DDS Output Termination

"A" sees a complex impedance, so R3 is necessarily a compromise. But it probably doesn't matter much. R1/2 is probably safe. Too bit an R3 could have compliance problems, which would matter.

You'd get less distortion if A and B drove a diffamp first. That's another interesting issue.

John

,

R1 kinda sorta serves as a pad to flatten the likely reactive input to the filter. A more comprehensive approach would be a pi-type resistive pad.

A more comprehensive example is a diplexer. View it as:A driving two paralleled loads: a LP filter to a flat load of R3, and a HP filter to a flat load of R3, with the crossover region chosen to keep the impedance seen to A quite flat.

A simple example of this diplexer approach can be seen, e.g. in W7EL's "Optimized QRP Transceiver". On the web at

If you look there, the HP is just a 0.1uF driving a 51 ohm load resistor, and the LP is a 100uH choke feeding a common base amp set to have a 50 ohm driving impedance. This is not perfectly flat but it pays off hugely in terms of termination of the unwanted products coming out of the DBM, making sure they are absorbed and not reflected back.

All that said... a little balanced broadband transformer is preferable and really quite painless unless you need to cover, say, more than 6 octaves or something. Few applications have to cover more than 6 octaves.

Tim.

How about a li'l emitter follower? A BFS17 is a lot cheaper than a broadband transformer :-)

--
Regards, Joerg

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7 cents for the broadband transformer core (Mouser 623-2643002402) quantity one. Add some trifiliar winding and viola, you're done.

BFS17 is circa 20 cents quantity one.

(Not that either cost matters much compared to the price of an AD DDS chip.)

And as for emitter followers... look closely at W7EL's common base amp at the LP leg of the diplexer. There is a lot of wisdom there.

And you still want a little transformer on the output of the DDS. AD is not joking when they say it is preferred.

Tim.

7 cents for the broadband transformer core (Mouser 623-2643002402) quantity one. Add some trifiliar winding and viola, you're done.

BFS17 is circa 20 cents quantity one.

(Not that either cost matters much compared to the price of an AD DDS chip.)

And as for emitter followers... look closely at W7EL's common base amp at the LP leg of the diplexer. There is a lot of wisdom there.

And you still want a little transformer on the output of the DDS. AD is not joking when they say it is preferred.

Tim.

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Thanks to all for the interesting replies to my query. Gary

Yeah, but now you have degraded the monetary value of your own work to next to nothing :-)

I've done multifilar windings a lot but never like the stench that develops when burning off the enamel. In the summer with a window open, well, ok, but not in the winter.

Was looking for it but only found a mention of it, no schematic:

I use common gate stages a lot if the source likes a hearty load. Despite the fact that one of my profs has publicly said common gate is stupid, one of the designs ran at around 35k units/year for a long time.

There I have become careful. I've found math bugs in datasheets. Also AD often proposed split grounds, engineers believed it and ended up with an EMI nightmare. Ok, I am not complaining since this also brought me income (to fix it).

--
Regards, Joerg

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