Ludens.cl FM stereo encoder

[richard.cs]

I have designed a PCB for the http://ludens.cl/Electron/fmtx/fmtx.html stereo encoder. Manfred published a PCB design with single-sided tracking and through-hole parts but I wanted an SMT one.

I only want one board for myself, before I order would anyone else like any?*

Changes from Manfred's design:
Nearly all 0603 SMT and SOIC ICs, a few through hole parts retained, particularly around the audio filters
19 and 38 kHz filters completely changed as I couldn't find suitable cores
160 x 100 mm double sided PCB
4.864 MHz crystal rather than 2.432 MHz due to availability.
Some component values rationalised.

I retained the obsolete RC4200 multiplier as they seem to be easy to get and the redesign to use another would have been significant.


I may also do a similar board for the exciter at some point, but perhaps not the PA. This isn't something I actually have a need for it just looked like it would be really fun to build. 

*Please respect Manfred's request that people don't use his design for pirate radio.

[CJay]

I've seen that page before but forgot all about the home made capacitors, wonderful.

I've got a sheet of copper and a half decent supply of mica to experiment with

[TimNJ]

This is just the encoder part?

Ludens.cl is a goldmine!

[richard.cs]

Hi Tim,

Currently just the encoder part but I am planning to do the synthesized exciter at some point (I have not checked component availability yet). There are 3 sections, plus it needs an overall PSU/control thing that wasn't finished. Manfred's board designs are published for all three anyway, I just fancied making a SMT version.

[TimNJ]

Hi Tim,

Currently just the encoder part but I am planning to do the synthesized exciter at some point (I have not checked component availability yet). There are 3 sections, plus it needs an overall PSU/control thing that wasn't finished. Manfred's board designs are published for all three anyway, I just fancied making a SMT version.

Gotcha. Are you using the RC4200 in thru-hole or SMT? Seems that the SMT one is largely unobtainable.

[richard.cs]

Through hole, I made the same conclusions on availability. There seemed a fair number of sellers for the through hole one, I figured I should be able to find a few even if I get some fakes on the way.

I've been looking at the exciter today, both the synth and the prescaler are obsolete and I'd like to avoid a custom crystal so I will look at redesigning the digital bit.

[mrf184]

IMO this is quite an antiquated (and expensive + labor intensive) way to build an FM transmitter; There are all-in-one chips with acceptable spurious performance and far better parts choices for the power amplifier that don't involve many decade old parts that have sourcing problems.

See thread on https://darkliferadio.proboards.com/thread/779/fm-transmitter-ics-review-question

it is a small fm transmitter with mp3 player and class F amplifier, powered from USB; it outputs between 2 and 4 watts of power depending on an inductor value, and includes the lowpass filter as part of the output matching.

[CJay]

IMO this is quite an antiquated (and expensive + labor intensive) way to build an FM transmitter; There are all-in-one chips with acceptable spurious performance and far better parts choices for the power amplifier that don't involve many decade old parts that have sourcing problems.

There are and some of them are really good and you're right it's silly to build something like the Ludens encoder if all you want is an FM stereo transmitter but the all in one chips are not much use if you want to learn how things work or alter them unless you happen to be able to decap the chip and probe the silicon...