Basic Principles of Signal Reproduction
Signal Reproduction: A communication receiver must be able to identify and select the desired signal from thousands of others present in the frequency spectrum (selectivity) and to provide sufficient amplification to recover the modulating signal (sensitivity). A receiver with good selectivity will isolate the desired signal in the RF spectrum and eliminate or at least greatly attenuate all other signals. A receiver with good sensitivity involves high circuit gain.
Selectivity in a receiver is obtained by using tuned circuits and/or filters. The LC tuned circuits provide initial selectivity; filters, which are used later in the process, provide additional selectivity.
Q and Bandwidth
Initial selectivity in a receiver is normally obtained by using LC tuned circuits. By carefully controlling the Q of the resonant circuit, you can set the desired selectivity. The optimum bandwidth is wide enough to pass the signal and its sidebands but also narrow enough to eliminate or greatly attenuate signals on adjacent frequencies.
As Fig. 9-1 shows, the rate of attenuation or roll-off of an LC tuned circuit is gradual. Adjacent signals will be attenuated but in some cases not enough to completely eliminate interference. Increasing the Q will further narrow the bandwidth and improve the steepness of attenuation, but narrowing the bandwidth in this way can be taken only so far. At some point, the circuit bandwidth may become so narrow that it starts to attenuate the sidebands, resulting in loss of information.
The ideal receiver selectivity curve would have perfectly vertical sides, as in Fig. 9-2(a). Such a curve cannot be obtained with tuned circuits. Improved selectivity is achieved by cascading tuned circuits or by using crystal, ceramic, or SAW filters. At lower frequencies, digital signal processing (DSP) can provide almost ideal response curves. All these methods are used in communication receivers.
The sides of a tuned circuit response curve are known as skirts. The steepness of the skirts, or the skirt selectivity, of a receiver, is expressed as the shape factor, the ratio of the 60-dB down bandwidth to the 6-dB down bandwidth. This is illustrated in Fig. 9-2(b). The bandwidth at the 60-dB down points is f4 – f3; the bandwidth of the 6-dB down points is f2 – f1. Thus the shape factor is ( f4 – f3)/( f2 – f1). Assume, for example, that the 60-dB bandwidth is 8 kHz and the 6-dB bandwidth is3 kHz. The shape factor is 8/3 = 2.67, or 2.67:1.
The lower the shape factor, the steeper the skirts and the better the selectivity. The ideal, shown in Fig. 9-2(a), is 1. Shape factors approaching 1 can be achieved with DSP filters.
A communication receiver’s sensitivity, or ability to pick up weak signals, is mainly a function of overall gain, the factor by which an input signal is multiplied to produce the output signal. In general, the higher the gain of a receiver, the better its sensitivity. The greater gain that a receiver has, the smaller the input signal necessary to produce a desired level of output. High gain in communication receivers is obtained by using multiple amplification stages.
Another factor that affects the sensitivity of a receiver is the signal-to-noise (S/N) ratio (SNR). Noise is the small random voltage variations from external sources and from noise variations generated within the receiver’s circuits. This noise can sometimes be so high (many microvolts) that it masks or obliterates the desired signal. Fig. 9-3 shows what a spectrum analyzer display would show as it monitored two input signals and the background noise.
The noise is small, but it has random voltage variations and frequency components that are spread over a wide spectrum. The large signal is well above the noise and so is easily recognized, amplified, and demodulated. The smaller signal is barely larger than the noise and so may not be successfully received.
One method of expressing the sensitivity of a receiver is to establish the minimum discernible signal (MDS). The MDS is the input signal level that is approximately equal to the average internally generated noise value. This noise value is called the noise floor of the receiver. MDS is the amount of signal that would produce the same audio power output as the noise floor signal. The MDS is usually expressed in dBm.
Another often-used measure of receiver sensitivity is microvolts or decibels above 1 mV and decibels above 1 mW (0 dBm).
There is no one fixed way to define sensitivity. For analog signals, the signal-to-noise ratio is the main consideration in analog signals. For digital signal transmission, the bit error rate (BER) is the main consideration. BER is the number of errors made in the transmission of many serial data bits. For example, one measure is that the sensitivity is such that the BER is 10-10 or 1-bit error in every 10 billion bits transmitted.
Several methods for stating and measuring sensitivity have been defined in various communications standards depending upon the type of modulation used and other factors.
For example, the sensitivity of a high-frequency communication receiver is usually expressed as the minimum amount of signal voltage input that will produce an output signal that is 10 dB higher than the receiver background noise. Some specifications state a 20-dB S/N ratio. A typical sensitivity figure might be 1-μV input.
The lower this figure, the better the sensitivity. Good communication receivers typically have a sensitivity of 0.2 to 1 μV. Consumer AM and FM receivers designed for receiving strong local stations have much lower sensitivity. Typical FM receivers have sensitivities of 5 to 10 μV; AM receivers can have sensitivities of 100 μV or higher. Common wireless transceiver sensitivities are in the -85 to -140 dBm range.
The Simplest Receiver Configuration
Fig. 9-4 shows the simplest radio receiver: a crystal set consisting of a tuned circuit, a diode (crystal) detector, and earphones. The tuned circuit provides the selectivity, the diode and C2 serve as an AM demodulator, and the earphones reproduce the recovered audio signal.
The crystal receiver in Fig. 9-4 does not provide the kind of selectivity and sensitivity necessary for modern communication. Only the strongest signals can produce an output, and selectivity is often insufficient to separate incoming signals. This receiver can only receive very strong local AM radio stations, and a very long antenna is needed. However, a demodulator like this is the basic circuit in any receiver. All other circuits in a receiver are designed to improve sensitivity and selectivity, so that the demodulator can perform better.
Super heterodyne Receivers
A sensitive and selective receiver can be made using only amplifiers, selective filters, and a demodulator. This is called a tuned radio frequency or TRF receiver. Early radios used this design. However, such a receiver does not usually deliver the kind of performance expected in modern communications applications. One type of receiver that can provide that performance is the superheterodyne receiver.
Super heterodyne receivers convert all incoming signals to a lower frequency, known as the intermediate frequency (IF), at which a single set of amplifiers and filters is used to provide a fixed level of sensitivity and selectivity. Most of the gain and selectivity in a super heterodyne receiver is obtained in the IF amplifiers. The key circuit is the mixer, which acts as a simple amplitude modulator to produce sum and difference frequencies. The incoming signal is mixed with a local oscillator signal to produce this conversion.
Fig. 9-5 shows a general block diagram of a super heterodyne receiver. In the following sections, the basic function of each circuit is examined. Although individual circuits are discussed in the following sections, keep in mind that today most receivers are made up of circuits fully integrated on a single chip of silicon or other semiconductor material. Such circuits cannot usually be changed or accessed.
The antenna picks up the weak radio signal and feeds it to the RF amplifier, also called a low-noise amplifier (LNA). Because RF amplifiers provide some initial gain and selectivity, they are sometimes referred to as preselectors. Tuned circuits help select the desired signal or at least the frequency range in which the signal resides.
The tuned circuits in fixed tuned receivers can be given a very high Q, so that excellent selectivity can be obtained. However, in receivers that must tune over a broad frequency range, selectivity is somewhat more difficult to obtain. The tuned circuits must resonate over a wide frequency range. Therefore, the Q, bandwidth, and selectivity of the amplifier change with frequency.
In communication receivers that do not use an RF amplifier, the antenna is connected directly to a tuned circuit, at the input to the mixer, which provides the desired initial selectivity. This configuration is practical in low-frequency applications where extra gain is simply not needed. (Most of the receiver gain is in the IF amplifier section, and even if relatively strong signals are to be received, additional RF gain is not necessary.) Further, omitting the RF amplifier may reduce the noise contributed by such a circuit. In general, however, it is preferable to use an RF amplifier. RF amplifiers improve sensitivity, because of the extra gain; improve selectivity, because of the added tuned circuits; and improve the S/N ratio. Further, spurious signals are more effectively rejected, minimizing unwanted signal generation in the mixer.
RF amplifiers also minimize oscillator radiation. The local oscillator signal is relatively strong, and some of it can leak through and appear at the input to the mixer. If the mixer input is connected directly to the antenna, some of the local oscillator signals radiates, possibly causing interference to other nearby receivers. The RF amplifier between the mixer and the antenna isolates the two, significantly reducing any local oscillator radiation.
Both bipolar and field-effect transistors, made with silicon, GaAs, or SiGe, can be used as RF amplifiers. The selection is made based upon frequency, cost, integrated versus discrete, and desired noise performance.
Mixers and Local Oscillators
The output of the RF amplifier is applied to the input of the mixer. The mixer also receives an input from a local oscillator or a frequency synthesizer. The mixer output is the input signal, the local oscillator signal, and the sum and difference frequencies of these signals. Usually a tuned circuit at the output of the mixer selects the difference frequency, or intermediate frequency (IF). The sum frequency may also be selected as the IF in some applications. The mixer may be a diode, a balanced modulator, or a transistor. MOSFETs and hot carrier diodes are preferred as mixers because of their low-noise characteristics.
The local oscillator is made tunable so that its frequency can be adjusted over a relatively wide range. As the local-oscillator frequency is changed, the mixer translates a wide range of input frequencies to the fi xed IF.
The output of the mixer is an IF signal containing the same modulation that appeared on the input RF signal. This signal is amplified by one or more IF amplifier stages, and most of the receiver gain is obtained in these stages. Selectively tuned circuits provide fixed selectivity. Since the intermediate frequency is usually much lower than the input signal frequency IF amplifiers are easier to design and good selectivity is easier to obtain. Crystal, ceramic, or SAW filters are used in most IF sections to obtain good selectivity. Some forms or receivers use DSP filters for selectivity.
The highly amplified IF signal is finally applied to the demodulator, or detector, which recovers the original modulating information. The demodulator may be a diode detector (for AM), a quadrature detector (for FM), or a product detector (for SSB). In modern digital superheterodyne radios, the IF signal is first digitized by an analog-to-digital converter (ADC) and then sent to a digital signal processor (DSP) where the demodulation is carried out by a programmed algorithm.
The recovered signal in digital form is then converted back to analog by a digital-to-analog converter (DAC). The output of the demodulator or DAC is then usually fed to an audio amplifier with sufficient voltage and power gain to operate a speaker. For non voice signals, the detector output may be sent elsewhere, to a TV, tablet, cell phone screen, computer, or some other device.
Automatic Gain Control
The output of a demodulator is usually the original modulating signal, the amplitude of which is directly proportional to the amplitude of the received signal. The recovered signal, which is usually ac, is rectified and filtered into a dc voltage by a circuit known as the automatic gain control (AGC) circuit. This dc voltage is fed back to the IF amplifiers, and sometimes the RF amplifier, to control receiver gain. AGC circuits help maintain a constant output voltage level over a wide range of RF input signal levels; they also help the receiver to function over a wide range so that strong signals do not produce performance-degrading distortion. Virtually all superheterodyne receivers use some form of AGC.
The amplitude of the RF signal at the antenna of a receiver can range from a fraction of a microvolt to thousands of microvolts; this wide signal range is known as the dynamic range. Typically, receivers are designed with very high gain so that weak signals can be reliably received. However, applying a very high-amplitude signal to a receiver causes the circuits to be overdriven, producing distortion and reducing intelligibility
With AGC, the overall gain of the receiver is automatically adjusted depending on the input signal level. The signal amplitude at the output of the detector is proportional to the amplitude of the input signal; if it is very high, the AGC circuit produces a high dc output voltage, thereby reducing the gain of the IF amplifiers. This reduction in gain eliminates the distortion normally produced by a high-voltage input signal. When the incoming signal is weak, the detector output is low. The output of the AGC is then a smaller dc voltage. This causes the gain of the IF amplifiers to remain high, providing maximum amplification.
As discussed in earlier chapters, frequency conversion is the process of translating a modulated signal to a higher or lower frequency while retaining all the originally transmitted information. In radio receivers, high-frequency radio signals are regularly converted to a lower, intermediate frequency, where improved gain and selectivity can be obtained. This is called down-conversion. In satellite communications, the original signal is generated at a lower frequency and then converted to a higher frequency for transmission. This is called up conversion.
Frequency conversion is a form of amplitude modulation or analog multiplication carried out by a mixer circuit or converter. The function performed by the mixer is called heterodyning.
Fig. 9-6 is a schematic diagram of a mixer circuit. Mixers accept two inputs. The signal fs, which is to be translated to another frequency, is applied to one input, and the sine wave from a local oscillator fo is applied to the other input. The signal to be translated can be a simple sine wave or any complex modulated signal containing sidebands.
Like an amplitude modulator, a mixer essentially performs a mathematical multiplication of its two input signals according to the principles discussed. The oscillator is the carrier, and the signal to be translated is the modulating signal. The output contains not only the carrier signal but also sidebands formed when the local oscillator and input signal are mixed. The output of the mixer, therefore, consists of signals fs, fo, fo + fs, and fo – fs or fs – fo.
The local oscillator signal fo usually appears in the mixer output, as does the original input signal fs in some types of mixer circuits. These are not needed in the output and are therefore filtered out. Either the sum or difference frequency in the output is the desired signal. For example, to translate the input signal to a lower frequency, the lower sideband or difference signal fo – fs is chosen. The local oscillator frequency will be chosen such that when the information signal is subtracted from it, a signal with the desired lower frequency is obtained.
When translating to a higher frequency, the upper sideband or sum signal fo + fs is chosen. Again, the local oscillator frequency determines what the new higher frequency will be. A tuned circuit or filter is used at the output of the mixer to select the desired signal and reject all the others.
For example, for an FM radio receiver to translate an FM signal at 107.1 MHz to an intermediate frequency of 10.7 MHz for amplification and detection, a local oscillator frequency of 96.4 MHz is used. The mixer output signals are fs=107.1 MHz, fo = 96.4 MHz, fo + fs = 96.4 + 107.1 = 203.5 MHz, and fs – fo = 107.1 – 96.4 = 10.7 MHz. Then a fi lter selects the 10.7-MHz signal (the IF, or fIF) and rejects the others.
As another example, suppose a local oscillator frequency is needed that will produce an IF of 70 MHz for a signal frequency of 880 MHz. Since the IF is the difference between the input signal and local oscillator frequencies, there are two possibilities:
fo = fs + fIF = 880 + 70 = 950 MHz
fo = fs – fIF = 880 – 70 = 810 MHz
There are no set rules for deciding which of these to choose. However, at lower frequencies, say, those less than about 100 MHz, the local oscillator frequency is traditionally higher than the incoming signal’s frequency, and at higher frequencies, that above 100 MHz, the local oscillator frequency is lower than the input signal frequency.
Keep in mind that the mixing process takes place on the whole spectrum of the input signal, whether it contains only a single-frequency carrier or multiple carriers and many complex sidebands. In the above example, the 10.7-MHz output signal contains the original frequency modulation. The result is as if the carrier frequency of the input signal is changed, as are all the sideband frequencies. The frequency conversion process makes it possible to shift a signal from one part of the spectrum to another, as required by the application.
Mixer and Converter Circuits
Any diode or transistor can be used to create a mixer circuit, but most modern mixers are sophisticated ICs. This section covers some of the more common and widely used types.
The primary characteristic of mixer circuits is nonlinearity. Any device or circuit whose output does not vary linearly with the input can be used as a mixer. For example, one of the most widely used types of the mixer is the simple but effective diode modulator described. Diode mixers like this are the most common type found in microwave applications.
A diode mixer circuit using a single diode is shown in Fig. 9-7. The input signal, which comes from an RF amplifier or, in some receivers, directly from the antenna, is applied to the primary winding of transformer T1. The signal is coupled to the secondary winding and applied to the diode mixer, and the local oscillator signal is coupled to the diode by way of capacitor C1.
The input and local oscillator signals are linearly added in this way and applied to the diode, which performs its nonlinear magic to produce the sum and difference frequencies. The output signals, including both inputs, are developed across the tuned circuit, which acts as a band pass filter, selecting either the sum or difference frequency and eliminating the others.
Doubly Balanced Mixer
Balanced modulators are also widely used as mixers. These circuits eliminate the carrier from the output, making the job of filtering much easier. Any of the balanced modulators described previously can be used in mixing applications. Both the diode lattice balanced modulator and the integrated differential amplifier-type balanced modulator are quite effective in mixing applications. A version of the diode balanced modulator shown in Fig. 4-24, known as a doubly balanced mixer and illustrated in Fig. 9-8, is probably the single best mixer available, especially for VHF, UHF, and microwave frequencies. The transformers are precision-wound and the diodes matched in characteristics so that a high degree of the carrier or local oscillator suppression occurs. In commercial products, the local oscillator attenuation is 50 to 60 dB or more.
FETs make good mixers because they provide gain, have low noise, and offer a nearly perfect square-law response. An example is shown in Fig. 9-9. The FET mixer is biased so that it operates in the nonlinear portion of its range. The input signal is applied to the gate, and the local oscillator signal is coupled to the source. Again, the tuned circuit in the drain selects the difference frequency.
Another popular FET mixer, one with a dual-gate MOSFET, is shown in Fig. 9-10. Here the input signal is applied to one gate, and the local oscillator is coupled to the other gate. Dual-gate MOSFETs provide superior performance in mixing applications because their drain current ID is directly proportional to the product of the two gate voltages. In receivers built for VHF, UHF, and microwave applications, junction FETs and dual-gate MOSFETs are widely used as mixers because of their high gain and low noise. Gallium arsenide FETs are preferred over silicon FETs at the higher frequencies because of their lower noise contribution and higher gain. IC mixers use MOSFETs.
One of the best reasons for using a FET mixer is that its characteristic drain currentversus gate voltage curve is a perfect square-law function. (Recall that square-law formulas show how upper and lower sidebands and sum and difference frequencies are produced.) With a perfect square-law mixer response, only second-order harmonics are generated in addition to the sum and difference frequencies.
Other mixers, such as diodes and bipolar transistors, approximate a square-law function; however, they are nonlinear, so that AM or heterodyning does occur. The nonlinearity is such that higher-order products such as the third, fourth, fifth, and higher harmonics are generated. Most of these can be eliminated by a band pass filter that selects out the difference or sum-frequency for the IF amplifier.
However, the presence of higher-order products can cause unwanted low-level signals to appear in the receiver. These signals produce bird-like chirping sounds known as birdies, which, despite their low amplitude, can interfere with low-level input signals from the antenna or RF amplifier. FETs do not have this problem, and so FETs are the preferred mixer in most receivers.
A typical IC mixer, the NE602 mixer, is shown in Fig. 9-11(a). An improved version is the SA612 mixer, which has roughly the same characteristics. The NE602/SA612, also known as a Gilbert trans conductance cell, or Gilbert cell, consists of a doubly balanced mixer circuit made up of two cross-connected differential amplifiers. Although most doubly balanced mixers are passive devices with diodes, as described earlier, the NE602 uses bipolar transistors. Also on the chip is an NPN transistor that can be connected as a stable oscillator circuit and a dc voltage regulator.
The device is housed in an 8-pin DIP. It operates from a single dc power supply voltage of 4.5 to 8 V. The circuit can be used at frequencies up to 500 MHz, making it useful in HF, VHF, and low-frequency UHF applications. The oscillator, which operates up to about 200 MHz, is internally connected to one input of the mixer. An external LC tuned circuit or a crystal is required to set the operating frequency.
Fig. 9-11(b) shows the circuit details of the mixer itself. Bipolar transistors Q1 and Q2 form a differential amplifier with current source Q3, and Q4 and Q5 form another differential amplifier with current source Q6. Note that the inputs are connected in parallel. The collectors are cross-connected; i.e., the collector of Q1 is connected to the collector of Q4 instead of Q3, as would be the case for a parallel connection, and the collector of Q2 is connected to the collector of Q3. This connection results in a circuit that is like a balanced modulator in that the internal oscillator signal and the input signal are suppressed, leaving only the sum and difference signals in the output. The output may be balanced or single-ended, as required. A filter or tuned circuit must be connected to the output to select the desired sum or difference signal.
A typical circuit using the NE602/SA612 IC mixer is shown in Fig. 9-12. Both R1 and C1 are used for decoupling, and a resonant transformer T1 couples the 72-MHz input signal to the mixer. Capacitor C2 resonates with the transformer secondary at the input frequency, and C3 is an ac bypass connecting pin 2 to the ground. External components C4 and L1 form a tuned circuit that sets the oscillator to 82 MHz. Capacitors C5 and C6 form a capacitive voltage divider that connects the on-chip NPN transistor as a Colpitts oscillator circuit. Capacitor C7 is an ac coupling and blocking capacitor. The output is taken from pin 4 and connected to a ceramic bandpass filter, which provides selectivity.
The output, in this case, the difference signal, or 82 – 72 = 10 MHz, appears across R2. The balanced mixer circuit suppresses the 82-MHz oscillator signal, and the sum signal of 154 MHz is filtered out. The output IF signal plus any modulation that appeared on the input is passed to IF amplifi ers for an additional boost in gain prior to demodulation.
Image Reject Mixer
A special type of mixer is used in designs in which images cannot be tolerated. All super heterodyne receivers suffer from images (see Sec. 9-4), but some more than others because of the frequency of operation, chosen IF, and interfering signal frequency. When proper choice of IF and front-end selectivity cannot eliminate the images, an image reject mixer can be used.
It uses Gilbert cell mixers in a configuration like that used in a phasing-type SSB generator. Referring to Fig. 4-28, you can see how this circuit can be used as a mixer. A balanced modulator is also a mixer. With this technique, the desired signal can be passed, but the image will be canceled by the phasing technique. Such circuits are sensitive to adjust, but they result in superior image performance in critical applications. This approach is used in some modern UHF and microwave IC receivers.
Local Oscillators and Frequency Synthesizers
The local oscillator signal for the mixer comes from either a conventional LC tuned oscillator such as a Colpitts or Clapp circuit or a frequency synthesizer. The simpler continuously tuned receivers use an LC oscillator. Channelized receivers use frequency synthesizers.
A representative local oscillator for frequencies up to 100 MHz is shown in Fig. 9-13. This type of circuit, which is sometimes referred to as a variablefrequency oscillator, or VFO, uses a JFET Q1 connected as a Colpitts oscillator. Feedback is developed by the voltage divider, which is made up of C5 and C6. The frequency is set by the parallel-tuned circuit made up of L1 in parallel with C1, which is also in parallel with the series combination of C2 and C3. The oscillator is set to the center of its desired operating range by a coarse adjustment of trimmer capacitor C1. Coarse tuning can also be accomplished by making L1 variable. An adjustable slug-tuned ferrite core moved in and out of L1 can set the desired frequency range. The main tuning is accomplished with variable capacitor C3, which is connected mechanically to some kind of dial mechanism that has been calibrated in frequency.
The main tuning can also be accomplished with varactors. For example, C3 in Fig. 9-13 can be replaced by a varactor, reverse-biased to make it act as a capacitor. Then a potentiometer applies a variable dc bias to change the capacitance and thus the frequency.
The output of the oscillator is taken from across the RFC in the source lead of Q1 and applied to a direct-coupled emitter follower. The emitter follower buffers the output, isolating the oscillator from load variations that can change its frequency. The emitterfollower buffer provides a low impedance source to connect to the mixer circuit, which often has a low input impedance. If frequency changes after a desired station are tuned in, which can occur as a result of outside influences such as changes in temperature, voltage, and load, the signal will drift off and no longer be centered in the passband of the IF amplifier. One of the key features of local oscillators is their stability, i.e., their ability to resist frequency changes. The emitter follower essentially eliminates the effect of load changes. The zener diode gets its input from the power supply, which is regulated,
providing a regulated dc to the circuit and ensuring maximum stability of the supply
voltage to Q1.
Most drift comes from the LC circuit components themselves. Even inductors, which are relatively stable, have slight positive temperature coefficients, and special capacitors that change little with temperature are essential. Usually, negative temperature coefficient (NPO) ceramic capacitors are selected to offset the positive temperature coefficient of the inductor. Capacitors with a mica dielectric are also used.
Most new receiver designs incorporate frequency synthesizers for the local oscillator, which provides some important benefits over the simple VFO designs. First, since the synthesizer is usually of the phase-locked loop (PLL) design, the output is locked to a crystal oscillator reference, providing a high degree of stability. Second, tuning is accomplished by changing the frequency division factor in the PLL, resulting in incremental rather than continuous frequency changes. Most communication is channelized; i.e., stations operate on assigned frequencies that are a known frequency increment apart, and setting the PLL step frequency to the channel spacing allows every channel in the desired spectrum to be selected simply by changing the frequency division factor. In some advanced digital receivers, a DDS synthesizer is used for the local oscillator and all tuning is digital.
The former disadvantages of frequency synthesizers—higher cost and greater circuit complexity—have been offset by the availability of low-cost PLL synthesizer ICs, which make local oscillator design simple and cheap. Most modern receivers, from AM/FM car radios, stereos, and TV sets to military receivers and commercial transceivers, use frequency synthesis.
The frequency synthesizers used in receivers are largely identical to those described. However, some additional techniques, such as the use of a mixer in the feedback loop, are employed. The circuit in Fig. 9-14 is a traditional PLL configuration, with the addition of a mixer connected between the VFO output and the frequency divider. A crystal reference oscillator provides one input to a phase detector, which is compared to the output of the frequency divider. Tuning is accomplished by adjusting the frequency division ratio by changing the binary number input to the divider circuit. This binary number can come from a switch, a counter, a ROM, or a microprocessor.
The output of the phase detector is filtered by the loop filter into a dc control voltage to vary the frequency of the variable-frequency oscillator, which generates the final output that is applied to the mixer in the receiver.
As stated previously, one of the disadvantages of very high-frequency PLL synthesizers is that the VFO output frequency is often higher in frequency than the upper operating limit of the variable-modulus frequency divider ICs commonly available. One approach to this problem is to use a prescaler to reduce the VFO frequency before it is applied to the variable-frequency divider. Another is to reduce the VFO output frequency to a lower value within the range of the dividers by down-converting it with a mixer, as illustrated in Fig. 9-14. The VFO output is mixed with the signal from another crystal oscillator, and the difference frequency is selected. With some UHF and microwave receivers, it is necessary to generate the local oscillator signal at a lower frequency and then use a PLL frequency multiplier to increase the frequency to the desired higher level. The position of this optional multiplier is shown by the dotted line in Fig. 9-14.
As an example, assume that a receiver must tune to 190.04 MHz and that the IF is 45 MHz. The local oscillator frequency can be either 45 MHz lower or higher than the input signal. Using the lower frequency, we have 190.04 – 45 = 145.04 MHz. Now, when the incoming 190.04-MHz signal is mixed with the 145.04-MHz signal to be generated by the synthesizer in Fig. 9-14, its IF will be the difference frequency of 190.04 – 145.04 = 45 MHz.
The output of the VFO in Fig. 9-14 is 145.04 MHz. It is mixed with the signal from a crystal oscillator whose frequency is 137 MHz. The crystal oscillator, set to 34.25 MHz, is applied to a frequency multiplier that multiplies by a factor of 4. The 145.04-MHz signal from the VFO is mixed with the 137-MHz signal, and the sum and difference frequencies are generated. The difference frequency is selected, for 145.04 – 137 = 8.04 MHz. This frequency is well within the range of programmable-modulus IC frequency dividers.
The frequency divider is set to divide by a factor of 268, and so the output of the divider is 8,040,000/268 = 30,000 Hz, or 30 kHz. This signal is applied to the phase detector. It is the same as the other input to the phase detector, as it should be for a locked condition. The reference input to the phase detector is derived from a 3-MHz crystal oscillator divided down to 30 kHz by a divide-by-100 divider. This means that the synthesizer is stepped in 30-kHz increments.
Now, suppose that the divider factor is changed from 268 to 269 to tune the receiver. To ensure that the PLL stays locked, the VFO output frequency must change. To achieve 30 kHz at the output of the divider with a ratio of 269, the divider input has to be 269 x 30 kHz = 8070 kHz = 8.07 MHz. This 8.07-MHz signal comes from the mixer, whose inputs are the VFO and the crystal oscillator. The crystal oscillator input remains at 137 MHz, so the VFO frequency must be 137 MHz higher than the 8.07-MHz output of the mixer, or 137 + 8.07 = 145.07 MHz. This is the output of the VFO and local oscillator of the receiver. With a fixed IF of 45 MHz, the receiver will now be tuned to
the IF plus the local oscillator input, or 145.07 + 45 = 190.07 MHz.
Note that the change of the divider factor by one increment, from 268 to 269, changes the frequency by one 30-kHz increment, as desired. The addition of the mixer to the circuit does not affect the step increment, which is still controlled by the frequency of the reference input frequency.
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