preparing a signal for analog to digital conversion?

Question

Im working on a project that requires analog to digital conversion. The signal comes from a 10.5GHz transceiver with a 20mA output. Im going to amplify the signal and filter out frequencys over around 10kHz and below 200Hz. Im trying to determine a good circuit to take the signal and interface it with a Motorola HCS12 microcontroller that uses a successive approximation 10-bit analog to digital converter. I want the frequency of the signal, which is the doppler shift from the transceiver. Im going to use this frequency value to determine speed in the microcontroller. I was thinking of using an LM2917 to convert frequency to voltage, hovwever it runs on 28V which seems high, Im going to power the device with batteries eventually and dont want to make the power supply more complicated. Has anyone done a similar conversion? How might you approach the problem? The ADC on the microcontroller has an input range of 0-5V, my frequency used range is going to be around 200Hz-8kHz. Thanks for any suggestions.

Answer 1

Use a comparator to convert the signal to a regular square wave at the frequency of interest. Now if you run that into a one-shot set for 100µs, you will have a train of pulses with a duty cycle varying from 100µs/125µs or 0.8 for a 8 kHz input, and 100µs/5000µs or 0.02 for 200 Hz. This can be implemented with a LM339 comparator and a 555 one shot.



If the signal is 5 volts, and you use a simple LP filter (RC), this will result in a DC level between 4 volts and 0.1 volts as the frequency varies over the range. And that you can apply to the ADC.





Other methods: Use a F/V converter as you mention, which will work on 5 volts. Or use the microcontroller to directly do the job.



For the latter, If the ADC in the microcontroller can convert in a µs or so, you can just let the microcontroller strobe (measure) the input signal as fast as it can, and use software to analyze the data to determine the zero crosses and thus determine the frequency. This will use up a good portion of the microcontroller's CPU, but if it has a good multitasking environment, this will work, tho it will require a lot of software.



Personally, I'd take the first approach above. It's the simplest.





PS, you should study the datasheet of any part you are interested in. Before you use it, make sure you understand every line of that datasheet.



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Answer 2

Actually, 28v is the MAX Vcc it can withstand. You can run all the way down to 5v Vcc. Have look at the datasheet for some suggested circuits.

Answer 3

I think you can just use some sort of clipper to clip the signal off at the proper voltage.