Stanley Reynolds wrote: > ________________________________ > > > 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 > From: Richard Moore <[EMAIL PROTECTED]> > To: [email protected] > Sent: Monday, December 1, 2008 3:41:48 PM > Subject: Re: [time-nuts] DAC resolution in GPSDO > > > The oscillator I think has a manual adjustment. You could be using the 5 volt > provided on board or the 8 volt provided by the OCXO for the ECC op amp. Note > the ECC voltage is dependent on the control board regulation some what even > using an external reference. You may want to manually adjust the OCXO closer > to zero and set the upper limit lower as well. By using a external voltage > divider on the ECC voltage or lowering the gain (R7 & R8). > > Bert's site: > " > Another contributing factor to stability/accuracy is voltage regulation as a > function of load current on the +5V regulator. A +5V variation will impact > the PWM output amplitude on the PIC micro. It will also impact the offset > applied to the operational amplifier to shift the tuning voltage negative in > the case where you control a VCXO with a -5V to +5V tuning range. > The single biggest contributor to load variation on the 7805 regulator is > change to the 10MHz output terminations. I have seen variations of a few > parts in 10-10 when disconnecting instruments form the system. The workaround > to this (assuming this variation affects you) is to always put 50 ohm > terminations on unused outputs. When adding an instrument, remove the BNC > termination and connect the instrument. This will maintain a nominal current > consumption on the output driver chip. Obviously, you will want to set the > controller so that the outputs are always on. Other current consumers, such > as the LED, the input driver and the decade counter do not affect stability > in a meaningful manner." > ... > "That being said, those of you who operate a tuning voltage from -5V to +5V > (and this is the case with the HP oscillators) - should try to center the > tuning voltage at or close to 0V. It can be mathematically demonstrated that > this is where the effects of the +5V voltage variations cancel out. For > applications that require a 0 to +5V tuning voltage, obviously, the closest > to 0V, the better." > _______________________________________________ > Its far better to use the PWM output to drive a fast SPDT analog switch(eg 1/3 of a 74HC4053) that switches between a precision reference and ground and low pass filter the SPDT switch output. Alternatively one can power the micro from a precision reference based low noise regulator.
Bruce _______________________________________________ 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.
