Re: Crystal drift

From: Mike Monett (no_at_spam.com)
Date: 09/24/04 

Date: Fri, 24 Sep 2004 01:13:08 -0400

Phil Hobbs wrote:
 
[...]

> Sideband locking is a possible strategy here. If you mix the two oscillators
> together, you can phaselock the beat note to a function generator with a
> frequency-phase detector, e.g. a 4046--it won't lock up to the image
> frequency, because the sign of the loop gain is opposite for the two
> sidebands.

Wouldn't the pll rail if it approached the image from the wrong direction?

> By making the loop narrow enough, you can avoid having the beat
> note modulate the VCXO significantly.

Aren't you locking to the beat frequency? With a phase/frequency detector and a
balanced charge pump that has zero deadband, there should be very little ripple
on the output.

> You do need a large enough frequency
> offset to make this work,

Heh - A frequency offset of 1e-3 or less would probably be quite difficult:)

> and although it won't lock up to the image
> frequency, it can lock to higher IM products if you don't take some care to
> prevent this (e.g. by acquiring lock at a high enough beat frequency that
> only one product is within the tuning range of the VCXO, then sweeping as
> desired).

Hmm... I'm not sure this came out as clear as you intended. Wouldn't a mixer
driving a phase/frequency detector ignore the higher IM products, or maybe I'm
not following you?
 
Also, depending on the frequency, a simple D-flop makes an excellent digital
mixer. Put one input on the clock, the other on the D. The Q output will switch
at the difference frequency.

> If you need to have the beat frequency go through 0, then John's quadrature
> idea is a good one--otherwise you could mix with a constant offset frequency,
> filter, then do the same sideband locking trick.
>
> Cheers,
>
> Phil Hobbs

Regards,

Mike