[time-nuts] Datum 4065A Cs Tube Response
actast at hotmail.com
Mon May 5 21:18:04 UTC 2014
Is this the right chip:
At $65 it is pricy but a bargain if it fixes your clock.
> Date: Mon, 5 May 2014 15:14:16 -0600
> From: ed_palmer at sasktel.net
> To: time-nuts at febo.com
> Subject: Re: [time-nuts] Datum 4065A Cs Tube Response
> 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 read
> > 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.
> > WB8TSL
> > 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 current
> >> 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 about
> >> 60 mV or a 'useful signal current' of about 6 nA. If your unit's circuitry
> >> can properly amplify that and keep it a clean signal, it should work.
> >> However, I would recommend setting the OCXO precisely on frequency with a
> >> 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 a
> >> 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 and
> >> the 'useful signal current' to 'background current' ratio. The 'background
> >> current' is what you see with no RF signal applied to the tube. Have you
> >> measured that? A ratio of 1 is EOL per the HP manual. If yours is about
> >> 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 wildly
> >> 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, but
> >> that could just be the specifics of the measurement.
> >> Thanks,
> >> Ed
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