FM Stereo Generator Alignment Instructions (2002/2/1) (c) 2002 G. Forrest Cook Alignment of the FM stereo generator should be done in several steps. An audio sine wave generator and a scope are required. This document is based on the Rev C-1.2 design. Oscillator: ----------- Make sure that you have a 38Khz square wave on the 4011 pin 10, and a 19Khz square wave on the 4013 pin 13. Connect the scope to U9B (LM833) pin 7. Adjust L7 for the highest amplitude waveform, it should be a 38Khz sine wave, approximately 1.5VRMS. Connect the scope to U9A (LM833) pin 1. Adjust L5 for the highest amplitude waveform, it should be a 19Khz sine wave, approximately 2.5VRMS. Left and Right 19Khz Notch Filters: ----------------------------------- Connect the sine wave generator to the left channel input on the board. Set the frequency to precisely 19Khz. Connect the scope to U1A (LM833) pin 1 and adjust the input potentiometer R12 for a non-clipping sine wave output on pin1. Move the scope to U2A (LM833) pin 1. Adjust L2 for the minimum 19Khz level. Perform the same adjustment on the right channel, adjusting L4. Left and Right Clipping Indicators: ----------------------------------- Connect the sine wave generator to both the left and right channel inputs on the board. Set the frequency to 1Khz. Connect the scope to U3B (LM833) pin 7 (TP1). Adjust input potentiometers R12 and R37 fully clockwise (on). Adjust the signal generator amplitude and input potentiometer R12 until the signal on the scope is just below clipping. A clipping waveform will have a flat top. Adjust R9 so that the Left Clipping LED turns on brightly. Connect the scope to U6A (LM833) pin 1 (TP2). Compare the TP1 and TP2 signals, clipping should occur at roughly the same input level. Adjust R34 so that the Right Clipping LED turns on brightly. Vary the signal generator amplitude below and above clipping, both clipping LEDs should turn on at the same point. 38Khz Subcarrier Null: ---------------------- Ground the two audio input signals. Connect a scope to U11 (LM1496) pin 12. Adjust R85 for the minimum 38Khz signal. If the null point is not within the range of the potentiometer, it may be necessary to adjust the value of R81 or R82. Multiplex Level Settings: ------------------------- This process could possibly be done better with a spectrum analyzer, here's how I did the test with the use of a decent quality stereo receiver. Connect the multiplex output (J3) to the input of a low power FM transmitter. The transmitter output should be connected to a dummy load, don't do this adjustment on the air. Make sure that your transmitter does not have any low pass filtering on the input. The transmitter should have an input level adjustment on the audio (multiplex) input. If it doesn't, add a 10K audio taper pot between the stereo generator and the transmitter audio input. The potentiometer should go between the stereo generator output and ground, the wiper (center) should go to the transmitter input. The goal is to achieve the correct balance between the multiplex level, the 38Khz subcarrier level, and the 19Khz pilot level. Mono Setup: ----------- Connect the sine wave generator to both the left and right channel inputs on the board. Set the frequency to 1Khz. Adjust the generator amplitude to the point just below where the clipping LEDs light up. Set the Fine Seperation Adjustment (R108) to the middle. Turn the 38Khz level (R68) to minimum. Turn the 19Khz level (R56) to minimum. You should be able to pick up the transmitter's signal on the receiver. Using headphones, adjust the transmitter input level about 10% below the point where you hear distortion on the 1Khz signal. Now, increase the 19Khz level (R56) until the receiver's stereo pilot light comes on. Increase the 19Khz signal a bit above this point. Typical 19Khz levels shoud be about 10% of the total signal. Stereo Setup: ------------- Connect two sine wave signals to the left and right inputs. Adjust the levels just below the clipping level. Set one generator to 800 Hz and the other to 1.2Khz. Frequency is not critical. Adjust the 38Khz level (R68) up until you hear one tone in one channel and the other in the second channel. Here's the non-scientific part: all three level controls interact, fine tune the input level and the 38Khz level for maximum level and minimum distortion in the receiver's headphones. Tweak the Fine Seperation Adjustment (R108) for the best seperation in the headphones, it may be best to look at one of the receiver's audio channels with a scope, adjust for the best sine wave. If you can't hear any difference, set the control to the center. Reduce the transmitter level to minimum so that the receiver is getting a noisy signal. You may need to move the receiver away from the transmitter's dummy load. Fine tune the 19Khz level until the pilot signal comes on. Having several brands of receivers around can be useful, make sure that they all pick up the 19Khz pilot. The Music Test: --------------- Lastly, connect some raucous music to the input of the stereo generator. Pick music with a high signal level and lots of different frequencies. My favorite choice is "Inca Roads" from Frank Zappa's "Make a Jazz Noise Here". Re-adjust the transmitter input level and 38Khz signal level for the best sounding signal in the receiver's headphones. You may need to repeat the 19Khz signal adjustment. Beware that if you change any of the adjustments, you will need to go through the stereo setup process again. It might be a good idea to mark the potentiometer settings once you are satisfied with the sound quality. That's it, you should now have a working high fidelity stereo generator. If you have a spectrum analyzer, compare your transmitter's signal to that of a local commercial station, they should be similar. Tune around the FM band to make sure that your transmitter does not produce any unwanted spur signals. It is better to undermodulate than to splatter. Once you are satisfied with the quality of your signal, it should be safe to connect the transmitter to an antenna.