by dare4444 Wed Dec 22, 2021 8:21 am
Joined: Mar Thu 01, 2018 12:30 am
Posts: 833 |
I did some calculations. It is a Switch mode self oscillating PWM transmitter. Here PWM really shines!
It will work at 700KHz. The 100pF NP0 capacitor sets the frequency. It's basically a self oscillating class D amplifier. The output of CA3130 varies with amplitude of the audio signal and when its output voltage varies and up or below the threshold set by the noninverting input, it turns on/off in accordance with the audio signal, thus generating PWM modulation. A complimentary transistor pair 2n3904 and 2n3906 buffers the PWM RF signal and a low pass filter then converts it into a sine wave. The CA3130 should work up to 1MHz. A NE5532 should work at 1.5 - 1.6 MHz and giving this miniature self oscillating PWM modulator the maximum range. Vary the +VCC supply or add two resistors in collectors of both output transistors to keep Pin to the final stage 100mW or less. I have paralleled all the gates of 40106. Probably an overkill but it doesn't matter to me.
With an antenna matching network and 9 foot wire antenna should cover your entire home & front/back yard with high fidelity audio transmissions on the MW band. My calculations showing a frequency of 700KHz.
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