Randy's Bench

A repository for projects past and present

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A Simple Spectrum Analyzer Part V

With this post we’ll finish up our Simple Spectrum Analyzer and draw up some conclusions based on it’s overall testing and construction. Here’s the noise amplifier I’ll use as an input source which was shown in an earlier post. This is good up to approx. 30MHz or so…


And below our analyzer’s output with this source. Note: were looking at an approx. 6MHz to 11MHz frequency span


I also promised a simple Log amplifier for the project which I’ll leave up to the reader to implement if desired.

So to wrap it up I think this project would be better suited as a core of a nice little single conversion AM receiver. Add a ‘386 amplifier, manual tuning pots and maybe a BFO and it would be a nice addition for the bench. Choose your band of interest and there you go…Enclosure worthy? I’ll leave that thought up to you. Come to think of it I may well do those additions myself!

Next time I’ll be revisiting a previous project that IS in need of a good sprucing up…and that is our Hobby Spectrum Analyzer!



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A Simple Spectrum Analyzer Part IV

Progress report

Our Simple Spectrum Analyzer has been moving along nicely so far. The basic circuitry is completed, the IF gain stage and 455KHz transformer are in place and tuned, and some basic testing is now underway…



Here’s the remaining back end schematic:

Find a higher resolution version here

1st mixer oscillator output (pin7) vs. Sweep ramp


Simple finger test at analyzer input showing 120Hz noise


9MHz signal generator input


Next time I’ll think about a simple logarithmic amplifier suitable for our analyzer project…



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A Simple Spectrum Analyzer Part III


It’s that time of year!

A nice day to be back at the bench I would say. Finished clearing off the driveway this Morning and it promptly began snowing again…great.
With some more manhattan pads now at my disposal it’s time to move on to the analyzer’s front end section. First we’ll look at the revisited schematic I’ve drawn up below:

Look here for a higher resolution version of the schematic…

I like to use these LM317T TO-92 regulators when working with these mixer IC’s, as well as other suitable projects. They are quite versatile and add just a few extra components. Of course an LM78L06 would be just fine. I’m adding an extra direct input to the analyzer which will employ a -20dB attenuator (not shown) which I lifted from the Hobby Spectrum Analyzer design. The two varactor diodes I’m as yet undecided about. I may start with two 1SV149’s which have a rather broad range (~30 – 450p) and see where it goes from there. I’m using two T50-2 cores for now but may migrate to 6 material down the road.
I think that’s about it for now till I advance a bit further in the build…


12/31/2017 Note: I made a mistake on above schematic – now correct…thanks


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A Simple Spectrum Analyzer Part II

With the sweep generator now completed and tested I’ve uploaded a short video to illustrate the generator adjustment ranges currently available (see below). I’ll leave the four trimpots in place ’till we have the completed project on the bench. We’ll then see if it’s enclosure worthy!

Here we have the completed sweep board and schematic below:

Find a higher resolution image of the sweep generator schematic here.

Before beginning the input and mixer portions of the design I need to punch out additional manhattan pads since my supply has gotten quite low. Also, its time for another toroid order from one of my favorite suppliers The Parts place.  Cecil has generally low pricing and I favor supporting a fellow ham. Check him out!

And then, what did I do with those varactors I neatly stored away?

’till next time

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A Simple Spectrum Analyzer Part I


Leafing through an old issue of Popular Electronics I came across an article by Joseph Carr describing a rather elementary spectrum analyzer for the bench I thought I would tinker with. The article is posted in December 1991’s issue which is located here.










Essentially a single conversion superhet built for the band of choice and instead of an output audio amp in place the output is monitored directly at the detector diode and low pass filter. All that is needed is simple sweep generator to drive the X-input of a scope and additionally the mixer VCO circuitry. I’ve put together a sweep generator based on Hans Summers offering for his Simple man’s Spectrum Analyser located on this page

Additionally one could add taps to the input and VCO transformers to give extra frequency range capabilities to the analyzer. Also, it would be simple to add a AD8307 log amplifier to the design without adding much complexity.
Here’s a look at the partially completed sweep generator card. I originally experimented with a slightly different design which proved to be unsuitable for this project hence the extra 8-pin DIP socket (555 timer).

Below we have a scope plot of the x output ramp. It’s frequency range covers ~8.8Hz to 119Hz. The P-P output covers from 4.96V and 10.5V (without waveform clipping )

I received a box of parts recently from Jameco which includes some nice enclosures one of which will fit the bill for this project…


Next time we’ll get the Sweep circuitry finished up and think about the superhet portion of our analyzer…


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News & Notes

Just a quick bench update…

I’ve decided to forestall the current Z80 D/A project for the time being now that I’ve essentially completed the breadboard wiring…go figure. My enthusiasm seems to have waned at bit. Actually I’d already begun and finished(?) another breadboard project which I’ve not as yet documented. This being a simple analog computer running the classic bouncing ball simulation using the X-Y inputs of my scope for output. I gleaned this project from a 1974 Electronics Experimenter’s Handbook available here. Please note that should the reader wish to duplicate this project there is an error in the published schematic that I identified. Comment and I’ll post the details.

So let’s see what else…Oh yes I brought out the dual conversion SWL receiver again. And yes It’s still plagued by horrific power line noise here at my QTH and likely no end in sight till we move. That’s still down the road a bit. That being said there are occasional moments when 40m (7MHz) comes through the mess and I’m able , as well to listen to 20 and 30m. I’ve added an S-meter to the receiver and it’s about time to chassis it up once my order arrives.

Oh and I’m considering upgrading my license by studying for the Amateur Radio General class exam, quite long overdue of course. I’ll see how that goes…Randy