[time-nuts] FE 5680A "new version" - Filtering the 10 MHz

Bob Camp lists at rtty.us
Wed Nov 16 12:49:46 EST 2011


Hi

I think that the issue is not so much harmonics, as spurious signals on the
output (signals not equal to N times 10 MHz). Harmonics by themselves are
not a big deal.

Bob

-----Original Message-----
From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On
Behalf Of shalimr9 at gmail.com
Sent: Tuesday, November 15, 2011 11:12 PM
To: Time-Nuts
Subject: Re: [time-nuts] FE 5680A "new version" - Filtering the 10 MHz

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?

Didier KO4BB
 
Sent from my BlackBerry Wireless thingy while I do other things...

-----Original Message-----
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

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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