TABLE OF CONTENTS The need to measure the frequency on each pulse motivated the development of the IFM receiver. The IFM is basically a crystal video receiver with the addition of a frequency sensing method. The usual frequency sensing method is to divide the signal into two paths with a short delay inserted in one path. The phase of the signals is compared and the phase shift is proportional to the carrier frequency
where:
? = phase shift (radians)
f = carrier frequency (Hz)
? = differential delay (sec)
The phase difference is proportional to the frequency, which means that for a given delay, the unambiguous band covered is that for which the phase change is 1/ ?. (For an unambiguous bandwidth covered equal to 1 GHz, the required delay is 1 ns.)
There are several problems in applying the IFM. Some are the same as for CV receivers. Perhaps the most pressing problem is caused by simultaneous signals. When more than one signal is present at the same instant, the phase shift above is not a meaningful measure of the frequency of any of them.
The IFM discriminator output is proportional to the signal amplitude. In view of its use to measure frequency, a natural addition (ahead of the discriminator) is a limiting amplifier. Adding a true limiter makes the discriminator output independent of the input signal level (which means that, in the absence of any signal,...
