In a message dated 12/14/2007 15:00:00 Pacific Standard Time, [EMAIL PROTECTED] 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 **************************************See AOL's top rated recipes (http://food.aol.com/top-rated-recipes?NCID=aoltop00030000000004) _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
