[time-nuts] Method for comparing oscillators

[David C. Partridge]

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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