FM Stereo Test Transmitter

This Test Project is a Stereo Multiplex (MPX) and FM Transmitter test, Instead of using special purpose ICs to produce the waveforms this design uses ordinary HCMOS logic. Audio Frequency (AF) signals are produced using binary counters and combinational logic and then filtered to obtain two very low distortion sine waves to serve as the test tones. These audio signals are then multiplexed by a 38 KHz Sub-Carrier to form a Left+Right and a Left-Right Double Side-Band Suppressed Carrier (DSBSC) signal to form the Stereo Multiplex (MPX). No fault and no measurement in the oscillator FM/ Modulation FM Block. The Signal Generator generates the 38 KHz sub-carrier and 19 KHz pilot tone. It uses an ingenious technique to generate 2 sine waves (audio) with very low distortion at around 300 Hz and 1.2 KHz. The 38 KHz sub-carrier is used to multiplex the audio signals and then summed with the 19 KHz pilot tone by the Multiplexer & Summer to form 16 different waveforms (Fig 4). The audio signals being encoded as Left or Right is based on the phase of the sub-carrier. The 16 different waveforms 300 Hz right/ 1.2 KHz left MPX no 19 KHz pilot 300 Hz right/ 1.2 KHz left MPX with 19 KHz pilot 1.2 KHz left only MPX no 19 KHz pilot 1.2 KHz left only MPX with 19 KHz pilot 300 Hz left only MPX no 19 KHz pilot 300 Hz left only MPX with 19 KHz pilot 300 Hz left/1.2 KHZ right MPX no 19 KHz pilot 300 Hz left/1.2 KHZ right MPX with 19 KHz pilot 1.2 KHz right only MPX no 19 KHz pilot 1.2 KHz right only MPX with 19 KHz pilot 300 Hz right only MPX no 19 KHz pilot 300 Hz right only MPX with 19 KHz pilot 300 Hz + 1.2 KHz mono audio only 1.2 KHz mono audio only 300 Hz mono audio only 19 KHz pilot only The state of the Control Logic is advanced by the Manual/Auto Clock. In manual mode it is done by a Push Button with a de-bouncing circuit. In Auto mode it is advanced every 3 seconds ± 0.5 sec. Jumper P2 selects the mode of advancing state of the Control Logic, Manual or Auto.

Step 1: Stereo Multiplex (MPX) and FM Transmitter test

This Test Project is a Stereo Multiplex (MPX) and FM Transmitter test, Instead of using special purpose ICs to produce the waveforms this design uses ordinary HCMOS logic. Audio Frequency (AF) signals are produced using binary counters and combinational logic and then filtered to obtain two very low distortion sine waves to serve as the test tones. These audio signals are then multiplexed by a 38 KHz Sub-Carrier to form a Left+Right and a Left-Right Double Side-Band Suppressed Carrier (DSBSC) signal to form the Stereo Multiplex (MPX). No fault and no measurement in the oscillator FM/ Modulation FM Block.

Fig. 2 Block Diagram of Stereo Multiplex (MPX) and FM Transmitter test

Fig. 3: Audio Signals, Sub-Carrier and Multiplexed (MPX) Signals

The Signal Generator generates the 38 KHz sub-carrier and 19 KHz pilot tone. It uses an ingenious technique to generate 2 sine waves (audio) with very low distortion at around 300 Hz and 1.2 KHz.

The 38 KHz sub-carrier is used to multiplex the audio signals and then summed with the 19 KHz pilot tone by the Multiplexer & Summer to form 16 different waveforms (Fig 4). The audio signals being encoded as Left or Right is based on the phase of the sub-carrier.

The state of the Control Logic is advanced by the Manual/Auto Clock. In manual mode it is done by a Push Button with a de-bouncing circuit. In Auto mode it is advanced every 3 seconds ± 0.5 sec. Jumper P2 selects the mode of advancing state of the Control Logic, Manual or Auto.