[time-nuts] fast switching quiet synthesizer

Anders Wallin anders.e.e.wallin at gmail.com
Tue Oct 7 15:43:00 EDT 2014


On Tue, Oct 7, 2014 at 8:32 PM, Magnus Danielson <magnus at rubidium.dyndns.org
> wrote:

>
>
> Some of the modern DDSes can take 10 MHz directly and step it up
> internally before hitting the DDS core, but it may be that you need to
> synthesize a higher clock from the 10 MHz first.
>

We've been using/testing an AD9912 eval-kit board. It can take 10MHz input
and has an internal 66x PLL and VCO for a 660MHz DDS sample-clock (just out
of spec actually, vco is min 700MHz if I read the datasheet correctly).
Output looks like so:
http://www.anderswallin.net/wp-content/uploads/2013/12/Fout100M_Span400M.bmp
DDS output is 0 to 2-400MHz with a resolution of ~few uHz since it has a
48-bit tuning word. We're programming it with an arduino+eth-shield using
modbus-ethernet. Changing frequency over ethernet takes about 60ms, but it
could be much much faster directly over SPI if you know the desired
frequencies beforehand and store them on the arduino.
zoom-in shows spurs at 50kHz
http://www.anderswallin.net/wp-content/uploads/2013/12/Fout100M_Span500k.bmp

The spurs improve when using the DDS with an external 1GHz sample-clock
(internal PLL+VCO on the DDS-chip disabled). We produced it from 10MHz with
an ADF4350 eval-board:
http://www.anderswallin.net/wp-content/uploads/2014/03/100MHz_AD9912_internal_vs_external_PLL.jpg
AD must use some nice&expensive spectrum-analyzer to produce that figure
with a -100 dBm noise floor! :)

even with the eval-boards there's a fair bit of building: multiple
powersupplies, cables, enclosure, etc.

Anders
PS. I could be tempted to join in if someone wants to make a PCB for these
chips - would save significantly compared to eval-boards..



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