# 100khz signal detector

I have an incoming 100khz ir signal. (from a well designed optical front end). I like to detect this signal even when it is very low. What is the best way to do this? I am only interested in the amplitude of the signal. I do not worry about the phase shift. Detection time is somewhat important too. (ie I have perhaps 10 20 usec to decide, so a fast detection is requird)

So far I have considered: Psd(lockin): too long to lock in and complex to implement Bp filter: if I design a really narrow filter the amplitude doesn't change as much ( perhaps a design error?). I play with various options and current thinking is to use this option if I can fix the amplitude problem

Other options ?

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You need to decide in 10-20 uS, but the pulse rate is 10 uS. This is very tricky! Most IR signaling schemes don't respond until about 4 to 10 pulses have been received.

Let's only consider the band pass filter approach (BPF) for the moment. There are two ways to look at this, but both ways are just different sides of the same coin.

1. The faster you need to detect the signal, the wider the BPF should be.
2. The higher the data rate (or bit rate), the wider the BPF should be. Even though you might not see this as a data communication system, it is.

This is because a simple "pulse is there, pulse is not there" modulation scheme is basically the same as amplitude modulation that is used for AM radio. It's a little different because you're carrier is a square wave and not a sine wave, and you're modulating it with a square wave and not sine. But the basic idea is the same. And that idea is this: while you might start with 100 KHz, the modulation causes frequencies higher and lower than 100 KHz. In extreme cases you might have frequencies of 50 KHz to 150 KHz (or worse)!

Of course, the wider the BPF is the easier it is for it to receive noise and falsely trigger!

Your choices are to increase your pulse rate to 500 KHz to 1 MHz, or change your signal encoding scheme and/or detection method. It might be possible to create a different signaling scheme based on multi-level amplitude modulation (never turning off the pulses entirely) and some sort of PLL to lock in on it. Even so it would be difficult.

So... The easy solution is to either raise your pulse rate or change your requirements for a 10-20 uS response time.

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 Thanks David. This is inline with simulations. I have a follow up question, if I make the BPF very narrow, I loose amplitude.. The presence of signal generates a sine wave but its amplitude doesn't change. Normal? – Ktc Apr 4 '12 at 15:22 @Ktc The narrower the BPF, the slower the output of that filter will respond to changes from the input. An infinitely (and impossibly) narrow BPF won't have any changes on the output. No yes, that is somewhat normal. – David Kessner Apr 4 '12 at 16:03