11242	2015-06-26 15:26:04	Will	AD2200 synchronous demodulator
Message	Date	From	Subject
Can anyone on this list explain how the ANALOG DEVICES ADA2200 actually works? It is called a synchronous demodulator but I have looked at the data sheet and am none the wiser. No practical circuit shown that gives further insight. Is it only useful for things like strain gauges where the same oscillator generated by the chip is used to excite them and then the output synchronously demodulated. Frequency limits are 25Mhz max and 2.56Mhz minimum. Cheers, Will
11259	2015-06-29 12:18:57	arfghans	Re: AD2200 synchronous demodulator
It's a lock-in amplifier, an instrument that we typically use for low-frequency signal recovery, mostly in physics research (that would be MY business). It's a bit like a direct-conversion receiver, but explicitly uses a phase-sensitive detector. In normal use, the signal of interest has a fixed carrier frequency and some modulation that we are trying to recover. The lock-in's reference frequency (LO, using radio terminology) is equal to the signal carrier frequency. In fact, we use a single oscillator for both applications. That way, the absolute phase as well as the frequency of the signal is known by the demodulator. This is literally "extra information" about the signal and results in an improved SNR at the output. Since the carrier/reference phase is fixed, the lock-in is also fully capable of measuring phase as well as amplitude. Using a pair of AD2200 chips, you can also do an I/Q measurement to obtain amplitude and phase at the same time. Then it's known as a two-phase lock-in. Or an IQ receiver... Most common use? A chopped laser beam passes thru an experiment (for instance, looking at optical absorption phenomena), and a photoreceiver picks up some weak response signal. The chopper is driven by the lock-in reference oscillator, so its phase is fixed. By narrowing the demodulator bandwidth (via lowpass filtering), spectacular SNR results are feasible in the face of large interfering signals. Often the BW is s a small fraction of 1 Hz! The datasheet does have a full explanation of the device, and even calls it a lock-in. Short on applications though. The lowfer guys might use it for the kHz bands. Gary, NA6O Instrumentation, measurement, and control systems at LLNL for 35 years!
11262	2015-06-29 15:19:38	Will	Re: AD2200 synchronous demodulator - Correction ADA2200
Thanks Gary, That confirms my thoughts, uses the same oscillator for sensor and demodulation. With its 2.5MHz to 5MHz range is it usable as a straight forward demodulator for CW/AM/SSB/FM with a fixed oscillator/BFO? Would it be possible to make it work as synchronous version? Cheers, Will
11263	2015-06-30 13:13:41	arfghans	Re: AD2200 synchronous demodulator - Correction ADA2200
I don't think this is a great building block for a radio. Everything about it is optimized for synchronous detection, which means the transmitter needs to share a clock signal with the receiver. Sounds like a VERY long run of coax when working DX! Also, the analog input is lowpass filtered at a frequency that's a fraction of the clock, so in fact it's only usable to a MHz or so. Using it asynchronously, you basically end up with a mixer with filters before and after. Then, to do SSB or other interesting demodulation, you need two of these devices set up with the 90-deg phase shift activated, followed by a suitable I/Q signal processor. Oh, and don't forget you need a computer to program the chip via SPI bus... I think I'd rather build something based on the various flavors of Rick's R2. Gary, NA6O