[time-nuts] Allan Deviation -> continuing saga...

Magnus Danielson cfmd at bredband.net
Sat Oct 28 09:23:41 EDT 2006


From: John Ackermann N8UR <jra at febo.com>
Subject: Re: [time-nuts] Allan Deviation -> continuing saga...
Date: Sat, 28 Oct 2006 08:36:43 -0400
Message-ID: <45434EDB.9090500 at febo.com>

> Didier Juges said the following on 10/28/2006 12:18 AM:
> 
> > The clock shaper uses a commercial 10.7 MHz IF transformer, readjusted 
> > down to 10 MHz (see schematic at 
> > http://www.ko4bb.com/ham_radio/Manuals/2_GPS_Stuff/Clock_Shaper.jpg).
> > It provides light loading to the OCXO through the 390 ohm resistor, yet 
> > provides about 2V p-p to the comparator, to get it out of the noise. I 
> > do not know the temperature stability of the transformer, or the loaded 
> > Q, I will check that later. I realize this is probably not an optimum 
> > design, but it's what I could do with what I have on hand. I am not 
> > using the fine tooth comb and tweezers yet, I am still with the rake and 
> > the fork lift :-)
> 
> To minimize jitter and tempco, you probably ought to stay away from a
> comparator if you can.  I've had good success using the input circuit
> from Brooks Shera's GPSDO which is a 74HC4046 PLL, but using only the
> input circuit and not the PLL section itself.  Alternatively, you might
> get away with a simple capacitor followed by 50/50 voltage divider to
> provide 1/2 Vcc bias going to the input of the divider chip.

Look at the AD8561 datasheet first. The 74HC4046 is much slower than the
AD8561 and the higher output slewrate of the AD8561 is certainly good to keep
jitter down. The 74HC4046 is worse for that aspect.

What I would rather critize Didiers schematic for is the lack of decoupling
caps. This is good for many reasons, but it includes keeping the rise and
fall times up and thus the slewrate up and thus the jitter down.

I would measure the actual phase-stability and phase-slope of the IF
transformer solution with aspect to temperature. You may want to lower its Q
value to flatten the phase-response. You loose in selectivity but gain in
phase-stability.

Cheers,
Magnus



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