long distance cell phone detector

AnalogKid

Joined Aug 1, 2013
10,170
As anyone who has built a phono preamp can attest, the front end of an opamp makes an excellent AM detector without any help from things like circuit components, design equations, etc. That is what is going on here imho. Each circuit has a tuned circuit at the input. After that it's just low frequency gain and drivers.

ak
 

Thread Starter

000SHREDDER000

Joined Apr 9, 2017
49
You will need a Faraday cage to do any meaningful testing on whatever you end up building.
Really:confused:, why?!

The '1458 opamp used in your post #17 linked circuit has a unity gain bandwidth of only 1MHz. I don't see how it can possibly operate up to even 50MHz, yet alone 500GHz as claimed.
The 1458 opamp inputs work like the diodes in a crystal radio. They detect any RF within the bandwidth of the coil and its capacitance.

That circuit is useless.
The op-amp gain has declined to (1) one at about 3 MHz and the lowest cell phone frequency is 700 MHz.
The gain resistors set no limit on the op-amp, it has no frequency filters, and the input impedance is so high (1 Terraohm) it would pick up geese flying overhead in the vast open tundra of the arctic circle. It is obvious that it won't shut off.

OK. I exaggerated, but you get the idea.. Without something to tune out the power line frequency and radio stations in a city, this "detector" is going to detect constantly.
the op-amp doesn't amplify at signal frequency, it just looks for rectified RF from the diodes.
 
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ian field

Joined Oct 27, 2012
6,536
Hey guys which kind of filter I must use RC filter or LC?
An RC would probably have to be a twin-T - but not at cell phone frequencies.

You're not far below the point where LC circuits start to look more like plumbing.

A correctly dimensioned dipole is probably the most practical approach.
 

AnalogKid

Joined Aug 1, 2013
10,170
Hereis 2 newer versions of that detector:
Which one can work?
It literally is impossible to say, since you have given no clue as to what "work" means. What is this for, what is the environment in which you are trying to discriminate cell phone carriers from background radiation, what does the output signal have to do, etcetcetcetc

ak
 

Kermit2

Joined Feb 5, 2010
4,162
You will have to build it to find out if it does what YOU need.

As has been pointed out. We still do not know the specifications required to fulfill the task. They remain frustratingly absent from your request.
 

Thread Starter

000SHREDDER000

Joined Apr 9, 2017
49
You will have to build it to find out if it does what YOU need.

As has been pointed out. We still do not know the specifications required to fulfill the task. They remain frustratingly absent from your request.
It literally is impossible to say, since you have given no clue as to what "work" means. What is this for, what is the environment in which you are trying to discriminate cell phone carriers from background radiation, what does the output signal have to do, etcetcetcetc

ak
Designer said it's cell phone detector, no more specifications unfortunately.
 

Kermit2

Joined Feb 5, 2010
4,162
Replace the diodes that are backwards with ones oriented properly.

Look at pictures of diode bridges to understand the required layout for proper operation.
 
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