Using microphone preamplifier IC with AGC to amplify Schottky diode mixer output?

I am currently thinking about using microphone pre-amp IC instead of an op-amp. Why? Because it contains AGC and reduces components count, allowing connect mixer output directly to 12-bit cheap ADC. Some microphone IC pre-amps include filtering at low frequencies, i think it can greatly improve dynamic range of signals could be received without using expensive external ADC. I believe it can't be used in motion sensors, as AGC and thresholding can't work well together (without knowing current AGC level). But for receiving data i think it can solve problem when transmitter is too close (signal is clipped) or too far (using only few bits of ADC resolution). What are advantages and disadvantages?

My main concern: what is minimal usable output of Schottky diode mixer? Would it be enough for such IC? For example MAX9814.

A Schottky diode mixer can produce outputs from almost nothing to tens of volts but they are generally used at higher frequencies than the MAX9814 would operate at.

The AGC also makes it pointless to use a 12-bit ADC as it will make the level virtually impossible to determine anyway. Trying to measure to mV precision after an amplifier with unpredictable gain makes little sense.

Brian.

microphone preamp will typically have a 20 KHz lowpass filter built in, so you can only use it if you know your mixer output will be below 20KHz.

A typical balanced microwave mixer will have 100 mV Peak output when the LO port is driven by +10 dBm, and the RF port driven by 0 dBm. IF you drive the RF port with less power (as in a receiver with varying power levels received) you can get dramatically lower output voltages.

AGC is done in RF receiver amplifiers and the level is called Received Signal Strength Indicator or RSSI and is calibrated in dBm with a lookup table or algorithm.

Some have 100dB range. http://www.digikey.com/catalog/en/pa...tion-rssi/6459

As Brian says, shottky ring mixers have a very wide dynamic range, but can also be very noisy compared to other types of mixers.

The reason for all the noise is that the local oscillator amplitude needs to be very high to make it switch properly. That would not be a problem, except making a completely (phase) noise free high power oscillator is not really possible. Just about all the noise in a shottky mixer comes from the oscillator not the actual mixer.

The mixer just modulates one signal with another signal, and if one of those signals is noisy, the output will be noisy. Needing such powerful local oscillator drive puts a shottky mixer at a severe disadvantage at the low signal level end. It will however have a very wide dynamic range, which still makes them useful for some applications.

The lowest noise mixer will probably be an active mixer of some kind, but that will also overload quite readily, especially if it has an untuned broadband input.

The trend these days is towards active H bridge mixer using mosfets. It has the high dynamic range of a shottky mixer, but requires vastly less oscillator power.

There is no such thing as a low noise mixer if you feed it from a very noisy oscillator.

The OP said "Schottky Diode Mixer" but maybe he means "Schottky Diode AM Detector"? The mixer works at an RF frequency and the detector works at audio frequencies.

I took it that he probably intends a Homodyne type mixer/detector which has two rf inputs and an audio output.

But why doesn't the AM radio already have AGC in the RF parts?

LO Input: any oscillator suitable for required task. It can be dielectric resonator oscillator (DRO) with narrow tuning range around 1..2MHz. Oscillator frequency suitable for required task 2.4GHz ... 100 GHz (or any other required frequency)
RF Input: signal from antenna suitable for required task (directional, omni)
IF Ouptut: output from mixer.
Mixer itself is can be double balanced microstrip mixer, or it can be all integrated circuit. On of the applications i see is data receiver. Frequency range of microphone amplifier IC is enough to recieve some quadrature modulated signals. There are many kind of modem-like signals that fit below 20khz. Cheap DSP processing then can be used with built-in 12-bit ADC.
Why using microphone amplifier?
1. cheap
2. reduces components count
3. allows to use 12-bit ADC without clipping and weak signals (unused high bits).
4. allows to eliminate tuning of sensitivity
although it have constraints on IF frequency (audio range).

Regardless of the pros and cons of mixer types, the OP was (I think) asking about increasing dynamic range by using an audio compressor to limit the measured signal or modulation level. The same answer still applies, there is no point in using a 12-bit ADC with it's inherently high resolution when the signal fed to it is subject to so many variables. If it was only a log compressor there may be some merit in it but the time constant factors plus added distortion and noise will negate any benefits.

Brian.