4-2
TRANSMITTER FREQUENCY
Whether of the fixed-frequency or tunable type, the radar transmitter frequency should be checked
periodically. If the transmitter is of the fixed-frequency type and found to be operating outside its normal
operating band, the problem is probably a defective part. The defective component must be replaced. If
the transmitter is tunable, the transmitter must again be tuned to the assigned frequency.
Each time a radar transmitter generates an rf pulse, it produces electromagnetic energy. You should
recall from your study of NEETS, Module 12, Modulation Principles, that the square wave used to
modulate the transmitter carrier wave has (1) the fundamental square-wave frequency and (2) an infinite
number of odd harmonics of the fundamental square wave frequency. When this square wave is used to
modulate the transmitter carrier frequency, both the fundamental and odd harmonic frequencies of the
square wave heterodyne with the transmitter carrier frequency. The heterodyning process produces in
each transmitted rf pulse the following frequencies:
1. The fundamental carrier frequency
2. The sum and difference frequencies between the carrier and fundamental square-wave
frequencies
3. The sum and difference frequencies between the odd harmonics of the square wave and the
carrier frequencies
For a complete discussion of this process, you should review module 12.
Actually, the radar energy is distributed more or less symmetrically over a band of frequencies. This
frequency distribution of energy is known as the FREQUENCY SPECTRUM. An analysis of frequency
spectrum characteristics may be made with a SPECTRUM ANALYZER. The spectrum analyzer presents
a graphic display of energy versus frequency. An extensive explanation of spectrum analyzer use can be
found in the Electronics Installation and Maintenance Book (EIMB), Test Methods and Practices,
NAVSEA 0967-LP-000-0130.
Spectrum Analysis
When properly performed and interpreted, a spectrum analysis will reveal misadjustments and
troubles that would otherwise be difficult to locate. Therefore, you should be able to perform a spectrum
analysis and understand the results.
You may be wondering why we are so interested in the frequency spectrum of an rf pulse. To better
understand why, look at the spectrum of a transmitter as compared to the response curve of a receiver in
figure 4-1. The receiver's response curve has a broader bandwidth than the transmitted spectrum, which
ensures complete coverage. But the receiver responds best to frequencies in the middle of the bandwidth.
This causes the receiver response to taper off from both sides of the center frequency until the response
passes through the half-power points, as shown on the curve. Usually the receiver response beyond these
points is too low to be useful and is not considered. Notice that the spectrum of the transmitter is centered
inside the response curve of the receiver, thus yielding maximum efficiency.
