Locked Loop Synchronous Demodulator

Posted on Oct 23, 2012

A simple, low component count phase locked loop that locks onto and detects the amplitude of an incoming baseband 7 bit Barker code using a switched resistor demodulator that is driven directly by a microcontroller's output pins. Balanced modulators using resistors and a microcontroller's output pins. Locking into oncoming Pseudo-noise (PN) stream. Microcontroller modulates its own crystal oscillator frequenc inside a Phase-Locked Loop (PLL). Extending the output range of a bipolar op-amp to opearte near ground.

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The only semiconductors in the circuit are the microcontroller and a quad opamp. This picture was taken before the offset adjustment for the signal level measurement circuit was installed. Some surface mount components are on the other side of the board. After a successful synchronous demodulator project, as described in Digital Lock-in Milliohmmeter and Experimental 1 kHz Synchronous Demodulator, I looked for a way to improve upon on the basic synchronous demodulator that I used in those projects. I was looking for a way to reduce the component count, eliminating the transmission gates if possible. I also wanted the demodulator to be able to lock to signal from a remote source, all the while maintaining the benefits of noise rejection and processing gain that one would expect from a synchronous demodulator, because such a circuit might lead to a simple but robust modulation and demodulation scheme for RF communications. The circuit in figure 1 is that of a synchronous demodulator that meets these requirements, and additionally, it operates from a single positive power supply. The circuit will be described in detail later in this page. The ATtiny2313 microcontroller is used as a voltage controlled crystal oscillator and Barker code generator, and could be replaced with small scale logic devices. The code generation is initiated by interrupts from the microcontroller's internal 8 bit timer, and there is plenty of...

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