[time-nuts] Setting Rubidium to match GPS source

CHazlitt chazlitt at mtaonline.net
Thu Sep 27 19:07:08 EDT 2007



Hello again,

Another question, has anyone here used an HP 3575A Gain Phase Meter (1Hz - 
13MHz) to set their Rubidium to match the GPS sourced 10 MHz clock? Would 
that method be more accurate to line the Rubidium up than using a 12 digit 
frequency counter clocked off of the GPS?


Signed, time newbie

Chris KL7FB


> Message: 4
> Date: Thu, 27 Sep 2007 06:45:28 -0700
> From: "Richard (Rick) Karlquist" <richard at karlquist.com>
> Subject: Re: [time-nuts] time-nuts, frequency counters
> To: Discussion of precise time and frequency measurement
> <time-nuts at febo.com>
> Message-ID: <46FBB3F8.4060800 at karlquist.com>
> Content-Type: text/plain; charset=ISO-8859-1; format=flowed
>
>>> The simple answer, is it introduces another place for drift
>>> to occur.  If the power supply that provides the current to
>>> create the C-field drifts with temperature, component aging,
>>> power line voltages, phases of the moon, ... your reference's
>>> frequency will also drift.
>>>
>>> -Chuck Harris
>>>
>> I must be missing something. Aren't all those bad things going to happen
>> whether you adjust
>> the C-field or leave it untouched? Isn't the C-field required for the Rb
>> to work dependably
>> and on freq? I think the question pertains to Rb sources that already
>> have a screw to make
>> the adjustment. Maybe you are saying the screw is there to put the
>> C-field back where it belongs.
>> I would have no way of knowing how to tell if it was right or wrong
>> other than frequency.
>>
>> Rick said it is not a good way to adjust the frequency. I don't really
>> understand why exactly either.
>
> There is an optimum range for the C-field.  Too little will result in
> insufficent Zeeman splitting and too much makes the frequency too
> sensitive to magnetic field.  Remember that frequency is proportional
> to the SQUARE of the C-field intensity.  The C field screw is not
> designed to "put the C-field back where it belongs" but rather to
> move it away from its comfort zone to pull the frequency.  I'm not
> an expert on Zeeman splitting, but I have heard people talking about
> finding a setting to avoid any spurious lines, as if the setting was
> like finding a clearing in a forest of lines.
>
> It is true that an adjustable C-field has the potential to drift more
> than a fixed C-field, but this is only one issue, that may or may not
> be pivotal.  Another issue with changing the C-field is that it may
> affect the magnetic hysteresis in the magnetic shields.
>
> With a synthesizer, you introduce no drift due to adjustability, you
> always operate at the optimum C-field value, the adjustment is linear
> and predictable, and the range of adjustment is not limited, so you
> can introduce any offset you want, and it is remotely, digitally,
> programmable.  And with modern synthesizers, this capability may well
> be virtually free.  Even the HP10816 Rb standard, designed in the
> 1970's, had a synthesizer that we set to get on frequency, however this
> synthesizer was not intended to be user programmable.
>
> BTW, "C-field" is a bastardized term borrowed from cesium beam
> technology where there were "A" and "B" magnets to select the state
> and the C-field to bias the magnetic field in the interaction region.
> Hence "A-field" "B-field" and "C-field".  Of course, Rb standards have
> no A or B fields.  It is sort of logical in an "inside baseball" sense.
>
> Rick Karlquist N6RK
>
>
>
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