any ne frequency and leads to more stabte amplifier. When over 120 dB o rf gain is imvolved, every little bit helps. The function of each item in Fig. 1. 1 can be explained as follows 1. RF amplifier It should have just enough gain, usually about 10 dB, to establish he overall noise figure of the receiver. The tuned circuits at the input and output need only be selective enough to reject image signals and other spurious signals that could intermodulate and appear at the imtermediate frequency. Some AGC may be needed to prevent overloading on strong signals. The RF amplifier may also be called on to suppress amy tenden for the local oscillator to radiate out to the antenna and interfere with other 2. Mixer and local ascillato The mixer has two inputs, one from the RF amplifier and one from the local oscillator. The nonlinearities of the mixer will create numerous demodulation products, and one of the sum or difference fre quency, will occur at the F frequency. Usually, there will be a second frequency, the image, that can also mix with the oscillator fre produce an output at the IF. Depending on the type of mixer used, conv- from-10 dB to +30 dB are common. The local oscillat must be tunable, yet have a low drift rate and relatively low sides noise, since this could increase the noise level of the receiver This section establishes the overall bandwidth and selectivity of the receiver. The bulk of the receivers gain will be concen- trated here and some type o automatic gain control will be included to ad- just for variations in received signal strength. The IF is usually at a lower frequency than the RF, but, in some special cases, the IF may be higher to reduce spurious intermodulation and image problems 4. Demodulators For each type of modulation used(i. e, AM, FM, SSB, PM),a number of different circuits exist. Some will have gin others a loss. 2
Some will require a reference input(i. e others wont. The demodulation may also red to produce output to aGC or AFC circuits. The recovered audio leved (or video, etc )will detemine the amount of gain required in the following audio or video am- 1.1.2 Specilications Before beginning the design of a receiver, it is necessary to consider the specifications required of the final result. In most cases this ands up as a compromise between what the designer would like and what is possi ble. The determining factor will usually be financial limitations, The fol lowing should then be considered before proceeding 1. Tuning range What range of frequencies must be tuned and will it be tuned contin- uously or in discrete chapels? A short-wave receiver, for example,must continuously tune from 3 to 30 MHz and will usually require some band switching. The locad oscillator will be a continuously tunable type. De modulators will be needed for AM, SSB, and CW. and IF bandwidths should correspond. For CB, a narrow range of frequencies from 26.965 to 27.405 are needed and will be trned as 40 discrete charnels, The local oscilator will therefore likely be a phase-locked loop synthesizer. Demod ulation could be either am or SSB Often, too much emphasis is put on sensitivity without attention to other details. For example, a 100-kHz navigation receiver will pick up so much atmospheric noise that a 100-gv desired signal from the antenna could be obscured at times. On the other hand, ao.1-uV signal at 150 MHz wil often be readily distinguishable from background noise When the modulation type and channel spacing are mown, it is pos- sible to determine the IF bandwidth and its skirt characteristics For FM- stereo broadcasting, a bandwidth of 350 khz is required. For AM aircraft communications, a bandwidth of 30 kHz is coumon--not to provide wide
bandwidth for high audio-frequency response but to accomodate frequen cy tolerances in the transmitters and receivers. The filter-skirt characteris tics will be set to reject adjacent channel signals as require 4.界 punzous signa An oherwise good design can be useless if unwanted signals can sneak into the reoeiver at the IF frequency(s) arough croag-modulatioa problems [J Typical specifications for several good receivers are as fallows (1)M stereo ner: frequency range 88-108 MHz Sensitivity: 1.8 uv acrose 300-0 input for 20 dB electivity: 100 dB for channels 400 kHz either side of center frequency Bandwidth 350kH敏-6- dB point Image rejection: 90 dB Spurious rejection: 90 dB I 90 dB 65 dB capture ratio: 1.5 dB/4) (2)Shortwave receiver: frequency nange 3.0-30 MHz Sensitivity: 0.5 gV for 10 dB S+N/N ratio(5) 2. 3 kHz at -6dB, 5.5 kHz at -60 dB (SSB mode) 60 dB IF rejection: 75 dB (3)CB receiver: frequency range 26.965-27.405 MHz Sensitivity 0.5 uV for 10 dB S+N/N ratio Bandwidth 6 kH at-6 dB 20 kHz at -60 dB age rejection: 60 dB Once the specifications are carefully determined, it is time to start
the design. But what is the best starting point? Generally, the most sensi tive points will be the two nonlinear circuits, the mixer and the detector The IF amplifier takes up the slack between the two, and the RF amplifier picks up the deficiencies of the mixer 1.1.3 Mxer The mixer section of the receiver should ideally produce an IF only at the difference (or sum, for up-conversion )of the two inp quencies. One of these inputs will be the local oscillator signal other will be the desired RF signal. Again, ideally, no other combination of input sigals should produce an IF output. If such frequencies do ex- t, filters must be provided to remove them before they reach the mixer The closest thing to an ideal mixer is any circuit In addition to the their second harmonics appearing at the output, the sum and the differ ence will also appear. The diierence is usually the one signal desired and so is selected by IF fltering. The amplitude of the difference signals will be proportional to the product of the ariginal RF signal level and the local oscillator level. any other two signals at the input oould also produce am output &t the IF if they are separated by an amount equal to the difference frequency. However, the output level they produce will be proportional to their signal levels be had if all RF imput levels to the mixer are kept as low as possihle and the local oscillator sigmal kept as high as poaaible. The one desired sig al will therefore be stronger than all the undesired ones. This is de- ibed in Fig. 1. 2. Thermixer circuit has four signals at its input, all of the same level. The local oscillator and is much higher than the other four. The IF filters only pass signals between0. 4 and 0. 6 MHz, RF frequencies C and d can mix with the illator and produce outputs at 0. 45 and 0. 55 MHz, respectively, well within the IF pasaband. One will be the desired signal and the other is the image, which should be removed by filtering before reaching the mixer
2.02.5 3.5540455 (a)Input spectr B-A 045050055 (MHz) g.1.2S《到s 盘m血whh能邛ps en to be separated by 0.5 MH they will also produce a mixing product (which contains the combined modulation of each)within the IF passband. However, the amplitude of this signal will be much lower than the desired IF signal. Therefare, best obtained by: (2)Using high local oscillator levels (3)Maintaining low RF signal levels (4)Providing proper filtering ahead of the mixer. 1.14 If the ideal square-law mixer can be built, what is the minimum fil ter that is required ahead of it? We have already seen that the image has to be removed and also any group of frequencies that could themselves mix The limiting case is shown in Fig ..3.The