[time-nuts] Tbolt disciplined LPRO Rubidium

William H. Fite omniryx at gmail.com
Fri May 13 18:09:19 UTC 2011


Thanks, Warren.  I might give  this a shot, if I can discipline my ten
thumbs.  :)



On Fri, May 13, 2011 at 12:16 PM, WarrenS <warrensjmail-one at yahoo.com>wrote:

>
> In response to request on how I set up my Tbolt with a long time constant
> to directly disciplined a LPRO 101 Rubidium;
>
> I use a standard LPRO 101 Rb as an external replacement for the Tbolt's
> internal OCXO,
> and connect the LPRO's external C_Field adjustment input to the Tbolt's Dac
> out thru a 1K ohm resistor (added as protection for the Tbolt.)
>
> Due to Tbolt's software limitations, an Extended_TC setting method needs to
> be used for setting Time Constants above 1000 sec.
> To calculate the Tbolt's Effective TC setting using this extended TC
> setting method
>
> a) The Effective_TC of the TBolt is Actual_TC setting times the
> Multiplier_Factor
> b) The Effective_Damping factor of a Tbolt is Actual_Damping setting
> divided by the square root of the Multiplier_Factor
> c) The Multiplier_Factor is equal to "Dac_Gain used / Actual_EFC_Gain"
>
>
> Example for setting Tbolt's Effective_TC = 20,000 sec,  Effective_Damping
> factor = 0.6 for a LPRO.
> a) Use Multiplier_Factor = 100
> b) Dac Gain = +0.90 Hz/V
> c) TC = 200 sec
> d) Damping = 6.0
> e) Set Initial Dac value to +2.5V,
>
> After adjusting  the LPRO's to 10 MHz using it's internal freq C field pot
> (with the external C_Field input floating  at 2.5V)     .
> f) It is a good idea to limit the Tbolt's Dac output swing by setting the
> Max_Dac out to + 3.0 V, and the Min_Dac out to + 2.0 V.
>
>
> Example of setting Tbolt's Effective_TC = 5,000 sec,  Effective_Damping
> factor = 0.7,   that I use to discipline a very stable external dual oven HP
> 10811.
> a) Use Multiplier factor = 10
> b) Dac Gain = -3.24 Hz/V    (Actual Gain for this 10811 is -0.324 Hz/V)
> c) TC = 500 sec
> d) Damping = 2.2
>
>
> The effective TC can be measured by how long of time that it takes for a
> Freq offset error to return to zero, best measured by using LH's Dac voltage
> plot.
> The effective Damping can be calculated by how much Freq error overshoots
> there is, which in turn determines how long of time the Phase error takes to
> return to zero.
> For a damping =  0.6  the Freq overshoot is 33% and Phase correction time
> is about 3.5 times the Freq correction time  (10%  phase overshoot)
> Using a damping of  0.7,  the Freq overshoot is 25%, & phase correction
> time is  5x the TC setting. (no phase overshoot)
> With a damping factor of 1.0, there will be very little Freq overshoot, but
> that causes the phase correction time to be very long. (overdamped Phase
> response)
>
> Using Lady Heather, the effective TC and Damping_Factor settings can be
> verified by plotting the offset freq recovery time to an intentional Dac
> step error.
> a) Disable the control loop (D D)
> b) Change the Dac voltage (D S ... ) by an amount equal to 1e-10 or 1e-9
> (verify on Osc plot)
> c) Enable the control loop (D E)
> d) Plot the Phase, Dac_Voltage and Osc_Freq over at least 5 times the
> effective TC setting.
>
> Before replacing a Tbolt's internal Osc with a cheap rubidium, one should
> understand the noise tradeoffs, especially at low taus.
> Lots of plots available on request
>
> ws
>
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>
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