[time-nuts] Unified VCXO Carrier Board
kb8tq at n1k.org
Thu Oct 22 07:44:27 EDT 2015
> On Oct 22, 2015, at 3:40 AM, Gerhard Hoffmann <dk4xp at arcor.de> wrote:
> I'd like to design a unified VCXO Carrier Board to these requirements:
> 1. It can host one of the the following VCXOs:
> 1.1. HP 10811A-6111 (as from 5370A)
> 1.2 Morion MV89A
> 1.3 MTI 260
> 1.4 CV-950
> 1.5 Timetech
> 1.6 Axtal
> 1.7 Pascall
Multiple footprints are fine and they don’t generally take up a lot of space. The 10811 is a bit of a hassle in that respect.
> 2. It provides unified tuning: 0V = lowest possible frequency, 3V3 or 5V = highest possible frequency, no matter of the VCXO tuning sense and range.
That immediately gets you into op amps and feedback resistor stability. With a number of combinations, the required resistors can get pretty expensive. It
also gets you into dual supplies with some OCXO’s.
> 3. provides a 5V tuning voltage reference for those VCXOs that don't have one of their own.
This gets you into the same sort of “is 1 ppm stability on the tuning good enough” set of questions.There are a number of OCXO’s out there that have odd
reference voltages.(10 V etc)
> 4. Frequency can be adjusted from external Vtune input and from a 10 turn pot.
Sounds good. Consider ground loops and offsets on the external input.
> 5.Board has 2 reference frequency inputs with LTC6957 receivers. One of them can interface the onboard VCXO to the CPLD.
I’m not sure these are needed if the destination is a CPLD.
> 6. Board has a 1pps input 3V3 CMOS level
Hopefully buffered with discrete logic. CPLD’s often are not very rugged in terms of over voltage on the inputs.
> 7. It can lock the VCXO to the reference frequency or the 1pps in. Provides LED lock indication.
Probably not easily with a 64 flip flop CPLD. For a full narrow bandwidth PLL you will need some more “stuff” on the board. With a
wideband loop, you will have a lot of noise on the oscillator.
> 8. It features a Xilinx Coolrunner2 2C64 CPLD, complexity 64 FlipFlops + combin. Logic. Unused Pins are brought out to Testpoints in 100 mil grid. The Coolrunner remembers its configuration and can be reprogrammed using the standard Xilinx USB dongle. It has a 10 pin 2mm header for this purpose. Small circuits can run at 200 MHz. This function exists already:
> < https://picasaweb.google.com/lh/photo/4Bpcfouj8WH0shNGIyuVUtMTjNZETYmyPJy0liipFm0?feat=directlink >
Given the number of much larger devices in the 10 to 100X larger range that are still under $10, I’d look at another device.
> 9. The Coolrunner provides a standard 1pps /20us out, maybe 10/100/1000 pps.
> 10. 2 Monoflops for 1pps LEDs in/out
> 11. There are 2 output buffers that drive valid 3V3 CMOS into 50 Ohms. They can be re-clocked to LTC6957 outputs with 1G74 flip-flops.
I would dedicate a couple of discrete (sot-23 logic) buffers to the 1 pps outputs.
> 12. Unbuffered VCXO output is available on SMA connector
> 13. One additional buffered output (LMH6702/AD8009 or discrete to avoid neg. supply). This is not meant to be a distribution amplifier.
> 14. Regulators for the voltages needed.
> 15. Requires soldering skills 0603 / sot23-5 / MSOP. No commercial interest. Could be TAPR or DIY.
With all this stuff on the board, layout will be “interesting”. Keeping it all from cross talking will be a challenge.
That gets even more complex as the number of external connections goes up. The more configurations, the
more things to worry about …
One simple example of the above:
1) pulls 3 ppm with a 5V tune.
2) has a stability over -30 to +70C of 1 ppb
3) has an ADEV at 1 second of 1 ppt
That comes out to:
Stability wise, that’s 10 ppt / C
Tune wise you have 0.6 ppm / volt or 0.6 ppt / uV
To maintain the 10 ppt/C your reference / tune voltage needs to be better than that. For instance, 5X better is a fairly
common design goal in many systems. That’s 2 ppt/C from the tune. Roughly 4 uV of stability would do it.
With a 5V reference the same 4uV is a 1 ppm / C reference. The same math applies to the stability of the resistors
in the tune circuits and the offset drift on the op-amps.
Lots to think about !!!
> I'm open to suggestions & ideas.
> Gerhard, DK4XP
> time-nuts mailing list -- time-nuts at febo.com
> To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
> and follow the instructions there.
More information about the time-nuts