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| Amplitude modulation of AD9834 DDS - failed, but why? |
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| Yansi:
So to make things bit more interesting - it works for low frequencies. At 50Hz mod., I can obtain what I came for. A very nice full-depth modulation, see picture below. But when if I switch to 1kHz, while keeping the mod. level, this happens: The FSADJ pin voltage becomes unstable and keeps changing states between proper 1.15V, 0V and 4V, producing severe distortion of the envelope and the modulation is only minute, not full. Just pure W T F ! |
| iMo:
When you sweep the modulation slowly from say 50Hz to 10kHz - do you see a changing pattern at the FSADJ pin? I would also disconnect the R11 from the REFout and feed the opamp bias 1.2V from an external source, it could be there is a parasitic feedback via Refout and it oscillates (C11 - add 100nF ceramics in parallel). |
| Yansi:
Yes, the higher the frequency, the shallower modulation is obtainable, then the same happens- FSADJ voltage starts going crazy from 1.2V to 0 then to 4V and again, see the image in the first post. It seems the FSADJ pin circuitry is somehow unstable internally. Makes me wondering, what effect would a cap to ground make? There is even one drawn in the AD9850 application note in the AN-423, meh. Let's try adding one. //EDIT: cap is doing nothing helpful here really. //EDIT2: Adding a damping resistor of 1k in series with FSADJ also changes nothing (apart from lowering the compliance range of the current sink). |
| Yansi:
--- Quote from: imo on April 07, 2019, 01:28:10 pm ---I would also disconnect the R11 from the REFout and feed the opamp bias 1.2V from an external source, it could be there is a parasitic feedback via Refout and it oscillates (C11 - add 100nF ceramics in parallel). --- End quote --- Not possible. REFOUT is rock solid all the time. Oscillation would be more likely at lower frequency, where the attenuation of R9-C11 is much less. (it is a 0.4Hz low pass filter!) Attenuation at 1kHz is like what, 60dB? |
| Yansi:
One last thing to try, before calling shit and crap on that IC: As suggested - using a 6k8 Rset resistor from FSADJ, but instead of grounded, fed from the bottom end from a signal source. Originally, I didn't like this idea, because that would lead to significant distortion when the modulation voltage will approach the VREF voltage. I thought using a current sink like this, I would be always able to set the current precisely as needed. |
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