# Low Frequency Downconverter

Discussion in 'The Projects Forum' started by GRC, Feb 7, 2012.

1. ### GRC Thread Starter Member

Apr 20, 2009
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I am trying to figure out how I can convert a 40khz signal from a ultrasonic receiver to a signal in the 100 - 1000 Hz range. I don't want to use digital ie divider, as I need to be able to hear the the strength of the signal change as my reflection gets larger.

Thanks
Geoff

2. ### crutschow Expert

Mar 14, 2008
13,475
3,362
You can multiply the signal together with a 39kHz reference signal using an analog multiplier. That will give you the sum and difference frequencies between the two and preserve the amplitude variation of the received signal. You use a low-pass filter to eliminate the sum frequencies and you are left with the difference frequency of 1kHz. Analog Devices makes many such multipliers.

3. ### praondevou AAC Fanatic!

Jul 9, 2011
2,936
489
Question: Does the received US sensor frequency change? I wouldn't think so for a static distance measurement, that would only happen if the target is moving.

Are you sending burst of 40kHz or a continuous signal?

Can you give more details of your application.

4. ### GRC Thread Starter Member

Apr 20, 2009
21
0
I was planning on sending a burst. The object could be moving but not concerned about the speed or distance, just want to able to aim the receiver by listening for a peak in the signal.

Thanks

Aug 21, 2008
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6. ### praondevou AAC Fanatic!

Jul 9, 2011
2,936
489
I'm not sure what you want. If neither speed of the object nor its distance to the US sensor matters then what is this circuit about?

For a moving object the reflected signal would experience a change in frequency. To transform this to an audible signal you can use Crutschows method. You measure speed of the object like this.

If you are sending short bursts you will be able to measure the distance to the object. The cheap US rangefinders work like this.

One I bought for \$6 from ebay needs a 10us pulse and the output signal width of the sensor depends on the distance to the object (that's almost zero to a about 20 to 30ms).

Just integrating this signal and modulate a 555 or a VCO with that signal shouldn't be so difficult.

If you post what you have done so far we can go from there.

7. ### GRC Thread Starter Member

Apr 20, 2009
21
0
Thanks guys for the help. I ran across this really simple circuit http://circuitos.cl.tripod.com/schem/r113.gif that Im going to try before going the mixer route as the mixers are not readily available. They are just using a FET as a mixer, not sure how that works but I guess I'll find out when I try the circuit.

Geoff

8. ### praondevou AAC Fanatic!

Jul 9, 2011
2,936
489
Oops, I misunderstood your requirements. You just want to convert 40kHz to say 1kHz. Somehow I thought there is a transmitter involved.