On Feb 4, 1:27 pm, wrote:

finally, how do you measure the freq of a received signal? what equipment do
you need?

There are many ways. It depends some on just what signal you are
receiving. If there's a carrier, you can use different techniques
than if there is not. From simple and not terribly accurate to
pretty good...you can simply look at the dial (analog or digital) on
your receiver. The old BC221 and LM military frequency meters were
basically just reasonably stable VFOs with a built-in crystal
calibrator and a calibration book, and you could use one of those to
zero-beat with the signal and read off the frequency. You can use a
frequency counter to measure the generator you've zero-beat with the
signal, or alternatively, the local oscillator(s) in your receiver.
For an SSB signal, you should be able to get within 10Hz or so if you
tune the receiver for the most natural-sounding audio; closer,
perhaps, if your ear is good. Then you have the frequency of the
carrier that was removed from the SSB signal. For best accuracy, I
like FFT techniques. I can connect an antenna directly to an FFT-
based spectrum analyzer, and measure carriers to a tiny fraction of a
Hertz in a relatively short time, better than a frequency counter if
you understand the filter shape of the FFT "bins". But if you're
receiving the signal via a path that is not absolutely stable, you are
only determining the frequency of the received signal, not the
transmitted one. During the ARRL frequency-measuring tests, I'm
limited by propagation from Newington to the West Coast. But another
advantage of FFT techniques is that you can see the spectral
"spreading" that accompanies a varying path (or a carrier that's not
stable), and estimate the error. If you measure during a time that
the path is changing monotonically, there may well be a shift that
makes it difficult to figure out just what the error might be; that
happens typically twice a day, whenever the receiving point and
transmitting point are not both in daylight or darkness. Of course,
you need a way to calibrate your frequency reference for the best
possible accuracy. A GPS-conditioned oven crystal standard is
probably about the best you'll be able to do economically most places.

Cheers,
Tom