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Old 25th Dec 2017, 4:35 pm   #81
ParcGwyn
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Default Re: Spectrum Analyser

Quote:
Dave, would you mind sharing the circuit of your set up to give me a few ideas how to go about the addition of the display?
The X and Y amplifiers were copied from the Y amplifier of a Radio Constructor design around 1975 by Robert Penfold, the tube was a 3RP1A and the biasing was from the data sheet the power supply was a simple series regulator. The schematic was hand drawn I will photograph it in the next few days and post here. A capture of the X and Y amplifiers is attached.

Dave
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Old 25th Dec 2017, 7:16 pm   #82
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Default Re: Spectrum Analyser

That’s quite a neat little circuit. Very minimalist.
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Old 25th Dec 2017, 8:03 pm   #83
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It is good for above 500kHz when driving the 3RP1A, not that it's needed for a spectrum analyser. I didn't thermally tie the fets together and the circuit does drift a bit for the first few minutes of operation. I used BC178s for the output pair and just a pot as the input attenuator everything else as per the circuit. A very simple circuit and more than enough gain when using the G4PMK circuit.

Dave

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Old 26th Dec 2017, 12:13 am   #84
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Default Re: Spectrum Analyser

Thanks for the info Dave, I'll look up the R/C article.

Regards
Symon.
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Old 3rd Jan 2018, 8:05 pm   #85
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Default Re: Spectrum Analyser

Here is a the circuit diagram for the display section. Hopefully it is readable, if you need a higher resolution I can PM it.

Dave

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Old 10th Jan 2018, 10:38 am   #86
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Hi.

Thanks for posting the display circuit Dave and sorry for the delay in replying. I've sent you a PM.

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Old 12th Jan 2018, 4:36 pm   #87
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Hi.

A message to the moderators. With reference to post #86 in this thread, I've tried contacting Dave (member ParcGwyn) on two occasions, last Wednesday and today but so far haven't received a reply. I wonder if there's a PM delivery issue in that he may not be receiving my messages?

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Symon
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Old 12th Jan 2018, 4:39 pm   #88
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Default Re: Spectrum Analyser

If there was a problem, like his in box being full, you'd have received an error message.
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Old 12th Jan 2018, 4:41 pm   #89
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No error messages apparent Graham.

Symon.
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Old 12th Jan 2018, 4:58 pm   #90
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Default Re: Spectrum Analyser

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Originally Posted by Radio Wrangler View Post
It's great to see that not only is there a home-made valve scope gang, there's also a small band of spectrum analyser builders.... a more extreme sect?
Hi

Well, I hope to test out my version of the Radcom G4PMK design over the coming weeks and look forward to using it if it's a successful build.
Talking of extreme sects, I suppose anyone whose contemplated building a PAL Vectorscope would also fit into this category . Seriously though, has anyone tried to build a vectorscope using a redundant oscilloscope for the basis of the design? I've always fancied having a play with one in PAL decoder alignment, and it would be even more fun to actually build one.

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Old 12th Jan 2018, 11:17 pm   #91
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Hi.

I managed to locate my spectrum analyser this evening, it was stored away in the garage in one of the many boxes.
It must have been around 2007 when I built it and it's not 100% complete but good enough for testing.

I made my own PCBs, the RF board being double sided and the other two boards, sweep/video and psu/marker being single sided. I regret not making these latter two boards using fibre glass copper clad board, never mind at least the RF board, the most critical one is.
The housing is an old Pace analogue satellite receiver box with a new modified front panel to accomodate the various pots, switches, and attenuator etc.

If my spectrum analyser turns out to be successful, then I hope to rehouse the electronics in a modified oscilloscope case so that I can have an integral display.
I hope to test my unit over the coming weeks as things have become busy once again plus there are a few outstanding projects that have to be completed before proceeding to this one.

Attached are a few pics.

Pic 3 is the RF Board
Pic 4 is the Sweep and Video Board
Pic 5 is the Power Supply Board which also includes the marker generator


Regards
Symon
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Old 13th Jan 2018, 12:15 am   #92
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Default Re: Spectrum Analyser

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Originally Posted by Philips210 View Post
Seriously though, has anyone tried to build a vectorscope using a redundant oscilloscope for the basis of the design? I've always fancied having a play with one in PAL decoder alignment, and it would be even more fun to actually build one.
I have a Tek 1421 which I have so far failed to sell as part of my charity sell-up (which unlovedness means it needn't cost much!) - feel free to PM me
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Old 13th Jan 2018, 10:21 am   #93
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Thanks Mark, PM sent.

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Old 14th Jan 2018, 12:26 pm   #94
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Default Re: Spectrum Analyser

Hi Symon,
Attached is a redrawn and higher resolution scan of the disply section of my spectrum analyser. you build looks very tidy.

Regards

Dave
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Old 14th Jan 2018, 4:57 pm   #95
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Default Re: Spectrum Analyser

Thanks very much for that Dave, it's much appreciated. I hope to do some initial setting up of the boards in my spectrum analyser later in the week and hope it goes to plan. I will report back with any good news, or request for help.

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Old 14th Jan 2018, 5:26 pm   #96
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Default Re: Spectrum Analyser

I did find that mine benefited from having the rf board screened in separate box, but I have seen others on the net with no screening around the board. Look forward to hearing how well your analyser works.

Dave
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Old 14th Jan 2018, 8:07 pm   #97
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That's a good point about a screening box for the RF board. It would be quite easy to make one from some tin plate soldered together. That's another little project to do. Thanks for the tip.
In my unit, the attenuator including slide switches has a tin plate screening box.

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Old 21st Jan 2018, 2:51 pm   #98
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Default Re: Spectrum Analyser

Spent a few hours on mine today finally.

Turns out 3mm red LEDs from Tayda (1.4 pence each) make better varactors than the official parts. There’s a far more linear response. Consistency is no good though so you have to go through a few until it will sweep the range required. Two in parallel works quite well too.

Got a problem with the output power from the VCO now though at the higher end of the sweep. I think the issue I have is lead lengths which is difficult to manage so I’m going to rebuild the VCO using a different construction method, design and parts.

The plan is to chicken out and integrate it with the first mixer so I will use an SA612AN with LED based varactors as the VCO and first mixer. Also moving to a slightly more robust construction method. Doing it dead bug causes some serious problems because anchoring scope probes seems to bend things and consequentially knock tetchy things way out of alignment. Also need to improve VCO shielding. To give you an idea how bad this got, it was shifting up to 10Mhz from last time I got it out.
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Old 21st Jan 2018, 3:50 pm   #99
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Default Re: Spectrum Analyser

There was homebrew design from the USA (Electronics Now Sept 1996) had a problem with VCO output falling at the low frequency end. The solution was to use the sweep waveform to increase the oscillator collector current at low frequencies.

Dave
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Old 21st Jan 2018, 5:21 pm   #100
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Default Re: Spectrum Analyser

I found out what it was in the end. I swapped the load resistor on the FET out for an inductor. Needed a current source really and the resistor that was there wasn't particularly ideal at the high end of the frequency range.

I'm doing this SMD next time. The average 1/4W carbon films aren't that great at that end of the frequency spectrum!
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