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Old 29th May 2019, 3:03 pm   #1
Steve G4WCS
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Default Poor man's spectrum analyser?

Was browsing radios for sale, and noticed a seller had used an sdr radio dongle to demonstrate the output. I thought this was a fantastic idea and wondered if these things are any use as a soectrum analyser , for setting up transmitters and the likes. I suppose if you also have a suitable signal generator it could also be used for peaking tuned circuits/adjusting filters and the like.

Thoughts ?
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Old 29th May 2019, 3:25 pm   #2
MrBungle
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Default Re: Poor mans spectrum analyser ?

Yes this is quite regularly used. You can get wideband noise generators (from aliexpress) and use one of them and the dongle to measure filter response.

I recently got a new kit radio and the alignment process does an "audio domain" version of this which was quite nice. You plug a noise source into the antenna port (I used an AWG's noise output) and then plug the audio output into your computer and use spectogram as an audio spectrum analyser. You adjust the BFO frequency on the radio and watch the 600Hz marker slip into the passband. Very neat.
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Old 29th May 2019, 4:01 pm   #3
G6Tanuki
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Default Re: Poor man's spectrum analyser?

There's various freeware out there which lets you use such things as a spectrum-analyser, yes. A "waterfall" display is also easy enough with the right software - potentially useful if you're looking to spot Sporadic-E openings!

I've also seen one of these used - in conjunction with a pulsed RF noise-source - as a tool for neutralising amplifiers.

You set the noise-source 'pulser' to work at a frequency of about 800Hz to 1KHz - then the amplitude of the tone at the receiver's audio output depends on the ratio of the noise-source signal to the receiver's internal noise-figure. It's a good way to demonstrate that - for something like a neutralised triode or JFET - the best noise-figure rarely if ever coincides with the best amplifier gain!
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Old 29th May 2019, 4:06 pm   #4
RogerEvans
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Default Re: Poor man's spectrum analyser?

They are cheap and fun and I used mine a lot when I was sorting out the code for a home made FPGA driven Arbitrary Function Generator with AM, FM and log/lin sweep. The highest spectral resolution is done with FFTs and you get very noticeable dips where the FFT segments join up. As Mr Bungle says, if you have a broadband white noise generator you can largely calibrate this out. I have the very, very cheap one which is just a radio/TV tuner dongle with some added software to drive it and it doesn't tune below something like 40MHz.

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Old 29th May 2019, 8:37 pm   #5
Steve G4WCS
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Default Re: Poor man's spectrum analyser?

They sound worthwhile having a play with. Yet another project to add to the list of projects I havent time for.
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Old 29th May 2019, 9:43 pm   #6
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Default Re: Poor man's spectrum analyser?

A while back I bought the popular RSP1A SDR. These are a bit bigger than a USB dongle and cost about £90 but you do get a higher performance compared to the cheapo 8 bit RTL SDRs.

https://www.sdrplay.com/rsp1a/

The RSP1A also supports a 'spectrum analyser' mode if you download their recently introduced spectrum analyser software. This works quite well but it is still at V1.0 and is very buggy. But it does have more of a spectrum analyser feel when run with the dedicated software. Otherwise, it can be run as a regular SDR with a spectrum display. But this doesn't look like a regular spectrum analyser display.

Note: I'd recommend fitting an external attenuator and diode limiter if using it to do conducted measurements via coax as I think the damage level is quite low compared to a regular spectrum analyser.
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