[time-nuts] FE 5680A "new version" - Filtering the 10 MHz
shalimr9 at gmail.com
shalimr9 at gmail.com
Tue Nov 15 23:11:47 EST 2011
I am trying to understand the problem with harmonics on the 10 MHz output.
If you use it to drive the external input of another instrument, it won't be a problem, more likely it will probably help since the instrument will convert that signal to a square wave more appropriate to driving digital logic anyway.
If you use it to drive a mixer, mixers are highly non-linear devices that will generate their own harmonics, and most mixers actually show improved performance (like reduced conversion loss) when driven with a square wave.
So what is the problem?
Sent from my BlackBerry Wireless thingy while I do other things...
From: "Charles P. Steinmetz" <charles_steinmetz at lavabit.com>
Sender: time-nuts-bounces at febo.com
Date: Tue, 15 Nov 2011 16:15:37
To: Discussion of precise time and frequency measurement<time-nuts at febo.com>
Reply-To: Discussion of precise time and frequency measurement
<time-nuts at febo.com>
Subject: Re: [time-nuts] FE 5680A "new version" - Filtering the 10 MHz
>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
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