[time-nuts] Method for comparing oscillators
David C. Partridge
david.partridge at dsl.pipex.com
Mon Aug 3 19:20:23 UTC 2009
Now if only there were a UK/European equivalent of that site ...
Dave
-----Original Message-----
From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On
Behalf Of Keith Payea
Sent: 03 August 2009 18:59
To: 'Discussion of precise time and frequency measurement'
Subject: Re: [time-nuts] Method for comparing oscillators
I like to use a site called www.findchips.com It searches 21 US
distributors for parts. It's billed as a finder of ICs, but it works well
for anything electronic with a part number. No advertising, no frills, just
gets the job done.
Unfortunately, there are a lot of parts that start with the string 74AC112
in this case.
Keith
-----Original Message-----
From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On
Behalf Of EWKehren at aol.com
Sent: Monday, August 03, 2009 10:46 AM
To: time-nuts at febo.com
Subject: Re: [time-nuts] Method for comparing oscillators
Thanks for the offer. I can find SMT in the US, I am looking N (dual in
line). For testing purposes I am going to use 74 S since I have quite a few.
Bert
In a message dated 8/3/2009 1:39:34 P.M. Eastern Daylight Time,
david.partridge at dsl.pipex.com writes:
I *may* have a one or two SMT 74AC112's left from my frequency divider
project - I sold most of them with the boards. Postage from the UK may be
non-trivial though.
Let me know if that is of interest and I will go digging in my parts bin.
Dave
-----Original Message-----
From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On
Behalf Of EWKehren at aol.com
Sent: 03 August 2009 17:37
To: time-nuts at febo.com
Cc: cdelect at juno.com
Subject: Re: [time-nuts] Method for comparing oscillators
The Dual Mixer seems to be for a long time a hot topic and I can see why. I
am also intrigued by it and after extensive discussions with Corby I have
come to the conclusion to duplicate it. Corby has one of the original NBS
units and after changing only the Op Amps sees Allen Deviation of
8.5X10-14
at one second. That is plenty good for me, so I have decided to copy the
NBS unit, replace Op Amps, ZCD and use an Optical Isolator instead of the
output transformer. I am also including a five channel 100 MHz counter.
This will give me a resolution of ten nsec. which is 1X10-15. The counter
talks to a PC via USB.
My goal is to keep the total material cost below $200. I have received
valuable suggestions from Bruce, some I am incorporating as long as it does
stay with in the $ 200.00. I am a strong believer in KISS and have almost
completed the total PC layout. All IC's are DIP! The board is partitioned
in such a way that it can be cut up in three sections or be left as one.
Two parts are Isolation amplifiers and mixer with all associated circuits
the third section separating the two sides is the counter. Price quote for
the board is $ 22.00 in fifty and $ 30.00 in ten quantity. When complete
and tested all information will be made available to every one. The only
component I have trouble buying is the 74 AC 112. Newark has them, but you
have to buy thousands. Material cost based on part searches is below $
200.00.
Let me make clear I am not trying to push the limits of technology but take
a conservative and KISS approach. Many will be able to make meaningful
tests where the limit will be their sources not the test set up.
What is missing is the two u Processor choices and the programming. I have
only limited knowledge in that field. I am looking for some one that is
willing to perform that task. The proper u Processors need to be picked
and they will have to be programmed. Two of the identical three may be
something like the PIC 1220. Once selected I can complete the board and
order prototypes. The programming should be simple, but that is easy to
say
for someone that never had to do so. Any body willing to help, please
contact me.
Obviously PC software also has to be created, to use the data from the
five counters. Any body that will do the programming will be a recipient
of a prototype board.
On a related subject there has been extensive discussion of delay
compensation. Cables and other devices are also contributors to errors do
to temperature. I have cheated in the past by just inserting an external
voltage to the tuning voltage to move the phase. What is wrong with that?
Bert Kehren
In a message dated 8/3/2009 11:31:34 A.M. Eastern Daylight Time,
df6jb at ulrich-bangert.de writes:
John,
I see you in the danger to confuse accuracy and stabilility. "Accuracy" of
an oscillator and "stability" of an oscillator are (albeit the fact that
our wishful thinking usually expects both from a good oscillator) two
completely different things that you should not mix against each other.
Your oscilloscope method (without proper handling of phase ambiguities)
measures a compound of both properties and is not well suited for
stability
measurements. You have to realize that one of the oscillators that you are
going to compare may be totally inaccurate so that you will see lots of
phase changes in time occuring. Nevertheless this inaccurate oscillator
may be perfectly stable running on its wrong frequency. Do you see the
difference?
Best regards
Ulrich Bangert
> -----Ursprungliche Nachricht-----
> Von: time-nuts-bounces at febo.com
> [mailto:time-nuts-bounces at febo.com] Im Auftrag von John Green
> Gesendet: Montag, 3. August 2009 16:59
> An: time-nuts at febo.com
> Betreff: Re: [time-nuts] Method for comparing oscillators
>
>
> I have studied the dual mixer approach and the consensus is that it
> is the most accurate method. However, it seems pretty difficult to
> obtain that accuracy. I do have some DBMs with IF response down to
> DC. I don't have a 10811 but do have a pretty good oscillator to use
> for the offset. The problem comes in with the time interval counter.
> The only thing we have is an old 5328A. I believe, at this time,
> DMTD is just not possible for me to do. My oscilloscope method seems to
> work pretty well. I can't produce graphs showing frequency stability
> but that isn't a big deal for me. I just want to be able to compare
> a Rb source to a GPSDO and look at several OCXOs either stand alone
> or in equipment we have here. If I figure correctly, an error of
> 1e-12 is 1 Hz every 27.7 hours if comparing 2 10 MHz sources. I don't
> have to wait for a full cycle to occur, I can see pretty small phase
> differences. Let's say I can see a 10 degree change. That would cut
> the observation time down to just over 3 quarters of an hour. Not
> bad. Most OCXOs will move a lot more than that so shorter times would
> work for them. I know from past experience that this works pretty
> well for looking at warm up performance. My first experiment will be
> the hardest. I am going to check a couple of Rb sources against a
> Tbolt. I'll let you know how this works out. Thanks for all the
> input.
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