CHAPTER 7
Communications Receivers
1. The simplest form of communications receiver is the tuned radio-frequency (TRF) receiver which provides RF and AF gain, selectivity, and a demodulator.
2. The two primary characteristics of a receiver are selectivity and sensitivity.
3. Selectivity is the ability to separate signals on different frequencies.
4. The bandwidth (BW) of a tuned circuit is the difference between the upper f2 and lower f1 3-dB-down frequencies (BW =f2 – f1).
5. The bandwidth of a tuned circuit is determined by the ratio of the resonant frequency to the Q where Q is XL/R of the inductor (BW =fr/Q). The Q can be computed from the bandwidth Q =fr/BW = fr / (f2-f1).
6. Sharper selectivity with steeper response curve skirts can be obtained by cascading tuned circuits.
7. The shape factor of a response curve is the ratio of the 6O-dB-down bandwidth to the 6-dB-down bandwidth. The lower the shape factor, the better the selectivity.
8. Sensitivity is the ability of the receiver to pick up weak signals and is a function of gain.
9. The disadvantages of a TRF receiver are tuning difficulties and selectivity varying with frequency.
10. These problems of a TRF receiver are eliminated by using a superheterodyne (superhet) receiver.
11. A superhet uses a mixer to translate the incoming signal to a lower frequency, known as the intermediate frequency (IF), where fixed gain and selectivity .can be obtained.
12. Most of the gain and selectivity in a superhet is obtained in the IF amplifier.
13. Low IFs are preferred because higher selectivity and better stability can be obtained with simple circuits.
14. Low IFs usually cause image interference problems.
15. An image is a signal on a frequency separated from the desired signal frequency by 2 times the IF value that interferes with reception.
16. The mixer will convert both the desired signal and the image to the IF.
17. Image interference is caused by poor receiver input selectivity that does not adequately reject the image.
18. Image interference can be reduced by using a double-conversion superhet that uses two mixers and IFs. The first IF is high to eliminate images, and the second is low to ensure good selectivity.
19. The most critical part of any receiver is the front end which consists of the RF amplifier and mixer as these circuits add the most noise to a weak signal.
20. Noise is any random interference to a weak signal.
21. A measure of a receiver's noise performance is its signal-to-noise (SIN) ratio.
22. External noise comes from industrial, atmospheric', and space sources.
23. Industrial noise sources are ignitions, motors and generators, switching devices, and fluorescent lights.
24. Atmospheric noise comes from lightning and other sources.
25. Space noise comes from the sun as well as other stars and extraterrestrial sources.
26. Internal noise from electronic components also interferes with reception and can totally mask weak signals.
27. Most internal noise comes from thermal agitation-the random movement of electrons and atoms in a material due to heat.
28. The thermal noise voltage across a resistor is proportional to the temperature, the bandwidth, and the resistance value and can be computed with the expression Vn = square root of 4kTBR where k is Boltzmann's constant 1.38 x 10-23,T is the temperature in degrees kelvin, B is the bandwidth in hertz, and R is the resistance in ohms.
29. Other types of internal noise are shot and transit-time noise in semiconductors.
30. Noise cannot be eliminated, but its effect can be minimized.
31. Noise performance of a receiver or circuit is expressed in terms of the noise figure F, also called the noise factor, which is the ratio of the SIN input to the SIN output. It is usually given in decibels.
32. Noise performance of microwave components is usually expressed in terms of noise Temperature in degrees Kelvin K = 290(F - 1).
33. Thermal noise is random and is often referred to as white noise or Johnson noise.
34. Since noise is a mixture of all random frequencies, its level can be reduced by narrowing the bandwidth.
35. Most receivers get their selectivity from double-tuned LC circuits.
36. Most receivers have AGC circuits so that a wide dynamic range of input signal amplitude scan be accommodated without distortion.
37. An AGC circuit rectifies the IF or demodulator output into dc to control the IF amplifier gain.
38. The gain of a bipolar transistor can be varied by changing its collector current.
39. In reverse AGC an increase in the AGC voltage decreases the collector current.
40. In forward AGC, an increase in AGC voltage increases collector current.
41. The gain of a dual-gate MOSFET in an IF amplifier is controlled by varying the dc voltage on the second gate.
42. Automatic frequency control (AFC) is a feedback system similar to AGC that is used to correct for frequency drift and instability in the LO of VHF, UHF, and microwave frequency receivers.
43. A squelch circuit is used to cut off the audio output to prevent annoying noise until a signal is received. Either the audio signal or background noise can be used to operate the squelch circuit.
44. Continuous tone control squelch (CTCS) circuits permit selective signaling by allowing only low-frequency tones to trigger the squelch.
45. A beat frequency oscillator (BFO) is used in SSB and CW receivers to provide a carrier that will mix with the input signal in the demodulator to generate the audio output.
46. A transceiver is a piece of communications equipment that combines a receiver and a transmitter in a common package where they share a common housing and power supply. Single-sideband transceivers allow sharing of the filter, LO stages, and other circuits.
47. Many new transceivers contain a frequency synthesizer that eliminates multiple crystal oscillators and LC tuned oscillators in the transmitter and receiver.
48. A frequency synthesizer is a signal generator usually implemented with a PLL that produces LO and transmitter carrier signals in a transceiver.
49. A frequency synthesizer has the stability of a crystal oscillator, but the frequency can be varied in small, equal increments over a wide range.
50. The frequency increments in a synthesizer are set by the frequency of the reference input to the phase detector.
51. The frequency of a synthesizer is changed by varying the divide ratio of the frequency divider between the VCO output and phase detector input.
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