Hi,

Well, actually there is two methods:

1) Switch reference clock - a switch or mux changes which clock is being
used.

2) Lock internal reference to external reference - a PLL lock of the
internal reference to the external reference is enabled.

For both, the counter core may need to heat up.

For the first, the internal reference heatup can be ignored completely.

For the second, the internal reference heatup cannot completely be
ignored, it takes time for it to be within lock range and the lock-in
behavior stabilizes. Still relatively quick, but an actual effect.

At the end of the day, it depends on how precise you attempt to measure.

Cheers,
Magnus

On 02/08/2018 03:16 PM, Scott McGrath wrote:
> Yes,   You have removed the reference from the warmup but the input circuits 
> still need time to reach thermal equilibrium for most accurate results
> 
> 
> 
> On Feb 8, 2018, at 3:34 AM, Azelio Boriani <azelio.bori...@gmail.com> wrote:
> 
> No warmup needed if the counter switches to the external reference. If
> the counter uses the external reference to lock the internal one then
> it is better to wait until the internal one is stable, maybe the
> counter has a standby mode where it appears powered off but the
> internal reference is active (usually an ovenized reference). For the
> best performance (stability of trigger levels, input amplifiers and so
> on) it is better to warmup the whole counter always.
> 
>> On Thu, Feb 8, 2018 at 8:33 AM, Chris Wilson <ch...@chriswilson.tv> wrote:
>>
>>
>>  08/02/2018 07:31
>>
>> Does  a  frequency  counter  connected  to a permanently running (Trimble 
>> Thunderbolt) GPS
>> disciplined frequency standard need to warm up after switch on before 
>> readings settle?
>> Just curious, thanks.
>>
>> --
>>       Best Regards,
>>                   Chris Wilson.
>> mailto: ch...@chriswilson.tv
>>
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