[time-nuts] Unified VCXO Carrier Board

Charles Steinmetz csteinmetz at yandex.com
Sat Oct 24 09:03:21 EDT 2015


Bruce wrote:

>The comparator circuit measured was the front end of David Partridge's
>divider. I merely measured the 10MHz output.

The MAX999 and ADCMP600 are the two comparator options noted on 
David's schematic.  Both parts suffer from a number of the design and 
die-level issues I noted in my previous message, and I have never 
obtained particularly good PN with either one.  Also, even the 
relatively direct path to the 10MHz output goes through two 'AC04 
inverters and an 'AC541 line driver, which contribute additional PN.

>One thing that I have found is that at low offset frequencies the measured
>PN is substantially reduced when air currents and other sources of thermal
>fluctuations are reduced. Even the effect of a thin piece of paper used as
>an air current shield can be easily seen.
>With careful shielding from thermal fluctuations I measure the low
>frequency offset PN to be substantially lower than the datasheet values.
>I've seen this effect with everything for which I've measured the PN.

Agreed.  Whether or not it is explicitly stated, I take "all 
circuitry to be enclosed and protected from drafts, and allowed to 
stabilize thermally before testing" as a given with any sensitive 
time or voltage circuit.

>One problem with comparators when attempting to measure their PN is
>that they don't have sufficient output to drive the TimePod input directly.
>An amplifier is required.

The spec sheet says both TimePod inputs accept -5 to +20dBm into 50 
ohms.  -5dBm is less than 0.4Vp-p, which requires less than +/-4mA 
from the source, so a 0-5v comparator output feeding a coupling 
capacitor and a 560 ohm series resistor should work fine as long as 
the comparator can source and sink at least 4mA.

Alternatively, a 0-5v comparator output could be buffered with three 
'AC04 inverters in parallel, or an 'AC line driver -- but that adds 
the PN of the gates.

>A resistor from point A to ground in the Wenzel style shaper you attached
>has little effect on the output symmetry due to C4.

It has just enough effect to correct the very small (<1%) asymmetry 
due to the unbalanced drive.  (With no resistor at Point A, the duty 
cycle is ~51%/49% high/low.)

>However it does allow the output amplitude to be adjusted.

According to the simulation, the resistor has no effect on the output 
amplitude until it is well below 1k ohms (at 1k ohm, the symmetry has 
been WAY overcompensated and the duty cycle is ~45%/55% high/low.

Best regards,

Charles




More information about the time-nuts mailing list