EMRFD Message Archive 624

Message Date From Subject
624 2007-04-07 18:12:18 Andrew homebrew sig gen
Following the pictures of Don's beautifully crafted sig gen - here is
my humble attempt! - it works...

Done as an exercise in getting my analog brain to cope with digital
circuitry - the AD9851 design is based on the AMQRP group DDS 60, and
some free software.

Havent done the post filter amp, need to design something as the AD op
amp specified in the original isn't available here in VK.

Rougha s guts, semi Manhattan style to see if it could be done - the
AD9851 is mounted on a carrier, which is mounted on a 28 pin IC socket
(machined) which in turn piggybacks to another one mounted on a piece
of veroboard - good RF construction NOT. But, does go surprisingly
well - 1Hz to 60Mhz in 1Hz steps....
625 2007-04-08 18:48:55 Gary Johnson Re: homebrew sig gen

Using a DDS kit a good approach, similar to what I did. I bought a little DDS3 kit from
Kanga (also based on the 9851) and buffered it with an LMH6703 wideband opamp. If
you're not familiar with modern, high-speed opamps, I wrote a report that includes a long
section about them. It's available at:


in the WB9JPJS folder, in a file called "IQPro + Amps Rev1". It coveres several opamps that
are of value in RF circutis. The LMH6703 and its evaluation board are available from
DigiKey. You do have to deal with SMT parts, but you end up with a nearly-ideal amplifier
that's flat to >300 MHz, with very low distortion, IIP3 > +35 dBM, +17 dBm output
capability, and essentially a perfect input and output match at all frequencies. I haven't
seen anything else like it. On my sig gen, I used a 100-ohm pot between the DDS and the
little opamp eval board and set the gain up to get about +15 dBm max output. I also threw
in a mixer at the DDS output to allow audio modulation.

Whole thing fit in a 2 X 3.5 X 1 Hammond box, and runs on an AC adapter. I'd write up an
626 2007-04-09 03:45:22 Andrew Re: homebrew sig gen
Gary, all noted - but, we are at the end of a very long supply chain
here in the SW Pacific - most of the u-beaut IC's that we can but read
about are just not available (except, if you want to buy a 100 or
so....) my copy of EMFRD cost me $75 Australian - a fair whack for a
book. And as for buying anything specialist or exotic, forget
it...."The Art of Electronics" a text I have long lusted after is
approx $120.00 here.

And the board(s) - same situation. Most US companies wont sell to
Australia, the customs/post 911 security stuff is just too onerous -
and postage charges are outrageous as well.........

Will try for the IC you menti
627 2007-04-09 03:52:53 Andrew Re: homebrew sig gen