[time-nuts] re low noise regulators
SAIDJACK at aol.com
SAIDJACK at aol.com
Sun Dec 16 15:26:46 EST 2007
In a message dated 12/14/2007 15:00:00 Pacific Standard Time,
bruce.griffiths at xtra.co.nz writes:
>> A simple RC filter of say 2 Ohms into 4700uF has a -3dB cut-off at
around
>> 17Hz (4700uF caps are getting quite small these days). That would take
care of
>> most of the 100Hz to 10KHz noise.
>
>It still helps to have as low a noise as possible before using such
>brute force filtering to get that extra few dB of noise reduction.
>> Using a typical current of 0.16A at 12V for a Euro-can OCXO we would only
>> have 0.32V voltage drop across the resistor.
>>
>However the resultant drop will depend on the ambient temperature.
>In principle the regulator output could be given a suitable temperature
>drift characteristic to compensate.
>However doing this without degrading the noise at the filter output may
>be challenging.
Hi Bruce,
the tempco effect of the series resistor may not be a factor at all for
typical OCXO's.
If we take a standard 1% resistor such as the Panasonic ERO-S2PHF2R00
(available on Digikey) with 100ppm/C tempco (that's a fairly bad tempco), then we
get:
0.32V * 100ppm = 32 microvolts per Degree C change (actually it's
+/-50ppm, so this example is worse than what we would see in real life).
If we now take the Supply-Voltage-versus-Frequency characteristic of a
typical standard single oven OCXO of 1E-08, at +/-5% variation for a 12V part we
get:
1E-08 for 1.2V change, then we get: 8.33E-09/V * 32E-06V = 2.66E-013
change per degree C.
For this particular OCXO with a stability of 1E-09 per degree C, the change
in frequency due to the change in the 2 Ohm resistor value is about 3700x
less than the temperature stability of the OCXO itself.
In other words the effect of the resistor is inconsequential.
For a good double-oven OCXO this is even less of a factor, for exampe the
MTI double oven we use only has 8.3E-11 per volt change - two orders of
magnitude less sensitivity than the example above.
The capacitors' microphonic sensitivity may actuallyy be more of a factor
here.
>It still helps to have as low a noise as possible before using such
>brute force filtering to get that extra few dB of noise reduction.
It's not just a few dB, it's -3dB at 17Hz already, and that drops at ~-20dB
per decade. At 170Hz it could theoretically give -23dB already, or in other
words if the noise floor was entirely due to the supply voltage noise, then
the difference would be between say -140dBc/Hz and about -160dBc/Hz at 170Hz -
this could be a very significant improvement.
You are absolutely right, it is very difficult to get low noise <10Hz.
bye,
Said
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