2-34
A sample of the transmitter energy is fed through the afc mixer and an IF amplifier to a
discriminator. The output of the discriminator is a dc error voltage that indicates the degree of mistuning
between the transmitter and the local oscillator. In this particular example let’s assume that the IF is 30
megahertz. If the output of the mixer is correct, the discriminator will have no output. If the mixer output
is above 30 megahertz, the output of the discriminator will be positive dc pulses; if the mixer output is
below 30 megahertz, the discriminator output will be negative dc pulses. In either case, this output is fed
through an amplifier to the control circuit. The control circuit adjusts the operating frequency of the local
oscillator so that no mistuning exists and the IF is 30 megahertz. In this example the local oscillator is a
reflex klystron and the control circuit provides he repeller plate voltage for the klystron; thus, the klystron
directly controls the local oscillator frequency. In this manner the local oscillator is maintained exactly 30
megahertz below the transmitter frequency.
Q34. What is the greatest limiting factor in a receiver’s detectable range?
Q35. What type of receiver is most often used in radar systems?
Q36. What IF frequencies are normally used in radar receivers?
Q37. Which component of the receiver produces the signal that is mixed with the received signal to
produce the IF signal?
Mixer
Many older radar receivers do not use a low-noise amplifier as the receiver front end; they simply
send the echo signal directly to a crystal mixer stage. A crystal is used rather than an electron-tube diode
because, at microwave frequencies, the tube would generate excessive noise. Electron tubes are also
limited by the effects of transit time at microwave frequencies. The crystal most commonly used is the
point-contact crystal diode; however, recent developments in the field of solid-state microwave devices
may soon replace the point-contact diode with devices that produce even less noise. The Schottky-barrier
diode is an example of a relatively recent development that produces less noise than the point-contact
crystal.
The simplest type of radar mixer is the SINGLE ENDED or UNBALANCED CRYSTAL MIXER,
shown in figure 2-24. The mixer illustrated uses a tuned section of coaxial transmission line one-half
wavelength long. This section matches the crystal to the signal echo and the local oscillator inputs. Local
oscillator injection is accomplished by means of a probe. In the coaxial assembly, the signal is injected by
means of a slot. This slot would normally be inserted in the duplexer waveguide assembly and be properly
oriented to provide coupling of the returned signal. In this application, the unwanted signals at the output
of the mixer (carrier frequency, the local oscillator frequency, and sum of these two signals) are
effectively eliminated by a resonant circuit tuned to the intermediate, or difference frequency. One
advantage of the unbalanced crystal mixer is its simplicity. It has one major disadvantage; its inability to
cancel local oscillator noise. Difficulty in detecting weak signals will exist if noise is allowed to pass
through the mixer along with the signal.
