On Dec 1, 2008, at 12:09 PM, Warren wrote: > Message: 4 > Date: Mon, 1 Dec 2008 11:22:42 -0800 > From: "WarrenS" <[EMAIL PROTECTED]> > Subject: Re: [time-nuts] V standards > > Concerning the Need for very high resolution Dacs in GPSDO. > >> Bruce said: "However there are low noise OCXOs with EFC adjustment >> ranges of 1E-6 or more, (that would benefit from >20 bit Dacs)
>> Question: Is the Aging rate of these low noise OCXO units poor >> enough > that you could not use a couple of fixed precision resistor and/or > a pot > for course adjustment and reduce the EFC range of the DAC by say > 1/100 ?. > Are there low noise OCXO (that are being used in GPSDO) whose long > term > drift over say 6 months would need more than 1E-8 or so of > 'automatic' turning? I'm using an Isotemp 134 OCXO that has an EFC sensitivity of around 1.62E-6 Hz/V near the 10MHz point. It is very stable and also well- aged, and its non-adjustable EFC V for 10MHz is near 3.318V. I'm running it in Bert Zauhar's FLL GPSDO, which has a PWM DAC with 14- bit resolution and an output range of 0 to +5V. This DAC resolution is not enough for tight frequency control of the Isotemp, given its very coarse Hz/V sensitivity. Combined with the DAC resolution, the system granularity is roughly 4.9E-11 Re 10MHz for it's finest step. This just isn't quite enough. > And concerning the 10811A. Should one consider reducing its EFC > tuning range by say 2 to 20 to ease the requirements of the Dac? > Even if the EFC tuning range was reduced by just a factor of two, > It could then be done with just a standard 0 to 5 volt Dac > instead of the -5 to +5 that I saw suggested by its spec. > I would think that a well aged unit would be much better than its > worse case > spec of 1E-7 per year, And even if not, I'm guessing that many of > the time-nuts, > would be very welling to trade off it needing a have few extra > manual adjustments > in order to get better performance. > > WarrenS Warren, that's exactly what I'm doing with the Isotemp, aiming for a division of about 6-10x. It's no problem for folks like us to tweak stuff when needed, so that's an effective solution. I'd like to have a higher resolution DAC to preserve a large tuning range, but the whole idea of this GPSDO was a simple, inexpensive, yet effective DIY GPSDO. See www.moorepage.net/gps2.html Dick Moore _______________________________________________ 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.
