2-33
As in all superheterodyne receivers, controlling the frequency of the local oscillator keeps the
receiver tuned. Since this tuning is critical, some form of automatic frequency control (afc) is essential to
avoid constant manual tuning. Automatic frequency control circuits mix an attenuated portion of the
transmitted signal with the local oscillator signal to form an IF signal. This signal is applied to a
frequency-sensitive discriminator that produces an output voltage proportional in amplitude and polarity
to any change in IF frequency. If the IF signal is at the discriminator center frequency, no discriminator
output occurs. The center frequency of the discriminator is essentially a reference frequency for the IF
signal. The output of the DISCRIMINATOR provides a control voltage to maintain the local oscillator at
the correct frequency.
Different receiving systems may vary in the type of coupling between stages, the type of mixer, the
detector, the local oscillator, and the number of stages of amplification at the different frequencies.
However, the receiver is always designed to have as little noise as possible. It is also designed to have
sufficient gain so that noise, rather than lack of gain, limits the smallest visible signal.
RECEIVER COMPONENTS
This section will analyze in more detail the operation of the receiver circuits mentioned above. The
circuits discussed are usually found in some form in all radar superheterodyne receivers.
Low-Noise Amplifier
LOW-NOISE AMPLIFIERS, sometimes called PREAMPS, are found in most modern radar
receivers. As previously mentioned, these amplifiers are usually solid-state microwave amplifiers. The
most common types are tunnel diode and parametric amplifiers. These amplifiers are discussed in detail in
NEETS, Module 11, Microwave Principles. Some older systems may still use a traveling-wave tube (twt)
as a low-noise first stage amplifier. However, the solid-state amplifiers produce lower noise levels and
more gain.
Local Oscillator
Most radar receivers use a 30 or 60 megahertz intermediate frequency. The IF is produced by mixing
a local oscillator signal with the incoming signal. The local oscillator is, therefore, essential to efficient
operation and must be both tunable and very stable. For example, if the local oscillator frequency is 3,000
megahertz, a frequency change of 0.1 percent will produce a frequency shift of 3 megahertz. This is equal
to the bandwidth of most receivers and would greatly decrease receiver gain.
The power output requirement for most local oscillators is small (20 to 50 milliwatts) because most
receivers use crystal mixers that require very little power.
The local oscillator output frequency must be tunable over a range of several megahertz in the 4,000-
megahertz region. The local oscillator must compensate for any changes in the transmitted frequency and
maintain a constant 30 or 60 megahertz difference between the oscillator and the transmitter frequency. A
local oscillator that can be tuned by varying the applied voltage is most desirable.
The REFLEX KLYSTRON is often used as a local oscillator because it meets all the requirements
mentioned above. The reflex klystron is a very stable microwave oscillator that can be tuned by changing
the repeller voltage.
Most radar systems use an automatic frequency control (afc) circuit to control the output of the local
oscillator. A block diagram of a typical afc circuit is included in figure 2-23. Note that the afc circuits
form a closed loop. This circuit is, in fact, often called the afc loop.
