Re: Bandpass filter measurement anomalies


A recent check with a spectrum analyzer on a Type-2 DDS module, whose on-board LPF had been removed, mounted on an Experimenter Board with the "push-pull" output transformer and a 50 ohm termination, showed the 2nd harmonic (of 10 MHz) down about 30 dB from the fundamental. This board does not include the wide band amp.

I'll see about pulling out another Type-2 DDS on a Type-2 PHSNA board with the MMIC amp and see what contribution the amp makes.

I saw the same effect when testing a bunch of our club's BPFs a couple of years ago with a Type-1 PHSNA. See the file "BRARC BPF Sweeps 17 Dec 2013.pdf" in the Files area. On those plots it looks like the 2nd harmonic was 28 dB or so down.

This is one of the few places where the interchangeable LPFs on the Type-2 PHSNA could be a useful feature. You can have a selection of them to use when testing filters at frequencies well below their pass band, where harmonics will give ghost responses. You can usually get another 20 dB or so of harmonic suppression.

Jim, N5IB

---In PHSNA@..., <sbdick@...> wrote :
Hi all. Have any of you observed any strange wiggles on the left edge of bandpass filter measurements?  This had been bugging me for awhile and I had attributed it to bad cabling, non-linearities in toroids, etc but I think I found the cause.  See writeup in the K1RF folder, file "bandpass Filter Measurementr Anomalies".  I think harmonics generated in the ERA-3+ output stane are causing wiggles and jiggles I see on the left side of a bandpass filter response. Has anyone else seen a similar problem?
"Digital Steve", K1RF

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