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Old 1st Apr 2017, 7:57 pm   #1
ParcGwyn
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Default Spectrum Analyser

I have just knocked up a simple spectrum analyser based on a design by A. Helfrick, Radio Design Jan 1998, and similar to the design by G4PMK in RadCom Nov 1989. I was surprised by how easy the circuit was get to get working even though I constructed my own 145MHz BPF. Signal shown is at 9MHz with second harmonic visible at 18MHz, RBW filter was around 280kHz. VCO was controlled with a triangle wave from a function generator, scope timebase was also triggered from this generator.
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Old 1st Apr 2017, 8:24 pm   #2
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Default Re: Spectrum Analyser

Wow - (roughly!) how does it work? I'm used to (using, not making) FFT-based AF analysers running huge amounts of code - which this clearly isn't.
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Old 1st Apr 2017, 8:42 pm   #3
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Default Re: Spectrum Analyser

That's very interesting! Curiously, the original article is availble as a PDF on this forum; post #12 https://www.vintage-radio.net/forum/...ad.php?t=83710.

Didn't you have problems sourcing parts for a circuit dating back so far?

B
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Old 1st Apr 2017, 8:46 pm   #4
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Default Re: Spectrum Analyser

Like a superhet receiver. The LO is swept by the signal applied to the X-input, and the IF is rectified and applied to the Y-input. So you get a graph of the signal amplitude at a given frequency, against frequency.
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Old 1st Apr 2017, 9:13 pm   #5
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Default Re: Spectrum Analyser

The noble art of breadboarding!

Sometimes the bits might not be available, but often you can find something similar enough that you can bend a design to use. If adds to the fun, and to the feeling of achievement.

In the early 1980s I breadboarded a 9-stage successive detection logger for an analyser like this. 280kHz was done with FM tuner filters. A crystal filter for a PMR radio gave me a 30kHz bandwidth. That logger can be done now by one chip from Analog Devices inc.

Al Helfrick taught Avionics at Embry-Riddle university in the USA - He wrote rather a good textbook. Good reading for the folk on here wanting to do an airband receiver.

Roger, G4PMK is now GM4PMK and does amateur radio by moonbounce. He's also the british agent for the german technical publication 'Dubus'

David
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Old 1st Apr 2017, 9:38 pm   #6
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Default Re: Spectrum Analyser

Very interesting design looking at the referred article and not too many difficult to obtain ICs.

Reckon I might have a bash at putting one of these together in the future. Certainly good enough for my requirements and cheaper than a nice HP/Agilent one
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Old 1st Apr 2017, 10:05 pm   #7
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Default Re: Spectrum Analyser

Ah, it's the thin end of a wedge.

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Old 1st Apr 2017, 11:27 pm   #8
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Default Re: Spectrum Analyser

Great stuff! My first homebrew analyser used a TV tuner (producing a 30~ MHz IF), then a second conversion (NE602) down to 10.7 MHz using FM broadcast filters and an NE604 producing an RSSI output driving the Y-input. It worked remarkably well although the response was not flat. Dynamic range was about 60dB. This would be in the early 90's I think.
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Old 2nd Apr 2017, 3:35 am   #9
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Default Re: Spectrum Analyser

Didn't one or both of the published analysers mentioned in the original post use cable TV tuners for their greater frequency range?

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?

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Old 2nd Apr 2017, 10:02 am   #10
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Default Re: Spectrum Analyser

Definitely an extreme sect. They are attractive as a proposition as a commercial unit is very expensive.

Well I ended up in bed at 2AM last night and have only just arisen but I have a working sweep generator on the breadboard. I've got an HP5328A arriving next week and some bits for the VCO so I'm going to see if I can measure sweep linearity to start with.

I'm going to attempt to use a SA612 as the first VCO and mixer. Not sure how successful that will be yet. Log amp will be an AD variety.
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Old 2nd Apr 2017, 11:12 am   #11
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Default Re: Spectrum Analyser

I look forward to your findings. I spent a lot of time setting up 'break points' to bend the ramp waveform so that the sweep would be linear. A real PITA...
I later ditched the TV tuner and used an NE602 as the front end, but it still needed quite a bit of bending. Obviously simple units like this don't have the stability to use narrow filters for the plotting of filter responses etc, but they can provide useful info on harmonic content and band occupancy etc. I had great fun with mine. I use a Signal Hound SA44b with its companion TG44 these days.
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Old 2nd Apr 2017, 12:14 pm   #12
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Default Re: Spectrum Analyser

Indeed. I've analysed the sweep circuit thoroughly now and the entire circuit actually seems rather complicated for what it does. I reckon I can do better and do away with the negative rail as well which would make it more friendly to running off a nice noise-free SLA battery.

As for redesign, I've got a load of TLC272 opamps which are two in one IC, have much lower bias current and can hit the negative rail. If I use a 7555 as the ramp generator for the above, with a 2n3906 based current source and control the bias with a pot, use a summing amp for centre frequency setting (also getting rid of the 555's low offset), the same X attenuation and subcontract the blanking out to the z input on my scope, things might be far simpler.

I reckon with a wideband noise generator you can do reasonably well to analyse filters and tuned amplifiers. I'm not expecting miracles, but being able to validate something approximately working without having to step a signal generator would be nice.

Fun is the most important thing though
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Old 2nd Apr 2017, 12:41 pm   #13
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Default Re: Spectrum Analyser

Blimey!

First an SSB transmitter and now a spectrum analyser
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Old 2nd Apr 2017, 12:45 pm   #14
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Default Re: Spectrum Analyser

Ah, I worked out a solution to breakpoint linearising a VCO some time ago. The circuit and explanation were published in HP journal April 1982 and Google will find it. The adjustments don't interact if you adjust them in order. And I don't look like Harry Potter any longer.

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Old 2nd Apr 2017, 1:29 pm   #15
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Default Re: Spectrum Analyser

Don't worry about that photo, David. We've all got some like that (or in my case far worse). That's clever stuff. When I first glanced at that breakpoint cct I thought 'but that's what I was using!' But it wasn't quite. Mine looked superficially the same with the series of diodes and pots, but mine interacted like there was no tomorrow.
I got there in the end, with all displayed freq's within a gnat's whisker.
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Old 2nd Apr 2017, 2:18 pm   #16
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Default Re: Spectrum Analyser

Without looking at it, I assume that is a series of diode clamps much like the naff sine shapers you get in your average function generators?
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Old 2nd Apr 2017, 4:17 pm   #17
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Default Re: Spectrum Analyser

Thanks for all the comments.

I don't intend to use this circuit as a completed instrument, it is just a trial to see how well the MC3356 and NE602 work in such an application. So far I am quite impressed. Incidentally there is mistake in both the Helfrick and RadCom designs, a dc blocking capacitor needs to be added in series with the oscillator coil of the NE602. I think that this was reported in a later RadCom. The pin numbers on the one of the op-amps in Helfrick's article are also incorrect. The only component I couldn't source that was used in the original articles was the Toko 145MHz filter although I have found that dip packaged ICs are becoming more expensive than SMD components, this was especiablly noticeable when I looked for a AD8307 log amp chip.

I haven't built a sweep generator but will probably use a 555. The linearity of the VCO was reported by Helfrick to be very good using the varicap diode specified, i haven't checked this as the output from my function generator isn't very high.

The next step will probably be to use a MC3356 as 2nd mixer, oscillator and log amp with a passive first mixer followed by amplification. Not sure about VCO yet.

I have been looking at Hans Summers http://hanssummers.com/spectrumanalyser.html design and like his modular construction technique.

This is the first time I have used the Manhatten breadboarding technique and I like it it alllowed for rapid prototyping but I did make a mistake with the layout, the MC3356 was meant to be turned through 180 degrees allowing a better layout.

Dave

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Old 2nd Apr 2017, 8:45 pm   #18
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Default Re: Spectrum Analyser

I've noticed that with the DIP ICs. In fact some of them are completely unobtainable in DIP form now. Same with some TO-92's! I've found that bitsbox.co.uk sell small carrier boards though which help:https://www.bitsbox.co.uk/index.php?...0355go1oloshl1

You can solder the ICs on them them in seconds with a flux pen.

Hans' analyser I've been reading about as well. The only killer I found is that short SMA patches are either expensive (read £15 each!) or are rubbish (from China). I'm trying to come up with how to work around that as I don't fancy either option.

S53MV has another independent and high performance design worth referring to as well: http://lea.hamradio.si/~s53mv/spectana/vco.html

I'm slowly being turned off on the Manhattan approach. I built an entire bitx20 receiver ( http://www.phonestack.com/farhan/bitx.html ) modified for 40m using it and it was a nightmare when it came to modify it. I'm more of a fan of the modular approach which involves making up lots of old fashioned small double-sided PCBs with a ground plane on the top. Prototypes for each piece are done deadbug and then transferred to PCB later.

Edit: linearization for the S53MV analyser is interesting: http://lea.hamradio.si/~s53mv/spectana/sa.html - about half way down

Last edited by MrBungle; 2nd Apr 2017 at 8:51 pm.
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Old 3rd Apr 2017, 12:03 am   #19
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Default Re: Spectrum Analyser

I have an article on a spectrum analyser that works to 100MHz and based on the SA605 FM receiver chip (that I have) and a couple of MAR-6 devices that I'm breadboarding at the moment.

Quote:
Ah, it's the thin end of a wedge.
...made me laugh as I've had to build a prescaler for my DFM and now have to make a simple sweep generator to plot/calibrate the 150MHz filter! Still, it's a very good learning curve and enjoyable - I'm just wondering what I'll have to build next just to finish this project!
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Old 3rd Apr 2017, 8:11 am   #20
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Default Re: Spectrum Analyser

I initially tuned the 145MHz filter using a 2m handheld nearby and a Bradley CT471 with rf probe. I would be interested to see how your analyser performs.

Dave

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