# Need to design a phase shift detector circuit... not sure how to start

#### hrstar24

Joined Apr 7, 2009
32
So I need to build a phase shift detector circuit for my project, and I don't really know where to start.

My project is basically dealing with phase shifts in a pulsing laser. I am going to pulse the intensity of the laser at a frequency of about 60 kHz in a sine wave. The laser then goes through a medium that slows it down, creating a phase shift. This phase shifted laser output then hits a photodiode which converts it back into an electrical signal. The phase shift detector will then recieve the original sine wave from the laser controller, and the shifted signal from the photodiode. I need the Phase Shift Detector to then translate the phase shift into some DC voltage that feeds into a micro controller.

Inputs:

A(t) = ACos(wt); Original wave form from laser controller
B(t) = BCos(wt - phi); Wave form from photodiode

My initial thinking was somehow multiplying these waves together, and then integrating them over 1 period of the wave. This way I can solve for phi, and use a simple function in the micro controller to convert the integration value into phi.

I am not really sure how to multiply the two signals together, is there some circuit I can use or some trick with the photodiode maybe? For the integrator, I was just going to use an op amp circuit.

Anyone got any suggestions on how to multiply the signals together? Or am I doing this phase shift detection in a complete wrong way? Is there a much simpler way to do this out there somewhere?

#### THE_RB

Joined Feb 11, 2008
5,438
Convert the incoming waves to digital using a comparator for each wave, adjust the Vref for each wave until both waves are a digital square wave of 50:50 duty cycle.

Then run both square waves through a AND gate, so output is high only when both waves are high. Then RC filter the output of the AND gate and DC voltage will be linear to phase shift.

#### Jaguarjoe

Joined Apr 7, 2010
767
I always did this stuff with XOR gates but that was a long time ago. Lots on the web about it.

If you're feeding a micro, skip the DC conversion and just feed the gate's digital output directly into the controller.

I think 74HC7046 chips have phase comparator circuits built in. Probably many more like it.

#### t06afre

Joined May 11, 2009
5,934

#### MrChips

Joined Oct 2, 2009
30,500
You can create a phase detector using a D-type flip-flop.

#### hrstar24

Joined Apr 7, 2009
32
Thanks for the help everyone

Convert the incoming waves to digital using a comparator for each wave, adjust the Vref for each wave until both waves are a digital square wave of 50:50 duty cycle.

Then run both square waves through a AND gate, so output is high only when both waves are high. Then RC filter the output of the AND gate and DC voltage will be linear to phase shift.
So the RC filter at the end functions as a voltage integrator, but it outputs a triangle wave w/ a square wave input. So do I just make the cutoff frequency much lower than the 60 kHz my waveforms are at? Will this make the output DC voltage constant?

#### THE_RB

Joined Feb 11, 2008
5,438
...
So the RC filter at the end functions as a voltage integrator, but it outputs a triangle wave w/ a square wave input. So do I just make the cutoff frequency much lower than the 60 kHz my waveforms are at? Will this make the output DC voltage constant?
Yes, with the right filter values (large R and C) the output will be close enough to DC so the final output is a DC voltage linearly proportional to the phase shift. It's a common system in laser rangefinders.

#### hrstar24

Joined Apr 7, 2009
32
Alright thanks.

I have run into a quick snag, though. Turns out we will probably be operating at ~6 MHz now, not 60 kHz like I planned before. The op amp comparator no longer works correctly on such a high frequency, so should I look into buying a comparator chip that works at high speeds?

Anyone got any recommendations for some relatively cheap chips I can get off digikey or sparkfun?

I found this chip on the TI website, will it work?

http://www.ti.com/product/tlv3501

#### vidhyavs

Joined Oct 10, 2012
1
Sir,
We are using D flip flop logic for implementation of phase detector in PLL. But after resetting the output is oscillating. Can you suggest any solution.

Thanking you.