Testing my Noise Source and 2m Band Pass Filter - update

Yesterday I caught up with Jack, VK6KDX's, and enjoy the pleasure of chatting about his projects while drinking his coffee. A great way to spend a few hours.

One of the activities we pursued was looking at the output spectrum of noise sources on some test gear Jack owns. Jack demonstrated his noise source and it works very well for testing 23cm filters. We then had a look at my noise source on the spectrum analyser. It had peaks and looked horrible. It occurred to me this morning that the reason for that was most likely stray RF being coupled into the noise source and being amplified by the mmic chain. I still haven't put my noise source into a metal box and the power supply decoupling could be greatly improved.

Despite the shortcomings of the test set-up it did allow me to see the 2m Band Pass Filter in action. A small shoulder on the left of the response curve could be either stray RF coupling or a result of this being a no-tune filter. But I'm really happy with both the noise source and the filter. Especially the filter. It's always pleasing when you build something that confirms your measurement technique for the inductors is really good. No trim-caps, no squeezing or expanding coils. Measure, design and build. Worked first time with no tweaking.

The spectrum analyser was set to 146MHz, 20Mhz per division. We didn't check or adjust for any drift in the spectrum analyser itself. As expected the response falls off faster on the high side. If you haven't read how I measure nH inductors then check out the post http://vk6tt.blogspot.com/2017/01/measuring-small-inductors.html.

Regards
Richard VK6TT

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