Frequency stability test on a dvb dongle and a "ham it up" using a GPS receiver


With the Ublox lea-6t GPS receiver you can use it to output a square wave from one to 10 MHz. It's derived from a 48 MHz Crystal but it's kept accurate with atomic clocks on the GPS satellites.

To set the frequency of say 871 kHz you can use u-center as the following picture shows.

Then using the ham it up and dvb dongle you can tune into the frequency. There's no need to attach any wires to the GPS module as the signal can be received even a few meters away from the GPS module.


After about 20 minutes I calibrated the received signal to 871 kHz

After leaving it for a little over an hour the reported frequency dropped by 220Hz.

This setup is incredibly sensitive to any change in temperature, air flow, capacitance, etc.

below is a screenshot over about 30 minutes, it has a distinctive slope and when I moved around the room it went haywire.

Comparison between using the ham it up and not using the ham it up

First I set the gps module to 3,579,545Hz (the color burst frequency). As the GPS module outputs a square wave odd harmonics are produced; using the 7th harmonic of 3,579,545Hz we get 25,056,815Hz. This frequency my dvb module can receive without the need for the ham-it-up.

Both the ham it up and the dvb module had about two hours to stabilize.

Tuning the dvb module the 25,056,815Hz signal I produced a waterfall output with a window of 30 Hz for about 30 minutes; this can be seen in the following figure.

As you can see the frequency varied little more than 1Hz over this time.

This I then compared by receiving the fundamental frequency of 3,579,545Hz; once again I used a 30 Hz window for about 30 minutes.

This time the frequency varied by 42Hz in 30mins (it went past the 30Hz window). It has a much more chaotic look to it and is sensitive to anything that happens in the room.

So at 25,056,815Hz without the ham it up the signal varied by about 0.04ppm while at 3,579,545Hz with the ham it up the signal varied by about 12ppm.

So it looks like the short term frequency stability of the dvb module on its own is quite good. However, when the ham it up is added the short-term frequency stability is shocking by comparison.



Jonti. Last modified Tue, 28 Jan 2014 02:05:50 GMT.