Evaluating RF filters?

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franko
Posts: 55
Joined: Mon Aug 28, 2017 9:33 pm

Evaluating RF filters?

Post by franko » Fri Jul 12, 2019 5:40 am

Are there instructions or a tutorial on how to use the spectrum analyzer software with an SDRplay to produce a frequency curve of an RF filter (i.e. low-pass, high-pass, band-reject, etc)?

I have some RSPduo's and RSP2's, as well as a nice Siglent waveform generator, but have never used the software and therefore could use any help that might be out there.

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Aerowerx
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Joined: Sun Dec 02, 2018 1:13 am

Re: Evaluating RF filters?

Post by Aerowerx » Sat Jul 13, 2019 12:33 am

That is exactly what I am planning on doing!

I suggest using a tracking generator, as described elsewhere in this forum.

in fact, I just now got mine working, but haven't looked at any filters yet.

You could use a broad band noise generator, but I don't think it would work as well.

Jim
N8EE

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franko
Posts: 55
Joined: Mon Aug 28, 2017 9:33 pm

Re: Evaluating RF filters?

Post by franko » Sat Jul 13, 2019 1:08 am

Hi Jim, thanks for the message. I did a search here on "tracking generator" and came up with nine pages of hits. It's sounding as though this will turn into a somewhat more involved project than I originally envisioned.

Meanwhile I happened to run across the following post on using a broadband noise source with an SDRplay to look at filter performance:

https://www.sdrplay.com/community/viewt ... f=7&t=1562

Coincidentally I have the same noise source that he used. So for my immediate purpose of making a quick hack to look at filters, that will probably perform acceptably. If there's a writeup anywhere on the tracking generator you put together, I'd be very interested in taking a look.

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glovisol
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Location: Piedmont, Italy

Re: Evaluating RF filters?

Post by glovisol » Sat Jul 13, 2019 5:49 pm


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Mike2459
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Re: Evaluating RF filters?

Post by Mike2459 » Sat Jul 13, 2019 5:56 pm

Will your waveform generator do frequency sweeps? If it can then it should work but you may have to some major attenuation of its output. I tried this with my Tektronix FG504 seemed to work pretty good with a fairly flat output from DC to 40MHZ. It tops out at 40.

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franko
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Joined: Mon Aug 28, 2017 9:33 pm

Re: Evaluating RF filters?

Post by franko » Sat Jul 13, 2019 7:13 pm

Thanks to glovisol for the links and Mike2459 re comments on frequency sweeps. I'll have to check my Siglent. As for attenuation, a while back I invested in a really useful 110 dB rotary attenuator that I use frequently when I want to very gradually introduce a signal into an SDR or similar.

For now, I found the broadband noise source that I bought on eBay a couple of years ago:

https://www.ebay.com/itm/SMA-noise-sour ... 1831285650

and connected it through the attenuator to the filter under test and then to an RSPduo, which is connected to a PC running SDRUno.

My first test was of a low-pass filter designed to pass the bands below 500 kHz but block the MW BC band. Here is the result on SDRUno:
low-pass-filter.jpg
low-pass-filter.jpg (176.38 KiB) Viewed 776 times
Interesting to see the filter characteristics. I definitely won't be using this filter when trying to hear WWVB on 60 kHz.

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Mike2459
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Re: Evaluating RF filters?

Post by Mike2459 » Sat Jul 13, 2019 8:47 pm

I am using a home built 2 pole elliptic LP filter with a response very similar to yours. Think the poles are slightly higher in frequency. I'm using it for NDB chasing. There is some noticeable attenuation starting around 500 KHZ.

As to WWVB reception this filter has not impacted it. It could be the antenna port, coupling transformers or even the antenna rolling off the lows. I have mine connected to the Hi-Z port. That should be easy to verify by removing the LP filter and comparing results.

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franko
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Re: Evaluating RF filters?

Post by franko » Sat Jul 13, 2019 9:41 pm

Mike2459 wrote:I am using a home built 2 pole elliptic LP filter with a response very similar to yours. Think the poles are slightly higher in frequency. I'm using it for NDB chasing. There is some noticeable attenuation starting around 500 KHZ.
My biggest challenge on the lower bands is the extremely strong MW AM BC environment we have here in the Los Angeles area -- 37 broadcasters nearby, including a number of very powerful stations 5 to 6 km from me. So one of my biggest priorities is to do everything possible to attenuate those signals. Ideally I'd like to fabricate a band-reject filter that strongly attenuates ~520-1700 kHz, but with sharp skirts so that the 630m (~475 kHz) and 160m (~1.84 MHz) ham bands are not impacted significantly. I talked to one guy who designs and sells RF filters, and he said he looked into this but gave up because the filter design was going to be so complex.
As to WWVB reception this filter has not impacted it. It could be the antenna port, coupling transformers or even the antenna rolling off the lows. I have mine connected to the Hi-Z port. That should be easy to verify by removing the LP filter and comparing results.
Actually I meant that mostly as a joke -- though I was surprised to see my filter's low-end skirt put 60 kHz about 45 dB below most of the LF spectrum. In practicality, WWVB is such a strong signal that I can almost always hear it well even when using this filter.

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Mike2459
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Re: Evaluating RF filters?

Post by Mike2459 » Sat Jul 13, 2019 9:50 pm

Ok I got it, WWVB has a fairly strong signal here in Ohio as well, about the same level as the 64 KHZ noise from the SMPS in my PC's power supply. :)

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glovisol
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Joined: Thu May 10, 2018 6:42 pm
Location: Piedmont, Italy

Re: Evaluating RF filters?

Post by glovisol » Sun Jul 14, 2019 9:53 am

Spectrum 0-1900 KHz with filter.jpg
Spectrum 0-1900 KHz with filter.jpg (183.38 KiB) Viewed 707 times
Franko, this is what you need to get rid of MW interference, it is effective and easy & inexpensive to build, just look at the thread.

https://www.sdrplay.com/community/viewt ... 973#p12847

glovisol
Attachments
Spectrum 0 -1900 KHz no HP Filter.jpg
Spectrum 0 -1900 KHz no HP Filter.jpg (196.96 KiB) Viewed 707 times
LF&MF Elim. Filter 1.RSP-1A.jpg
LF&MF Elim. Filter 1.RSP-1A.jpg (143.4 KiB) Viewed 707 times

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