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Broadband RF Field Strength Probe

using Atmel AT90S1200A AVR controller

auto-zero assembly code

This broadband probe has a small antenna (about a 15 cm length of insulated wire). Radio Frequency energy coupled to the antenna is detected and made available to drive millivolt level signals to the input of a DVM (Digital Volt Meter). Its battery powered for convenience with very low current drain and automatic shutdown for long battery life.

I've used the circuit shown below to check the output of transmitters at 4 MHz, 35 MHz, 55 MHz, 100 MHz, 900 MHz, a cell phone, and a microwave oven. It really is broad band, though I don't make any claims for the flatness of response. Since the collectors and emitters of the detector transistor are both at RF ground, choice of transistors isn't all that critical. A low base-collector capacitance will enhance the VHF and UHF sensitivity. All transistors should be of the same type to minimize thermal drift. The DC gain of the detector is about 50X (estimated by multiplying the voltage drop across the collector load by 38)Assembly is not critical and mine was built on punched fiberglass board without a ground plane. The 10k pot is a rough offset adjustment. Set it for about a +50 mV offset (assuming that the one you build doesn't experience an offset drift of more than 50 mV). Push the reset/recal button (after power is applied, of course) and the auto zero circuit will bring the offset down of a millivolt or two and turn out the LED. The meter will continue to operate for 10 to 20 minutes (depending upon actual battery voltage) before shutting itself off.

An Atmel AT90S1200A micro controller is used to take advantage of its ability to operate with the internal clock rather than needing an external crystal. The advantages of using the internal clock over the crystal are a lower parts count, less radiated RF from the controller, and low cost. Note that the low frequency operation of several hundred kilohertz results in low power supply current demands from the controller.

The circuit is battery powered. I used two AA cells and put a power switch in series with them. The switch is only a comfort factor since the circuit has a timer that automatically shuts everything off after about 15 minutes (depends on battery voltage). When shut down, current drain drops to a few tens of micro amps. The auto-shut off feature has been well tested. In the first year since I built this, I have left with the power switch in the on position in my desk drawer while I was out of town twice, for a period of two months each time. Everything still works fine when the reset/recal button is pressed even though its still using the original pair of batteries.

The circuit is intended to drive the input of a high impedance DVM. Making the circuit battery powered gives the additional advantage of being able to float the circuit so that one of the DVM terminals can be grounded.

I use this detector to drive a DVM set to the 200 mV scale since the signals I am interested in seeing result in an output of a few tens of millivolts. The flexible wire antenna provides flexibility in use in that it can be bent and shaped to control the sensitivity. The wire antenna can also be formed to make a "sniffer" probe to help in finding areas with high RF levels.

If you want, you can build the detector without the AT90S1200 AVR Micro controller, but you will have to zero it meter manually every time you use it, and of course, there won't be an auto shutoff timer. Details of the AT90S1200A AVR Micro controller's operation are given in the assembly code file.

Here is the auto-zero assembly code .

I built mine in a plastic pencil box. Being made of clear plastic, I avoided the extra hole that would be required for the LED, since the LED shines right through.

Schematic diagram of the detector. Its simple and fills the bill as a tune-up meter and RF detector/sniffer
over a wide range of frequencies.


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