Electronics > Projects, Designs, and Technical Stuff

Maintaining phase accuracy for kHz power measurements

<< < (4/5) > >>

ogden:

--- Quote from: KaneTW on January 14, 2020, 03:03:00 am ---I'm generating a sine wave using selective harmonic elimination PWM, which switches at some multiple of the required frequency (depending on how many harmonics you need eliminated). It's then passed through a LC filter to get rid of the rest, and into an output transformer.

The C2000 is needed due to its HRPWM capability. I've not found an alternate part that can do its job

--- End quote ---
Virtually any DDS with >=10Msps, like ad9833 will do the job.

KaneTW:
Seems like a huge pain to implement on a DDS---just the timing seems like a nightmare. I basically have a list of phase angles that I need to switch at (or, equivalently, a list of duty cycles and periods). If I use the AD9833 duty cycle seems to be fixed and I can only adjust phase/frequency, and using the 2nd register for it doesn't seem to be fast enough.

Also, it's expensive. A C2000 MCU with 4 sufficiently powerful HRPWM channels is $2.50/1k, whereas a single channel AD9833 is $4.13/1k. Even if you use symmetry to mux the AD9833, that's at least $8 in DDS alone for what a single MCU can do.

DSPics are a good call. I'll see if they can handle my requirements.

ogden:

--- Quote from: KaneTW on January 14, 2020, 06:21:37 pm ---Seems like a huge pain to implement on a DDS---just the timing seems like a nightmare. I basically have a list of phase angles that I need to switch at (or, equivalently, a list of duty cycles and periods). If I use the AD9833 duty cycle seems to be fixed and I can only adjust phase/frequency, and using the 2nd register for it doesn't seem to be fast enough.

--- End quote ---
What you mean by "AD9833 duty cycle seems to be fixed"? You want to change duty cycle of sine waveform? (attached. pic from web search, LOL obviously).


--- Quote ---Also, it's expensive. A C2000 MCU with 4 sufficiently powerful HRPWM channels is $2.50/1k

--- End quote ---
Ok. Fair point. Then what about "software DDS" using precomputed waveform buffer you send into DAC using DMA? Even 1Msps DAC of mentioned STM32F334 with same filter you use for PWM, can reach quite clean 100KHz output. On C2000 MCU you are doing essentially the same, just you use "hi-speed 1-bit DAC" (PWM).

KaneTW:
SHE-PWM works by calculating optimal switching angles to eliminate harmonics up to some defined limit. For example, the waveform https://i.kane.cx/RmKAlS eliminates all harmonics up to the 11th: https://i.kane.cx/DAdCTi . This can then be used via a full-bridge to have a high-efficiency, clean single-tone amplifier.

*After* the full-bridge, it gets filtered down to a sine wave.

ogden:

--- Quote from: KaneTW on January 14, 2020, 09:25:05 pm ---SHE-PWM works by calculating optimal switching angles to eliminate harmonics up to some defined limit. For example, the waveform https://i.kane.cx/RmKAlS eliminates all harmonics up to the 11th:

--- End quote ---
Seems like SHE-PWM needs my further attention, thanx :) Agreed - *if* you use "switching" amplifier, then DDS is no use. You need "1-bit DAC" with PWM or PDM(DSD) modulation.

Navigation

[0] Message Index

[#] Next page

[*] Previous page

There was an error while thanking
Thanking...
Go to full version
Powered by SMFPacks Advanced Attachments Uploader Mod