Hi Mark, from my experience even a resolution of 0.25C is several orders of magnitude too low for use in holdover temperature compensation on an OCXO. This would result in fairly massive EFC changes on the Thunderbolt ovens, especially if they are single ovens with significant thermal sensitivity. The average error for a typical single oven OCXO due to this would be ~1.25E-010, see below. To make this work somewhat smoothly, one would need at least 0.001C or better 0.0001C resolution. I can't get into too much detail, but our units allow for down to ~0.000025C resolution by using a 24 bit ADC for the temperature sensing. Keep in mind that for temperature compensation the absolute temperature is not of interest, but rather the change of temperature over time. This kind of resolution is not possible with I2C based absolute temp sensors, one needs analog circuitry feeding high-resolution Delta-Sigma ADC's. These circuits are not accurate on the absolute temperature, but they do have very high resolution for relative temperature changes. A resolution of 0.25C would give you only 40 individual DAC settings for a 10C change as could be expected on a typical diurnal temperature change on a lab bench. This would make the frequency jump around and really sink the ADEV performance of the unit during holdover. To give you an example: a typical single-oven OCXO has about 1ppb per degree C change. This would mean the unit could only be adjusted by 2.5E-010 steps with the Dallas Temp sensor as a reference. So the average error would be 1.25E-010, which results in a massive 12.5 microseconds average drift error in 1000s intervals due to the temperature quantization error! The math looks better for double oven OCXO's, but not much. I am pretty sure the Dallas chip is only used for environmental monitoring of the absolute temperature, and not for compensation anywhere inside the PLL loop. Any Thunderbolt designers on this list?? bye, Said In a message dated 2/4/2009 12:45:51 Pacific Standard Time, [email protected] writes:
My bet is Dallas Semi changed their chip in a way that is not compatible with the Tbolt firmware. If Trimble changed their firmware to not do the high resolution read cycle, they would not have done the first steps of the high res read cycle (mask off the LSB, subtract 0.25C) which actually reduces the temperature resolution from nine bits to eight bits. Trimble could have easily missed the problem if they did not plot the temperature curves. The unit reports a proper temperature, it just does not have the fine resolution of the earlier units... which could easily affect holdover performance. _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
