AVR LED RF Field Strength

Posted on Nov 22, 2012

Useful as a transmitter tune-up meter or an RF sniffer, this is an RF field strength indicator that is loosely based on the Broad Band RF Field Strength Probe, described elsewhere. It detects RF via a square law detector, basically its a crystal set with gain. The bias circuit for the detector is significantly modified from the probe to reduce the effects of battery voltage droop during use. The circuit contains a small analog section and LED driver, tied together by the micro controller. A two-transistor circuit provides a stabilized voltage and serves as a detector that serves an A-to-D converter based on the comparator contained in the micro controller.

AVR LED RF Field Strength
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The voltage from the RF detector is measured once every 160 milliseconds and after removing the zero offset, correcting the polarity (higher field strength results in lower voltage from the detector), overflow detection and formatting to BCD, the micro controller provides drive signals to the three digit LED display, which is refreshed once every 16 ms, with a dwell time of about 4 ms per character.This is a little slow for a handheld device as when the meter is moved, a trail of digits is left on the user's retina, so if this bothers you, increase the refresh rate. Q1 is a combination shunt regulator and bias source for Q2. As a shunt regulator, it provides a stabilized voltage for the A-D converter circuit. This shunt regulator has low loop gain, thus the display may jitter because this voltage affects the RF detector's output voltage and power supply fluctuations resulting from driving the LEDs may result in feedback that causes the readings to fluctuate. This could be improved by using a band gap reference ahead of the shunt regulator, but it drives cost and complexity up. I have found the circuit to work erratically when using some batteries -these were some very cheap carbon-zinc and (supposedly) Ni-MH cells that were counterfeits of name brands I bought in S.E. Asia. I suspect these cells have a relatively high impedance, thought I did not confirm this experimentally. Name-brand (real ones) alkaline and Ni-Cad...

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