[time-nuts] Datum 4065A Cs Tube Response

EWKehren at aol.com EWKehren at aol.com
Mon May 5 22:49:22 UTC 2014

The original price of the STEL1173CL was $ 125 in quantity 10 so $ 65 is a  
bargain.  I have no idea as to how it is used in the 4065A but if it is  
limited to 13 frequencies it may be worth wile to take a look at the AD 9913 
it  is capable to produce very precise frequencies exceeding its 32 bits, tvb 
did  run tests on a board we build and I have extra boards available. Has a 
PIC on it  that may help to emulate. Most likely will it not be the only 
STEL  failure.
Bert Kehren
In a message dated 5/5/2014 5:15:01 P.M. Eastern Daylight Time,  
ed_palmer at sasktel.net writes:

I know very  little about today's DDS chips, but I think that emulating 
the STEL-1173  would be a challenge.  It provides 48 bit frequency 
resolution.   I counted 13 different frequencies that are used to monitor 
the signal to  make sure that it's on frequency. Based on the manual, 
they were shifting  it many times per second. I'm not surprised the chip 
died - they worked it  to death!


On 5/5/2014 6:49 AM, paul swed wrote:
> I  will agree with Joe. I have a CS tube thats darn near impossible to  
> the beam current and yet it still locks. That truly amazes me. I  seem to
> recall other comments ages ago about that chip failing. There  should be a
> way to emulate it these days with all of the DDS chips and  such that are
> available.
> Good luck.
> Regards
>  Paul.
> On Mon, May 5, 2014 at 8:20 AM,  J. L. Trantham <jltran at att.net> wrote:
>>  Ed,
>> If I have the math correct, and you are measuring  the voltage to ground
>> through a 10 MegOhm input impedance DMM, you  have about 7.5 nA beam 
>> which seems a bit low compared to  what I remember of the HP 5061A.
>>   However,
>>  you still have a definable 'peak' with a 'peak to valley' voltage of  
>> 60 mV or a 'useful signal current' of about 6 nA.  If  your unit's 
>> can properly amplify that and keep it a  clean signal, it should work.
>> However, I would recommend setting  the OCXO precisely on frequency with 
>> GPSDO before trying to  close the loop and 'locking' the signal to the CS
>> tube.  It  will dramatically lower the work load of getting everything
>>  adjusted properly, particularly in a setting of low beam  current.
>> Somehow, the value of 40 nA sticks in my mind  from the 5061A.  The 5061A
>> manual says end of life of the HP  CS tube is a peak beam current of 8 
nA or
>> less.  However, I  have units with less current and they still lock.  
The HP
>>  manual also says to measure the voltage at the output of the tube with  
>> 100
>> MegOhm or higher input impedance DMM.  If  yours is less, that may
>> artificially lower your  values.
>> EOL of the tube is a multifactor issue,  including Signal to Noise ratio 
>> the 'useful signal current' to  'background current' ratio.  The 
>> current' is what  you see with no RF signal applied to the tube.  Have 
>>  measured that?  A ratio of 1 is EOL per the HP manual.  If yours is  
>> 4.5 nA, as suggested by the 'off peak' values shown, or  less, you still
>> have
>> a useful signal and, hopefully, a  useful tube.
>> I'd recommend continuing with the  repair.
>> Good luck.
>>  Joe
>> -----Original Message-----
>> From:  time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On
>>  Behalf Of Ed Palmer
>> Sent: Monday, May 05, 2014 12:46  AM
>> To: Discussion of precise time and frequency  measurement
>> Subject: [time-nuts] Datum 4065A Cs Tube  Response
>> I'm playing with my first Cs standard.   It's a Datum 4065A which 
appears to
>> have a dead STEL-1173  synthesizer.  Before I put too much effort into
>> replacing  that, I thought I'd check the tube and see if it has any life
>>  left.  I've attached a chart showing the response of the central  peak.
>> My methodology was similar to TVB's as shown  here:
>> http://www.leapsecond.com/pages/cspeak except that I  measured the tube
>> output directly with a digital voltmeter.   The system is reporting 
>> varying levels for the beam current  so I didn't want to use any of it's
>>  circuitry.
>> Does this look like a usable tube?   Healthy or on it's last legs? What
>> response levels are typical for  a Datum 7504A tube?  I see that these
>> levels
>> are  somewhat lower than those shown on leapsecond for the 5061A tube,  
>> that could just be the specifics of the  measurement.
>> Thanks,
>>  Ed
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