[time-nuts] 10 MHz reference switching
Bob Camp
lists at rtty.us
Mon Mar 8 23:51:54 UTC 2010
Hi
Or put another way:
If you have a structure that uses series resistors and shunt diodes:
The diodes are reverse biased when the switch is "on".
The diodes are forward biassed when the switch is "off".
No current when on = no noise.
Bob
On Mar 8, 2010, at 6:43 PM, Bruce Griffiths wrote:
> life speed wrote:
>> Hello Time Nuts,
>>
>> I'm back again. I designed bandwidth-limited (30 MHz) ADA4899-1 opamp circuits to buffer the 10 MHz reference, with theoretical noise of less than 3 nV/rtHz. Oscillator noise is about -155 dBc/rtHz from 1 KHz to 10 KHz, and -160 dBc/rtHz from 10 KHz to 100 KHz. Unfortunately the rest of the design is holding up PCB manufacture, so results on these won't be available for awhile.
>>
>> Which brings me to the other design-stopping considerations: Is anybody aware of low-noise analog switches that can be used to reconfigure the 10 MHz reference paths without significantly degrading the phase noise? As always, I believe 1/F noise would be the concern. Non-reflective is great, but I can accomplish that by using more switches and 50 ohm resistors if need be. Isolation is a key consideration as well. Mechanical relays are out of the question.
>>
>> An alternative to switching is phase-locking the OCXO to an external reference. I have thoroughly looked into this possibility, and it is likely my preferred approach. But I need to understand how the switching approach compares, hence my question. I do not need help with phase-locking; this feature is readily available from OCXO vendors who have dedicated years of research to optimizing their products.
>>
>> Any switch experience out there?
>>
>>
> Why would flicker noise be a concern if there is no dc current flowing in the switches?
>
> Bruce
>
>
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