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Note: I'm primarly a 4th year Computer Science student, but I have also interest in electronics but I have very limited knowlege.

Is it possible to measure the signal quality or level of a RF transmitter with a RF reciever (like this: https://www.sparkfun.com/products/10533)? I need to know if I am getting close to the source. I mean is it possible to use these RF transmitter/recievers for not data transfer but signal level calculation?

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  • \$\begingroup\$ ... so you want to do direction finding / emitter location? \$\endgroup\$ – vicatcu Mar 25 '13 at 1:37
  • \$\begingroup\$ Yes, basically this is what I want to achieve. Am I on the right track? \$\endgroup\$ – Mustafa Mar 25 '13 at 1:46
  • \$\begingroup\$ The product you linked is a transmitter, not a receiver. \$\endgroup\$ – The Photon Mar 25 '13 at 2:43
  • \$\begingroup\$ @ThePhoton fixed it, I have posted wrong link before. \$\endgroup\$ – Mustafa Mar 25 '13 at 3:25
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What you are looking for is a receiver with an RSSI function (Received Signal Strength indication); this is usually an analog output proportional to the log of the signal strength.

The datasheet does not give particularly good specs, as is common with the Sparkfun datasheets I have looked at. If you could identify the real manufacturer, they might have better data.

In this case, that takes us to a page on www.rf.net.tw which has exactly the same data sheet. However it does have an "Ask Question" form where you could ask if this module has an RSSI function. (Be prepared for the question to be ignored or misunderstood, but it may be worth a try).

The datasheet DOES say "Modulate Mode : ASK" which means "Amplitude Shift Keying" which means the linear output MAY vary in voltage to some extent with signal strength, but probably not enough to be useful, because AGC (automatic gain control) will keep the output signal almost constant.

One of the comments on the Sparkfun page talks about using the linear output for RSSI and links to this page about it. Doesn't look very reliable to me.

This page describes digging into a similar module (different manufacturer) and finding an undocumented RSSI signal, if you are brave enough.

All of which probably leaves you looking for another receiver : this time, search for "315 MHz receiver with RSSI" and you may find something more useful.

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  • \$\begingroup\$ +1, actually I have the schematic for the same reference design (it's used by a few manufacturers) and while under NDA so I can't share exact details you're right it does include AGC before that stage so probably won't be useful. \$\endgroup\$ – PeterJ Mar 25 '13 at 10:42
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The part you linked has an output pin labeled "Linear Out". The word linear implies the output of this pin should be somehow linearly related to the rf signal. But there doesn't seem to be any further information provided about this pin. Does it deliver the full 315 MHz signal? Does it deliver a demodulated version of the signal? It might even provide a slowly varying output indicating the received signal strength, which would be the most convenient for you. If you have access to a 500 MHz scope, you should be able to see if any of these are good guesses about the function of the pin. If one of them is, you should be able to use this signal to determine the received signal strength.

On the other hand, with nothing more than a pin name to go by, it's possible that this pin is not connected to anything at all.

Even in that case, there's probably somewhere on the circuit board you hook up a bit of wire-wrap wire and pick off a signal proportional to the input signal. But finding it would depend on either knowing the circuit schematic or reverse-engineering it somewhat.

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  • \$\begingroup\$ I think the Linear out just has more drive strength than the Digital out. I've scoped signals on modules like these and they look virtually identical on a trace. \$\endgroup\$ – vicatcu Mar 25 '13 at 13:58
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There is a chance you could use a loop of wire (probably two turns maximum with a small tuning capacitor to broadly tune 315MHz) and an "RF" diode detector and decoupling capacitor. This will give you a dc signal that rises as you approach close to the transmitter BUT it won't work at much of a range - maybe a few inches or a couple of foot maximum. This is based on an expected low or sub mW output from your transmitter.

I know folk have used this before but if your looking at more than a couple of feet it'll run out of steam.

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The device that you are looking for is called a field strength meter. RF engineering is one of the most complex specialties within the field of electrical engineering. Here's a link to an article that discusses the construction of an analog field strength meter.

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