[PPRAANet] FM Transmitter Fingerprinting

[Paul Signorelli]

I wonder how many other guys do FM Transmitter 'Finger Printing'?

I have had a 'Finger Printer' for some time.

It consists of an FM Receiver and a discriminator pick off that goes to an oscilloscope.

Really simple stuff.

Here is how it works:

When a transmitter is keyed up the radiated signal varies depending on what type of radio it is.

An old crystal controlled radio will come right up on frequency with little overshoot deviation or drift.

There are not many of those on the air today.

All of the other newer radios us a PLL/VCO to generate the transmit frequency and when it starts

there can be positive or negative overshoot, sometimes up to 1 KHz.

All this takes place with in a few milliseconds so you don't hear it but you can see it on a scope.

The polarity and amplitude of the overshoot gives you the fingerprint of the transmitter.

After the transmitter is on there can be additional drift depending on the length of the transmission.

Of course if there is CTCSS/PL on the carrier that adds another fingerprint element.

I have a list of a 'prints' for guys in COS.

It is a fun addition to our hobby.

If you are unfamiliar with it you can read a paper on Finger Printing at:

<http://onlinelibrary.wiley.com/doi/10.1029/2000RS002345/pdf>.

Paul   w0rw