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Registered Member #1024
Joined: Sun Sept 23 2007, 10:56AM
Location: Northern NSW, Australia
Posts: 95
I am currently tacking this problem, and have a question, but first some background.
I am using a TORX177 reciever, the state of which is undefined when no pulse train is incoming. Whilst the RX does seem stable at low with a stable (high or low) dc input, it does let out a 20us pulse high approx 20us after the end of any pulse train 'burst'. I currently am driving the RX with a 3.6Mhz modulated pulse for my interrupter ontime burst. I am therefore simply using a re-triggerable monostable set to a pulse out time of approx 30us to reliably square up the signal, and cover this spurious pulse. This however means the minimum burst time is close to 90us once other delays are added (won't go into it in too much detail). What I really want is a minimum burst time of about 20us.
So it seems a constant pulse train is the way to drive the TORX177, thereby giving you down to approx 4Mhz burst resolution (not sure the max RX frequency of the TORX177).
Therefore my intention is to send it a constant pulse train , alternating between approx 1.8Mhz (for logic low), and 3.6Mhz (for logic high). And decode these two pulse trains somehow.
This setup should also make the RX less succeptable to noise, at least noise below 3.6Mhz., whereas currently ANY noise during burst off time will cause a minimum 30us high signal.
My question is : What would be the simplest , fastestest , most reliable way to detect a minimum frequency (approx 3.6Mhz)? A simple frequency to voltage RC circuit seems unlikely to be stable and reliable. A PLL chip detector circuit seems like overkill. Perhaps someone knows of a small , simple , frequency to volatge chip?
Basically I need a simple circuit that will go high when fed with a minimum (say 3.6Mhz) signal, and otherwise be low.
All ideas welcome!
============= EDIT ============== May have answered my own question: Keep my retriggerable monostable, set it to a pulse duration of just over 8 cycles at 3.6mhz , use this pulse to drive the reset of a decade counter (clocked with the Toslink signal) and take output from counter at 8th count pin. I've got the parts so looks like I'll do this , but any other suggestions still welcome.
============= EDIT ============ Nope , that idea is rubbish it would need to be a non-retriggerable monostable.
Registered Member #1024
Joined: Sun Sept 23 2007, 10:56AM
Location: Northern NSW, Australia
Posts: 95
Well Blow me down! Thanks, I had not read my datasheets properly before ordering! To quote from the TORX173 datasheet: "High level output when optical flux is received. Low level output when optical flux is not received"
Simple.
To quote from the TORX177 datasheet: "When modulated optical high level signal is received, output signal is high. When modulated optical low level signal is received, output signal is low. When non- modulated signal (optical all high or all low level signal) is inputted, output signal is not stable."
Mmm , not so simple.
I guess the moral is read the datasheets
And for interrupter signal sending , BUY THE TORX173 .
Excellent , thanks for pointing that out Dr C , I had read that thread , but had overlooked that small and vital portion.
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Reliable, stable, Toslink RX problem.
