[time-nuts] Method for comparing oscillators

Mon Aug 3 16:36:40 UTC 2009

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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