RF Power Level Detector?

[mindctrl]

Hi there,

Was wondering if any of you have ever had an idea on how to do this:

I'd like to develop a simple RF power detector that takes a signal passed to it through bandpass filters (so to single out a certain frequency), and then detect to see if the RF frequency exceeds a certain threshold. Basically, the practical application is to simply detect if a transmitter is close to the receiver (provided there are no other high wattage transmitters on that specific frequency.)

I'm sure this has been done before. Anybody have any ideas/plans/diagrams?

Thanks in advance,
mindctrl

 

[Nigel Goodwin]

So basically you want to listen before you transmit?, to make sure the channel is clear. What you really need is a transceiver (a transmitter and receiver in one, that tune together). Your control system can check the S meter output to ensure it's clear before transmitting. However, this isn't by any means fool proof, as someone else might start to transmit at the same time as you.

 

[stevez]

What you have described might be a receiver with a signal strength (S) meter as implied in Nigel's response. Usually the S meter reading is higher when the signal in from the front end is higher. The response may or may not be linear, calibrated or uniform across the bands. The signal for the S meter can/is obtained from various places between the front end and the audio output.

A couple of things to keep in mind:

The S meter will respond to the voltage (millivolt) at the antenna terminals. The signal that is of interest to you may not be constant so the reading on your S meter will be changing - rapidly or slowly depending on a number of factors.

You will have to calculate backward to get to power - given that the S meter responds only to voltage at the antenna terminals. The receiver antenna, transmitter antenna, etc all affect transmitter power.

There is noise to be received even with no transmitter nearby - and there is the noise created in your receiving equipment. The background RF noise level is not constant on many frequencies. An example: on 40 meters (7 mHz) thru the course of several hours, and all other things being equal, the background noise level might be at nearly S0 but climb to S9 or more. As might be obvious - less powerful signals can be swamped by background noise.

 

[stevez]

And of course I didn't finish before posting this- there is a lot of good information on receiver design to be found in amateur radio publications. There is a device called a dip meter - the older version was called a Grid Dip Meter - sometimes it's called a Gate Dip Meter now, referring to the transistor "gate" that took the place of the grid. Many dip meters had a switch that allowed them to be a tuned field strength meter - providing some selectivity.

 

[mindctrl]

Actually, the receiver will need to do no transmitting at all. It simply needs to passively listen to a certain frequency and determine if the power levels exceed a certain strength. This way, I can rule out normal background noise and determine if there is a genuine signal coming through.

Once the power levels reach their threshold level, they are admitted through to the PIC, which will listen for a certain series of tones on that specific frequency.

This way, my transmitter can be 50 feet away, and still transmitting its series of tones (the key), and the base receiver MIGHT get them, but if the signal is too weak, it won't admit the frequency through to the PIC.

However, if I'm say, 10 feet from the base receiver, obviously my signal strength would be stronger on the preset frequency, therefore reaching the threshold level preset on the receiver and allowing the signals to be transmitted into the PIC where they are confirmed or not. Sound a little clearer now?

 

[Nigel Goodwin]

I suggest you try telling us EXACTLY what you are wanting to do, and not how you think it should be done - you're still giving confusing ideas!.

From your last post I'm imagining you simply want to transfer data to a PIC via a radio link?.

 

[stevez]

Just a few comments or thoughts:

On VHF FM receivers, for commercial/govt/amateur use there is a squelch. The squelch mutes the receiver audio opens up until a signal is received that is strong enough to be coherent. I constructed a simple squelch that sampled audio via a capacitor. The capacitor passed the higher audio frequencies that were the junk or noise that you don't want to hear. When coherent audio was on the channel the capacitor had less high frequency stuff to conduct and the squelch opened allowing the coherent audio to be heard.

An improvement over the squelch, that is in use today is called PL - the proprietary term was Private Line (GE I think). This used a set of low frequency audio tones that were added to the transmitted audio. It was low enough in frequency (250 Hz and below) that it could be filtered - leaving the rest of the audio available for listening.

 

[mindctrl]

Alright, I'll tell you EXACTLY what I'm doing... and I'll try to make this as clear as possible!


Basically, what we have is this: I wish to build a receiver that detects if a transmitter is within an immediate radius (15ft). I would like to do this through a two part system:
1) Base Receiver
2) Transmitter

I carry the transmitter on my person. The receiver stays in a fixed spot. When the receiver detects that the transmitter (presumably carried by me in my pocket) is within a close enough range, it will perform a certain action (turn on a light, unlock a door, whatever). When I leave the immediate area of the receiver, it detects the transponder is no longer nearby, and performs another action (lock a door, turn off the light, etc).

Think of this as a "proximity detector." Imagine the practical use of this. When you approach your home door, it unlocks automatically for you and turns on the light inside. When you leave your house, the door automatically locks behind you. No more leaving your keys lying around, locking yourself out, etc. It is an autosensing system that takes care of performing menial tasks. Think of it, perhaps, as a type of long range RFID system.

The way I had imagined this set up was that you would have the transmitter consistently sending a tone coded pulse on a specified frequency. The receiver would listen on the same frequency and filter out all background noise, similar to the function of a squelch. When the transponder comes within sufficient range, the RF power levels would obviously increase, eventually able to transmit clearly through the background noise. This takes care of the proximity issue. Theoretically, the closer the transmitter gets to the base station, the more powerful the RF signal appears to be.

Eventually, when the RF levels get to a preset amount, they are allowed to talk to a decoder which determines if the transmitter is sending the correct tone code. If it detects the correct algorithm, it does whatever action it has been preprogrammed to do in this case. If not, the PIC simply ignores the signal and waits for the correct one.

Is this a little more clear? I hope I didn't manage to mess that one up... I re-read it several times to myself.

 

[Nigel Goodwin]

Yes it's clear, but it's a very 'unreliable' idea - the signal strength will vary greatly depending on many factors, not just proximity.

You might try checking the licence free radio modules I use in my tutorials?, the receiver has a voltage output that shows the signal strength.

 

[mindctrl]

Thank you for your input and opinions, Nigel.