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!