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May 2021 - Spectrum Analysers on the cheap

For the most part, test equipment for me has been a relatively inexpensive adjunct to my radio hobby. Perhaps surprisingly so because I spend most of my time building radio gear than operating it.

The process is that I take a fancy to a particular project and perhaps take a detour to build a bit of test gear that makes the first project possible. Often times even simple test gear is all you need.

For years I found that an old 40MHz Tektronix oscilloscope did what I needed. No doubt there are test procedures that exceed my knowledge that it couldn't do - but that was fine. That old scope served me for a long time. Then it died in 2016 beyond economical repair - and after a lot of review reading I replaced it with a Siglent SDS1102CML which was in the budget hobby class.It has so much more capability than the old Tektronix that I will very likely keep it as my main 'scope for many years.

Decades ago (where did that time go?), I was converting ex-taxi radios to work on 2 metres and I built my first device that could show signals across the spectrum. That allowed me to easily peak up multiplier stages. That was the "Spectrum Wavemeter".


It was basically, an absorption wavemeter with a sweeping variactor tuning.You would employ an oscilloscope to provide a display.


It was useful inasmuch that on one occasion I could see that my 160m transmitter was also an 80m transmitter at the same time. It wasn't capable of displaying any fine detail. You couldn't actually "measure". It was what is sometimes called "indicating equipment".

I could work out what frequency I was looking at by using the second input to tune a signal generator through - and when it beat with the unknown signal - you had your frequency.

I called it a "Spectrum Wavemeter" because I didn't have the cheek to call it an analyser.

Moving forward, I have used an "SDR dongle" with some free software that far exceeds the above spectrum wavemeter. In this article you can see it being used with a return loss bridge to display the resonant points of a vertical tri-band antenna covering 20/15/10 metres..


Bringing the story up to date there are the two budget options that still rather amaze me.

Firstly there is the TinySA. Buyers in the UK should note the address of the authorised reseller Mirfield Electronics. You will be certain to get a genuine TinySA from Mirfield instead of one of the many knock offs that are known to fail. It is where I got mine.

So popular are these that stock can be an issue. It is worth the wait though at the bonkers price of £70.

The little screen is pretty good but if you like something bigger, just like with the NanoVNA there is independent software to run on your computer and that allows operation, screengrabs and extra measurement options. If that was all I had I would be very happy but there is another option, the excellent SDRPlay receiver.

What makes it exciting though is the free SDRPlay supplied Spectrum Analyser Software by Steve Andrews. The RSP1A (which I have) is currently selling for £112 and do beware for knock-offs of these too. Go direct to SDRPlay or an authorised dealer.

One downside is that Steve's software (or the radio itself) produces odd artifacts that are not real signals. In the following example, there is a signal just below the 12MHz point (just to the left) that does not exist when measuring with the TinySA. It is there even when the device is disconnected.

Which of the above is a better device I can't comment, but I like Steve Andrews software very much and he continues to refine it.

In the snapshot below I am injecting 12MHz into the LO port of a Mini-circuits TFM-2 mixer and 3MHz into the RF port and the I.F. port is connected to a diplexer. 

 

Here you can see a screenshot from the SDRPlay Spectrum Analyser. Measuring the output from the diplexer which has a 50 ohm terminated output produces the above result.



 

You can see that the two input signals produce mixing products and the intention is to select the 9MHz product to use as the receiver Intermediate frequency. There is quite some difference between the input signal amplitude (which you don't want in your intermediate frequency) and the products. The 15MHz product would normally be removed by a later crystal filter.

I have included some more measurements in this album that show a +4dBm drive signal - and also what it performs like without a diplexer (a lot of noise products).

A Rigol or Siglent Spectrum Analyser would cost in excess of a £1000 pounds and there is no doubt that it would perform far better and measure more accurately than the SDRPlay or TinySA. If I thought I had a real need for all that extra wondefulness, and if I understood the technicalities to the level of a professional, well I might have a word with Santa.

But George Dobbs G3RJV often quoted William of Occam who said - "It is vain to do with more - what you could do with less". For many years George managed without a spectrum analyser at all. If he can turn out project after project - as he did, then that way of working suits me too.
If I can test mixers and characterise filters with devices like this (and the NanoVNA), then I feel that I do not need to spend the family budget on fancy equipment.

One last comment brings me back to the Siglent oscilloscope that I began the article with. It also has a "poor man's spectrum analyser" function in the guise of "Fast Fourier Transform" (FFT). Is that any good? Well back when I bought the scope it was pretty crude. Here is a couple of snapshots from my 'scope of a similar measurement as the one above.


Well obviously it works. And the one measurement that you can take accurately is frequency. But as for measuring signal levels, it is pretty hopeless. But if you just needed to check that the mixer was actually mixing, well you can do that.
But then I did buy this oscilloscope several years ago. Has the equivalent budget priced model got better?  I would say so - definitely. Charlie  ZL2CTM has that successor to my budget Siglent and this is what his oscilloscope can do.

Still not a "proper spectrum analyser", but good enough for Charlie to test his mixers and do some basic measurements. On my 'scope, FFT is completely useless if I enable measurement in dB. It kind of works if you select milli-volts. That's what Charlie's more recent 'scope seems to be also displaying and maybe the dB function is still as pants as with my older 'scope. His 9MHz signal is showing as 19.7 on the vertical scale, and if that is millivolts, then that represents around -30dB. There are plenty of charts available on the internet that do the conversion for you.

There are a vast array of hobby oscilloscopes available to us. I would probably buy a Rigol or a SIglent because I know that there are UK companies that act as agents and who have staff that know about the products. I bought mine from Labtronix UK. My friend Nick G8INE got his Rigol spectrum analyser from Telonic and we had a day out visiting their offices to collect it (and then a pub lunch).

Telonic were very welcoming and are now also Siglent agrents. You can probably buy cheaper of both brands on EBay but were there later to be a problem I know that a UK based authorised dealer would be my preference. Back in "olden times" when we had ham radio rallies in the UK is was not unusual to see Telonic there. I don't know of other brands (which might be perfectly acceptable) have the same level of support and availability.
 

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