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 <[email protected]> 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: [email protected] [mailto:[email protected]] 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 > > > _______________________________________________ > time-nuts mailing list -- [email protected] > To unsubscribe, go to > https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. > _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
