High Fidelity FM Stereo Modulator

High Fidelity FM Stereo Modulator (Rev E)

This page was last modified on April 14, 2008.

Rev E Design

This circuitry is used for generating a high quality FM stereo multiplex signal that is suitable for driving mono FM transmitters. The Rev E FM stereo multiplex generator is the most recent design in a series of circuits that have appeared on this site. The older designs should now be considered obsolete. This version features improvements to the audio mixer stage, better gain adjustments in the audio filter stage and a rework of the multiplexer stage. When it is adjusted properly, the Rev E design produces an FM stereo signal with excellent fidelity. The Free Radio Berkeley 1W "No Tune" PLL synthesized unit (year 2000 version) was used as the transmitter for this project. If you use the FRB board, these modifications should be performed on the transmitter. Wavematch Communications has a 2W mono FM transmitter that looks like it would work with this circuit, some audio front-end mods may be required. The modifications involve the removal of the audio input filtering components and the addition of an input level control. The input filter mod allows the 19Khz and 38Khz signals to pass through to the transmitter's modulation stage.
For the best signal coverage, an efficient and properly tuned antenna is a must. This J-Pole antenna works quite well and can be grounded for some protection against lightning.
It is a good idea to have a stereo compressor between the signal source and the transmitter, when properly set, it can save the operator from having to "ride" the volume level between the point of good modulation and the point of distortion. The Alesis 3630 compressor looks like it would be a good choice, although this has not been tried. This circuit does not perform pre-emphasis (high frequency boosting), a graphic equalizer in front of the compressor can be used if desired.
This is a fairly high-level project, it should only be attempted by someone with a fair amount of electronics experience. A decent oscilloscope and an audio signal generator are necessary for aligning this circuit.
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.

Theory

The mixer stage involves a passive circuit for the two stereo inputs. The microphone signal is first boosed with two stages of op-amps, then split into the left and right channels with a double 10K audio taper potentiometer. The two stereo inputs are filtered for RF energy with LC Pi filters. The outputs of the stereo input and mic volume controls are padded with 5.1K resistors, this prevents the control from one channel pair from changing the volume of another channel pair at the top settings. The left and right signals are fed through individual master volume controls, these can be used to set the maximum signal level and balance the channels. The 2K pad resistors on the master volume controls prevent scratchy sounds that show up when turning the controls all the way up. The stereo audio filter board has two filter stages and transistor buffer stages that provide isolation and gain between the filter stages. The audio signal is amplified, run through a three pole low pass filter, buffered again, then run through a 19Khz notch filter. The 19Khz filter enhances the cutoff of the low pass filter in the frequency range between 15Khz and 19Khz. The LM833N driver amplifiers increase the audio signal level and provide DC drive to the following LM833N sum/difference amplifiers.
The FM stereo modulation waveform consist of a mono (Left + Right) baseband audio signal plus a (Left - Right) AM-modulated 38Khz subcarrier signal plus a 19Khz sine wave pilot tone.
The L-R mixer amplifier is a difference amplifier, it produces the signal that is used to modulate the 38Khz wave via the LM13700 modulator. The L+R+19K+38K mixer amplifier combines the Left and Right baseband audio signals with the 19Khz pilot tone and the 38Khz AM modulated subcarrier to produce the multiplex output signal. It should be noted that the 38Khz signal should really be double sideband supressed carrier (DSB-SC) modulated instead of AM modulated. The next version of this circuit will probably involve replacing the LM13700 IC with a double balanced mixer such as the LM1496 or possible the NE602. Meanwhile, the circuit shown performs well, and is sufficient for low power FM transmitters.
The 38Khz oscillator produces a square wave signal, that is fed to the 4013 flip-flop IC which divides the frequency in half to produce 19Khz. The 38Khz and 19Khz square wave signals are fed to two L-C resonant circuits to produce sine wave signals. The two sine waves are amplified by the sine wave buffer ICs to produce 38Khz and 19Khz sine waves. The 19Khz sine wave is fed to the L+R+19K+38K mixer. The 38Khz sine wave is fed to the carrier input of the LM13700 modulator.
The VU/clipping meter combines the left and right audio signals from the driver amplifier outputs in the TL071 summing amplifier. The combined signal is amplfied by the TL071, then fed to the LM3916N VU meter IC. The colored LEDs are wired as a 5 stage VU meter, the upper 5 stages are connected to the white Clipping indicator LED. The LM3916N is wired to be in the dot display mode, the parallel connections to the white LED cause the LED to light for all levels above clipping, making the clipping indication more visible. Power to the LEDs is taken from the unregulated 12V supply to keep the LED switching noise off of the audio amplifier supply.
The power supply is not shown on the schematic. A 12VDC source is required, this can be anything from a lead acid battery to a wall-wart to a standard analog power supply. Beware that most wall-warts have misleading voltage ratings. The power supply should be rated to produce at least 2 amps. This source is tied to the +12V unregulated inputs.
The +/- 12V regulated supplies come from a MeanWell model DKE15A-12 DC-DC converter supplied by the +12V unregulated supply. Jameco.com sells these devices. This allows the transmitter to be run from a single 12V source such as an automobile or a battery. One +12V/-12V regulated line powered power supply could be used to power the whole transmitter, just tie the +12V regulated and +12V unregulated lines together.
See the Wikipedia article on FM modulation for reference information.

Alignment

Alignment of this circuit requires putting two sine waves of differing audio frequencies into the audio input. This Stereo Test Tone Generator circuit works well for the job. Other sine wave sources can be a pair of audio test generators, one or two PCs running sine wave generator software, or a test tone CD. The sine waves should be connected to the left and right inputs of the mixer board and the levels should be adjusted to 0.2V on the audio input of the multiplexer board. An oscilloscope should be connected to the various test points (tp-#) on the low pass and notch stages, the gains of each stage should be adjusted so that the peak-to-peak sine wave values match the values shown on the schematic for tp1, tp2 and tp3. If properly adjusted, tp1 through tp3 (left and right) should show distortion-free sine waves.
The 38Khz and 19Khz coils should be adjusted for peak values on tp9 and tp8. The scope should be put on tp12 and the carrier level should be adjusted a bit below the point where the 38Khz wave shows no clipping. The carrier bias adjustment and 38K modulation controls are interactive, both should be set so that the modulated 38Khz signal is just below 100% modulation on tp12. All waveforms should be sine, not square. The 19Khz pilot level should be set to 3V p-p on tp8. A properly adjusted modulator should produce only "pretty" sinusoidal waveforms on tp12 and tp4. If the waveforms look square or non-sinusoidal, the transmitter's output will be distorted.
The audio peak level meter should be adjusted so that the white clipping LED just starts to light when the rest of the circuit is running at 100% modulation.
Lastly, the input level adjustment shown in the FRB modification schematic should be adjusted. Run the transmitter into a dummy load and listen to the signal in a nearby FM stereo radio through headphones if possible. With dual sine waves input to the mixer and modulation at 100%, adjust the transmitter input level to the point of audible distortion, then back off a small amount. It can be helpful to increase the sine wave input to purpously cause audible distortion, then tweak the transmitter input volume to the point below where the broadcast sounds "harsh".
Connect a stereo music source to the audio inputs. The signal on the FM receiver should be stereo. If the alignment steps were carried out correctly, the received signal should not be distorted as long as the clipping indicator is off.

Sumber : Low power FM Stereo Radio Station