in parallel to give a load of 150/3, or 50 ohm and 3 x 2, or 6, W. Standard composition carbon resistors can be used. The tolerance is not critical, and resistors with 5 or 10 percent tolerance will work well.

For low-power transmitters such as CBS and amateur radios, an incandescent light bulb makes a reasonably good load. A type 47 pilot light is widely used for transmitter outputs of several watts. An ordinary lightbulb of 75 or 100 W, or higher, can also be used for higher-power transmitters.

The best dummy load is a commercial unit designed for that purpose. These units are usually designed for some upper power limit such as 200 W or 1 kW. The higher-power units are made with a resistor immersed in oil to improve their heat dissipation capability without burning up. A typical unit is a resistor installed in a 1-gal can filled with insulating oil. A coaxial connector on top is used to attach the unit to the transmitter. Other units are mounted in an aluminum housing with heat fins to improve heat dissipation. The resistors are noncritical, but they must be noninductive. The resistor should be as close to pure resistance at the operating frequency as possible.

Standing Wave Ratio Meters

The SWR can be determined by calculation if the forward and reflected power values are known. Some SWR meters use the mono match or directional coupler circuits described above and then implement the SWR calculation given in Example 22-2 with op-amps and analog multiplier ICs. But you can also determine SWR directly.

Communication Test Equipment | RF Voltmeters |  Power Meters

Fig. 22-3(a) shows a bridge SWR meter. A bridge is formed of precision, noninductive resistors, and antenna radiation resistance. In some SWR meters, resistors are replaced with a capacitive voltage divider. The meter is connected to measure the unbalance of the bridge. The transmitter is the ac power source.

Fig. 22-3(b) shows the circuit rearranged so that the meter and one side of the bridge are grounded, thereby creating a better match to unbalanced coaxial transmission lines. Note the use of coaxial connectors for the transmitter input and the antenna and transmission line.

The meter is a basic dc microammeter. Diode D1 rectifies the RF signal into a proportional direct current. If the radiation resistance of the antenna is 50ohms, the bridge will be balanced and the meter reading will be zero. The meter is calibrated to display an SWR of 1. If the antenna radiation resistance is not 50 ohm, the bridge will be unbalanced and the meter will display a reading that is proportional to the degree of unbalance. The meter is calibrated in SWR values.

Signal Generators

A signal generator is one of the most often needed pieces of equipment in communication equipment servicing. As its name implies, a signal generator is a device that produces an output signal of a specific shape at a specific frequency and, in communication applications, usually with some form of modulation. The heart of all signal generators a variable-frequency oscillator that generates a signal that is usually a sine wave. Audio-frequency sine waves are required for the testing of audio circuits in communication equipment, and sine waves in the RF range from approximately 500 kHz to 30 GHz are required to test all types of RF amplifiers, filters, and other circuits. This section provides a general overview of the most common types of signal generators used in communication testing and servicing.

Function Generators

A function generator is a signal generator designed to generate sine waves, square waves, and triangular waves over a frequency range of approximately 0.001 Hz to about 3 MHz. By changing capacitor values and varying the charging current with a variable resistance, a wide range of frequencies can be developed. The sine, square, and triangular waves are also available simultaneously at individual output jacks. A function generator is one of the most fl exible signal generators available. It covers all the frequencies needed for audio testing and provides signals in the low-RF range.

The precision of the frequency setting is accurate for most testing purposes. A frequency counter can be used for precise frequency measurement if needed.

The output impedance of a function generator is typically 50 ohm. The output jacks are BNC connectors, which are used with 50- or 75-ohm coaxial cable. The output amplitude is continuously adjustable with a potentiometer. Some function generators include a switched resistive attenuator that allows the output voltage to be reduced to the millivolt and microvolt levels.

Because of the very low cost and flexibility of a function generator, it is the most popular bench instrument in use for general testing of radio amplifiers, filters, and low-frequency RF circuits. The square wave output signals also make it useful in testing digital circuits.

RF Signal Generators

Two basic types of RF signal generators are in use. The fi rst is a simple, inexpensive type that uses a variable-frequency oscillator to generate RF signals in the 100-kHz to the 500-MHz range. The second type is frequency- synthesized.

These simple RF signal generators contain an output level control that can be used to adjust the signal to the desired level, from a few volts down to several millivolts. Some units contain built-in resistive step attenuators to reduce the signal level even further. The more sophisticated generators have built-in level control or automatic gain control (AGC). This ensures that the output signal remains constant while it is tuned over a broad frequency range. Most low-cost signal generators allow the RF signal is generated to be amplitude-modulated. Normally, a built-in audio oscillator with a fixed frequency somewhere in the 400- to the 1000-Hz range is included. A modulation level control is provided to adjust the modulation from 0 to 100 percent. Some RF generators have a built-in frequency modulator.

Such low-cost signal generators are useful in testing and troubleshooting communication receivers. They can provide an RF signal at the signal frequency for injection into the antenna terminals of the receiver. The generator can produce signals that can substitute for local oscillators or can be set to the intermediate frequencies for testing IF amplifi ers. The output frequency is usually set by a large calibrated dial. The precision of calibration is only a few percent, but more precise settings can be obtained by monitoring the signal output on a frequency counter.

When any type of generator based on LC or RC oscillators is being used, it is best to turn the generator on and let it warm up for several hours before it is used. When a generator is first turned on, its output frequency will drift because of changes in capacitance, inductance, and resistance values. Once the circuit has warmed up to its operating temperature, these variations cease or drop to a negligible amount.

Newer generators use frequency synthesis techniques. These generators include one or more mixer circuits that allow the generator to cover an extremely wide range of frequencies. The great value of a frequency-synthesized signal generator is its excellent frequency stability and precision of frequency setting.

Most frequency-synthesized signal generators use a front panel keyboard. The desired frequency is entered with the keypad and displayed on a digital readout. As with other signal generators, the output level is fully variable. The output impedance is typically 50 ohms, with both BNC- and N-type coaxial connectors being required.

Frequency-synthesized generators are available for frequencies into the 20- to the 70-GHz range. Such generators are extremely expensive, but they may be required if precision measurement and testing are necessary.

Sweep Generators

A sweep generator is a signal generator whose output frequency can be linearly varied over some specific range. Sweep generators have oscillators that can be frequency-modulated, where a linear sawtooth voltage is used as a modulating signal. The resulting output waveform is a constant-amplitude sine wave whose frequency increases from some lower limit to some upper limit (see Fig. 22-4).

Communication Test Equipment | RF Voltmeters |  Power Meters

Sweep generators are normally used to provide a means of automatically varying the frequency over a narrow range to plot the frequency response of a filter or amplifi er or to show the bandpass response curve of the tuned circuits in equipment such as as a receiver. The sweep generator is connected to the input of the circuit, and the upper and lower frequencies are determined by adjustments on the generator. The generator then automatically sweeps over the desired frequency range.

At the same time, the output of the circuit being tested is monitored. The amplitude of the output will vary according to the frequency, depending on the type of circuit being tested. The output of the circuit is connected to an RF detector probe. The resulting signal is the envelope of the RF signal as determined by the output variation of the circuit being tested. The signal displayed on the oscilloscope is an amplitude plot of the frequency response curve. The horizontal axis represents the frequency being varied with time, and the output represents the amplitude of the circuit output at each of the frequencies.

Fig. 22-5 shows the general test setup. The linear sweep from the sweep generator is used in place of the oscilloscope’s internal sweep so that the displayed response curve is perfectly synchronized with the generator.

Communication Test Equipment | RF Voltmeters |  Power Meters

Most sweep generators also have marker capability; i.e., one or more reference oscillators are included to provide frequency markers at selected points so that the response curve can be actively interpreted. Marker increments maybe 100 kHz or 1 MHz. They are added to (linearly mixed with) the output of the RF detector probe, and the composite signal is amplified and sent to the vertical input of the oscilloscope. Sweep generators can save a considerable amount of time in testing and adjusting complex tuned circuits in receivers and other equipment.

Most function generators have built-in sweep capability. If sweep capability is not built in, often an input jack is provided so that an external sawtooth wave can be connected to the generator for sweep purposes.

Arbitrary Waveform Generators

A newer type of signal generator is the arbitrary waveform generator. It uses digital techniques to generate almost any waveform. An arbitrary waveform generator stores binary values of the desired waveform in memory. These binary words are fed sequentially to a digital-to-analog converter that produces a stepped approximation of the desired wave. Most arbitrary waveform generators come with preprogrammed standard waves such as sine, rectangular, sawtooth, and triangular waves, and amplitude modulation. These generators are set up so that you can program a waveform. The arbitrary waveform generator provides a fast and easy way to generate almost any signal shape. Because digital sampling techniques are used, the upper-frequency limit of the output is usually below 10 GHz.

Frequency Counters

One of the most widely used communication test instruments is the frequency counter. It measures the frequency of transmitters, local and carrier oscillators, frequency synthesizers, and any other signal-generating circuit or equipment. It is imperative that the frequency counter operates on its assigned frequency to ensure compliance with rules and regulations and to avoid interference with other services.

A frequency counter displays the frequency of a signal on a decimal readout. Counters are available as bench instruments or portable battery-powered units. A block diagram of a frequency counter is shown in Fig. 22-6. Almost all digital counters are made up of six basic components: input circuit, gate, decimal counter, display, control circuits, and time base. In various combinations, these circuits permit the counter to make time and frequency measurements.

Frequency Measurement

Frequency is a measure of the number of events or cycles of a signal that occur in a given time. The usual unit of frequency measurement is hertz (Hz), or cycles per second. The time base generates a very precise signal that is used to open, or enable, the main gate for an accurate period of time to allow the input pulses to pass through to the counter. The time base accuracy is the most critical specification of the counter. The counter accumulates the number of input cycles that occur during that 1-s interval. The display then shows the frequency in cycles per second or hertz.

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The number of decade counters and display digits also determines the resolution of the frequency measurement. The greater the number of digits, the better the resolution will be. Most low-cost counters have at least five digits of the display. This provides reasonably good resolution on most frequency measurements. For very high-frequency measurements, the resolution is more limited. However, good resolution can still be obtained with a minimum number of digits by an optimum selection of the time base frequency.

In most counters that have a selectable time base, the position of the display decimal point is automatically adjusted as the time base signal is adjusted. In this way, the display always shows the frequency in units of hertz, kilohertz, or megahertz. Some of the more sophisticated counters have an automatic time base selection feature called auto-ranging.

Special auto-ranging circuitry in the counter automatically selects the best time base frequency for maximum measurement resolution without over-ranging. Overranging is the condition that occurs when the count capability of the counter is exceeded during the count interval. The number of counters and display digits determines the count capability and thus the overrange point for a given time base.

Prescaling

All the techniques for measuring high frequencies involve a process that converts the high frequency to a proportional lower frequency that can be measured with conventional counting circuitry. This translation of the high frequency to the lower frequency is called down conversion.

Prescaling is a down-conversion technique that involves the division of the input frequency by a factor that puts the resulting signal into the normal frequency range of the counter. It is important to realize that although prescaling permits the measurement of higher frequencies, it is not without its disadvantages, one of which is loss of resolution. One digit of resolution is lost for each decade of prescaling incorporated.

The prescaling technique for extending the frequency-measuring capability of a counter is widely used. It is simple to implement with modern, high-speed ICs. It is also the most economical method of extending the counting range. Prescalers can be built into the counter and switched in when necessary. Alternatively, external prescalers, which are widely available for low-cost counters, can be used. Most prescalers operate in the range of 200 MHz to 20 GHz. For frequencies beyond 20 GHz, more sophisticated down-conversion techniques must be used.

Spectrum Analyzers

The spectrum analyzer is one of the most useful and popular communication test instruments. Its basic function is to display received signals in the frequency domain. Oscilloscopes are used to display signals in the time domain. The sweep circuits in the oscilloscope deflect the electron beam in the CRT across the screen horizontally. This represents units of time. The input signal to be displayed is applied to deflect the electron beam vertically. Thus, electronic signals that are voltages occurring with respect to time are displayed on the oscilloscope screen.

The spectrum analyzer combines the display of an oscilloscope with circuits that convert the signal to the individual frequency components dictated by Fourier analysis of the signal. Signals applied to the input of the spectrum analyzer are shown as vertical lines or narrow pulses at their frequency of operation.

Fig. 22-7 shows the display of a spectrum analyzer. The horizontal display is calibrated in frequency units, and the vertical part of the display is calibrated in voltage, power, or decibels. The spectrum analyzer display shows three signals at frequencies of 154.7, 157.8, and 160.2 MHz. The vertical height represents the relative strength of the amplitude of each signal. Each signal might represent the carrier of a radio transmitter.

Superheterodynes

Perhaps the most widely used RF spectrum analyzer is the superheterodyne type (see Fig. 22-8). It consists of a broadband front end, a mixer, and a tunable local oscillator. The frequency range of the input is restricted to some upper limit by a lowpass filter in the input. The mixer output is the difference between the input signal frequency component and the local-oscillator frequency. This is the intermediate frequency (IF). As the local-oscillator frequency increases, the output of the mixer stays at the IF.

Each frequency component of the input signal is converted to the IF value by the varying local-oscillator signal. If the frequency components are very close to one another and the bandwidth of the IF bandpass filter (BPF) is broad, the display will be just one broad pulse. Narrowing the bandwidth of the IF BPF allows more closely spaced components to be detected. Most spectrum analyzers have several switchable selectivity ranges for the IF.

Communication Test Equipment | RF Voltmeters |  Power Meters
Communication Test Equipment | RF Voltmeters |  Power Meters

Spectrum analyzers are available in many configurations with different specifications, and they are designed to display signals from approximately 100 kHz to approximately 70 GHz. Most RF and microwave spectrum analyzers are superheterodyne. Spectrum analyzers are usually calibrated to provide relatively good measurement accuracy of the signal. Most signals are displayed as power or decibel measurements, although some analyzers provide for voltage level displays. The input is usually applied through a 50-V coaxial cable and connector.

FFT Spectrum Analyzers

The fast Fourier transform (FFT) method of spectrum analysis relies on the FFT mathematical analysis. FFT spectrum analyzers give a high-resolution display and are generally superior to all other types of spectrum analyzers. However, the upper frequency of the input signal is limited to frequencies in the tens of megahertz range.

In addition to measuring the spectrum of a signal, spectrum analyzers are useful in detecting harmonics and other spurious signals generated unintentionally. Spectrum analyzers can be used to display the relative signal-to-noise ratio, and they are ideal for analyzing modulation components and displaying the harmonic spectrum of a rectangular pulse train.

Despite their extremely high prices (usually $10,000 to $50,000), spectrum analyzers are widely used. Many critical testing and measurement applications demand their use, especially in developing new RF equipment and in making final tests and measurements of manufactured units. Spectrum analyzers are also used in the field for testing cable TV systems, cellular telephone systems, fiber-optic networks, and other complex communication systems.

Network Analyzers

A network analyzer is a test instrument designed to analyze linear circuits, especially RF circuits. It is a combination instrument that contains a wide-range sweep sine wave generator and a CRT output that displays not only frequency plots as does a spectrum analyzer but also plots of phase shift versus frequency.

Network analyzers are used by engineers to determine the specific performance characteristics of a circuit they are designing, such as a filter, an amplifier, or a mixer. They are also useful in analyzing transmission lines and even individual components. The network analyzer applies a swept-frequency sine wave and measures the circuit output. The resulting measurement data is then used to produce an output display such as amplitude versus frequency plot, a phase shift versus frequency, or even a plot of complex impedance values on a Smith chart display.

Network analyzers completely describe the performance or characterization of a circuit. This type of information is useful not only to engineers creating the circuits but also to those in manufacturing who have to produce and test the circuit. Despite their very high cost, these instruments are widely used because of the valuable information they provide and the massive amount of design and test time they save.

Vector Signal Analyzers and Generators

As wireless signals have become so complex and varied over the years, it has been necessary to create test instruments that can analyze and generate these signals. Vector signal analyzers and generators are those instruments. They can create or analyze OFDM, QPSK, QAM, FSK, WCDMA, and other signals that are common to widely used wireless technologies like Wi-Fi, WiMAX, 3G WCDMA, HSPA, and LTE, as well as unique radar and satellite standards.

Vector Signal Generator

A vector signal generator (VSG) is a variation of an arbitrary waveform generator. A simplified example is shown in Fig. 22-9. The complex signal, such as OFDM, is generated in the DSP circuitry, which produces the in-phase (I) and quadrature (Q) components. The I and Q signals then go to DACs that produce the equivalent analog signals. These are then sent to a quadrature modulator along with a final carrier signal from a local oscillator (LO) PLL. The modulator output is usually amplified and applied to an attenuator that allows the signal level to be set. In some instruments, the signal may be generated at a lower intermediate frequency (IF) and then mixed by an upconverter to the desired higher output frequency.

Most VSGs are provided with preprogrammed signal formats for the most common wireless standards, such as 802.11 or LTE. VSGs may also be user-programmed for special formats or variations of existing standards. A commercial VSG is shown in Fig. 22-10.

Vector Signal Analyzer

A vector signal analyzer (VSA) is an instrument designed to capture and provide a way to disassemble a signal to see its detail. A VSA is similar to a spectrum analyzer but with more output options. The VSA is useful in design to ensure that new circuitry meets the desired standards. It is also useful in troubleshooting to locate problems and standards variations. A VSA can usually display a frequency spectrum, a modulation constellation diagram, a digital code, or even standard analog time-domain signals like an oscilloscope.

Fig. 22-11 shows a generic block diagram of a VSA. The RF signal to be analyzed is fed to the amplifier and/or an attenuator that sets the level. The signal is applied to a mixer that downconverts the signal to an IF of a specific bandwidth. A bandpass filter sets the bandwidth, and a low-pass filter serves as an anti-aliasing filter for the analog-to-digital converter (ADC). The ADC produces a time-sampled version of the signal that will be used by the remainder of the VGA to do its analysis. The sampled data is stored and further groomed for additional processing.

The digitized input signal then goes to a quadrature demodulator along with a local oscillator signal where the I and Q components of the signal are recovered. The I and Q signals are stored and corrected as needed, and then sent to one of several processors, depending upon the selected output. If a frequency spectrum output is desired, the signals go to a fast Fourier transform (FFT) processor, where the frequency components of the signal are determined and then displayed in traditional frequency spectrum format.

Another option is demodulation that recovers the specific data in the form of a constellation diagram display. This is especially useful for PSK and QAM signals. This type of output lets you measure the error vector magnitude (EVM) of the signal. For CDMA analysis, the processor can generate and display the unique coding associated with the signal. Alternately a standard time-domain output may be selected. Some VSAs can also accommodate MIMO signals.

Fig. 22-12 shows some typical outputs. Fig. 22-13 shows a commercial VSA. Some manufacturers package the VSA and VGA modules together to form a vector signal transceiver.

With most wireless equipment using OFDM and other advanced forms of signal technology today, the VSA and VSG have become the test instruments of choice for design, manufacturing tests, and field troubleshooting.

Field Strength Meters

One of the least expensive pieces of RF test equipment is the field strength meter (FSM), a portable device used for detecting the presence of RF signals near an antenna. The FSM is a sensitive detector for RF energy being radiated by a transmitter into an antenna.

It provides a relative indication of the strength of the electromagnetic waves reaching the meter.
The field strength meter is a vertical whip antenna, usually of the telescoping type, connected to a simple diode detector. The diode detector is exactly like the circuit of a simple crystal radio or a detector probe, as described earlier, but without any tuned circuits so that the unit will pick up signals on any frequency.

The field strength meter does not give an accurate measurement of signal strength. In fact, its only purpose is to detect the presence of a nearby signal (within about 100 ft or less). Its purpose is to determine whether a given transmitter and antenna system are working. The closer the meter is moved to the transmitter and antenna, the higher the signal level.

A useful function of the meter is the determination of the radiation pattern of an antenna. The field strength meter is adjusted to give the maximum reading in the direction of the most radiation from the antenna. The meter is moved in a constant-radius circle around the antenna for 360°. Every 5° or 10°, a field strength reading is taken from the meter. The resulting set of readings can be plotted on polar graph paper to reveal the horizontal radiation pattern of the antenna.

Other types of field strength meters are available. A simple meter may be built to incorporate a resonant circuit to tune the input to a specific transmitter frequency. This makes the meter more sensitive. Some meters have a built-in amplifier to make the meter even more sensitive and useful at greater distances from the antenna.

An absolute (rather than relative) field strength meter is available for accurate measurements of signal strength. The strength of the radiated signal is usually measured in microvolts per meter (μV/m). This is the amount of voltage the signal will induce into an antenna that is 1 m long. An absolute field strength meter is calibrated in units of microvolts per meter. Highly accurate signal measurements can be made.

Other Test Instruments

There are hundreds of types of communication test instruments, most of which are very specialized. The ones described previously in this chapter, plus those listed in the above table, are the most common, but there are many others including the many special test instruments designed by equipment manufacturers for testing their production units or servicing customers’ equipment.

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