Help finding a specific RF chip

[mu42]

The basic premises of what I am trying to do, is build a transmitter receiver circuit that has a signal strength identifier so I can calculate the distance between the transmitter and receiver. For example, if the chip is receiving 100% of signal strength (2mW) the distance equals 40 feet, and if it is receiving 50% signal (1mW) the distance equals 20 feet. I cannot find a chip like this anywhere... Is this an on track thought? Any other suggestions or better ways for finding the distance between receiver and transmitter would be appreciated if this idea is trash. I need to keep it small scale. It is going to be used within 500 feet.

I am going to be using the MC9S12 (Dragon12) as my microprocessor etc.

 

[fss]

Signal strength doesn't depend only on linear distance... your environment has a fairly significant effect on received signal strength.

 

[berkeman]

The basic premises of what I am trying to do, is build a transmitter receiver circuit that has a signal strength identifier so I can calculate the distance between the transmitter and receiver. For example, if the chip is receiving 100% of signal strength (2mW) the distance equals 40 feet, and if it is receiving 50% signal (1mW) the distance equals 20 feet. I cannot find a chip like this anywhere... Is this an on track thought? Any other suggestions or better ways for finding the distance between receiver and transmitter would be appreciated if this idea is trash. I need to keep it small scale. It is going to be used within 500 feet.

I am going to be using the MC9S12 (Dragon12) as my microprocessor etc.

As fss points out, there are lots of things that affect received RF field strength. Depending on the frequency used, you have multipath issues (reflections from stuff adding or subtracting from the straight-line signal), and attenuation issues independent of distance (like people walking through the line of sight between TX --> RX).

Better choices are laser rangefinding or ultrasonic radar (for shorter distances, like room sized), or radar (if you can put an appropriate reflector on the object to be tracked).

Another option suggested in a recent thread is just to use GPS and even an iPhone application.,

 

[Antiphon]

This is basic ham radio stuff but if you want a short path, try some of the 802.15.4 chipsets. All of them report rf signal level. To keep from getting overwhelmed I suggest looking at Snap OS by Synapse. They have hidden the hardware under an easy-to-use Python bytecode interpreter.

Search for a development kit at digikey or maybe Future Electronics.

 

[berkeman]

This is basic ham radio stuff but if you want a short path, try some of the 802.15.4 chipsets. All of them report rf signal level. To keep from getting overwhelmed I suggest looking at Snap OS by Synapse. They have hidden the hardware under an easy-to-use Python bytecode interpreter.

Search for a development kit at digikey or maybe Future Electronics.

But what about the multipath and fade issues? He may buy the kits and do lots of work, only to find that it will not work for his application. I know from personal experience how problematic the RSSI is on 802.15.4 systems...

 

[mu42]

Hmmm... I am not completely experienced in all of this. The 802.15.4 systems are practically a whole kit with micros and everything right?

To add to what the circuit is suppose to accomplish, is if say.. an ambulance turns on its lights and siren, it starts transmitting at a certain frequency, and the receiver located inside a civilian car picks that up, and an LCD inside the car displays how far away the response vehicle is.

I know that in the public domain, you need a license to transmit, but you can detect freely, which is why I am doing this on a small scale up to 500 feet, in a parking lot, or in a building. As far as 100% accuracy, I am aiming for about 70% accuracy on range detection for now. I am capable of handling the code and hardware, except for the transmission part of things. I just spent $160 on other devices to pair with my dragon12, so anything compatible with that would be optimal. I had also considered going with a wireless modem route, but then again, there is still the issue with finding the distance away. Gah... I feel like I hit a huge roadblock.

EDIT: OMG, is this what I am looking for? It has a built in RSSI.
http://www.sparkfun.com/commerce/product_info.php?products_id=9581

Here is another...
http://www.sparkfun.com/commerce/product_info.php?products_id=691

And another...
http://www.sparkfun.com/commerce/product_info.php?products_id=152

 

[berkeman]

Hmmm... I am not completely experienced in all of this. The 802.15.4 systems are practically a whole kit with micros and everything right?

To add to what the circuit is suppose to accomplish, is if say.. an ambulance turns on its lights and siren, it starts transmitting at a certain frequency, and the receiver located inside a civilian car picks that up, and an LCD inside the car displays how far away the response vehicle is.

I know that in the public domain, you need a license to transmit, but you can detect freely, which is why I am doing this on a small scale up to 500 feet, in a parking lot, or in a building. As far as 100% accuracy, I am aiming for about 70% accuracy on range detection for now. I am capable of handling the code and hardware, except for the transmission part of things. I just spent $160 on other devices to pair with my dragon12, so anything compatible with that would be optimal. I had also considered going with a wireless modem route, but then again, there is still the issue with finding the distance away. Gah... I feel like I hit a huge roadblock.

EDIT: OMG, is this what I am looking for? It has a built in RSSI.
http://www.sparkfun.com/commerce/product_info.php?products_id=9581

Here is another...
http://www.sparkfun.com/commerce/product_info.php?products_id=691

And another...
http://www.sparkfun.com/commerce/product_info.php?products_id=152

Those are the kind of modules you use for ISM band communication (where licensing is generally not an issue, as long as you keep the power down). However, you are NOT going to be able to get any useful range information out of the RSSI data. Multipath and fading totally mess up the RSSI data. You literally can move just a few feet, and go from a very strong signal to a very weak one.

I think your best bet is to go with GPS receivers, in order to figure out the separation of two circuits. You can still communicate via the ISM band, but you will be communicating GPS coordinates in order to calculate the separation of the modules.

 

[davenn]

However, you are NOT going to be able to get any useful range information out of the RSSI data. Multipath and fading totally mess up the RSSI data. You literally can move just a few feet, and go from a very strong signal to a very weak one.
I think your best bet is to go with GPS receivers, in order to figure out the separation of two circuits. You can still communicate via the ISM band, but you will be communicating GPS coordinates in order to calculate the separation of the modules.

agreed, there is nothing that I have ever heard of that can relate signal strength to distance
Ultrasonics, radar, or the GPS mentioned are your best choices :)

Dave
VK2TDN

 

[mu42]

Blah... Gps is so messy. Do you have any suggestions as to what modules I should be looking for?

 

[davenn]

Blah... Gps is so messy. Do you have any suggestions as to what modules I should be looking for?

the accuracy of GPS over up to 500 ft is not going to be overly good unless you use some of specilist GPS measuring techniques.

ultra-sonics prob not very good over 500ft worl better over 100ft or less
best bet is laser ... many (most these days) surveyors use lasers units mounted on tripods for accurate land area measurments.

Dave