[time-nuts] Allan Deviation -> continuing saga...
cfmd at bredband.net
Sat Oct 28 11:36:41 EDT 2006
From: Didier Juges <didier at cox.net>
Subject: Re: [time-nuts] Allan Deviation -> continuing saga...
Date: Sat, 28 Oct 2006 08:44:27 -0500
Message-ID: <45435EBB.3030107 at cox.net>
> Magnus Danielson wrote:
> > 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.
> There are decoupling caps everywhere, I kept the schematic simple.
Implicit components is obviously a bit hard to comment on.
> Same, the counters actual wiring is not depicted, it is straight from the
> Texas Instrument data sheet.
Those where a bit more obvious, since there isn't that much variations to hook
those up to acheive the /128 functionality. You could get /256, but that
output is not synchronous so you would expect higher jitter. A 74F74 would
solve that thought.
> I also have a 200 ohm pot across the OCXO
> output to adjust the drive level into the 390 ohm resistor not shown.
In series I assume.
> > 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.
> I have planned to do that. The transformer I am using is quite
> temperature stable, and the Q is controlled via the 390 ohm resistor,
> which may or may not be the optimal value from a Q standpoint. It is
> close to the best value from a drive level standpoint, to make sure I do
> not overdrive the comparator. I did not observe a measurable change in
> resonance frequency when blowing a hot air gun at it from a distance,
> even though my measurement method was crude (adjust the drive frequency
> using a SG503 and peak the voltage, use scope to monitor).
Good to hear. I have a decent network analyzer to help me out for such
experiments. On the other hand I have a lack of space in the lab which
effectively prohibits such experiments so...
> I could use the 5370 to measure the phase shift while I blow hot air on
> the circuit (transformer and comparator), or I could make a small hot
> box with a light bulb and a variac, put the OCXO and clock shaper in it
> and use my HP3478B voltmeter and a thermistor to record temperature, and
> plot delay and temperature via the GPIB while I crank the temperature,
> then I could plot delay vs. temperature.
> The opportunities to experiment are endless... I will never have time
> for all that, I better retire now :-)
Indeed. I get more toys^H^H^H^Hinstruments next week.
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