[time-nuts] DAC resolution in GPSDO
stanley_reynolds at yahoo.com
Mon Dec 1 22:31:33 UTC 2008
On Dec 1, 2008, at 12:09 PM, Warren wrote:
> Message: 4
> Date: Mon, 1 Dec 2008 11:22:42 -0800
> From: "WarrenS" <warrensjmail-one at yahoo.com>
> 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
> 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
> 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.
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
From: Richard Moore <richiem at hughes.net>
To: time-nuts at febo.com
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).
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."
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