Impressed by the qualities of the David III stereo coder (already shown here before), I decided to build a close replica of the Inovonics 531 FM modulation analyzer.
The only changes are the controller board, which I replaced with an ATmega2560, and the RF frontend block. The firmware closely follows the original features, although my inspiration came only from the manual and a short presentation video on YouTube.

Of course, this project has no commercial purpose – it’s intended purely for personal use in aligning stereo coders and FM transmitters.

Demo:

Fabulous work! Well done!

It must be good if Albert says so

It is just amazing. I can't find words that can level it 100x higher than that.
Only thinking about getting the firmware done, figuring out all the schematics and long hours making the PCB's and getting the components, it is just daunting.
And then the case to fit it all.
Wow!

Really nice work, especially those panels. Looks like a commercial product (in a good way)

Will you be publishing design files? I've wanted to build a modulation analyser for a while

Agree with the above - really really nice work.

It's difficult to get hold of a decent modulation analyser at a sensible price. I did manage to get a copy of the old Breakaway Broadcast software based modulation analyser which can be used with an SDR dongle for off-air reception but it seems this is no longer available and I don't have the installation files any more.

Bomba , fabulus ...

I would like to make an analoque tuned rf filed meter for some purposes , that i can dial some frequency , with an external or step attenuator , any help , bros.
i have one from an old electronic magazine , but have no att and it is for 1 specific fr , if i can replace the tuned circuit with another wideband one ?

The PCB layout files, firmware, and other details can be found here:
https://www.qsl.net/yo4hfu/FM_Mod_Analyzer_531.html

The page is under construction; I need some time to transcribe all of my notes.

This project was quite interesting. The main challenges were developing the software to control the bargraphs, multiplexed LEDs, and PLLs over a single SPI bus, and performing the alignment to reach performance close to the specifications.

Although I used only precision components (1% resistors and 2.5% capacitors), the amplitude and phase matching of the low-pass filters is not perfect above 10 kHz. Adjusting them according to the manual is nearly impossible, as many of the settings influence each other. The solution was to use a tracking generator sweeping between 50 Hz...15 kHz.

Stereo separation (external MPX) is incredible, over 60 dB, fed by a homemade Inovonics 718 (David III).

It all started as a curiosity, and then I kept adding one section after another.
A bit of madness...

Wow that's a huge level above what I am used to seeing in the hobbyist community!

Thats a nice one!

reverend wrote: ↑Fri Sep 05, 2025 2:12 pm Agree with the above - really really nice work.

It's difficult to get hold of a decent modulation analyser at a sensible price. I did manage to get a copy of the old Breakaway Broadcast software based modulation analyser which can be used with an SDR dongle for off-air reception but it seems this is no longer available and I don't have the installation files any more.

I don’t know if its any use, the pira p275 https://pira.cz/fm_broadcast_analyzer/

radionortheast wrote: ↑Sun Sep 07, 2025 9:21 am
I don’t know if its any use, the pira p275 https://pira.cz/fm_broadcast_analyzer/

That's like trying to compare a Rolls Royce and a Trabant!

Albert H wrote: ↑Sun Sep 07, 2025 5:19 pm

radionortheast wrote: ↑Sun Sep 07, 2025 9:21 am
I don’t know if its any use, the pira p275 https://pira.cz/fm_broadcast_analyzer/

That's like trying to compare a Rolls Royce and a Trabant!

I take that Trabant! You can keep the RR!

Yeah a rolls royce would problalby get nicked, I did used to like them when I was little

I was thinking of this, the only way I can think of doing this could be done, don’t think it would work, it would be more or less like the a team getting back together, assuming theres noway to zoom in on the sdr look at the audio spectrum, which would be the easiest thing do, would surprise theres no program out there that could do it, maybe some ham program?.
Maybe record the audio from the tx it would have to be unfilttered above 15khz, then open in audacity, I only remember doing this from a microphone, you see the audio frequencies, you could maybe see the 19khz tone, very much a long shot. I suppose you’d have to have something to compare it too, recordings of other stations.. if it worked it would not cost anything to do

I've used the "FM MPX Spectrum View" feature on SDRSharp as a wannabe modulation meter. 0dB on the graph seems to align with 75kHz deviation, so set your pilot level to -20dB etc

Theres a mod level on my tef86 radio, it is green up to 70, then goes red, I just realised this the other day many people have this radio it could be quite useful, i’m not sure were the 75 is since there are no markings after 70, afew stations do go way beyond it though. (it is below the s meter) no pilot level though

video below shows differences between different models incase anyone is interested, I got the black one shown 2nd in the video

Since the experiment of cloning the 718 and 531 was interpreted by Inovonics as a possible copyright infringement, and they were concerned that I might sell these devices — although I never had such an intention — I decided to remove the related posts and additional files.

Regards.

Hmm, how they did find about it? Do they read this forum