[time-nuts] Allan Deviation -> continuing saga...
Dr Bruce Griffiths
bruce.griffiths at xtra.co.nz
Sat Oct 28 17:35:44 EDT 2006
Didier Juges wrote:
> 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. Same,
> the counters actual wiring is not depicted, it is straight from the
> Texas Instrument data sheet. I also have a 200 ohm pot across the OCXO
> output to adjust the drive level into the 390 ohm resistor not shown.
>> 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
> 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).
> 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 :-)
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Comparators are fine with a good layout.
If the input signal is low noise and has a sufficiently high slew at the
switching threshold hysteresis is unnecessary.
Older ultrahigh speed TTL comparators like the AD9696 and AMD686 have an
output stage that is inherently unstable in the transition region.
If the input to the output stage remained too long in this region the
output stage oscillates.
Even these work well with a good layout and a nice clean low noise high
slew rate input signal.
Hysteresis is mandatory when using these comparators with low slew rate
inputs to maintain stability.
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