Testing my Noise Source and 2m Band Pass Filter

Well it turns out I couldn't readily find anyone with the test gear to look at my homebrew noise source. I will keep asking around but in the meantime I thought I'd see if it could be used to determine the response of the 2m bandpass filter I described in this post.

The first thing I did was look at the noise source output across the spectrum in 10MHz bites with my spectrum analyser. It wasn't flat. The was about +/- 2dB from 10MHz to 1GHz.  So any measurements would have to be on a before and after basis.

So I put the 2m filter between the noise source and the spectrum analyser and used the peak hold function as a surrogate for an average over time.  I measured the response every 10MHz from 85MHz to 205MHz. Then I  removed the filter and measured the levels again. Subtracted the difference and I had a response curve.

Here then is the measured results against what was modelled:

If you look closely the measured response is indicated by the 'dot' at each marker. Clearly something is not quite right. Somehow the filter has "gain" at 135MHz and 145MHz. That somehow is not possible. And the measurement at 125MHz looks odd.

I had previously measured the filter response into a good 50 ohm power meter and it was a very close match to what was modelled. Hence, the distortion in the measured results was due to either the spectrum analyser not being a true 50ohm resistive input or the patch leads were not 50 ohms. Either way it shows that the noise source can be used for measurement, but only when the test gear is not influencing the result.

Perhaps the reason for the +/- 2dB response a cross the spectrum is due to the spectrum analyser input impedance. Once I find another piece of test gear to measure the noise source on I will know more. In the meantime, I'm using the mythbusters approach and calling the noise source project finished.

Since this noise source costs peanuts I urge you to build one, or something similar, and comment on how it went.

Regards
Richard VK6TT