• Detect a weak signal; the minimum level, which may be as low as 0.25 microvolts, defines the receiver sensitivity.
• Amplify a received signal and maintain the information contained in a minimum strength signal at a minimum of 12 dB above the electrical noise level (signal-to-noise ratio). If the audio distortion produced in the receiver is also taken into account the above figure becomes the signal-to-noise+ distortion (Sinad) ratio. As the signal is increased, the ultimate Sinad should attain 50 to 55 dB.
• Separate the wanted signal from any unwanted ones which may be very close in frequency (the adjacent channel may be 12.5 kHz away at UHF); the selectivity.
• Recover the information from the carrier; demodulation.
• Amplify the audio information to a level suitable for operating a loudspeaker; the audio power output. The audio amplification must introduce the minimum distortion.
• Disenable the audio amplifiers in the absence of signal to cut out the electrical noise. This is done by the mute or squelch circuit.
It is possible to amplify directly the incoming RF signal to a level suitable for demodulation. This is done in a tuned radio frequency (TRF) receiver, but these are seldom used today because of the problems of obtaining sufficient selectivity and gain at one radio frequency, and the difficulty of retuning a number of RF stages to change frequency. Almost all receivers designed for analogue communications now operate on the superheterodyne principle where the incoming radio frequency is converted to a lower, more manageable, intermediate frequency (IF). The fixed IF means that only the oscillator and, possibly, one RF amplifier stage need retuning for a change of channel. The lower frequency of the IF facilitates the acquisition of adequate gain with stability and selectivity.
There is little difference in the layout of receivers for AM and FM except that the circuits perform their functions differently. Figure 10.3 is a block diagram of a typical FM receiver with a crystal controlled local oscillator.
When two frequencies are applied to a non-linear circuit such as the mixer, they combine to produce other frequencies, their sum and difference being the strongest. The superheterodyne mixes a locally generated frequency with the received signal to produce a, usually lower, frequency retaining the modulation of the received signal. Commonly used values for this intermediate frequency (IF) are 465 kHz for MF and HF receivers and 10.7 MHz for VHF and UHF. At these fixed frequencies the necessary high amplification with low noise and stability and the required selectivity are easier to obtain. The local oscillator (injection) frequency = signal frequency ± IF frequency.
Superheterodyne receivers are susceptible to a particular form of interference. Assume a local oscillator frequency of 149.3 MHz is mixed with a wanted signal of 160 MHz to produce the IF frequency of 10.7 MHz. A signal of 138.6 MHz would also combine with the local oscillator to produce 10.7 MHz. The frequency of this spurious response is the image, or second channel frequency. The IF amplifier cannot discriminate against it so some degree of selectivity must also be provided in the RF amplifier and input circuitry. Intermediate frequencies are chosen which are a compromise between ease of obtaining adjacent channel discrimination and image frequency rejection.
The IF amplifier contains a block filter, either crystal or ceramic (see Figure 7.7), necessary to discriminate between channels adjacent in frequency. The design of the filter is crucial. It must be wide enough to accommodate the band of frequencies present in the modulation plus an allowance for frequency drift and its response over this band must be uniform with minimal ripple, particularly if data is to be received, yet its response must be of the order of −100 dB at the frequency of the adjacent channel.
Apart from the demodulator the main difference between AM and FM receivers lies in the operation of the IF amplifier. The IF stages in an AM receiver are linear, although their gain is variable. Part of the IF amplifier output is rectified and used to control the gain to provide automatic gain control (AGC). An increase of signal above a predetermined level, with delayed AGC, causes a reduction in IF gain maintaining a sensibly constant audio output level.
RFMixer IF filter amplifier~ ~
IF DeAFAF amplifier
Local oscillator (xtal or synth.) Filter
Amplifier
The IF amplifier in a FM receiver possesses a very high gain, some 100 dB, and is non-linear. On receipt of a signal, or even with only the receiver noise, it runs into limitation, cutting off both positive and negative peaks of the signal or noise. This gives FM its constant level audio output over a wide range of signal levels. It also produces the capture effect where a strong signal, fully limiting, completely removes a weaker signal. A signal difference of some 6 dB is required to provide effective capture.
Most receivers employ only one change of frequency, single superheterodynes, but double superheterodynes are occasionally used at VHF and above. A double superheterodyne changes the frequency twice, perhaps to 10.7 MHz for the first IF and then, using a fixed frequency crystal second local oscillator, to a lower, often 1.2 MHz or thereabouts, second IF. The result is greater gain and selectivity but the incorporation of a second oscillator and mixer increases the number of possible spurious responses.
A variation of a very old receiver circuit, the Autodyne, forgotten in about 1914, is now finding favour in receivers for digitally modulated signals. Under its new names of Homodyne or zero-IF receiver it lends itself to the purpose. In this type of receiver the local oscillator runs at the same frequency as the incoming signal, hence the zero-IF. Frequency or phase shifts of the incoming signal representing the data emerge from the mixer at base band and are applied directly to the processing circuits.
In a communications receiver the noise generated in the aerial and RF stages in the absence of a signal is amplified to what may be, when demodulated, an unacceptable level. To eliminate the annoyance the loudspeaker is switched off during no-signal periods by a squelch or mute circuit. The mute circuit rectifies the high frequency noise at the demodulator and uses it to switch off the audio amplifier. When a signal is received the limiting action of the IF amplifier of an FM receiver depresses the noise in favour of the signal. Some AM mobile receivers use additional FM circuitry to provide improved mute action.
An ideal receiver would generate no noise and the signal-to-noise ratio, in a receiver of given bandwidth, would be determined by the level of the signal at the base of the antenna compared with the noise produced in the antenna. The noise factor of the ideal receiver is the number of times the signal power must exceed the antenna noise power to produce a 1:1 ratio at the receiver. It is given by (see Section 1.5.1):
e2 (e.m.f.)antenna4kTBR
When the receiver input impedance is matched to the antenna impedance, half the power is dissipated in the antenna and the noise factor is 2 (3 dB). In a practical receiver, the noise generated in the RF amplifier is the most significant, and the noise figure is the sum of the RF amplifier noise plus all the preceding losses. Figures between 4 and 6 dB are common and the higher the noise figure, the worse the receiver sensitivity.
Receivers are evaluated for quality on the basis of signal-to-noise ratio (S/N or ‘SNR’), sometimes denoted SN. The goal of the designer is to enhance the SNR as much as possible. Ultimately, the minimum signal level detectable at the output of an amplifier or radio receiver is that level which appears just above the noise floor level. Therefore, the lower the system noise floor, the smaller the minimum allowable signal.
The noise performance of a receiver or amplifier can be defined in three different, but related, ways: noise factor (FN), noise figure (NF) and equivalent noise temperature (TE); these properties are definable as a simple ratio, decibel ratio or Kelvin temperature, respectively.
Noise factor ( FN). For components such as resistors, the noise factor is the ratio of the noise produced by a real resistor to the simple thermal noise of an ideal resistor.
The noise factor of a radio receiver (or any system) is the ratio of output noise power (PNO) to input noise power (PNI):F
N
=
PNO
PNI T=290/K
In order to make comparisons easier the noise factor is usually measured at the standard temperature (To) of 290 K (standardized room temperature); although in some countries 299 K or 300 K are commonly used (the differences are negligible). It is also possible to define noise factor FN in terms of the output and input signal-to-noise ratios:
FN
=
SNI
SNO
where
SNI is the input signal-to-noise ratio
SNO is the output signal-to-noise ratio
Noise figure (NF). The noise figure is the frequency used to measure the receiver’s ‘goodness’, i.e. its departure from ‘idealness’. Thus, it is a figure of merit. The noise figure is the noise factor converted to decibel notation:
NF = 10 log(FN)where
NF is the noise figure in decibels (dB) FN is the noise factor
log refers to the system of base-10 logarithms
Noise temperature (Te). The noise ‘temperature’ is a means for specifying noise in terms of an equivalent temperature. That is, the noise level that would be produced by a resistor at that temperature (expressed in degrees Kelvin). Evaluating the noise equations shows that the noise power is directly proportional to temperature in degrees Kelvin, and also that noise power collapses to zero at the temperature of Absolute Zero (0 K).
Note that the equivalent noise temperature Te is not the physical temperature of the amplifier, but rather a theoretical construct that is an equivalent temperature that produces that amount of noise power in a resistor. The noise temperature is related to the noise factor by:
Te = (FN − 1)Toand to noise figure by
Te =KTo log−1 NF − 110
Noise temperature is often specified for receivers and amplifiers in combination with, or in lieu of, the noise figure.
A noise signal is seen by any amplifier following the noise source as a valid input signal. Each stage in the cascade chain amplifies both signals and noise from previous stages, and also contributes some additional noise of its own. Thus, in a cascade amplifier the final stage sees an input signal that consists of the original signal and noise amplified by each successive stage plus the noise contributed by earlier stages. The overall noise factor for a cascade amplifier can be calculated from Friis’ noise equation:
FN = F1 + F2 − 1 + F3 − 1 +···G1 G1G2FN − 1+ G1G2···Gn−1
where
FN is the overall noise factor of N stages in cascade F1 is the noise factor of stage-1
F2 is the noise factor of stage-2
FN is the noise factor of the nth stage
G1 is the gain of stage-1
G2 is the gain of stage-2
Gn−1 is the gain of stage (n− 1).
As you can see from Friis’ equation, the noise factor of the entire
cascade chain is dominated by the noise contribution of the first stage or two. High gain, low noise radio astronomy RF amplifiers typically use low noise amplifier (LNA) circuits for the first stage or two in the cascade chain. Thus, you will find an LNA at the feedpoint of a satellite receiver’s dish antenna, and possibly another one at the input of the receiver module itself, but other amplifiers in the chain might be more modest (although their noise contribution cannot be ignored at radio astronomy signal levels).
The matter of signal-to-noise ratio (S/N) is sometimes treated in different ways that each attempts to crank some reality into the process. The signal-plus-noise-to-noise ratio (S+ N/N) is found quite often. As the ratios get higher, the S/N and S+ N/N converge (only about 0.5 dB difference at ratios as little as 10 dB). Still another variant is the SINAD (signal-plus-noise-plus-distortion-to-noise) ratio. The SINAD measurement takes into account most of the factors that can deteriorate reception.
To obtain the maximum signal-to-noise ratio the bandwidth of every circuit must be designed to admit its operational band of frequencies only. The wider its bandwidth the more noise a circuit admits, and the more the bandwidth exceeds that needed, the worse becomes the signal-to-noise ratio. There is a linear ratio between bandwidth and noise power admitted: doubling the bandwidth doubles the noise power.
Improvements in local oscillator crystal frequency stability has resulted in improved signal-to-noise ratios by reducing the necessary width of the IF filter.
Demodulation, the recovery of the audio from the IF bandwidth affects the signal-to-noise ratio. The relationship is complex, but consider two examples. First, for 12.5 kHz channel spacing FM:
The audio frequency range is 300 Hz to 3000 Hz
Bandwidth, b = 2700 Hz
The modulation index M = fd(max)/fm(max)
Signal/noise out = 3M2/2b× signal/noise in
For a 12.5 kHz PMR channel:
M = 2500/3000 = 0.83
b = 2.7kHz
3M2/2b = 0.38
A 0.38 times reduction in power is −4.8 dB. Demodulation in this case worsens the signal-to-noise ratio by some 5 dB, and if a signal-to-noise ratio of 12 dB is required at the loudspeaker, 17 dB is needed at the input to the demodulator.
When channel separations were 25 kHz, and deviation 5 kHz, the situation was:Audio frequency range, 300 Hz to 3000 Hz
Bandwidth, b = 2700 Hz
For a 50 kHz channel:
M = 5000/3000 = 1.66
b = 2.7kHz
3M2/2b = 1.54
A 1.54 times gain in power is +1.8 dB. In this case the demodulation improved the signal-to-noise ratio slightly. Reducing the bandwidth would have the same effect.
For AM, the signal-to-noise ratio is dependent on the modulation depth. The demodulation process reduces the signal-to-noise ratio by 6 dB but the recovered audio is less than the IF bandwidth by a factor of 3:1 which compensates for the demodulation loss. Reducing the modulation depth degrades the signal-to-noise ratio by 6 dB for every halving of the modulation depth.
• Sensitivity. The minimum signal to which a receiver will respond. For an AM receiver, the generally accepted standard is the signal (30% modulated with sinusoidal tone, either 400 Hz or 1 kHz) required to provide an audio output of 50 mW. For an FM receiver, the standard is the unmodulated signal required to produce a 20 dB reduction in noise. A typical figure is 0.25µV (p.d.) for 20 dB quieting. However, sensitivity is often quoted in terms of either the signal-to-noise ratio or Sinad so, in modern parlance, sensitivity and signal-to-noise ratio are sometimes considered to be synonymous.
• Signal-to-noise ratio (may be quoted as Sinad, signal-to-noise and distortion). Typically 0.3µV (p.d.) for 12 dB Sinad.
• Spurious response attenuation. Typically better than 80 dB.
• Adjacent channel selectivity. Better than 65 dB at 12.5 kHz channel spacing.
• Cross modulation. The modulation, in the receiver, of a wanted signal by a stronger, unwanted signal. It is usually caused by nonlinearity in the receiver RF stages.
• Blocking and de-sensitization. The reduction in sensitivity of a receiver due to overloading of the RF stages when a strong signal is applied. A blocked receiver may take an appreciable time to recover.
• Audio frequency response. Typically within +1dB to −3dB of a 6 dB/octave de-emphasis curve from 300 Hz to 3000 Hz (2.55 kHz for 12.5 kHz channel spacing, above which the response falls more rapidly).
• Audio output. Typically 3–4 W. For hand-portables, 100–500 mW. (Distortion may also be quoted, typically better than 5%.)
• Switching bandwidth. As for transmitter.
• Duplex separation. For a receiver/transmitter combination, the minimum separation between the receiving and transmitting frequencies which will permit duplex operation with minimal degradation of the receiver performance.