// draft // 11-Mar-2006
Keywords: Symmetricom Datum Sigma-Tau Hydrogen Maser Cavity-Auto-Tuned Short-term Long-term Stability Allan deviation
The Register Speed Multiplier (RSM) of the Sigma Tau MHM-2010 hydrogen maser is configured by two small toggle switches labeled "4x" and "2x" located behind the front panel. Interpreting these as two binary digits allows one to choose one of four RSM rates: 1x, 2x, 4x, or 8x. The expectation is that 1x gives better short-term stability and 8x better long-term stability (?)
It has been quite difficult for me to accurately measure the difference in maser performance for each switch setting. It requires, over the span of many days, a very stable lab environment and a very stable reference; down to the levels around 1×10-15.
My lab is still not temperature stable to 1 ºC levels (goal is 0.1 ºC) and the reference in this case is a KVARZ CH1-75 active hydrogen maser; which during this time had its own set of performance challenges. Nevertheless, below are two runs made in October 2005 which give a hint of the performance difference in the RSM rate. I believe the auto-tuning of the CH1-75 reference was disabled during these runs (mode FrunM rather than AFCC-I).
As seen in the photo below the RSM toggle switches are located under the top right corner of the front panel. The entire front panel must be removed in order to access the switches (they are not intended to be changed often; but the operation can be done live while the maser is operating).
The labels aren't totally clear but the email below clarifies the situation.
> Are the 4x and 2x RSM switches behind the front > panel "on" when pushed to the left or right? I.e., > is 1x (slow) when both are left and 8x (fast) when > both are right? Your assumption is correct, right is multiply (Fast), left is X1 (slow) > Approximately what is the time constant at either extreme? I don't remember an exact time constant without going through the system and doing a calculation, however, in the slowest speed the action of the tuner is usually seen as a slight bump in the stability curve around 10000s, and at the fastest the bump moves to the left and peaks around 1000s.
OK, now for the plots.
|Both RSM switches set LEFT (1x)|
|Both RSM switches set RIGHT (8x)|
Although it would have been nice to have had longer runs (tau out to a day, 105 s, or even 1 week or 106 s), or multiple runs (to give some confidence of run-to-run consistency), or runs at each of all four switch positions, this pair of runs gives a good idea of the difference between the 1x and 8x positions.
These plots show that the ADEV most affected by the change is tau 200 s to 4000 s. In particular, at around tau 1 hour, the ADEV gets about 2½ times better (about 2.5×10-15 vs. 6×10-15) in the RSM 1x position.