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phase comparator

This can be done, various phase detectors will handle the level issue differently.

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.

Yes, as is a double-balanced mixer.

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.

"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).

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Tim Wescott
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Tim Wescott
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I should add that the 74HC4046 has three different types of phase detectors. Fairchild Semiconductor has a nice description of the phase comparator action in their datasheet:

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Tim Wescott
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Tim Wescott

[snip]

Correct! You can only get a signal proportional to *phase* when both frequencies are the same.

...Jim Thompson

-- | James E.Thompson, P.E. | mens | | Analog Innovations, Inc. | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | Phoenix, Arizona Voice:(480)460-2350 | | | E-mail Address at Website Fax:(480)460-2142 | Brass Rat | |

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Jim Thompson

do

phase

The Philips 74HCT9046 has a rather better phase-frequency detector - see the data sheet for details

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----------- Bill Sloman, Nijmegen

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bill.sloman

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)

Is there a common circuit for comparing the frequencies of two signals as well (and outputting a voltage proportional to the frequency difference)?

Is an XOR gate with a lowpass filter on its output essentially a phase comparator?

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?

cheers, Jamie Morken

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Jamie Morken

Hi,

I think I see now that if the two input signals have different frequencies then the output of the phase comparator will cycle at the beat frequency of the two input signals.

cheers, Jamie

Tim Wescott wrote:

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Jamie Morken

In article , Jim Thompson wrote: [...]

unless:

If you define the difference between the frequencies as the rate that the phase is shifting, then the XOR phase detector correctly shows the changing phase angle for half of a cycle of the beat frequency. On the other half, it shows a decreasing phase shift instead of the correct increasing phase shift.

If the phase detector is part of a PLL, the nature of this output for unequal frequencies is important in considering whether the circuit will lock.

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kensmith@rahul.net   forging knowledge
Reply to
Ken Smith

I read in sci.electronics.design that Ken Smith wrote (in ) about 'phase comparator', on Wed, 5 Jan 2005:

At that point, it's a 'half-full or half-empty' thing....

....but at that point it isn't; the apparently decreasing phase shift initiates a frequency change in the wrong direction. I suppose it can be overcome with extra circuitry.

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Regards, John Woodgate, OOO - Own Opinions Only. 
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John Woodgate

If the frequency difference is much less than the loop bandwidth, the loop locks the next time the phase comes around.

The extra circuitry you suggest is fairly simple. If you XOR F(out) with

2F(out) you get a version of F(out) that is shifted by 90 degrees. If you XOR this with the input signal and low pass filter the result, you get a signal that goes positive to say that the phase comparitor's output can be trusted. This can be used to vary the resistance of a JFET or better yet modulate pin 5 of a CA3080 to make the VCO on the agerage go towards the right frequency.

XOR gates come 4 to a package and the circuit leave one left over. Perhaps you could just invert the output of the phase detector when the signal goes negitive.

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kensmith@rahul.net   forging knowledge
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Ken Smith

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