FM Stereo Encoder

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FM radio hobbyists will love this classic FM stereo encoder circuit. The encoder converts any monophonic FM transmitter into a stereo one.

Technology evolves year after year, but FM radio is still on the air. The digital age has not yet conquered the radio and FM radio is still based on classical analogue technology. The majority of commercial FM radio stations broadcast in stereo mode. That is, they transmit a pair of audio channels through a single carrier. This is achieved with the use of a stereo encoder.

The encoder multiplexes the left (L) and right (R) audio channels and produces a composite stereo signal which then, modulates the transmitter. At the receiver side, the composite signal is decoded and the two audio channels are retrieved to 2 speakers. For more details on the FM stereo broadcasting technique, please refer to a tutorial, here.

In the current article we will only deal with a stereo encoder circuit project and its construction details and we will assume that readers of this article are aware of the basic principles of FM stereo broadcasting.

FM-Stereo Generator Block Diagram
Figure 1. Block diagram of an FM-Stereo Encoder


The FM stereo encoder circuit

The block diagram and the actual electronic schematic of the stereo encoder are shown in figures 1 and 2, respectively. The electronic circuit is relatively simple. It uses two digital integrated circuits, a classical voltage regulator, a balanced modulator IC, and some operational amplifiers.

FM stereo encoder circuit
Figure 2. The FM stereo encoder circuit

AC voltage is applied on a classical diodes bridge (BR1). The AC is rectified and then filtered from C8 and C9. IC1 then, stabilizes the voltage at 12V and supplies the rest of the circuit.

The T1 transistor operates as a reference oscillator and generates a reference signal at 456KHz. It is actually a crystal oscillator, but instead of using a crystal, it uses a ceramic filter at 455KHz. C5 is used to slightly offset the ceramic filter from its center frequency and the oscillator finally generates a signal at 456KHz. The 456KHz signal is then fed to IC2 which divides the frequency of the reference signal by 6. This way, at pin 5 of IC2 (at test point A) a signal of 76KHz is produced. Then, IC3 is used to divide the frequency of 76KHz again. A double division by 2 and 4 takes place on IC3 and results in a 38KHz signal on pin 9 of IC3, and in a 19KHz signal at pin 7. The 19KHz and 38KHz signals are to be used as the pilot signal and as the modulating carrier, respectively, for the stereo encoder. These signals are appropriately filtered from two low pass filters. These filters are based on inductors, capacitors and resistors (L-C and RC filters) and produce a pair of pure harmonics on 19 kHz and 38 kHz, at the test points C, B, respectively.

The left (L) and right (R) audio channels are applied to the inputs of the encoder via R24 and R37 potentiometers. R24 and R37 serve as the volume adjusters for each audio channel separately. The stereo audio signal is then passed to IC5-A and IC5-B, to be properly amplified and pre-emphasized for FM. The time constant of the pro-emphasis is set to 50μs, according to FM stereo regulations in Europe.

IC6-B and IC6-A operate as an adder and as a subtractor, respectively. IC6-B is used to add R and L signals, in order to produce the main channel R + L. By the other hand, IC6-A is used to produce the differential L-R signal. The L-R signal is then multiplied by the 38 KHz carrier, on IC4, to produce the sub-channel of the FM stereo signal. IC4 is a classic balanced modulator which is used as a multiplier. The sub-channel is the product of the L-R and the 38KHz signal, and it is actually a DSB-SC (double-side band, suppressed carrier) signal. The sub-channel, from pin 6 of the IC4, is then summed with the pilot signal and the main channel R + L, on IC7. The IC7 acts as an adder and sums up the three signals (sub-channel, pilot signal and main channel) to produce the composite FM stereo signal (at control point D) which then, is used to modulate any FM transmitter on a stereo mode.

How to build the FM stereo encoder

In order to build the FM stereo encoder, you will need some components and the appropriate printed circuit board. The printed circuit board artwork is provided below. The required electronic components, as well as their values, are summarized in the electronic schematic (figure 2).

All resistors are of the 1/4W through-hole type and of low tolerance. For R1 to R21, a tolerance of 5% or better is adequate. The rest of the resistors, however, should be of 1% tolerance or better.
Capacitors C23 to C50 are also required to be of low tolerance and of good quality. Otherwise there will be an imbalance on both the intensity and the pro-emphasis between the two audio channels. You may use any type of small sized and low voltage through-hole capacitors.

The components should be mounted and soldered on the printed circuit board according to the assembly guide of Figure 3. There are also some components that are not to be mounted on the circuit board but they must be wired to the circuit board through cables. These components are the R24 and R37 potentiometers and the D1 LED.  You must also use appropriate connectors for the inputs and the output, and these connectors must also connect to the board through cables. A small transformer of 15V/1A must also be used to provide AC power to the encoder. The 15V output of the transformer should be connected to the AC input of the circuit (on the BR1 bridge).

FM Stereo Encoder electronic board
Figure 3. How to assemble the FM Stereo encoder

Optionally, external VU-meters may also be connected to the encoder. For this reason, there are two outputs available on the encoder. These outputs can be connected to level indicators that may be placed on the front panel of your encoder, to visualize the sound volume level. You may use any modern LED-bar VU-meter or any analogue VU meter, in order to add a modern or a vintage feel in your encoder, respectively.


How to calibrate the FM stereo encoder

In order for the encoder to operate properly, some minimal initial settings are required. To make these settings, you need to use an audio frequency generator, an oscilloscope, an FM stereo receiver and a frequency meter.

You should apply some settings on C5 capacitor, and on R10 and R46 potentiometers. In more detail, the settings should be as follows:

  • Connect the frequency meter to the A test point and set C5 to achieve a frequency equal to 76KHz. Then, verify that there are 19KHz and 38KHz at control points C and B, respectively. If you connect an oscilloscope to C and B, you should observe pure harmonics (sinusoidal signals).
  • Connect the audio signal generator to the right (R) channel input and apply an 1 KHz signal at 1 Vp-p. Connect the oscilloscope to the encoder’s output and set the potentiometer R46 until you observe a signal like the appropriate one showed in figure 4.
  • Connect the FM stereo encoder to the transmitter. Use an FM Stereo radio in order to receive the signal of the transmitter. Connect the audio frequency generator or apply audio to both channels (R and L) and adjust the sound volume (from R24 and R37) for 100% modulation. Set the potentiometer R10 to the minimum level required, so that the FM - Stereo indicator of the receiver is up. R10 sets the level of the pilot signal and you should set this signal to be as weak as possible - to the minimum level required for FM stereo broadcasting. Alternatively, you may perform this configuration by using an oscilloscope instead of the receiver. By observing the composite FM stereo signal of the encoder on an oscilloscope, you may adjust the pilot to about 10% in respect to the maximum amplitude of the FM - Stereo composite signal.

Balance of main and sub-channel
Figure 4. Right Channel only - pilot off; Used to Balance Main and Sub-channel

For more information on how to best configure the FM-Stereo encoder, please refer to the FM Stereo Broadcast Measurement Guide.


Attachments

Printed Circuit Board Artwork for the FM Stereo Encoder

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