[time-nuts] Linear voltage regulator hints...
bob at evoria.net
Tue Dec 9 11:31:13 EST 2014
I can't speak for Dan, but since he hasn't responded, I can speak a bit about the issue. As you know, my GPSDO uses a constant voltage TIC and the PWM output from a dsPIC33 to perform the DAC function. During testing we've found that one critical point is that the PWM voltage from the PIC changes with temperature. Dan managed to solve that for us by buffering the PWM with a 125 gate powered by a stable voltage. So, the temperature sensitivity issue is essentially solved. I would guess that he's looking around for a canned solution rather than using a good reference with an op-am.
We need very stable voltages for both 2.5 and 3.3 unless I revisit the TIC's RC to make it useable at 3.3. On my prototype, I'm using an ADR-291G as a VREF for the ADC on the PIC. I had originally used LF33 regulators for the board voltage, but recently switched to using another ADR-291G with an op-amp and a suitable divider to get 3.3V to power the rest of the board, including the 125 gate. My prototype board was made with through-hole components, but I'm about convinced to do the next board on SMT. Good regulators would have been nice, but it sounds like we're going to be using references and op-amps to get what we need.
Here's an overnight ADEV plot against the new Cs of where we are in the project. Red is ADEV. Green is the TIC. Blue is the output of the GPSDO to Channel A and the Cs to Channel B of my 5335A measuring TI, using the 1PPS from my GPSDO to trigger the external gate. I see that the Cs phase has drifted down slightly vs my GPSDOe. I wonder if this is an indication of a calibration problem with the Cs or some subtle issue on my board? This test will run throughout the day, so maybe that question will be answered.
From: Bob Camp <kb8tq at n1k.org>
To: Discussion of precise time and frequency measurement <time-nuts at febo.com>
Sent: Monday, December 8, 2014 6:48 PM
Subject: Re: [time-nuts] Linear voltage regulator hints...
Hopefully the issue (and question) is about stability of the EFC voltage. Any decent OCXO should have a voltage stability that’s well below it’s temperature stability when run off of a fairly standard regulator. As an example, An OCXO that does 5x10^-9 over 0 to 70 C probably has a voltage stability below 5x10^-10 for a 1% change. One percent is more than what a modern regulator should be moving when running an OCXO.
> On Dec 8, 2014, at 7:33 PM, Dave M <dgminala at mediacombb.net> wrote:
> Dan Kemppainen wrote:
>> In playing with some oscillators and a GPSDO here, I think I'm seeing
>> a voltage sensitivity issue.
>> So, I started looking at the output voltage of various regulators vs.
>> temp. Using standard LM/UA type linear regulators and some LDO's, they
>> all appear to be pretty sensitive to temperature. (millivolt / few
>> degrees Fahrenheit sort of sensitivity). Most of the datasheets seem
>> to ignore temp sensitivity. Almost like they are so bad they don't
>> want to publish it...
>> Does anyone have hints on TO-220 or D-Pak type regulators that have
>> really good temp coefficients and good line regulation? A few PPM/Deg
>> C might be nice, if possible. Or am I into a 'roll you own' type
> Yes, in order to get low PPM stability, you are going to "roll your own". No 3-terminal regulator or reference that I know of can run an OXCO (heater and oscillator).
> What are your voltage and current requirements?
> Dave M
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