Good receiver module for RSSI?

desertstorm

Hey,

First time in the radio forum, but hopefully someone can help me out

Have a summer project that involves rf, and designing a simple circuit from scratch which is pretty new to me.

Basically going to need to be able to measure a transmitted signal strength accurately over a short distance (<10m)

I've been trying to find appropriate receiver modules and I've come across the Linx LR series : Linx LR datasheet

It's available in the 433 MHz mark, which I believe makes it okay to use (License free etc.?)

My main question is how accurate is using RSSI pin data for getting signal strengths from a specific transmitter? I believe the big issue is it picks up any in band rf energy, but is there any way around this interference?

Also any suggestions for a better receiver, considering the short ranges involved? I'm just interested in this currently as it seems relatively easy to integrate and has a corresponding transmitter chip that looks equally alright to implement, and are both pretty cheap.

Thanks

Fuzzy Clam

Are you wanting to take accurate measurements in microvolts, millivolts etc.?

Doing this isn't as easy as just measuring the output of a receiver. The gain of the circuit and the receive antenna must be known also at the exact frequency to be measured. It would also need to be callibrated against a "known" signal source.

If the power of the RF source you want to measure is many watts, then a simple passive tuned circuit may suffice. This makes construction a lot easier but you still will have no way of translating the output into meaningful figures.

brownmini

desertstorm wrote: »

My main question is how accurate is using RSSI pin data for getting signal strengths from a specific transmitter? I believe the big issue is it picks up any in band rf energy, but is there any way around this interference?

Also any suggestions for a better receiver, considering the short ranges involved? I'm just interested in this currently as it seems relatively easy to integrate and has a corresponding transmitter chip that looks equally alright to implement, and are both pretty cheap.

1. According to te datasheet on the chip it reacts to RF signal levels and not data levels received.
This to me will do what you appear to want to do.
Calibration of the meter to a known signal level is going to be the awkward part.
Have a read of this too.
http://www.qsl.net/n/n9zia//wireless/appendixF.html#2


If you really want a wideband version then I found three pictures on an American website.
For you and others, here's the links.
http://www.qsl.net/n/n9zia//wireless/pics/fs_meter.png
and
http://www.qsl.net/n/n9zia//wireless/pics/LED_sig_meter.png
and
http://www.qsl.net/n/n9zia//wireless/pics/LED_sig_meter-1.png


2. Not really without adding extra stuff. If I understand you properly - you want to monitor the signal strength on 433.xxx MHz (give or take 140kHz) then this will do.
If you want to monitor a wider bandwidth then a non-specialised and non-tuned front end is what you want.

From that datasheet, down the end they give you suggestions for various ways of making an aerial for it.
The simplest is the sticky-up aerial. and might be the best for version 1 of this project.
Get the thing working first then think about putting the 'go faster' stripes on it later.


The other way would be to buy a scanner with an S-meter on the front panel!! Calibrated S-meter too.
And when you're finished sniffing RF on 433MHz, wander off to listen to various other things like RTE1 or RTE2.



73

Fuzzy Clam

brownmini wrote: »

The other way would be to buy a scanner with an S-meter on the front panel!! Calibrated S-meter too.
And when you're finished sniffing RF on 433MHz, wander off to listen to various other things like RTE1 or RTE2.



73

You definately will not get accurate readings using an S meter. The equipment required for accuracy costs over 10,000 Dollars. Even calibrating the gear is at least 1000 Dollars.

The OP hasn't replied yet so maybe he isn't serious about what he wants to do. Some clarification on how accurate his readings need to be would be useful.

desertstorm

thanks for the replies so far, was just waiting to see what people had to say!

Accuracy wise, as I said it'd be over a very short range application, <10m, I basically want to use it to measure a distance so the idea would be to calibrate it based on an inverse squared relationship and then tweaking for whatever other losses

Ideally an accuracy of +/- <10cm would probably be sufficient for now

I'm hoping to be able to programatically work out when I'm measuring background rf and when I'm measuring the strength of the received signal, but I've a little background in op amps so was half considering trying to make a filter, but it's been a while so that's plan b for now...

watty

Using Signal Level to Measure < 10m at 1% accuracy (10m) to 0.1% accuracy 1m.

That won't work.

Unless you are far above the ground, or maybe outdoors in a field. At that distance an Ultrasound echo is better. If you want a Beacon rather than echo there are ways too. But at 10m, you need quite short microwaves. 10GHz = 3cm wavelengh. Maybe use 40GHz to 200GHz. Sound is Million times slower, hence 40KHz is same resolution as 40GHz RF.


Unless you are using GHz, you are too close to Near Field at 10m for Inverse Square law.

At < 10m there is huge variation of signal from reflections and aerial orientation, terrain etc.

I've used 10.2GHz TX/RX and the TX /RX too directed and too much reflections. Reflecting off a workbench 3m away (6m path) was similar signal level to 1m direct link.

What is the MINIMUM distance as well as Maximum 10m?
What accuracy at minimum distance?

As Fuzzy Clam says, you need special gear, very linear wide dynamic range tuned amp and then detector. Not cheap. A Consumer 433MHz Chip with RSSI might be some use for 10m to 500m outdoor, not at distances under 10m.

desertstorm

Ahh thanks wattsy, hadn't even thought of that! Have been looking into ultrasound as well but that advantage hadn't even struck me

So if I did stick with the rf route, would I be right in saying a 2.4 ghz signal would give a resolution of around 12 cm, but there's going to be a lot of interference issues?

I can't really find any easily available shf IC's, 2.4 ghz seems easier to come by

Will look into ultrasound more thanks

watty

Some movement detectors are 10.450GHz, and that band is reserved for Movement detectors

But Ultrasound will work better.

Either way you need to work with measuing delay of a pulse or an accurate time code on a signal. Getting better than 1m accuracy hard with RF and it needs Atomic clocks (or GPS derived clock). Ultrasound is simpler.

brownmini

desertstorm wrote: »

I basically want to use it to measure a distance so the idea would be to

Ah...this is something that wasnt clear from your previous message.
Using Sig Strength to gauge distance.
You could argue that it should work, produce the formula etc, but it won't.
Too many variables in a practical way for it to stand a chance.

Back to square one.

brownmini

watty wrote: »

Using Signal Level to Measure < 10m at 1% accuracy (10m) to 0.1% accuracy 1m.

How about using the VLF/OMEGA phase/cycle comparison but at 433MHz to plot position and therefore distance ?

Instead of having a 'Lane' of 10,000 metres , each 'Lane' would be 70cm wide.

A serious amount of circuitry required and perhaps not a project for one summer.

The RF Tech Guy

desertstorm wrote: »

Basically going to need to be able to measure a transmitted signal strength accurately over a short distance (<10m)

This is achievable.

What is your expected range of transmitted powers? Is it in mW or Watts that you wish to measure.

You've said its for a summer project. So I have a question, is it just to demonstrate that you have researched, implemented and tested your idea? If that is the case then a simple fixed tuned receiver may very well suit your idea and near field or far field won't really enter into it to be honest with you if its for a fixed purpose as you simply design accordingly.

If you wish to measure transmitter signal strength over a broad range of frequencies this is not so easy unfortunately although can be done crudely (a calibration table can be made from also for such a device).

desertstorm wrote: »

My main question is how accurate is using RSSI pin data for getting signal strengths from a specific transmitter? I believe the big issue is it picks up any in band rf energy, but is there any way around this interference?

Not as inaccurate as you would think.Once calibrated against a known source the output from the RSSI pin via an appropriate buffer amp can be a reasonably accurate indicator if a few precautions are taken.

It all depends on the complexity of your end use.

For example, you could use such a device for checking the output of key fob transmitters such as car remote locking fobs etc as a "good / no good" test with good accuracy.

desertstorm

Hey RF Tech Guy,

Missed your post there! I've had to put this to the side for a short while, been a bit sidetracked by exams at the moment

From what I've gathered so far it'll be measuring at a fixed (high) frequency while trying to figure out how to handle interference

Will be using xBees as a starting platform as they seem the easiest starting point so it'll just be in the mW

Was wondering what would be the easiest way to make a small uni-directional antenna? or can small ones be bought?

The RF Tech Guy

Interesting stuff.

You can get the xBees with a chip aerial already on it. Otherwise the simplest omni-directional aerial you could make or buy would be a halfwave dipole.

It you're making or buying an aerial for the xBees just be aware that the connector is a reverse polarity SMA rather than the conventional SMA.

Such an aerial for 2.4GHz should be available in the likes of the Maplin stores or any number of on-line retailers. They're typically under 10 Euro. If you're stuck give me a shout and I'll point you in the direction of a few suppliers.

Very curious as to the statement about "measuring at a fixed (high) frequency while trying to figure out how to handle interference"

Do you mean something along the lines of a communications protocol that will be resilliant in the presence of a potentially interfering signal from other 2.4GHz ISM devices?

brownmini

desertstorm wrote: »

Was wondering what would be the easiest way to make a small
uni-directional antenna? or can small ones be bought?

With a "uni-directional" aerial ?

I presume - you're talking 2.4GHz and not 432MHz.
Perhaps a short-ish Cantenna would be the easiest/neatest and easiest to
stick the rest of your 'project onto to make it into a carry-able object'.
For guidance - see http://www.turnpoint.net/wireless/cantennahowto.html

They're a bit bigger than the little Aerial that springs out the of an Xbee
but if you want 'uni-directional' then you'll have to build something close to
the active element of the aerial to get the 'uni-directional' property.
The common or garden directional aerial most people will think of is a Yagi
(YAGI-UDA if you're really picky)
See http://en.wikipedia.org/wiki/Yagi-Uda_antenna for starters.
Please note that when you are very close to a signal source, most directional
aerials will be useless for telling the direction of that signal.