Sense rf frequency and power

[Lantz]

Greetings all,

I looking for a few pointers on a small rf hobby project I've been thinking about. I've got zero experience with analog circuits, I've only ever done some basic digital stuff. So there's alot of things I'm gonna need to learn.

Basically, what I want to do is to receive a short vhf pulse using a simple loop antenna (as small as possible), and sense received power and frequency. I'm not going to be receiving any data, and it doesn't need to be all that sensitive, as I'm only looking for a limited range (of course this depends on the transmitters as well).

What would be the simplest and cheapest way of doing this? I've been thinking of using a range from lets say 130 to 250 MHz. Where do I start? Frequency-to-voltage converters? How would I sense the AC current?

Also, what kind of currents/voltages can I expect to get from a small loop antenna? Are we talking mV, nV?

I'm sure it shows I'm new to this! Any pointers in the right direction would be greatly appreciated!

Thanks,
Lantz

 

[Externet]

Hello Lantz.
Your receiving antenna signal strenght/amplitude should be measured with an RF voltmeter/proper oscilloscope and the power calculated from the antenna impedance, not measuring current.
Power measurements may be more accurately done at the transmitting end than at receiving.
Depending how 'short' the pulse is in the operating mode, you may not be able to capture its magnitude and for measuring only, a longer 'pulse' may be convenient if the oscilloscope is not a storage-type one.
A frequency counter or the oscilloscope can tell the frequency/period.

At close range and depending of power, it may be in the range of hundreds of millivolts/milliwatts up to whatever.

Miguel

 

[Lantz]

Thanks for your reply! Here's a ton of follow up questions.

Will the amplitude and impedance both change in a similar way if I turn the antenna to/away from the transmitter? Which is easiest/best to measure? Are there any IC's that does this?

I found some really basic rf voltage measuring technique here:
**broken link removed**
Would that be of any use for me? Perhaps I could use that and then amplify the voltage a bit, send it over to an ADC and then away to a fpga/pic.

Another thing, I suppose I would need to use some filters to filter out undesired frequencies, any hint on what would be best for that?

 

[JimB]

Will the amplitude and impedance both change in a similar way if I turn the antenna to/away from the transmitter?

The impedance will not change, but the amplitude of the received signal will change as the antenna is turned.

Are there any IC's that does this?

Yes, have a look at this:
https://www.analog.com/static/imported-files/data_sheets/AD8307.pdf
that will be far more sensitive than this ever will be:

I found some really basic rf voltage measuring technique here:
**broken link removed**
Would that be of any use for me? Perhaps I could use that and then amplify the voltage a bit, send it over to an ADC and then away to a fpga/pic.

Another thing, I suppose I would need to use some filters to filter out undesired frequencies, any hint on what would be best for that?

Any antenna is frequency selective to some extent, depending upon its length, is you are using an antenna which is short compared with the wavelength of the signal you are probing, will be "wide band" and accept a wide range of frequencies with little selectivity.

A simple LC tuned circuit at the frequency of interest would be a good start.

JimB

 

[Lantz]

Ah, nice, that looks like just the thing I need. RSSI!

I suppose a LC tuned circuit like you mention won't work if I need to receive a range of frequencies? What would you suggest then?

What would be a simple way of measuring the frequency? Can I use a down converter or frequency divider? I will probably have a FPGA in the system, perhaps I could use that?

 

[Lantz]

About filtering out unwanted frequencies, let's say I choose to limit myself to 200-210 MHz. Could I do that by mixing the input with a 200 MHz reference, then use a 10 MHz low pass filter?