Re: Bandpass filter measurement anomalies

Nick Kennedy

That's a very interesting observation, Steve.  

I've not done a good job of keeping plots I've made where I can find them, nor having them properly labeled. Also, most filter plots were done only wide enough to see the shape down to a certain floor, so the half-frequency point wasn't covered.

However!  I do have one plot of a 14 MHz filter that goes below 7 MHz and I see the effect you mention. I see a "mini-me" response in the half frequency area.

It's interesting to try to quantify the harmonic output with this. My peak at 14 MHz is at about -3 dBm and my 7 MHz peak is at about -45 dBm so it looks like my 2nd harmonic at 7 MHz is 42 dB down.

This plot is also wide enough for me to view the 1/3 frequency point, but I don't see evidence of any 3rd harmonic content.


Nick, WA5BDU

On Mon, Jun 20, 2016 at 7:15 AM, sbdick@... [PHSNA] <PHSNA@...> 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?

Also see, in the writeup how the output amplifier is not needed if the type II DDS with its push-pull transformer drives the antialias filter directly with no attenuator, providing a 6dB power increase compared to ther single ended version as shown in the AD9851 data sheet, page 11. With the antialias filter driven directly bu the transformer output and a single 6dB attenuator, you can get anout -9dBm output with no amplifier, eliminating any harmonics created in the output stage. I will try this and report back.

"Digital Steve", K1RF

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