[time-nuts] Some additional RTFG learnings

Tue Feb 20 19:42:49 UTC 2007

I believe that there is more to the interface cable than a simple
crossover -- I wonder if there are any pins that are jumped on one or
the other of the connectors.

By the way, in the experiment I reported on earlier, the XOs seem to be
running undisciplined as well, and are about 1 Hz off nominal frequency.

There's definitely something else needed to make these things work properly.

John
----

TheInfamousFlavio at hotmail.com said the following on 02/20/2007 02:33 PM:
> I got similar results in that the rb oscillators are not GPS disciplined 
> after a 72 hr run.
> 
> I started off by manually offsetting the frequency of a warmed up rb unit by 
> doing a manual adjustment to the rb oscillator via the manual adjustment pot 
> so that it would be approximately 0.0050Hz off of my GPSDO.  Then I let it 
> run for 72hrs and no change to the oscillator.
> 
> A few interesting things I discovered:
> 
> 1. there is the unit is putting voltage to the electronic cfield connector 
> of the rb oscillator.  I discoved this when I tried adjusting the oscillator 
> when it was outside of the case.  When I put it back in the case an entirely 
> different frequency displayed then when I put it back in the case.  When I 
> manually adjusted the cfield pot on the rb oscillator, the unit did NOT 
> compinsate for the manual change...which lead me to believe that there is no 
> disciplining happening on the RB unit side. Although there might be 
> potential for it to happen because of the  voltage to the cfield pin ... 
> perhap some sort of initialization command is needed.
> 
> 2. Both the XO and the RB unit can be in operational mode simulataneously. 
> On the Interface xover cable, I simply disconnected pins 1 and 5 (the 
> outside pins of the top row).  A simple +seems to control the A/B behavior 
> of the XO and RB units.  So both units display "NO GPS" off and "ON" lit.
> 
> 3. There a little BCA shorting wires a few places on both RB and XO units. 
> The RB unit has BC jumped at each of these locations and the XO unit has AC 
> jumped.  W201 determines from which points the unit takes the signal from to 
> and from the oscillator.  W202 and W203 determine the to and from for the 
> GPS receivers TX and PPS signal at the interface.
> 
> 4. You can xover the interface with only pins 2 and 6 on the XO to pins 4 
> and 9 on the RB. This simply send the GPS receiver's TX and PPS in the XO to 
> the RB unit. By doing this both units will have 'NO GPS' off and 'ON' on.
> 
> 5. Grounding the CPURESET doesn't seem to make a difference.
> 
> Does anyone have an idea of what PLDENB might mean?
> 
> 
> 
> 
> 
> ----- Original Message ----- 
> From: "John Ackermann N8UR" <jra at febo.com>
> To: "Discussion of precise time and frequency measurement" 
> <time-nuts at febo.com>
> Sent: Tuesday, February 20, 2007 10:00
> Subject: Re: [time-nuts] Some additional RTFG learnings
> 
> 
>> I started data gathering of XO vs. XO about three minutes after yanking
>> the interconnects as described below (but after the units had been
>> running in GPS locked mode for well over a day).
>>
>> There is definitely a stabilization period required after the XO becomes
>> operational; on this measurement, the one second tau is around 4x10e-9!
>> I'll try to get an idea how long it takes to stabilize and report back.
>>
>> John
>> ----
>>
>> John Ackermann N8UR wrote:
>>> Hi --
>>>
>>> I'm doing some stability comparisons of the RTFG-m-RB and RTFG-m-XO 
>>> units.
>>>
>>> I fired up two RTFG pairs (thanks to Jim Miller for lending me his
>>> units) with the 10 MHz and cross-over interface cables in place.  The
>>> units fired up normally.
>>>
>>> I did a 24 hour frequency stability run of the RB units measured against
>>> each other, and all I learned in that time is that either (a) the RBs
>>> are not actually GPS disciplined, or (b) the loop time constant is
>>> longer than a 24 hour data collection will show.  After doing the XO
>>> run, I will rerun the RBs for a longer period to see if there is any
>>> sign of discipline.
>>>
>>> Then, I disconnected the RB units so I could activate the XOs to run the
>>> same test.
>>>
>>> I thought the behaviour on doing that was worth noting:
>>>
>>> 1.  Disconnect the 10 MHz reference cable.  RB stays in "ON" mode, XO
>>> goes to "FAULT."
>>>
>>> 2.  Disconnect crossover interface cable.  RB stays in "ON" mode, XO
>>> stays in "FAULT" and "NO GPS" comes on.  But there is signal at the RF
>>> OUT connector.
>>>
>>> 3.  In about 30 seconds, XO "FAULT" and "NO GPS" go off, "ON" comes on.
>>>
>>> So, the XO is definitely testing for the presence of the 10 MHz
>>> reference input signal, and becomes unhappy when it goes away.
>>>
>>> John
>>>
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>>> time-nuts at febo.com
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>>>
>>>
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