Wideband (<30MHz) AM modulator circuit, how to make one?

[CJay]

Will be interesting to see how you get on with modulating FS

[Yansi]

I have designed my circuit based on this and some common sense reading the AD9834 datasheet, to get an idea, what might be sitting at the FSADJ pin - as I've said, probably a cirrent mirror, sourcing current from the REFOUT internal reference. Supported by the fact, that the voltage at FSADJ pin is slightly below the internal ref voltage suggest this may be true.

AS the FSADJ current SHOULD scale linearly with output current, one probably might use this to obtain the AM.

I will try making (yet another) test PCB for this.

I just need to figure out how to implement the output (filter + buffer). Filter probably a 7th order elliptical @ 30MHz, but I don't know whether to use the differential output of the DDS, or whether not. And where and how is it best to obtain the impedance transformation from 200 to 50 ohm.

If I have calculated correct, the DDS should produce about -12dBm maximum with 3mA full scale current set (4mA is the limit by the datasheet). (full scale current being 18 times what is pulled from the FSADJ pin).

[MT]

I have designed my circuit based on this and some common sense reading the AD9834 datasheet, to get an idea, what might be sitting at the FSADJ pin - as I've said, probably a cirrent mirror, sourcing current from the REFOUT internal reference. Supported by the fact, that the voltage at FSADJ pin is slightly below the internal ref voltage suggest this may be true.

As the FS adj current SHOULD scale linearly with output current, one probably might use this to obtain the AM.

It will swing between 32mV and 624mV with 200ohm load on Dac outs within the given current min-max range at Fs pin.  RELAX! it has been done before, many times! AD9102 uses switched resistor network on the Fs Iset..

[Yansi]

In which way should i RELAX?  I am kind of a non-believer when it comes to strange hacks like this one. What has been done before specifically?