# Specific frequency signal detector

How does for example a generic signal detector (E.G. this cellular frequency detector circuit) actually pick up on a specific range of frequencies? (for 4g im assuming its around 900Mhz?)

In regards to that, could this circuit be changed so that it picks up other ranges/specific frequencies (E.g. 500Mhz)?

P.S Please bear in mind I am very new to this.

Thanks!

OK, so after at first taking this circuit seriously, I now realize that it's actually a nonsense circuit. It appears here:

https://www.seattleu.edu/scieng/ece/laboratory/cellphone/ with the explanation:

"R1 and R2 form a voltage reference connected to U1-A Pin3. R3 and C2 form a RC circuit (You must understand basic RC circuits). Any sudden energy picked up by the antenna connected to Pin2 causes U1-A output Pin1 to change (you must read up on the basics of Operational Amplifiers and Comparators). Pin1 output remains changed during the period of RC or the antenna energy (cell phone radio waves) is removed."

... and the rest of the circuit is just a threshold detector that Pin 1 does or does not exceed a threshold.

OK, so that explanation is somewhat BS. The time constants for R1 to R2 and C1, C2 are very far away from doing anything resembling tuning at 800MHz or whatever, and LM358 is completely useless at that frequency.

So my guess is that if this circuit actually does flash an LED when a (very) nearby cell phone is used, it's a result of the relatively high power picked up by the antenna, rectified by some subsidiary aspect of this circuit (such as the input protection diodes of the LM358), causing a temporary imbalance on the + and - inputs. It's possible that R1..3 and C1, C2 so that RF data bursts repeating at a much lower frequency cause a more prolonged LED flash.

In short, there's nothing about this circuit that tunes it to the RF frequency of cell phones, with the possibly exception of the antenna length.. If it does anything at all it's more or less an accident of secondary features of the circuit.

• Thanks for the well explained answer, that's where I think my friend found the circuit then probably when I asked for some help, now that I know it's not good to learn from. On that note, where would you suggest I go to learn how to design a signal detector that I could control what frequency it responds too? – Kira Aug 8 '15 at 7:29
• Also you mention the time constant for the for R1 and C1 don't make it tuned too 800Mhz, are the ac time constants usually determine the frequency your trying to pick up? – Kira Aug 8 '15 at 7:37
• @JohnDoe Take a look at band-pass and band-reject filters, those are actually used to tune a circuit to a specific frequency. – Golaž Aug 8 '15 at 14:24
• @John Doe: Circuits for radio frequencies (especially high 100's MHz) are very different from those of <100kHz, so you can often tell immediately what ballpark freq range a particular circuit operates in. Op amps are decreasingly useful above 100kHz and certainly not above 10Mhz, so right away we know that in this circuit the cell signal per se can't get processed by the first op amp. But maybe there's something clever about the antenna, plus the input R's and C's? Well, T = 100nF x 6.8M = 1E-7 x 6.8E6 = 0.68 seconds, so no arrangement of those R's and C's does anything useful at cell freqs. – gwideman Aug 9 '15 at 8:40
• @JohnDoe To get some idea about this topic, you can google for discussions of "radio tuning circuits" or similar. But be aware that for high 100's of MHz through GHz, the capacitance and inductance involve are very tiny. A few squiggles on a circuit board may act as an antenna or inductor. So you would want to follow with research specifically on circuits for that range ('UHF'). You may also sniff around for chips which (along with their application notes) solve this problem in a compact fashion. Not my area, but the LT5504 and friends seem close, though not quite. – gwideman Aug 9 '15 at 9:20