Hi Glen,
Yes, the part we're presently using is $15 quantity 1 at Digi-Key, and
goes down to $13 in quantity.
Precision is just /one/ issue. We're also trying to keep phase noise low
across the entire design.
We already have one part we have to do software temperature compensation
upon: The I/Q modulator in the CMX991 has a balance-vs-temperature
graph, so after we balance the device with the built-in detector, we
have to track the temperature. So we have a sensor chip on the same pad
as the CMX991.
This is all enough to sell us on digital I/Q modulators and mixers for
some future design.
We are going to make it as good as we can for the first run, and then
experiment with making it cheaper.
Thanks
Bruce
On 02/08/2015 04:59 PM, glen english wrote:
Bruce
I'd suggest living with a +/- 2ppm oscillator design and have the modem
capable of AFCing and pulling the oscillator against a table. You will
striggle to keep it cheap searching for a 0.5pm oscillator- don't foget
aging !
The receiver can of course build, over time, a lookup table of pulling v
temp , also.
Have a temperature lookup table that is written at manufacture time.
Watch out for the TCXOs with SAWTOOTH (!) internal correction that leads
to big jumps at times....
the receiver just needs , over aging of the oscillator, some means of
tweaking its table.
On 9/02/2015 11:47 AM, Bruce Perens wrote:
On 02/08/2015 02:40 PM, Gregory Maxwell wrote:
Hopefully there will be room to add (or already included) a GPS receiver.
Hi Greg,
We have a few approaches for precise frequency control. First, there is
a 10 MHz reference input with a signal conditioner chip. If you have a
small GPSDO, you can connect it there. The current sub-1PPM oscillator
is this one:
http://www.conwin.com/datasheets/tx/tx355.pdf
It doesn't have voltage control, and perhaps we should consider that.
We're out of connections on the instrumentation DAC, so we'd need to
change that, too.
Another alternative is to steer the digital oscillator in the SDR.
This really is a problem, because now that we have GMSK digital voice in
less than 1.2 kHz, our sub-1PPM oscillator can drift the width of an
entire simplex channel at our highest frequency.
For a TDMA repeater, the repeater should be on GPSDO or Rubidium, and
the users should phase-lock to the repeater's frequency.
Thanks
Bruce
Beyond the very useful direct applications for positioning (APRS;
automatic repeater directories; blind signal triangulation) and time,
a GPS receiver would allow phase locking a VCTCXO to get good
frequency accuracy for modes where this is required.
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