| Electronics > Projects, Designs, and Technical Stuff |
| Amplitude modulation of AD9834 DDS - failed, but why? |
| << < (5/6) > >> |
| Yansi:
I have not taken an image from the scope and do not remember xactly, but for low frequency modulation, the cap voltage varied only slightly, say 5 to 10% of its steady DC voltage, with full modulation applied. |
| iMo:
9851: DAC Bypass Connection. This is the DAC voltage reference bypass connection normally NC (NOCONNECT) for optimum SFDR performance. 9951: DAC Band Gap Decoupling Pin. A 0.1 μF capacitor to AGND is recommended 9831/32/35: COMP: Compensation Pin. This is a compensation pin for the internal reference amplifier. A 10 nF decoupling ceramic capacitor should be connected between COMP and AVDD 9833/34/37/38: COMP: DAC Bias Pin. This pin is used for decoupling the DAC bias voltage. FSADJ pin modulation: https://ez.analog.com/dds/f/q-a/29200/amplitude-modulation-on-ad9838 --- Quote ---The FSadjust pin is controlled internal and the feedback loop BW is limited to guarantee loop stability. If you are interested in AM modulation, You will need to add a VGA or similar externally. --- End quote --- |
| Yansi:
To explain why I am trying this all: Using any kind of external amplitude varying circuitry like VGA, multiplier, gilbert cell completely voids the point of using DDS in the first place - because of high harmonic content generated that way. Making a good linear multiplier that can work with high modulation depths and wide range of frequencies costs MONEY. I have already experimented with various kinds of modulators, and all of them are bad when used for wide range of RF, because of harmonic content generated. Directly modulated DDS is probably the lowest cost of them all, with best results achievable. (well... if it happens to work correctly) :) |
| iMo:
It looks like the various 98xx DDS differ in how the Rset/FSADJ is internally handled. The 9851 appnote shows the fet modulation instead Rset, claiming it works fine up to 50KHz. The 9838 (similar to 34) AM issue in the above link ended up with "no go advice" while the OP there had some progress with removing the COMP cap. |
| Yansi:
I can also confirm, that by omitting the COMP pin cap on that AD9834, it proceeds to work stable and being able to fully modulate up to 10kHz (haven't tested further, not interested there). Am thinking right now, if the former solution with the precision programmable sink will work now, when the cap was removed from there. ??? |
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