Improved FM Stereo Modulator

This circuit is used for generating a high quality FM stereo multiplex signal that is suitable for driving high quality mono FM transmitters such as the Free Radio Berkeley PLL synthesized unit. Be sure to download the modifications file below if you are using the newer FRB "No-Tune" 1W kit.
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.

New Design

In a true internet style collaboration, Daniel Sill took my circuit and a bit of advice from me, added a lot of work, and produced an amazing collection of schematics, pictures, and PC board artwork. Here is the schematic for this revision, dubbed rev C12, stereo separation has been improved and some of the more unusual parts have been eliminated from the circuit. Don't even attempt to build this unless you have access to a decent oscilloscope.

New Schematic in Gif format(82kb)
New Schematic in PostScript format(250kb)
Important Construction Notes This is a MUST READ.
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) This can take 15 minutes or more to download with a modem. Be sure to download the Construction Notes above.
Alternate Site(3.5mb) for the Bundle
Another Alternate Site(3.5mb) for the Bundle

"Lord Milson" sent me two CAD files for this circuit board in Protel format, I have not tested them out, but you may find them of use.
protel1.zip (380K)
protel2.zip (460K)

FRB 1W No Tune Xmitr Modifications that are required to use this circuit (PostScript format)
Dual stereo mixer plus mic front end for the stereo modulator (Jpeg format)
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.
It may be necesary to tweak the values of the two capacitors on the crystal if it fails to oscillate, the values shown worked for me.

Old Design

The Rev C version of the circuit stabilized around 11/98
This version works, but is only here for reference, use the new design.

README file with detailed circuit description
Notes on FM transmitter modulation.
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)

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