phase difference measurement

[pisoiu]

Hi,
Does anyone have any idea how can I measure the phase difference between two digital signals? Nothing special so far, but one signal is a PECL 200Mhz clock and the other is a trigger signal (low to high edge) which may appear anywhere between those two raising edges of the clock signal...and I want to know where. I had some ideas ( track and hold circuit followed by an ADC, but I haven't found a fast enough ic). An discrete track and hold circuit with a switch which charge/discharge a cap from the clock source continuously untill the trigger occur is also a dream, because the switch must turn into off state in a period <1ns to preserve a acceptable error between the ideal point and the real point. The fastest switch I found have toff=5ns...which is the period of my signal....so....any ideas? Thx.

 

[flatulent]

Here is one conceptual method. Use an edge triggered RS flip flop so you get an output pulse whose width is equal to the time offset. Put this to a RC network so you get a ramp that goes up during the pulse. Select the value so that the output is about 80% of the full value for a maximum width pulse. Put a schottky diode in series with the resistor so that when the input goes down the capacitor is not immediately discharged. Follow this by a peak detector and then your adc. You can put a resistor across the capacitor so that it slowly discharges before the next time you want to make a reading. You will have to process the output of the adc to take the diode drop effects into account.

 

[bastos4321]

The average output of a edge triggered RS flip flop is a measure of the phase. Just use an RC to filter it and then process. This only works for periodic signals. Its the principle of a PLL.

Bastos