TTL Phase shift/delay

[edhaslam]

Hi all,

I'm designing a lock-in amplifier and I need to generate a TTL reference signal (no problem) but also a 90 deg phase-shifted version of that signal also. The Frequency of the signal will be either set to 10kHz, 50kHz or 100kHz, so the way it is done would have to be frequency independent.

Does anyone have any good ideas of what might be the best way of doing this?

I could use a small PIC to perform the phase shifting, by sample the incoming reference signal and applying the appropriate delay and output the phase shifted signal. Or, would it be easier to do this using glue logic?

Any ideas? Been scratching my head over this one!

Cheers,
Ed

[Ttelmah]

Personally, if the frequency is at all variable, 'best' solution would probably be to use a PLL, generating source*4, and then divide this by four. Use the incoming frequency as a gate, so the divider uses the edge 'after' the one on the incoming tone. 10:1, is a reasonably simple range for a PLL.
Obviously at this sort of rate, you could do it in software. Use the CTC, to measure the time period of the incoming signal, divide this by four, and use this as a count on another timer, to time the edge change from detecting the incoming signal change.
Both are pretty simple.
However since you are making the oscillator in the first place, why not just run this at 2* the required frequency, and have two /2 dividers?. Run one off this signal, and the other off it's inverse. Two signals 90 degrees phase shifted, and very easy/reliable....

Best Wishes

[Steve H]

Here is a link that discusses some tried and true methods of producing quadrature conversions...

members. tripod. c o m / michaelgellis / mixerscom.h t m l

In FM detection quadrature demodulators are formed using a LC tank tuned to the IF frequency. Since you only have three frequencies and they are low value there are many simple methods that may be used to make a switched LC tank that may work for you.


HTH - Steve H.

[SherpaDoug]

You need a 2 bit Johnson ring counter, made from a pair of flip-flops. Feed it 4X your desired frequency and it gives all four quadrature signals.
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[edhaslam]

Many thanks guys, good food for thought.