Re: phase comparator

From: Tim Wescott (tim_at_wescottnospamdesign.com)
Date: 01/04/05 

Date: Tue, 04 Jan 2005 09:42:14 -0800

Jamie Morken wrote:

> Hi,
>
> I would like to compare the phase of two 1MHz sinewave and/or
> squarewave) signals and get a output voltage proportional to their phase
> shift, with a small phase angle error ie. 1% if possible. Also what
> happens when the amplitudes of the two input signals are different, and
> what happens if the frequency isn't exactly the same? (ie. from a
> doppler shift)

This can be done, various phase detectors will handle the level issue
differently.
>
> Is there a common circuit for comparing the frequencies of two signals
> as well (and outputting a voltage proportional to the frequency
>

Discriminator, as used in FM radio. However, a three-state
phase/frequency detector with an appropriate loop filter will often do
well enough without a bunch of fussy resonant circuits.
>
> Is an XOR gate with a lowpass filter on its output essentially a phase
> comparator?

Yes, as is a double-balanced mixer.
>
> I am slowly learning more about modulators/demodulators and upconverters
> and downconverters, and am a bit confused on how these devices can be
> used for FM and PSK etc..
>
> Is there a reason why an IQ modulator/demodulator can be made to operate
> over a large frequency range without needing external tuning caps/coils
> (ie. AD8346/AD8347) while a downconverter/upconverter (ie.
> max2680/max2660) requires external tuning passives depending on the
> frequencies of operation? Is this because an IQ modulator/demodulator
> is "balanced" from the I and Q being 90degrees out of phase?

There's no fundamental reason -- a downconverter that's going to a
non-zero IF ("image reject") may need better phase matching, but it's
probably mostly the manufacturers.
>
> cheers,
> Jamie Morken

"Phase Lock Loop Circuit Design" by Dan Wolaver is a good book. Not a
classic, but I think it could be if it caught on. It seems to be pretty
clear for self study (but I wouldn't know -- I took the class from Dr.
Wolaver). 

-- 
Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

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