The following circuits are used for generating a high quality FM stereo multiplex signal that is suitable for driving mono FM transmitters such as the Free Radio Berkeley PLL synthesized unit.
Be sure to download the transmitter modifications file below if you are using the newer FRB "No-Tune" 1W kit.
Construction of this circuit should not be attempted without the use of a decent oscilloscope and an audio signal generator.
It is the operator's responsibility to run the transmitter in accordance
with the frequency regulating authority of their country, it may be necessary to
use an output attenuator on some transmitters.
For the next generation of the modulator circuit (call it Rev D), I intend to get rid of all of the op-amps in the audio path, and replace them with quieter discrete transistor circuits. I also plan on replacing the input low-pass filter circuits with discreet coil/capacitor filters. The tunable coils introduced in rev C1.2 were definitely an improvement over the original design, I intend to work with the same parts.
For the Rev D design, the "simpler is better" design rule will prevail. stay tuned to this space...
As of May, 2003, I have started working on the rev D FM stereo multiplex generator. The audio filter stage is complete. It uses all transistors and sounds a lot cleaner than the op-amp based circuit. The L+R and L-R matrix generator circuit also uses transistors. The only remaining op-amp in the circuit is the final mixer stage which should be easily replaced with a transistor.
Before going further, I decided that it was necessary to have a decent test signal generator, see my new Stereo Test Tone Generator project for details.
Here's the new rev D Audio Filter and Matrix circuit, it's not quite finished yet, expect changes.
This Stereo Clipping Indicator circuit connects to the Audio Filter.
The rest of the circuit will include the 38Khz and 19Khz oscillator, the 38Khz modulator, and the final mixer stage, hopefully coming soon. Except for the final mixer, the rest of the circuitry should be fairly similar to the D. Sill design.
D. Sill Schematic in
Gif format(82kb)
D. Sill Schematic in
PostScript format(250kb zoomable)
Important Construction
Notes This is a MUST READ.
Alignment Instructions for
the rev C1.2 modulator.
Top photo of home-made
circuit board(95kb)
Bottom photo of home-made
circuit board(90kb) Wires are used in place of top PC layer.
Zip file bundle
containing the new schematic, docs, boards, pix(3.5mb zip) Modem downloads
will be slow.
Be sure to download the Construction and Alignment Docs above.
Alternate Site(3.5mb
zip) for the Bundle
Another Alternate
Site(3.5mb zip) for the Bundle
Construction notes and scope traces(1.9mb PDF) From J.A. Álvarez Amorós
"Lord Milson" sent me two CAD files for this circuit board in Protel format, I have no way to test them out,
but you may find them to be useful.
protel1.zip (380K)
protel2.zip
(460K)
FRB 1W No Tune
Xmitr Modifications that are required to use this circuit.
Useful Notes concerning FM
transmitter stereo modulation.
Dual stereo mixer plus
mic front end for the stereo modulator.
Dual stereo mixer
(PostScript format)
I have located a source of hard to find 38khz crystals, Digi-Key,
1-800-344-4539, part number SE3314-ND, Epson America C2 type. DigiKey's web site.
Digi-Key also carries
the Toko coils that are used in this circuit.
README file with
detailed circuit description
Hand Wired Circuit
Board (76kb)
Front End circuit in GIF
format (10kb)
Front End circuit in
PostScript format (19kb)
Audio Low Pass Filter in
GIF format (10kb)
Audio Low Pass Filter in
PostScript format (18kb)
Oscillators in GIF
format (10kb)
Oscillators in
PostScript format (19kb)
Stereo Matrix circuit
in GIF format (10kb)
Stereo Matrix circuit in
PostScript format (18kb)
FRB 1W No Tune Xmitr
Modifications that are required to use this circuit.
