[time-nuts] OCXO Soft-Start

David davidwhess at gmail.com
Fri Apr 14 10:40:07 EDT 2017

On Wed, 12 Apr 2017 20:18:38 -0700, you wrote:

>On 4/12/17 7:14 PM, Richard (Rick) Karlquist wrote:
>> 10.9 MHz is likely the B-mode of the SC cut.
>> (It's a different mode, not a different overtone).
>> This mode has a tempco of 20 ppm and is used
>> to do thermometry.
>> IMHO, there is NO excuse for the oscillator
>> designer to design an oscillator that doesn't
>> oscillate unconditionally in the right mode.
>> NONE!  What was the actual manufacturer of
>> the OCXO? (AFAIK, Trimble doesn't make their
>> own).
>> My old boss at Zeta Labs (a really great boss)
>> used to walk up to people who were testing
>> their latest circuit and momentarily turn
>> down the current limit on each of the power
>> supplies to see if the circuit recovered
>> correctly.  It often didn't, and then it
>> was back to the drawing board...
>These days, a common trap is when you've got a DC/DC converter on the 
>input - with a soft start, the current goes huge, causing all kinds of 
>(I'm in the middle of closing a failure report on just such a 
>sensitivity - an unexpected interaction between the source DC/DC 
>converter and the load DC/DC converter..)

Linear regulators with foldback current limiting and sometimes safe
operating area protection can do the same thing with unfriendly loads.
Safe operating area protection becomes a problem when the input to
output voltage difference is large lowering the current limit at low
output voltages.

A more insidious problem and design mistake has sometimes occurs with
thermal protection.  If the thermal protection does not have enough
hysteresis, then the regulator cannot "hard start" into a heavy load
at high temperature and turns itself into a temperature controlled

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