Hi As you do the tweaks, the frequency changes should go from parts in 10^8 to 10^9 to 10^10 per turn. A lot depends on the pot setup and the crystal in terms of how high it starts. The flip side to that is your counter and local reference standard need to be able to measure at least parts in 10^10 to get it set. Is it worth getting it super close? Probably not without a temperature test setup. Bob > On Feb 29, 2016, at 11:14 PM, Nigel Vander Houwen <timenuts-nigelvh@nigelvh.com> wrote: > > Howdy All, > > This thread has split into a couple, but I’ll try to respond here to the various things. > > Based on how this crystal warms up, it does appear that it is a BT type crystal, as it warms up frequency goes up, and as it gets hotter eventually turns around and heads downhill again. > > I’ve managed to tune the oven to where the peak seems to be. It’s a many turn potentiometer (something like 21 turns), that plays a small portion on the resistance, so +/- about a quarter turn at the peak didn’t seem to really impact the frequency. I left it in the middle of that range. > > My replacement thermistor has a lower beta than what it seems the original had. The original thermistor being specified at 9.93K @ 80C, and the calculator shows mine at about 12K @ 80C. Since the circuit for this crystal has a 10K + 9K & the 2K POT on one side of an op-amp comparator, and a 10K + the thermistor on the other side, I added a 2.2K resistor to the POT leg of that voltage divider to bring the tuning range about to where it was stock, as I found that unaltered, with the new thermistor I couldn’t set the oven temperature low enough, every frequency was on the downhill slope past the peak. > > I appreciate Frank’s offer for a close to original thermistor, however I’m not in the netherlands (despite my name), and I’ve already got a reasonably close replacement epoxied into the oven like the original was. > > Now I’ll leave it run for a while and see about tuning the frequency. I’ve got it pretty close at the moment, but the coming days/weeks will show their own colors. > > I would like to thank everyone for the insight around getting this thing tuned up. It’s sincerely appreciated. > > I’ll probably start designing a DAC + Phase comparator board to GPS discipline this old crystal. Have to see how that ends up comparing to my thunderbolt. > > Nigel > >> On Feb 28, 2016, at 15:03, Bob Camp <kb8tq@n1k.org> wrote: >> >> Hi >> >> Ok a bit more of the story. >> >> It’s easy to simply turn on the device and see how it warms up. Back when it was made, the >> SC did not yet exist. The only thing it could be was a BT. With an X-ray setup you can absolutely >> tell it’s a BT. With the blank and a pair of calipers you can make a darn good guess it’s a BT. >> >> Since HP did not make their own blanks, the “competition” was the source of their blanks. No need >> for those guys to guess about anything. >> >> Despite all of this logical and rational thinking, the BT remained a “top secret” sort of thing as far >> as (at least certain people at) HP were concerned. Those who were concerned also had the route >> to the HP PR machine so that’s the story that went out to the world. >> >> Those involved left HP long ago. The whole thing became a non-issue once the 10811 came out. >> What is the most amazing thing to me is that 30 years after it became a non-issue there still is >> confusion about the topic. >> >> Bob >> >>> On Feb 28, 2016, at 11:48 AM, jimlux <jimlux@earthlink.net> wrote: >>> >>> On 2/28/16 6:23 AM, Bob Camp wrote: >>>> Hi >>>> >>>> The whole “BT Cut” issue was a big top secret in HP. They spent a lot of time obscuring >>>> the fact that they used BT’s. The belief was that if any of the other outfits figured out that >>>> was what they were doing, the competition would have better OCXO’s. >>>> >>> >>> >>> Can you tell what the cut is if you have the blank in front of you? >>> >>> Wouldn't the competition just buy an instrument with the oscillator, saw it open, and measure it? >>> >>> >>> _______________________________________________ >>> time-nuts mailing list -- time-nuts@febo.com >>> To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >>> and follow the instructions there. >> >> _______________________________________________ >> time-nuts mailing list -- time-nuts@febo.com >> To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >> and follow the instructions there. > > _______________________________________________ > time-nuts mailing list -- time-nuts@febo.com > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there.

On 3/1/2016 4:13 AM, Bob Camp wrote: > Is it worth getting it super close? Probably not without a temperature test setup. > > Bob > Right. It is entirely possible that if you did a temperature test in an environmental chamber, you would find that you could get a better tempco by adjusting oven set point to have a slight offset from the turnover. In this regime, the crystal drift would cancel out the tempco of the electronics. This all depends on the crystal tempco, the electronics tempco, and the respective thermal gains to the crystal and the electronics. But there will be an adjustment giving zero tempco (at least around a single ambient temperature). My old boss at Agilent used to call this sort of thing a "hero experiment", which he used as a pejorative term. Rick