[time-nuts] Sorry for the unedited post: Counter Internal Oscillator Importance with ExternalReference?

[Tom Knox]

Hi All;

I used my speech recognition software last night and reading my post this morning I was embarrassed.  Hopefully this email will make sense.
Thanks everyone for their input. Tom, i have a question; was there any difference between the two 53230A oscillators when locked to an external reference? I have also experienced poor results with my 53230A HS phase locked to an external reference. My issue is constant pop-ups showing reference errors. I wonder if that is related to issues you encountered. I would like to compare oscillator options in the same counter model and I have several each  53230A's and MCA3027's but sadly they have the same oscillator option in each respectively so I cannot directly compare PLL oscillator performance. My thinking is a phase locked internal reference may not be the best option for those of us with ULPN, Ultra Stable house references so I am trying to reverse engineer the Tektronix FCA/MCA counter to determine if there is away to directly feed an external reference instead of phase locking the internal reference. My plan is to find a way (if possible and it is encouraging) to do this while keeping the rest of the functionality intact, Which does appear possible on the Tektronix, Fluke, Pendulum instruments (All the same base units branded differently). It appears depending on the oscillator option U11B sends an off/on signals to switch between internal oscillator options; Std Pin3, Oven Pin5, or Rubidium Pin4. This switching then has the installed reference option provides 10MHz signals to  U11B pin204 RB, Pin205 Std, Pin206 Oven respectively, most likely changing PLL parameters at the same time. Whether this is auto sensed or manually controlled  I have yet to determine. The selection may be in the menu. My thinking is this may allow simply adding a connector and cable to the rear panel to apply an external reference directly.  In these counters U9A seems related to the PLL functions including control, sensing, and switching using the standard External Reference input. If my idea pans out this feature would remain unchanged so the internal oscillator can still be used (When the direct feed is not appropriate) to clean up a dirtier external reference. My plan is (Unless someone else has already looked into this is to next see if the same is also possible with the 53230A. The BIG question does this improve performance. Any thoughts? I spoke with a friend Fred Walls today and asked if he though the internal reference was much of a factor when Lock to a good external reference and he did not thin so. So I will keep anyone interested informed with my progress since I am sure many of you have reference that may benefit from this modification if it does improve performance.

Thanks again for everyone's input.

Thomas Knox

________________________________
From: time-nuts <time-nuts-bounces at febo.com> on behalf of Tom Van Baak <tvb at LeapSecond.com>
Sent: Tuesday, May 9, 2017 9:03 AM
To: Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] Counter Internal Oscillator Importance with ExternalReference?

Hi Thomas,

About the 53230A -- like many of us I figured there was no point in buying the expensive OCXO versions since I have plenty of good 10 MHz references around here. So years ago Keysight loaned me two of them for a month: one with, and one without OCXO. Like most counters, the 53230A has an ext-ref input. But it is horrible. And the ref output is even worse. I think we've discussed this on the list before, including ADEV and PN plots. I decided not to buy one, ever, until they fixed the problem.

So it could be a nice counter. Someone at Agilent / Keysight needs to re-design the int/ext/mux/pll clock handling of the 53230A; someone who understands phase noise and short-term stability; someone who honors the intent of ext-ref-in and ref-out; someone who keeps stupid microprocessor / LCD / comms / power supply noise out of the clock path. Meanwhile I keep my old 53132A's and older SR620's and stay away from the 53230A.

If you have your own measurements, please share. Maybe I just got a bad sample, I don't know. But the ext-ref performance smelled like poor engineering to me. My hope is that someone on the list will hack a 53230A, fix the problem, and share the solution with us, or even with Keysight.

/tvb


> Hi All;
>
> How important is the standard, medium, or high stability reference in counters like the 53230A, or FCA3120 when locked to an ultra low phase noise external reference? Particularly when making measurements and adjustments on other ultra high performance references?
>
> I know Agilent sold 53132A counters with an option (H01 I believe) that bypassed the internal reference completely when and external reference is applied, but I think standard configuration on most counter is to discipline the internal reference.
>
> Thanks for your thoughts.
>
> Thomas Knox


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