Author Topic: transponder peek detector.  (Read 957 times)

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Offline kiwidave72Topic starter

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transponder peek detector.
« on: January 20, 2025, 10:10:03 am »
Hi all, anyone able to help me with a RF Peak detector with a RP2040.

I have a RF transponder that sends out packets of data. for now im not worried about demodulating the data, but just having two Led's triggered based on the signal strength.
when the transponder is over the loop its set an output pin (Red) Led to high, and the transponder is near it set (Orange) Led to high. and when there is no signal than all are OFF.

I have a loop antenna and a impedance circuit in line than an amp to august its output gain for here i want to connect it to the RP2040.

But im not sure what circuit need to have to connect the RF signal from the amp to an analog pin on a RP2040.
Ive seen example that using two 10k resistors to both ground and vss of the microcontroller, but not sure if this is right.

anyone here got any suggestions.

ive included three pictures of the osilloscope of the signal for when the transponder off, near and over the loop.
 

Offline jwet

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Re: transponder peek detector.
« Reply #1 on: January 22, 2025, 04:17:16 am »
The most expedient with no additonal components, other than 2 R's and one C would be to make a divider of say two 47k resistors to each rail into an adc input.  You then take this signal through a .1 uF capacitor into the midpoint of this divider and into the same adc.  What you'll get is a an average value (approximately) that have excursions of almost nothing (no signal), 170 mV (medium) and 350 mv (high).  You could run the ADC in the background at 2 Khz and each 10 mS or look at the data.  This would be 200 samples.  These spikes come in at about 1Khz, you'd expect to get 10 samples that are offset from the mean in a 10 ms sample- they could be high or low.  Your adc will be giving you 1/2 scale with no excursions with no signal, if the spikes are +-85 mV, you'd expect to see this happen almost 10 times for the mid signal and likewise for +-175 mV.  I don't know what the PICO's adc resolution is but you can work out these details.  You can have this little 2Khz sampler going on in an ISR and just log the number of times you're greater or less that these thresholds away from the average.
 


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