Phase difference measurement circuit...

I have been fiddling around with some various phase measurement circuits, and I am trying to figure out one I can build cheaply, with minimal components, and get a good result. Ideally, I would like something I can run from a 5V single supply. I have two 10mhz signals...one is a nice reference oscillator...4V pk/pk, sawtooth wave...single ended. The other is a differential sine wave, 50mV pk/pk. I want to measure the phase difference of these two signals. I have one circuit I built using this old line receiver I had laying around....an SN75107AD. I am feeding the differential signal into it, and strobe/gating it with the reference oscillator. It works, but it is not very tolerant to any signal noise, and to really work well, it needs a negative supply voltage, which adds more cost/components.

Anyone have any suggestions on a different way to do this, that is cheap, and can deal with 10mhz signals?

Thanks,

-Ted

Depending on what sort of accuracy you want, one way to do it is to first turn each signal into a square wave (use a divide by two) then add the two square wave together using precision resistors or AND gate them, in either case measure the mean DC voltage output and do the calculations.
Frank

chuckey said:

Depending on what sort of accuracy you want, one way to do it is to first turn each signal into a square wave (use a divide by two) then add the two square wave together using precision resistors or AND gate them, in either case measure the mean DC voltage output and do the calculations.
Frank

Click to expand...

I thought about using a differential comparator like an AD8561 or something like that to square up the signal, run it into an AND gate, and then feed that into an R/C circuit to mean up the DC voltage like you are talking....the challenge there is comparators that can run at 10mhz seem to be $3-5 or more. Any idea who I could get around the expensive comparator? I suppose I could build a comparator with an op-amp, but the design would have to be spot-on, or I would have a whole new set of issues to deal with.

-Ted

Consider putting a divide-by-10 (or so) on each signal.

I don't know for certain which IC will do what you want, but the IC's that come to mind are 4017, 4020, 4060. There are counterparts in the TTL/LS/HC/families.

Then it might work to use an ordinary comparator, or PLL, or 567 IC.

if you use a JK flip flop or such like to do your divide by two, the output is always a square wave. The only inaccuracy could be if the two chips don't provide the same level when going to a "1". Just calibrate their pulses using inphase signals for both and a pot in the one that has the greater level.
Frank

I think the challenge with some of these divide-by solutions are that one of the signals is only a 50mV pk-pk signal. I could amplify it...but, then I have the extra cost of amplifying it....just so I could use a flip-flop....when in that case maybe I just use a high-speed comparator. Or maybe just stick with the line receiver type chip, which will take zero crossings on a 20mV signal.

-Ted

You'll notice that the hysteresis of line receivers causes a level dependent phase error. A similar effect is caused by propagation delay versus overdrive characteristic of comparators. I would try with cascaded self-biased unbuffered CMOS inverters.

The cheapest way would be to use the 4 V signal via a 200 resistor to provide the drive to a four diode balanced modulator and measure the DC output. The trouble is that you will have to calibrate it by putting various lengths of coax in series with the LO to find the relationship between the phase and the DC.
Frank