High Quality Stereo Encoder with NJM2035
This stereo encoder is the perfect solution for those looking for transmitting high quality stereo sound with low cost. This stereo encoder produces crystal clear stereo sound very good and very good channel separation that can match many more expensive stereo encoders that are available in the market.
It's all possible thanks to 38KHz quartz crystal that controls the 19kHz pilot tone, so you'll never go back to calibrate or adjust sirkuit.NJM2035 offer superb quality and manufactured by NJR CORPORATION (JRC), a subsidiary of New Japan Radio, a company known as the world's best manufacturers of high end professional audio semiconductors. This transmitter will work with mono FM transmitter including the TX300 and TX500 are available on our website. The entire series can easily fit on a small "x 1.5" printed circuit board that allows to adjust in a place where space is limited. While building your stereo encoder please take your time and always check with the scheme to ensure that all connections are done correctly.
Supply Voltage: 1.2V - 3.6V MAX
Current Draw:> 3mA
Channel Separation: <25 dB
Signal to Noise Ratio: 67 dB
Operation Temperature: -20 - 75 ° C
Frequency Range: 20Hz - 15KHz
R 2x 47K
R 1x 10K
R 1x 82K
VR 1x 50K POT
C 1x 33uF
C 1x 10uF
C 3x 100nF (104)
C 1x 100pF (101)
C 1x 10pF (10)
1x IC IC NJM2035
1x XT 38 KHz Crystal
How it Works
Stereo encoder consists of three main stages, pre-emphasis, digital encoder and mixer stages.
Pre-emphasis stage is achieved by using two 47K resistors and two capacitors 1NF. This helps to eliminate noise generated during the FM transmission of your audio signal.
The second stage is built around NJM2035 is a digital encoder. All internal blocks except for two audio amplifiers (pins 1 & 14) which acts as a separator made using digital circuits. The first digital circuit is a 38KHz oscillator generated by using external 38KHz crystal (pin 7), 10pF capacitor (pin 6) and 100pF bypass capacitor (pin 5). After 38KHz frequency generated is then buffered and divided into two 19KHz signals with a 180 degree phase difference. Once it is done two frequencies associated with two time division switching MPX digital alternating, one for each audio channel. Here's the audio channel is switched between each other with a total frequency of 38KHz. If you will be able to slow this frequency to 1Hz per second you will be able to hear that this trick is all but. During the first half of the two you will hear the left audio channel and in the second half the second you hear the right audio channel. Due to the fact that the channels are activated by a rapid frequency of 38KHz per second our brain can not recognize that this channel is really switched on and accept this as a continuous audio signal. At the same time another signal from the 38KHz oscillator is divided half to 19KHz. This signal is called a PILOT tone because it will help the stereo decoder in the receiver to slice MULTIPLEX signal (mixed L and R audio channels) and separate them back to the left and right channel audio.
The third stage is a mixer which consists of 33uF and 100nF capacitors and resistors are 82K and 10K. The role of this circuit is to mix the multiplex subcarrier and pilot signals together. The subcarrier multiplex signal coming out of Pin 9 of IC NJM2035 is the sum and difference of both left and right audio channels are activated at 38Khz. PILOT signal coming out of the pin 8 is the frequency of 19KHz that is used to distinguish what channel is currently switched on and without decoding stereo that will not be possible.