FFT/spectrum analyzer module for old CRO - Page 1
Forum section: electronics / beginners

author: edy
author_rank: Super Contributor
timestamp: April 26, 2022, 09:58:12 pm
content: I'd like to build a spectrum analyzer (something rudimentary) that I can plug in to my old CRO which I can use as the display, perhaps using an Arduino. I found some examples online (seehttps://create.arduino.cc/projecthub/shajeeb/32-band-audio-spectrum-visualizer-analyzer-902f51). I thought of trying to possibly adapt it to output to an X/Y display on the CRO, or using a trigger mechanism to reset the beam to the left, and then output the spectrum as the beam sweeps to the right.Has anyone tried this? Would you recommend using an Arduino or do you think building this out of simpler components is going to be way too much effort? I'll have to pick apart the Arduino code in the page above, as it seems to map it to LED display. I'll have to instead create an algorithm to make it display to an old CRO. If I use the trigger/sweep method, I'd just send a "blip" to trigger the CRO and then have it sweep at 1/100th second (for example) and have the Arduino send out the level at each frequency (from the FFT calculation) from low to high so that outputs the entire set of data within that 1/100th second sweep, drawing the spectrum from left to right side of the CRO display. For this I'll need to use a R-2R ladder across several digital output pins to create my DAC to draw different levels.Alternatively if I use X/Y methods I'll need to "draw it" on the display, will need two R-2R ladders for fine control of both coordinates and can probably do some more sophisticated stuff but at this point I don't want to get into "feature creep" as I'm sure this project has serious room to expand.Any thoughts would be welcome. I haven't played around with the code yet but just trying to understand what is going on. Thanks.

author: Whales
author_rank: Super Contributor
timestamp: April 26, 2022, 10:52:52 pm
content: That could work. The real question is your application: just for aesthetic purposes?Not sure of the noise performance of an ATMEGA's ADC running at 38KHz, I wonder if you get the full 8 bits or something less?You could get away with a single signal cable if you output a tall sync pulse at the start of the graph and design the width of the graph to match a timescale setting on your scope. That way two wires (for X and Y) are not needed.

author: edy
author_rank: Super Contributor
timestamp: April 28, 2022, 01:30:50 pm
content: Quote from: Whales on April 26, 2022, 10:52:52 pmYes, it is purely for aesthetic purposes and something simple and fun to try and display on my old CRO. I could display both original audio signal (time domain) and spectrum (frequency domain) as channels 1 and 2 on the scope, one above the other. Doesn't have to be accurate. It is inspired by all these spectrum analyzer videos that I see on the Techmoan YouTube channel. It doesn't have to be that accurate, just look interesting and somewhat match the music. It will be mono as I don't think the Arduino has nearly enough power to handle that, let alone stereo.Correct, as I mentioned I could either "draw" the FFT output using a trigger sync pulse to set the beam left and then time it to match the timescale, or do X/Y. The first method (and what you mentioned as well with the tall sync pulse) seems to be easier, needs less components (e.g. only 1 R-2R ladder) and one wire to the scope.Since the code is readily available already, this is something that hopefully isn't too complex to understand and hack around with and I can try something hopefully simple enough to actually finish (which is my biggest problem).

author: Terry Bites
author_rank: Super Contributor
timestamp: April 28, 2022, 04:54:11 pm
content: Essentailly its an AM superhet radio with a swept local oscillator- you might be able to derive the sweep from your scope. The big problem will be band switching- this requires the Tuner LO, RF amp to track.Many aeons ago I buit a crude analyser around an SA605 ic. The SA615 is the newer version. The RSSI output can be used as the detector output- it works quite well, not bad at all. I used a counter and DAC for the sweep. Scope sync/ trigger uses the counter reset. Long lost schematics I'm affraid. You need a multi range linear VCO to drive the lo input.

author: RoGeorge
author_rank: Super Contributor
timestamp: April 28, 2022, 09:43:47 pm
content: Tektronix used to have SA adapters for oscilloscopes, fully analogue,w2aewmade 2 videos about 1401A:https://www.manualslib.com/manual/1529406/Tektronix-1401a.html?page=103#manual

author: sahko123
author_rank: Frequent Contributor
timestamp: April 28, 2022, 11:53:18 pm
content: I think for something running on an arduino a superhetorodyne converter is a bit much. An arduino as well might not be able to process an fft at the same frequency as it can take in data. For something purely aesthetic fft would be the way to go for like audio frequency stuff. A superhetorodyne converter would work better for higher frequencies.Funnily enough my first post on this forum was about a similar project idea i had!