Hi Glen et al,
This has been a really valuable discussion, thanks for all of the feedback
and ideas.
On Sun, Feb 8, 2015 at 5:22 PM, glen english <g...@cortexrf.com.au> wrote:
> yeah, you've probably got to run at least 10 of them before getting
> overly committing to any single item. Of course that is a design luxury you
> may not have, and it does cost money ....
>
> $15 is alot for an oscillator for that sort of radio...but if it solves a
> hard problem, then it is cheap.....
>
> Steve I use those DV75Cs that you've recommended- they are quite good.
>
>
This was the best specs I could find available on digi-key that met the
requirements.
I do not have a VCTCXO because the cheapest SmartFusion II doesn't have any
more PLLs to use, which is how the Hermes does it. I wanted to avoid
having yet another PLL on the board...
> Phase noise I reckon will be dominated by your integer VCO and ability to
> keep noise off the power supplies, which is always a challenge in a small
> space.....
>
Agreed that this is the messy part, because you really can't know
everything about how it will turn out until you take the plunge and make a
board turn...
> Use the same oscillator to drive the CPU, also, ----go to hell and back to
> minimize the number of oscillators in the box- otherwise harmonics and
> beats will drive you insane.
>
>
All of the RF sections and DSP in the FPGA run off of the same 10MHz clock
via a clock distribution chip. There's other crystal oscillators on-board
for the USB (24 MHz) and Ethernet (25 MHz) transceivers. The SmartFusion 2
has a 32KHz RTC and a 12MHz main crystal. A number of PLLs on-chip to
derive its 166MHz clock, and the other fabric clocks. Too much? I'm
thinking it's possible to get rid of the 12MHz main oscillator and just use
the 10MHz one, but it's going to mean disabling the one in the Emcraft
M2S-SOM directly.
> Be sure to lock/sync ALL switching regulators .
>
>
This is a question I actually had... I have 3.3V and 5V switching
regulators... do they both get synced together? From your comment it
sounds like that's the right idea, or should they be 90 degrees out of
phase?
> I apologise if I am telling a chook how to lay eggs.....
>
>
In truth, thank you very much for sharing your experience and wisdom here.
It's incredibly valuable for a software-guy turned hardware-guy like me.
Regards,
Chris KD2BMH
> g
>
>
> On 9/02/2015 12:14 PM, Bruce Perens wrote:
>
> 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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> --
> -
> Glen English
> RF Communications and Electronics Engineer
>
> CORTEX RF
> &
> Pacific Media Technologies Pty Ltd
>
> ABN 40 075 532 008
>
> PO Box 5231 Lyneham ACT 2602, Australia.
> au mobile : +61 (0)418 975077
>
>
>
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