Harold E. Johnson wrote:
Would that be the DDS controled receiver design from the UK?
His DDS-Micro design was in an ap-note on the Analog Devices
web site (pdf download), but source code wasn't published.
The specific pic micro he used is now obsolete, though similar chips
are available (require software changes). Also the AD9850 dds
chip he used is now somewhat long in the tooth.


The very one. Source code available on the Picastar website.

URL please? Thanks!
(He must have been a dedicated hacker to fit all that he did
in just 1k! I've seen a similar design that would calibrate
itself, and it took almost 2k of code! Actually given all
the glue required to expand 13 i/o lines to drive everything
it might be easier and cheaper just to use a MegaAt16 AVR
micro, program in "C", and have the jtag to debug.)

I am considering building a DDS vfo for a homebrew xcvr, but
I am looking at using one of the AD9951 series chips. These clock
to 400mhz and have a 14bit D/A vs the 10bit in the 9850. Spurs
and phase noise should be quite a bit lower with the 9951 series
chips over the 9850. You will need a surface mount to .1 mill
spacing proto board to hand solder it (not to mention good eyes
and/or a strong magnifier, and steady hands). Such proto boards
are available on ebay sometimes, and can also be found for sale
via a google search.


I've been running a 9951 overclocked at 600 MHz (It'll do a bit over 700
before it REALLY doesn't like it) Waterproofed in the shielding department,
feedthru decoupled and on a 2 layer board with PTH copied from the AD proto
board. BTW, the 9951 is still running cool at 600 MHz (External LO) with the
center post solder grounded to a large PTH.

Waterproofed? Ya liquid cooling it?!!
I've got a small proto board to change from the small surface mount
pattern to a .100" through hole pattern so I can bread board with
the 9951. There isn't any provision for the ground on the bottom
of the chip, so I'd have to add a small square of copper foil to the
proto board, drill a hole in the middle, and solder through the
back of the board to the chip, bringing the ground lead out the back
to the circuit ground.

Are you driving the chip AT 600mhz or using the frequency multiplier
on the chip with a lower frequency drive? What kind of oscillator
is driving the chip? (noise in the oscillator might matter).
Seven pole band pass filter, also in the DDS box. Phase noise is
non-existent. Spurious, while at the AD specs is still objectionable.

W4ZCB

I had this idea of not using a low pass filter, but rather a band
switched band pass filter with a separate section for each band.
Might even use a varicap to tune the filter for max output at
the lo frequency via a d/a driven by the micro tuning the dds.
I wonder if such a filter would reduce the spur output over using
even a seven pole low pass?