[time-nuts] FRS-C TTL / sine outboard filter question

Didier Juges didier at cox.net
Thu Jun 24 03:40:37 UTC 2010


I agree with you that for SatCom application, long term stability is not as much of a concern, phase noise typically is the main issue.

But I suspect that for a time nuts looking at stability over periods of hours or days, temperature effects cannot be ignored. Passband ripple usually goes with group delay ripple, and that delay will change with temperature. For instance, on the Tek 494P I have, the 30Hz crystal filter is thermostatically regulated. When the instrument is cold, there is no signal at all...


------Original Message------
From: Mike Feher
To: Time-Nuts
To: Didier Juges via Cox
Subject: RE: [time-nuts] FRS-C TTL / sine outboard filter question
Sent: Jun 23, 2010 8:41 PM

I do not understand why the passband ripple would be of any consequence in
the big (or small as we typically talk about) picture. During any
measurement interval, it will be a constant, for all practical purposes,
including ours. A measurement at a different time, at a different
temperature that may adversely affect where the ripple is, it will certainly
create a slightly different amplitude and phase delay, however, again, a
constant through the measurement process, not affecting the result. When we
were designing the system I mentioned previously, both Dave and Fred of NIST
were in assistance, as well as the staff of the relevant departments of
MIT's LL, and no one saw any issues. Regards - Mike

Mike B. Feher, N4FS
89 Arnold Blvd.
Howell, NJ 07731


-----Original Message-----
From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On
Behalf Of Leigh L. Klotz, Jr. WA5ZNU
Sent: Wednesday, June 23, 2010 6:15 PM
To: didier at cox.net; Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] FRS-C TTL / sine outboard filter question

Thank you, Didier.

That pretty much sums it up. I was aware of the signal level issue but
didn't consider that passband ripple would be temperature sensitive.  It's
fun learning to think in the long time domain.

In the meantime I've found a 10 mbit ethernet ISA card in my office and
will follow Robert G8RPI's suggestion to use G4HUP's document to obtain a
ready-made 10 MHz filter from it.  Plus as a bonus the box had a BNC T
connector in it.


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