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Old 27th Jan 2018, 10:05 pm   #101
Join Date: May 2007
Location: Redruth, Cornwall, UK.
Posts: 1,525
Default Re: Spectrum Analyser


I am making some progress with the spectrum analyser. I initially checked the power supplies which were fine and then some quick checks that the ramp generator was functioning. I found that I didn't need the inverter IC7 for my oscilloscope. It was simply unplugged and the wire link fitted as per the PCB legend. I left resistors R22 and R23 in place.
When testing, I found that I had problems with the VCO amp that feeds the VCO/first mixer stage. Voltage checks revealed there was no +12V on pin 7 of IC9. On inspecting the track layout I found the problem, there was no track going to pin7. I somehow must have missed this when I made the PCBs! A small TCW wire link was all that was required. Fortunately, there were no other errors or damage caused. The sweep was now working OK.

The first pic is the RF board in isolation with an RF feed close to the 145MHz filter. The sweep input at R4 was shorted to 0V.

The second pic shows it in operation without a sweep feed to the VCO/first mixer and the marker generator off.

The third and fourth pics show the display with the 10MHz marker generator on.

There is a degree of non linearity as can be seen but I think I should be able to improve upon this as I have only done an initial test and a brief setup to see if everything is working.
At present, I don't have a reliable and accurate 145MHz source to check the 145MHz filter though I do get the expected upward deflection of the trace if I place the output of my RF signal generator close to the filter and tune it to around the 145MHz point. My frequency counter is not resolving the RF signal very well from my RF generator so I can't be sure of the generator's accuracy. I believe the problem lies with the RF generator as the waveform is somewhat distorted. It looks like I need to get this sorted out before proceeding further with the spectrum analyser. As Radio Wrangler says, it's the thin end of the wedge! Never mind it's all good fun and I'm pleased so far with the results.

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Last edited by Philips210; 27th Jan 2018 at 10:16 pm. Reason: typo
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Old 28th Jan 2018, 4:58 am   #102
Radio Wrangler
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Join Date: Mar 2012
Location: Fife, Scotland, UK.
Posts: 11,653
Default Re: Spectrum Analyser

Once you're happy with your analyser, a small voice in the dead of night will keep whispering in the back of your mind....

"Tracking generator..... tracking generator..... tracking generator...."

There's no escape.

Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done
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Old 28th Jan 2018, 12:55 pm   #103
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Join Date: Jun 2005
Location: Aberystwyth, Wales, UK.
Posts: 293
Default Re: Spectrum Analyser

Glad the analyser is working, I keep thinking tracking generator. I have got as far as a block diagram, using the local oscillators from the spectrum analyser should remove phase noise enabling a narrow RBW filter.


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Old 28th Jan 2018, 1:35 pm   #104
Join Date: May 2007
Location: Redruth, Cornwall, UK.
Posts: 1,525
Default Re: Spectrum Analyser


There's an interesting description of a tracking generator here:

I didn't realise how complicated they are and that it's basically a reverse process to the way the spectrum analyser functions.

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Old 15th Jul 2018, 6:32 pm   #105
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Join Date: Jun 2016
Location: London, UK.
Posts: 2,958
Default Re: Spectrum Analyser

Been a few months now. Thought I better pop in and say that I have got absolutely nowhere with mine since I last posted. Haven’t had the time unfortunately so have taken on some slightly less ambitious projects.

With respect to tracking generator you should just be able to mix your LO with the 2nd IF frequency and low pass it and that will give a tracking output. Getting it to stay in the passband of the spectrum analyser’s IF filters isn’t easy apparently if you have a relatively low RBW. Also the TG really needs some levelling to get rid of any fixture or cable side effects so you’re actually measuring the DUT. This is easier if you have a computer in the loop.

However I’m becoming slightly disheartened with all of this and the associated performance once I’ve expended the effort and money and I reckon I’m actually going to be up a lot of time if I just buy one. Rigol do some cheap ones (for a spectrum analyser anyway) with TG. I’m considering shelving buying a new car for a bit and just footing that. The wife spent more on clothes last year so there’s no complaining to be done
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