This shows you the differences between two versions of the page.
precision_timing:lpro_disciplining_with_thunderbolt [2013/01/08 19:00] |
precision_timing:lpro_disciplining_with_thunderbolt [2013/01/08 19:00] (current) |
||
---|---|---|---|
Line 1: | Line 1: | ||
+ | ===== LPRO disciplining with Thunderbolt ===== | ||
+ | |||
+ | // From the [[Time Nuts Mailing List]]: | ||
+ | // | ||
+ | |||
+ | In response to request on how I set up my Tbolt with a long time constant to directly discipline 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: | ||
+ | |||
+ | - The Effective_TC of the TBolt is Actual_TC setting times the Multiplier_Factor | ||
+ | - The Effective_Damping factor of a Tbolt is Actual_Damping setting divided by the square root of the Multiplier_Factor | ||
+ | - 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.** | ||
+ | |||
+ | - Use Multiplier_Factor = 100 | ||
+ | - Dac Gain = +0.90 Hz/V | ||
+ | - TC = 200 sec | ||
+ | - Damping = 6.0 | ||
+ | - 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), 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** | ||
+ | |||
+ | (I use this setting to discipline a very stable external dual oven HP 10811.) | ||
+ | |||
+ | - Use Multiplier factor = 10 | ||
+ | - Dac Gain = -3.24 Hz/V (Actual Gain for this 10811 is -0.324 Hz/V) | ||
+ | - TC = 500 sec | ||
+ | - 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: | ||
+ | |||
+ | - Disable the control loop (D D) | ||
+ | - Change the Dac voltage (D S ... ) by an amount equal to 1e-10 or 1e-9 (verify on Osc plot) | ||
+ | - Enable the control loop (D E) | ||
+ | - 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 | ||