SUPERHETERODYNE RECEIVER SUBSYSTEMS
THE "FRONT END"
First, we will look at the stages that select and convert the RF input to the IF. These stages are known collectively as the "Front End" of a superheterodyne receiver. The front end includes the following stages:
We begin by revisiting the issue of superheterodyne images. Consider a receiver designed to receive AM broadcasts. Its tuning range is 540 to 1700 KHz. The standard IF for AM receivers is 455 KHz, so our LO must have an output ranging from 995 KHz to 2695 KHz. We already know that the image, LO, and input frequency are related as follows:
Image = LO + RF Input
Thus the range of image frequencies is 1460 - 2610 KHz. The image frequencies are quite far removed from the desired RF input frequencies, so the tuned circuit used to select the input frequency should have sufficient selectivity to reject the image signal. For example, when the radio is tuned to 1000 kHz, the image frequency is nearly double, 1910 kHz, which will be rejected by the tuned circuit on the RF input. Suppose the same receiver can also receiver shortwave broadcasts in the 5 - 16 MHz range. If the receiver is now tuned to 10 MHz, the image is 10.910 MHz. The image frequency is only ~ 9% greater than the desired input frequency. In this situation it is quite possible that images will be received with the desired signal, creating problems at the receiver output.
How does one resolve this problem? One answer is simple: select the highest IF frequency possible. The IF should be at least 10% of the highest RF frequency, in order to minimize the problems caused by images. Sometimes this is not possible and the selectivity of the input circuit must be improved. This can be done by inserting an RF amplifier between the antenna and the mixer. The RF amplifier, if properly designed offers the following improvements:
Improved image rejection (the addition of an RF amplifier puts another tuned circuit between the antenna and mixer, increasing image attenuation)
Improved sensitivity (signals below the mixer's noise floor are amplified to a higher level prior to entering the mixer) Improved noise figure (mixers are often relatively noisy). As we learned in chapter 1, the first stage's noise ratio is the major contribution to the receiver's entire noise figure. Adding a low noise pre-amplifier can significantly improve the NF for the receiver as a whole.
We will consider the mixer and LO together. The mixer can be any non linear device. It can be as simple as a diode into which the LO and input signals are fed. Diode mixers using Schottky diodes can be used at frequencies up to 10 GHz, but the diode mixer provides no gain. Mixers using transistors provide some gain, but their use above 500 MHz is limited by their internal parasitic capacitances.
It is not necessary to have a separate oscillator circuit that generates the LO signal for the mixer. It is possible to use the same transistor for both. The transistor oscillator is biased to operate in a non-linear mode so that it can function as both a mixer and an oscillator. This type of circuit is known as an autodyne mixer or self-excited mixer.
One problem with the autodyne mixer is that of "pulling". If the input frequency is too close to the LO frequency, the LO frequency will be "pulled" to the input frequency and there will be no IF output. This problem limits the use of the autodyne mixer to relatively low frequencies where the RF and LO frequencies are well separated.
There are integrated circuits designed specifically for mixer applications. One of the most widely used is the NE 602 linear IC. It can be used with RF frequencies of up to 500 MHz.
Regardless of the design used for the mixer/LO combination, the final element in the circuit is always a filter that selects the desired IF frequency. When the IF is below 2 MHz the filter is usually made from a resonant RF transformer. At higher frequencies, either an RF transformer or some type of crystal filter or ceramic resonator may be used. The choice depends on the required bandwidth. The bandwidth of transformers is limited by their Q. Above 2 MHz it is difficult to manufacture a transformer with sufficient Q to provide the narrow bandwidth required for certain modes. For wideband modes such as broadcast FM, RF transformers provide sufficient selectivity.
THE IF SECTION
The IF section determines the receiver's selectivity and provides most of the gain for the receiver. As a result, the IF stage has a large influence on the sensitivity of the receiver, The selectivity of the IF amplifier is determined by the filters used at its inputs and outputs. Doubly-tuned RF transformers are often used and special types of crystal filters or ceramic resonators may be used to provide the extremely narrow selectivity required for certain modulation techniques.
The number of IF amplifiers used in the IF section is determined by the sensitivity required, cost considerations and in some cases, selectivity requirements. High selectivity filters may have significant losses, necessitating additional IF amplifiers to make up for the signals lost in the filter.
The active device used in an IF amplifier may be a FET, BJT or a linear IC. IC's such as the CA3028 are particularly attractive because they provide high gain in a small package with a limited external parts count.
In addition to providing signal gain, the IF stage may be used to perform some signal processing. In an FM receiver, it is desirable to eliminate any amplitude variations from the IF signal. This is done through a special IF stage known as a limiter. The limiter has a constant output amplitude over a wide range of input amplitudes. It removes any amplitude modulated noise from the FM signal before it goes to the FM detector.
It is possible to externally control the gain of most IF amplifier stages. This is often done automatically, using an automatic gain control circuit (AGC). Variations in input signal strength due to changing propagation, which the listener would perceive as changes in loudness can be eliminated in this manner. Some receivers permit disabling of the AGC, in order to run the IF section at maximum gain in order to receive very weak signals.
In most AM receivers the detector is a diode, similar in function to the cat's whisker diodes used in the earliest crystal sets. The RF signal is rectified and put through a low pass filter, which leaves only the audio that was originally modulated onto the AM carrier. Although the diode detector is very simple, it suffers from two drawbacks:
Some high performance receivers use a circuit known as a synchronous detector. The IF signal is applied to one input of a mixer and a local oscillator signal whose frequency and phase are identical to the IF carrier is applied to the other. The mixer has several outputs, including the signals whose frequency is the difference between the sideband frequencies and the carrier frequency. This is the original audio signal. The mixer's output is passed through a low-pass filter, which removes all mixer products except for the audio signals. The cost and complexity of the synchronous detector limit its use to military communications gear and high end shortwave receivers.
CONTROLLING THE GAIN OF THE RECEIVER - AUTOMATIC GAIN CONTROL (AGC)
At first glance, it would seem that the best way to run a receiver would be at maximum possible gain, so that the weakest signals could be intercepted. For some applications this is the case, but for casual listening, this is generally a poor operating choice. The strength of the RF input signal varies greatly over time. AM signals can fade as much as 30 dB in a few seconds as propagation changes. Radios installed in vehicles see wildly varying input levels as the vehicle's position changes.
To provide control of gain, an automatic gain control (AGC) circuit is used. The AGC generally gets its input from the detector. The detected signal is put through a long time constant low pass filter. All audio variations in the detected signal are removed and only the slow variations in signal strength due to fading remain. This slowly varying voltage is used to vary the bias on the IF amplifier stages, reducing or increasing the gain as necessary to maintain a constant audio level at the output of the receiver.
The gain of a BJT is approximately proportional to the bias current. By designing the AGC circuit to have a negative output voltage, the AGC voltage can be applied to the base to lower the forward bias of the BJT, and therEfore its gain.
More sophisticated receivers used for other modulation techniques may have a faster AGC time constant that allows the gain of the IF stages to follow more rapid variations in signal strength. Some receivers use AGC circuits with variable "attack" and "recovery" times (which determine how quickly the AGC reduces and then increases the gain).
AF AMPLIFIER SECTION
AM RECEIVER SYSTEMS
The diagram above shows the block diagram for a typical AM superheterodyne receiver. An RF amplifier is generally not used; AM signals are generally strong enough to provide good reception without the additional amplification before the mixer. Note that both the mixer and the IF gain are controlled by the AGC. This is necessary because it is not possible to get good AGC action by controlling only the gain of the IF stage. Controlling the mixer gain also prevents strong local signals from overloading the mixer.
A tremendous amount of gain is required to boost a signal from the microwatt level at the antenna to the watt level at the output of the receiver. Consider an AM receiver whose sensitivity is 10 ÁV for a 10 dB SNR into a 50 ohm input impedance and whose output power is 1 W into 8 ohm load. How much gain is needed to accomplish this?
The input power and output powers expressed in dB are:
The required receiver gain is 30 - (-87) = 117 dB.
The figure below shows a typical division of gain between the various stages of the receiver.
Notice that most of the gain comes from the IF stages. There are two reasons for this:
It is much easier to design a high gain amplifier to work at just one frequency, over a narrow bandwidth.
Too much gain in the RF stages can cause the mixer to be overloaded by strong signals.
The second consideration brings us to a third measure of receiver performance known as dynamic range. Dynamic range measures the difference between the sensitivity (the faintest signal that can be received) and the largest signal that can be received without being distorted. Dynamic range is measured in dB and it is varies from 70 dB to over 100 dB. High dynamic range is good; the receiver can respond to signals over a wide range of input level without causing distortion in the output. As signal levels change due to varying propagation and the AGC adjusts the gain to maintain constant output, the distortion levels in the output will not change as a result of overload.
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