I'm using one of those Analog Device DDS ICs that has a differential
current-mode output. On the Analog Devices evaluation board schematics, they
usually take the outputs through a fully differential LC filter for
anti-aliasing and then through a balun to end up with a single-ended output.
This of course takes a few more parts than immediately taking the output
through a balun and then using a traditional single-ended filter. I've been
debating the pros and cons of the two approaches and so far the reasons I can
think of for going differential filter and then the balun are...

1) Somewhat more "ideal" filter in that a ground plane (or worse, ground
trace) isn't being used for return currents, it's just current flowing from
one side to the other of the differential output. (Seems minor, although from
past filter designs I've done at UHF this might be a much larger improvement
than I'm guesstimating here.)
2) Filtering out the high-frequency scunge before it hits the balun allows the
balun to not handle quite as much energy and hence perform a little bit better
(more linearly). (Also seems minor.)

Am I missing anything else here?

Thanks,
---Joel

On Tue, 1 Apr 2008 13:59:35 -0700, "Joel Koltner"
yahoo.com> wrote:

Hi John,

"John Larkin" highNOTlandTHIStechnologyPART.com> wrote in message
news:dug5v31cac60g06e8ht0qvvh1uguf6u1nq@4ax.com...

The output frequency range I care about is 275-300MHz (it's one of their 1GSps
DDS chips), so I'm generally avoiding op-amps here. :-)

I am just after sine waves. Grounding one output seems reasonable enough,
although Analaog Device's advice in this case is to use a doubly-terminated
filter on one output (say, 100 ohms at each end) and then terminate the other
output with the same effective load (100 ohms double terminated --> 50 ohms).
(This is from
http://www.analog.com/UploadedFiles/Tutorials/450968421DDS_Tutorial_rev12-2-99.p... ,
page 47.)

"Joel Koltner" yahoo.com> wrote in message
news:ZmxIj.117926$ng7.16077@en-nntp-05.dc1.easynews.com...

they

output.

been

can

from

from ...

Hi John,

So what were you planning to build out of the AD9851?

(I'm building a simple OOK-like frequency hopping transmitter.)

I'm thinking I'll drop Analog a note and see if they have anything to say
about this. Unlike, say, Texas Instruments, they tend to actually respond to
e-mail without having to remind them many times of getting a rep on the phone!

---Joel

On Tue, 1 Apr 2008 17:29:06 -0700, "Joel Koltner"
yahoo.com> wrote:

"John Larkin" highNOTlandTHIStechnologyPART.com> wrote in message
news:f5t5v39oj6fmd8bjga1uubmffi2ftah79r@4ax.com...

In this case, is it "this is just unnecessary and doesn't really help so it's
a waste of parts"-nonsense or "this is actually harmful compared to the
simpler approach"-nonsense? (Or possibly "ADI knows something they're not
telling us..." -- buried in some application note somewhere for the ADF436x
PLLs there's a note about (paraphrasing), "You really ought to have both
outputs, V+/V-, terminated in the same load since, umm, if you just ground V-
and use V+ in certain corner cases the PLL will fail to work..."

On Wed, 2 Apr 2008 09:41:57 -0700, "Joel Koltner"
yahoo.com> wrote:

Joel Koltner wrote:

With a $20 DDS chip? Couldn't you guys use one of the ChipCon ICs from
TI plus a uC to do the hop instructions? Just don't count on much tech
support there ...

What is it with TI these days? Where do they think the design-ins for
the next years will come from? Or to say it more like Wall Street would,
what will keep the stock price up?

--
Regards, Joerg

Hi Joerg,

"Joerg" removethispacbell.net> wrote in message
news:6GSIj.4009$p24.3371@nlpi061.nbdc.sbc.com...

Yep. We're only building one or two prototypes at the moment, so the cost of
parts is nothing compared to the cost of development time.

Not at the rather, uh, ...rapid... hop rates we're using. That 275-300MHz is
just the beginning of the transmitter chain; much higher frequencies are
involved by the time we're through.

---Joel