Re: [time-nuts] Beginners GPS locked frequency counter question

[Jim Lux]

On 10/31/15 7:32 PM, Bob Camp wrote:

Hi

So … how good is the “calibrate and go” (not the tone on second channel) 
approach likely to be?

If it’s a bare crystal or normal XO (not a TCXO) that is supplying the clock, 
the crystal will follow some
fairly well known curves. Which one of the curves it follows will depend 
entirely on the individual sample
you have. What I see on my sound card will not be what you see on a different 
sample of exactly the same
sound card installed in a similar machine.



having just done a bunch of measurements over 4 days on some crystals of 
this type (16 MHz, nothing special), I can shed some light on it.


over half a day sitting on my desk, they fluctuated about 0.1 ppm  (peak 
excursion)


AVAR is about 1E-9 at tau=10sec, rising to 2e-7 at 10,000 seconds (after 
taking out the temperature dependence)


There is a strong temperature dependence.. with temperature changes of 6 
degrees (C) the frequency change is on the order of 6ppm (90 Hz out of 
16 MHz)






It’s a pretty good bet that your device will change by 0.1 to 1 ppm per degree 
C. Yes it can be a bit better. It
can also be a whole lot worse. Your room environment is likely to move 1 to 3 C 
per hour if you have the heat
or air-conditioning turned on. If you are in an unheated garage … who knows. If 
you take the most likely 0.5 ppm/C tempco
and the most likely 2C change you get a “wobble” of about 1 ppm. The period of 
this wobble probably will be
in the 30 to 90 minute range.



yes, that's about right..
Although my office doesn't have that kind of fluctuation..
And I ran the 4 day test with the oscillators in a cardboard box over 
the weekend, so that damps short term variations.



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Re: [time-nuts] Beginners GPS locked frequency counter question

[Nick Sayer via time-nuts]

> On Oct 31, 2015, at 3:29 PM, Magnus Danielson  
> wrote:
> 
> Some people is very fond of using the frequency measure of counters, I've 
> grown more and more sceptic to it for a number of reasons when doing ADEV and 
> friends, then I use TI that avoids a number of issues.
> But that is a much more involved story.
> 

In doing my evaluations of my GPSDO, I’ve tried two configurations:

1. Feeding a reference into one input and the test signal into the other and 
asking my 53220A to give me the time difference between the two, and then 
having TimeLab fetch that.

2. Feeding a reference into the external reference input and the test signal 
into input 1 and asking for the frequency, and having TimeLab fetch that.

So far, #2 has given me (very) slightly lower AVARs. My guess as to why is that 
if you’re reading phase differences, then that is very sensitive to the 
accuracy with which you time the sampling. I’m interrogating the TIA with a 
virtual machine over WiFi, so I don’t have any reason to expect that that 
timing is going to be exceptionally accurate.

If anyone has any additional insight or corrections to my understanding, I’m 
all ears.

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Re: [time-nuts] Beginners GPS locked frequency counter question

[Bob Camp]

Hi

So … how good is the “calibrate and go” (not the tone on second channel) 
approach likely to be?

If it’s a bare crystal or normal XO (not a TCXO) that is supplying the clock, 
the crystal will follow some
fairly well known curves. Which one of the curves it follows will depend 
entirely on the individual sample 
you have. What I see on my sound card will not be what you see on a different 
sample of exactly the same
sound card installed in a similar machine. 

It’s a pretty good bet that your device will change by 0.1 to 1 ppm per degree 
C. Yes it can be a bit better. It
can also be a whole lot worse. Your room environment is likely to move 1 to 3 C 
per hour if you have the heat
or air-conditioning turned on. If you are in an unheated garage … who knows. If 
you take the most likely 0.5 ppm/C tempco 
and the most likely 2C change you get a “wobble” of about 1 ppm. The period of 
this wobble probably will be 
in the 30 to 90 minute range. 

On top of the temperature induced effects there are also voltage issues and 
long term aging. Unless you are in 
a very unusual setting, both should be well below the temperature related 
effects until you get out to time periods
of many months to several years. 

At 130 KHz, 1 ppm is 0.13 Hz. You will need to run a pretty good / long FFT on 
the sound card to get that sort of 
resolution. With some coding, you can do various things to speed the process up 
a bit.  Depending on what 
you are doing, 1 ppm may indeed be “good enough”. 

A typical TBolt GPSDO when locked and running normally will supply an output 
that is good to better than 0.1 ppb
99% of the time. It will hit 0.01 ppb 90% of the time. With some care, it could 
be even better. Both assume you 
are using a counter gate in the 1 to 10 seconds range to check it.

That moves you up 10,000 to 100,000 times more accurate than the 1ppm drift on 
the sound card time base. 
Getting *useful* accuracy at that level out of a straight sound card reading is 
quite a challenge. You also get into a lot 
of fancy math surrounding things like 6 hour long measurement times. 

My *guess* would be that the GPSDO is not really needed in this case ...

Bob

> On Oct 31, 2015, at 8:58 PM, Tom Van Baak  wrote:
> 
>>  As an aside, I work low frequency RF transmissions on 136 Mhz, and
>>  very narrow bandwidth. Can a soundcard be locked to GPS instead of
>>  its own internal crystal for precise frequency output?
> 
> Chris,
> 
> It might first be interesting to see how far off the frequency is before you 
> worry about disciplining it with GPS.
> 
> One trick that I use is to make the soundcard generate 1 Hz pulses and 
> compare that against GPS with a TI counter. The tool is 1hz.c 1hz.exe in my 
> tools directory (http://leapsecond.com/tools). This avoids having to open up 
> the computer and probe or buffer the soundcard oscillator.
> 
> If you collect a long enough data set you will also get a feel for how stable 
> the oscillator is; something you will need to know to tune your GPSDO 
> algorithms.
> 
> But wait, there's more. If you find that your soundcard oscillator is, say, 
> 12.34 ppm fast -- you don't actually have to tune or discipline or replace 
> the physical oscillator. Instead just change the software that's writing to 
> the soundcard and have it generate waveforms that it thinks are 12.34 ppm too 
> slow. In other words, instead of defining PCM_RATE 48000 as a constant, you 
> set pcm_rate = 48000 * (1 + 12.34e-6) as a variable. One line of code.
> 
> It gets even better. If all you need is one channel of output, then generate 
> your waveform on the L channel and generate 1PPS on the R channel. Use a 
> TBolt and TIC to continuously measure R vs. 1PPS and send those readings back 
> to the PC for averaging. As the time interval grows (indicating a frequency 
> offset) beyond acceptable levels, then make corresponding changes your 
> pcm_rate variable. This essentially becomes a software GPSDO. No h/w changes 
> are required to the board; you don't even have to open the case. The output 
> waveform will always be spot on-frequency, regardless of soundcard oscillator 
> frequency offset and drift.
> 
> /tvb
> 
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Re: [time-nuts] Beginners GPS locked frequency counter question

[Mod Mix]

Chris,

Can a soundcard be locked to GPS instead of its own internal crystal for 
precise frequency output?
I do not know which soundcard you have in mind - hope to be not totally 
off-topic.
I use a KS-24361 10MHz signal at the Mutec MC-3+ reference input to feed 
MC-3+ generated word clock with better (short time) stability into my 
sound cards.
Sound quality improvement of course depends on the world clock handling 
on the given card - works fine with both, RME AES PCI and Lynx L22.

hth
Ulli
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Re: [time-nuts] Beginners GPS locked frequency counter question

[Graham / KE9H]

Chris:

If by "purely mathematical" you mean with infinite precision and infinite
accuracy in the calculations, then the answer is "No."

The counter is using integer mathematics, in a world with propagation
delays and clock jitter, and uncontrolled phase relationships.
So resulting rounding errors and lost remainders in division processes.
If you read the specs on any counter, you will find that there is a
absolute accuracy PLUS an error window of (typically) plus or minus one
count.  Both error sources are very real.

I assume you mean 136 kHz.

But the answer to your sound card question is that you can't just GPS lock
the data converter on the sound card.  If it is running in a different
"clock domain" than the synchronous signal processing that follows it, then
you usually start having buffer underflows and buffer overflows which
introduce signal integrity problems.  The sound card sampling clock is
usually derived from some master clock in the computer, such that the sound
card is running in the same clock domain as the rest of the computer, so
you more likely need to GPS lock the whole computer.  This capability is
normally not provided for consumer grade computers, so you will be on your
own for the circuit modifications.

If you are doing signal processing in a desktop or laptop computer, they
are not "real time" operating systems, so understand the implications of
that, too.

--- Graham

==



On Sat, Oct 31, 2015 at 5:50 AM, Chris Wilson  wrote:

>
>
>   31/10/2015 10:46
>
>I have a Racal counter locked to 1 MHz on its rear panel external
>input socket from my Trimble Thunderbolt GPS. I derive the 1 Mhz
>from a David Partridge divider board. If I also feed the counter
>with the 10 Mhz direct output from the same GPS it reads one or two
>Hz out. As I assume the counter is working purely mathematically
>why would that be please?
>
>
>As an aside, I work low frequency RF transmissions on 136 Mhz, and
>very narrow bandwidth. Can a soundcard be locked to GPS instead of
>its own internal crystal for precise frequency output?
>
>Thanks.
>
> --
>Best Regards,
>Chris Wilson.
> mailto: ch...@chriswilson.tv
>
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Re: [time-nuts] Beginners GPS locked frequency counter question

[Jim Lux]

On 10/31/15 3:50 AM, Chris Wilson wrote:



   31/10/2015 10:46

I have a Racal counter locked to 1 MHz on its rear panel external
input socket from my Trimble Thunderbolt GPS. I derive the 1 Mhz
from a David Partridge divider board. If I also feed the counter
with the 10 Mhz direct output from the same GPS it reads one or two
Hz out. As I assume the counter is working purely mathematically
why would that be please?


As an aside, I work low frequency RF transmissions on 136 Mhz, and
very narrow bandwidth. Can a soundcard be locked to GPS instead of
its own internal crystal for precise frequency output?




you could probably get a DDS of some sort to take the high quality 10MHz 
and turn it into the clock frequency of your sound card. You'd need to 
figure out how to pull the XO off the board and feed an appropriate 
signal in.


There's a fair number of people interested in this, so I suspect someone 
has already done it.


The other approach is to replace the XO on sound card with a VCXO, and 
"discipline" it against the 1pps from the GPS.  It's a matter of using a 
different divisor than 10,000,000.



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Re: [time-nuts] Beginners GPS locked frequency counter question

[Magnus Danielson]

Hi Chris,

On 10/31/2015 11:50 AM, Chris Wilson wrote:



   31/10/2015 10:46

I have a Racal counter locked to 1 MHz on its rear panel external
input socket from my Trimble Thunderbolt GPS. I derive the 1 Mhz
from a David Partridge divider board. If I also feed the counter
with the 10 Mhz direct output from the same GPS it reads one or two
Hz out. As I assume the counter is working purely mathematically
why would that be please?


Your assumption is wrong.

There is a +/- 1 error in counting cycles.
Also, if the start and stop does not derive from the same trigger, 
differences in the triggers can result in a time-shift which bias the 
value. Difference in delay for start and stop events in the counter also 
produces error. Also, the white phase noise contribute.


In essence, a number of practical design limitations makes measurements 
somewhat noisier and somewhat biased compared to what theory says. It's 
important to understand these so that their effect can be estimated and 
sometimes mitigated.





As an aside, I work low frequency RF transmissions on 136 Mhz, and
very narrow bandwidth. Can a soundcard be locked to GPS instead of
its own internal crystal for precise frequency output?


There is sound-cards you can lock to a 48 kHz clock.

Cheers,
Magnus
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Re: [time-nuts] Beginners GPS locked frequency counter question

[Florian Teply]

Am Sat, 31 Oct 2015 10:50:36 +
schrieb Chris Wilson :

> 
> 
>   31/10/2015 10:46
> 
>I have a Racal counter locked to 1 MHz on its rear panel external
>input socket from my Trimble Thunderbolt GPS. I derive the 1 Mhz
>from a David Partridge divider board. If I also feed the counter
>with the 10 Mhz direct output from the same GPS it reads one or two
>Hz out. As I assume the counter is working purely mathematically
>why would that be please?
> 
> 
>As an aside, I work low frequency RF transmissions on 136 Mhz, and
>very narrow bandwidth. Can a soundcard be locked to GPS instead of
>its own internal crystal for precise frequency output?
> 
In principle yes. In practice this is a bit more involved, but still
possible. The easy way would be professional audio equipment, where
A/D-Converters quite often can be fed externally with a Master clock.
This is often used in studio arrangements, where one doesn't want
sampling clocks of different modules to drift relative to each other.
But the obvious downside is that professional audio equipment that
supports this kind of operation out of the box is "slightly" more
expensive than standard consumer stuff and usually isn't made to fit
into a computer but rather a 19 inch rack.

With Consumer sound cards, this is also possible, but requires some
hardware hacking in order to replace the onboard oscillator with
something that allows for locking to a frequency standard.

In both cases, the frequencies used there are somewhat odd compared to
what the average time nut has available: common master clock frequencies
are on the order of 16, 24, 32, 48 or 64 times the sampling frequency
for said professional equipment, consumer stuff often is clocked with
something like 6.144MHz (=128 times 48 kHz) or something similar.

That's nowhere near a clean 5 or 10 MHz a time-nut is likely to have
around... Which doesn't mean this would be difficult, in the end such a
setup is no different from usual time-nuts or lab setup, just the PLL
division ratio is different to lock the oscillator to a 1PPS signal.
But still, mod'ing consumer equipment takes some time to research the
actual circuit used and come up with a way to couple this to a known
good frequency source...

Florian

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Re: [time-nuts] Beginners GPS locked frequency counter question

[paul swed]

Chris welcome to the group.
Several comments.
The 1 or 2 Hz most likely is trigger and jitter. Its somewhat odd that you
are off by 2 Hz. Typically its 1 Hz.
I suspect you are also working 136 KHz. With respect to sound cards the
internal clock could be replaced and locked if its at all accessible. With
todays integration thats hard to say.
Regards
Paul
WB8TSL

On Sat, Oct 31, 2015 at 6:50 AM, Chris Wilson  wrote:

>
>
>   31/10/2015 10:46
>
>I have a Racal counter locked to 1 MHz on its rear panel external
>input socket from my Trimble Thunderbolt GPS. I derive the 1 Mhz
>from a David Partridge divider board. If I also feed the counter
>with the 10 Mhz direct output from the same GPS it reads one or two
>Hz out. As I assume the counter is working purely mathematically
>why would that be please?
>
>
>As an aside, I work low frequency RF transmissions on 136 Mhz, and
>very narrow bandwidth. Can a soundcard be locked to GPS instead of
>its own internal crystal for precise frequency output?
>
>Thanks.
>
> --
>Best Regards,
>Chris Wilson.
> mailto: ch...@chriswilson.tv
>
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Re: [time-nuts] Beginners GPS locked frequency counter question

[Bob Camp]

Hi



> On Oct 31, 2015, at 6:50 AM, Chris Wilson  wrote:
> 
> 
> 
>  31/10/2015 10:46
> 
>   I have a Racal counter locked to 1 MHz on its rear panel external
>   input socket from my Trimble Thunderbolt GPS. I derive the 1 Mhz
>   from a David Partridge divider board. If I also feed the counter
>   with the 10 Mhz direct output from the same GPS it reads one or two
>   Hz out. As I assume the counter is working purely mathematically
>   why would that be please?


Is it always out in the same direction ( = broken counter) or does it
bounce back and forth to each side of “correct” ( = noise) ?

> 
> 
>   As an aside, I work low frequency RF transmissions on 136 Mhz, and
>   very narrow bandwidth. Can a soundcard be locked to GPS instead of
>   its own internal crystal for precise frequency output?

Yes and there are threads on the list about doing this. The quick wrap up is:

1) Find the clock source (oscillator or crystal) on the sound card.
2) Pull it off the card and attach a cable.
3) Synthesize the correct frequency off of your GPSDO’s output (generally 
with some sort of PLL / VCO arrangement) 
4) Feed the signal into the cable to the sound card 

Bob 

> 
>   Thanks.
> 
> -- 
>   Best Regards,
>   Chris Wilson.
> mailto: ch...@chriswilson.tv
> 
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Re: [time-nuts] Beginners GPS locked frequency counter question

[Chris Caudle]

On Sat, October 31, 2015 10:03 am, Graham / KE9H wrote:
> The sound card sampling clock is
> usually derived from some master clock in the computer, such that the
> sound card is running in the same clock domain as the rest of the
computer, so
> you more likely need to GPS lock the whole computer.

Generally never true.  Sometimes on integrated (on motherboard) converters
the sample clock may be derived from a system clock, but that is a
convenience factor, not consequential to the design, and every add in card
(PCI or PCI Express) has an independent oscillator providing the sample
clock.  The samples are buffered and delivered asynchronously to and from
the processor over PCI or PCI Express, with bus operation not related to
the sample clock.
USB is slightly more complicated because there are multiple ways clocking
is handled.  Asynchronous isochronous mode is essentially the same as
described above.  USB adaptive isochronous mode throws some wrinkles in
that are probably not worth discussing in this context (unless it turns
out the original poster is trying to modify a USB sound interface and not
a PCI or PCIe interface).

-- 
Chris Caudle


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Re: [time-nuts] Beginners GPS locked frequency counter question

[Tom Van Baak]

>   As an aside, I work low frequency RF transmissions on 136 Mhz, and
>   very narrow bandwidth. Can a soundcard be locked to GPS instead of
>   its own internal crystal for precise frequency output?

Chris,

It might first be interesting to see how far off the frequency is before you 
worry about disciplining it with GPS.

One trick that I use is to make the soundcard generate 1 Hz pulses and compare 
that against GPS with a TI counter. The tool is 1hz.c 1hz.exe in my tools 
directory (http://leapsecond.com/tools). This avoids having to open up the 
computer and probe or buffer the soundcard oscillator.

If you collect a long enough data set you will also get a feel for how stable 
the oscillator is; something you will need to know to tune your GPSDO 
algorithms.

But wait, there's more. If you find that your soundcard oscillator is, say, 
12.34 ppm fast -- you don't actually have to tune or discipline or replace the 
physical oscillator. Instead just change the software that's writing to the 
soundcard and have it generate waveforms that it thinks are 12.34 ppm too slow. 
In other words, instead of defining PCM_RATE 48000 as a constant, you set 
pcm_rate = 48000 * (1 + 12.34e-6) as a variable. One line of code.

It gets even better. If all you need is one channel of output, then generate 
your waveform on the L channel and generate 1PPS on the R channel. Use a TBolt 
and TIC to continuously measure R vs. 1PPS and send those readings back to the 
PC for averaging. As the time interval grows (indicating a frequency offset) 
beyond acceptable levels, then make corresponding changes your pcm_rate 
variable. This essentially becomes a software GPSDO. No h/w changes are 
required to the board; you don't even have to open the case. The output 
waveform will always be spot on-frequency, regardless of soundcard oscillator 
frequency offset and drift.

/tvb

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Re: [time-nuts] Beginners GPS locked frequency counter question

[Magnus Danielson]

Hi Chris and Bob,

On 10/31/2015 02:13 PM, Bob Camp wrote:

Hi




On Oct 31, 2015, at 6:50 AM, Chris Wilson  wrote:



  31/10/2015 10:46

   I have a Racal counter locked to 1 MHz on its rear panel external
   input socket from my Trimble Thunderbolt GPS. I derive the 1 Mhz
   from a David Partridge divider board. If I also feed the counter
   with the 10 Mhz direct output from the same GPS it reads one or two
   Hz out. As I assume the counter is working purely mathematically
   why would that be please?



Is it always out in the same direction ( = broken counter) or does it
bounce back and forth to each side of “correct” ( = noise) ?


Does not have to be a broken counter, not at all.

For instance, the SR620 displays this kind of behavior if you have not 
calibrated it correctly.


Counters measure elapsed time and elapsed events.
If you divide time with events you get average period measure.
If you divide events with time you get average frequency measure.

While the time-base tries to give you a second worth of measures, in 
actual life it only delays the trigger of the "stop" in relation to the 
"start" trigger that time, and then the "stop" even is recorded and the 
elapsed time occurs.


If "start" and "stop" triggers does not trigger at the same phase (due 
to voltage offsets or time offsets), then there will be a time-bias. 
Some counters calibrate this bias out, where as others try hard to avoid 
it. If you don't calibrate it properly, this bias can cause a shift up 
or down in perceived frequency measure. This is not necessarily a sign 
of a broken counter, it's a sign of an uncalibrated counter.


For instance, the SR620 auto-calibration does not cancel this effect, 
you have to adjust the calibration manually to zero it out.


Another flaw could be missed or added cycles, as it adjust the event 
count in the above formula. Those usually show themselves as larger error.


Some people is very fond of using the frequency measure of counters, 
I've grown more and more sceptic to it for a number of reasons when 
doing ADEV and friends, then I use TI that avoids a number of issues.

But that is a much more involved story.

Cheers,
Magnus
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Re: [time-nuts] Beginners GPS locked frequency counter question

[Bob Camp]

Hi

> On Oct 31, 2015, at 6:29 PM, Magnus Danielson  
> wrote:
> 
> Hi Chris and Bob,
> 
> On 10/31/2015 02:13 PM, Bob Camp wrote:
>> Hi
>> 
>> 
>> 
>>> On Oct 31, 2015, at 6:50 AM, Chris Wilson  wrote:
>>> 
>>> 
>>> 
>>>  31/10/2015 10:46
>>> 
>>>   I have a Racal counter locked to 1 MHz on its rear panel external
>>>   input socket from my Trimble Thunderbolt GPS. I derive the 1 Mhz
>>>   from a David Partridge divider board. If I also feed the counter
>>>   with the 10 Mhz direct output from the same GPS it reads one or two
>>>   Hz out. As I assume the counter is working purely mathematically
>>>   why would that be please?
>> 
>> 
>> Is it always out in the same direction ( = broken counter) or does it
>> bounce back and forth to each side of “correct” ( = noise) ?
> 
> Does not have to be a broken counter, not at all.

There’s a fine line between “broken” and “not calibrated” …. To me “broken” = 
needs to be fixed. That
can easily include calibration adjustments. It can also include an input that 
has drifted far enough that
the adjustments no longer bring it back into proper operation. The distinction 
here is between a counter
that is operating properly (bouncing each side of zero error) and one that 
exhibits a bias. The ones that
exhibit a bias need to be “fixed”. 

Bob

> 
> For instance, the SR620 displays this kind of behavior if you have not 
> calibrated it correctly.
> 
> Counters measure elapsed time and elapsed events.
> If you divide time with events you get average period measure.
> If you divide events with time you get average frequency measure.
> 
> While the time-base tries to give you a second worth of measures, in actual 
> life it only delays the trigger of the "stop" in relation to the "start" 
> trigger that time, and then the "stop" even is recorded and the elapsed time 
> occurs.
> 
> If "start" and "stop" triggers does not trigger at the same phase (due to 
> voltage offsets or time offsets), then there will be a time-bias. Some 
> counters calibrate this bias out, where as others try hard to avoid it. If 
> you don't calibrate it properly, this bias can cause a shift up or down in 
> perceived frequency measure. This is not necessarily a sign of a broken 
> counter, it's a sign of an uncalibrated counter.
> 
> For instance, the SR620 auto-calibration does not cancel this effect, you 
> have to adjust the calibration manually to zero it out.
> 
> Another flaw could be missed or added cycles, as it adjust the event count in 
> the above formula. Those usually show themselves as larger error.
> 
> Some people is very fond of using the frequency measure of counters, I've 
> grown more and more sceptic to it for a number of reasons when doing ADEV and 
> friends, then I use TI that avoids a number of issues.
> But that is a much more involved story.
> 
> Cheers,
> Magnus
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Re: [time-nuts] Beginners GPS locked frequency counter question

[Bob Stewart]

Tom,
It's been awhile since I did soundcard SSTV, but the program MMSSTV uses the 
time ticks from WWV to do a calculation similar to what you describe.  You tune 
in WWV on your receiver, which is coupled to your computer's soundcard.  The 
MMSSTV calibration routine puts a raster scan on the screen.  You adjust the 
calibration factor so that the time ticks are lined up vertically from top to 
bottom.  The code is proprietary, but I suspect that he is merely dumping bytes 
from the soundcard on the screen, and the calibration value is some sort of 
multiplier for the soundcard's clock rate.  Someone who has done soundcard FFT 
programming might have a better idea.

Bob

  From: Tom Van Baak <t...@leapsecond.com>
 To: Discussion of precise time and frequency measurement <time-nuts@febo.com> 
 Sent: Saturday, October 31, 2015 7:58 PM
 Subject: Re: [time-nuts] Beginners GPS locked frequency counter question
   
>  As an aside, I work low frequency RF transmissions on 136 Mhz, and
>  very narrow bandwidth. Can a soundcard be locked to GPS instead of
>  its own internal crystal for precise frequency output?

Chris,

It might first be interesting to see how far off the frequency is before you 
worry about disciplining it with GPS.

One trick that I use is to make the soundcard generate 1 Hz pulses and compare 
that against GPS with a TI counter. The tool is 1hz.c 1hz.exe in my tools 
directory (http://leapsecond.com/tools). This avoids having to open up the 
computer and probe or buffer the soundcard oscillator.

If you collect a long enough data set you will also get a feel for how stable 
the oscillator is; something you will need to know to tune your GPSDO 
algorithms.

But wait, there's more. If you find that your soundcard oscillator is, say, 
12.34 ppm fast -- you don't actually have to tune or discipline or replace the 
physical oscillator. Instead just change the software that's writing to the 
soundcard and have it generate waveforms that it thinks are 12.34 ppm too slow. 
In other words, instead of defining PCM_RATE 48000 as a constant, you set 
pcm_rate = 48000 * (1 + 12.34e-6) as a variable. One line of code.

It gets even better. If all you need is one channel of output, then generate 
your waveform on the L channel and generate 1PPS on the R channel. Use a TBolt 
and TIC to continuously measure R vs. 1PPS and send those readings back to the 
PC for averaging. As the time interval grows (indicating a frequency offset) 
beyond acceptable levels, then make corresponding changes your pcm_rate 
variable. This essentially becomes a software GPSDO. No h/w changes are 
required to the board; you don't even have to open the case. The output 
waveform will always be spot on-frequency, regardless of soundcard oscillator 
frequency offset and drift.

/tvb

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