FM Stereo Multiplex Generator Rev D Alignment Instructions (c) G. Forrest Cook 2003 Revision Date: June 4, 2003 The overall alignment of this circuit is performed in the following order, these are approximate instructions, expect them to change. You will need two sine wave test signals and an oscilloscope to perform this alignment. This circuit can be used for aligning this circuit: http://www.solorb.com/elect/stereoosc/index.html Audio Stage Adjustments ----------------------- Connect an audio sine wave generator to the audio inputs. Align the gain adjustments of the first two left channel amplifiers. Align the left channel clipping indicator so that it comes on just before the left input amplifier starts flat-topping the sine wave signal. Align the gain adjustments of the first two right channel amplifiers. Align the right channel clipping indicator so that it comes on just before the left input amplifier starts flat-topping the sine wave signal. Both channels should be set to clip at the same input level. If the two input transistors are not exactly matched, set both inputs so that the clipping indicator comes on when the first of the two channels starts to clip. Feed a 19Khz sine wave into the audio inputs, adjust the input levels to below clipping. Tune the left and right channel 19khz notch coils for maximum rejection of the 19Khz audio signal on tp3-L and tp3-R. Align the gain adjustments of the third left channel amplifier. Align the gain adjustments of the third right channel amplifier. The audio stage gains should be set so that when distortion happens on the first stage, the following stages are below the distortion level. Center the L-R and L+R balance controls for a starting point. Adjust the L-R mixer gain below the point where tp5 shows distortion. Adjust the L+R mixer gain below the point where tp6 shows distortion. Connect a single test tone generator to the left and right inputs, adjust both input levels for identical signal levels on TP3-L and TP3-R. Adjust the L-R balance control for a minimum signal on tp5. Connect a test tone generator to the left input, connect an inverted signal of the same signal to the right input. adjust both input levels for identical signal levels on TP3-L and TP3-R. Adjust the L+R balance control for the minimum signal on tp6. You may want to repeat the L-R and L+R gain and balance adjustments. Connect an audio amplifier to tp5 and tp6 and verify that the signals have decent sound quality with music fed into the inputs. Except for the mixer gain settings, the preceeding adjustments should be set and left alone before adjusting the modulator section. Oscillator and Modulator Adjustments ------------------------------------ Tune the 38Khz and 19Khz oscillator coils for maximum sine wave levels. Connect the circut to a transmitter that is feeding its RF into a dummy load. Feed sine wave test tones into the stereo inputs and adjust the input volume level just below clipping. The transmitter's input level should be set just below where distortion is heard. Once this level is found, it shoud be set and left alone. Tune a good quality FM receiver into the signal and listen to the signal with headphones. Adjust the 38Khz carrier level to the maximum level below the point where distortion is heard. Adjust the 19Khz pilot level slightly above the point where the FM receiver lock light turns on. Adjust the carrier bias control so that the 38Khz modulated signal (tp9) always has a small amount of carrier. Adjust the 38Khz subcarrier level below the point where distortion is heard. Slightly tweak the 19Khz oscillator coil (Phase Adjust) for the best stereo separation. If you have a dual trace scope, connect the channels to tp7 and tp8, both sine waves should cross each other at zero on the rising edge of the 19Khz wave. Fine-tuning by ear is still advisable to compensate for phase delays in the LM13700. Repeat all of the adjustments from the mixer gains up to this point, these controls are interactive. Final Adjustments ----------------- Connect a high quality music source to the input of the multiplexer and adjust the inputs to below the clipping level. Perform the following adjustments for minimum distortion and maximum stereo separation: L-R gain, 38Khz carrier level, Carrier bias, 38Khz subcarrier level, and phase adjust. It may help to turn the input levels up to heavy clipping, then adjust the various 38Khz controls to the point where the least distortion is heard. If you make it this far without going nuts, the circuit should be producing a very good FM stereo signal. Print these instructions out and keep a copy with the modulator.