[time-nuts] FE 5680A "new version" - Filtering the 10 MHz
Charles P. Steinmetz
charles_steinmetz at lavabit.com
Tue Nov 15 21:15:37 UTC 2011
Robert wrote:
>One "off the shelf" or more likely out of the junk box option is to
>use the filter components from a 10Mb/s ethernet card. They work
>quite well but might introduce some temperature related phase shifts.
As others have pointed out here in the past, to minimize phase and
amplitude shifts with temperature changes, you would like the phase
and amplitude responses to be changing as gradually as possible in
the vicinity of the oscillator frequency. However, this is not the
case with a bandbass filter centered on the oscillator frequency or a
low-pass filter with a cutoff near the oscillator frequency.
If the undesired output components are harmonics, you can reduce the
first few with traps (i.e., notch filters). This may be all you need
(for example, if the harmonics were generated by a buffer amp with
simple nonlinearities). Even if there is an extended harmonic
series, as may be the case with the 5680A, it will allow you to
filter the higher harmonics using a low-pass filter with relaxed
specifications (higher cutoff frequency, fewer poles, and/or better
damping), thus preserving gentle phase and amplitude response at the
oscillator frequency.
Best regards,
Charles
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