GSM Detector (operating and standby modes)
My objective is to design a GSM detector that detects mobile phones even if they are in standby mode. The first step is converting the GSM signal to IF signal using a downconverter. My IF signal is between 20 Mhz and 130 Mhz. My question is: how can I detect the GSM signal in this 20Mhz-130Mhz signal?
I only need to detect the presence of GSM signal in that IF signal and I don't need any processing.
You could use some sort of RSSI system with a settable threshold level. The tx signals from a nearby phone would be larger than that from a base station (assuming you are not near a BS).
I believe even in standby, the mobile phones are polled regularly by the BS, so be on the lookout for short bursts of transmission as well.
But GSM signals are variable carriers so the handset changes its carrier frequency depending on BS to which it has connected.(There are many BS around the handset)
So locking a carrier won't be continuously, after a while this carrier will have a different frequency in GSM band.You should track this frequency hopping but its data is sent in GSM datastream as encrypted so you cannot know this information.That's why locking to a single carrier is not a proper solution.
Instead, a banpass ( GSM band which you're interested in) system following a RF detector will give you more flexibility.There are many RF detector in the market that gives you Power Level of the signal strength.
http://www.analog.com/en/rfif-compon...#RMS_Detectors
@ kripacharya: Can you give me more detail about RSSI and how can I use it with my 20Mhz-130Mhz IF signal?
This is all correct of course. However imho OP requires a more simplistic solution, which I inferred from the fact thats he's down converted the WHOLE gsm band, and is neither trying to decode a particular handset, or even lock onto a single carrier.
In addition, he wants to detect passive/ standby phones as well, which (i think) only talk on the control channel frequencies.
I think his best bet would be to further narrow the bandwidth to only the control channel frequencies, and monitor the signal strengths there.
What say you ? Good solution ?
Can you give me more detail about RSSI and how can I use it with my 20Mhz-130Mhz IF signal?
Please, Which circuitry shoud I add after the downconverter stage in order to be able to detect the existance of a GSM signal in my 20Mhz-130Mhz IF signal?
Is it OK to use an RF power detector to this purpose?