My FM stereo encoder project

based on the Pira CZ stereo encoder

This project started some years ago..

One of my passions is fixing old vintage radios.
To make things easier I had to buy some good test equipment’s.
So I bought a nice Marconi 2022E RF generator like this one:

Marconi 2022E RF generator

This RF generator could do nicely AM and FM modulation
on all the broadcast bands.
For the AM side there were no problems so far.

But when you talk FM radio repair you need some kind

of FM stereo generation to get the radio’s demodulation section works correctly.

Fortunately my new acquired Marconi had an external modulation input.

So I needed some kind of FM stereo multiplexer unit
that I could connect to my generator…

The search started around the net..

My quest for the very best and not too expensive kit of this kind

brought me to some available solutions.

Like this one based on the BH1417 Stereo Encoder chip

or this one based on the NJM2035 chip

Both of these solutions were ok but when I started to go further

in their circuit specifications I noticed that the audio frequency spectrum

for both of these chips was not quite good and worst of it

there was lots of harmonics generated by the inside multiplexer !!!
And their filter sections were not very sharp, neither efficient..

So I browsed deeper on the net until I heard about

a very nice and accurate stereo MPX FM encoder available in kit.

Introducing the Pira CZ Stereo Encoder for FM Broadcasting

Here is a snapshot of the web site description of the product:

This stereo encoder is a halfway between analogue and digital processing.
It combines the best from both domains to provide high-quality and easy to build device.
The sampling frequency used in this stereo encoder is 97 times (!!!) higher than the pilot tone frequency.
This makes very easy to reject all spectral residues around the sampling frequency without affecting the main signal characteristics.
Using of a microcontroller allows to build this stereo encoder with reduced part count and get excellent results in real operation.

But since this web site did not sell any of these boards

I had to figure out where I could buy one..

Here comes eBay to the rescue !

You can buy a bare PCB of this encoder for less than $18.00US + s/h.

Here is how it looks:

FM Stereo Generator Encoder MultiPLEXER BroadCAST PCB (PIRA) by moutoulos ™

The only thing that is sad about this PCB is that the seller

did not provided any parts kit or even a single parts list..

So I now had to check the original PIRA Encoder web site

for any parts list and description of the actual circuit behaviour.

Fortunately the PIRA CZ original web site

did provide me all the needed infos. I then ordered all the necessary parts.

After testing the PIRA encoder board I noticed that each audio inputs

had impedances of around 2k ohms ! This was not quite handy

for most of the useful audio external devices. Earphone devices (iPhone, Small FM radios)

would be ok but it appeared that these device’s audio levels were not enough loud..

So I needed some high impedance

interface that would bridge the external audio to the PIRA inputs,

raise the overall gain and also thought that 500hz tones would be nice to have

when testing stereo channel separations.

The following schematic shows the frontend that had been

added just between the external source audio signal

and the PIRA original audio inputs.

Frontend circuit that amplifies the external audio and add 500hz tones

So I started to build both PCB’s and test them all connected together.

After some tests I decided to remove both R31,38 2k ohms PIRA’s

Input resistors and replaced them with 10k ohms to ease my two TLC2274

Opamps from driving too much current to the PIRA’s inputs.
Doing so I also replaced C22,23 for 10uf capacitors instead of 220uf.

Now I have plenty of gain on the external audio inputs
AND I can use separate L-R 500hz tones for further tests.
All of these are finally connected to my Marconi 2022E
external modulation input.

The project’s final steps..

First, I started to draw the front panel layout and placed

All the components that would be useful: