I can use an Efinix FPGA and a bunch of cheap fast DACs to make some DDS clock sources, specifically four. The pain is the lowpass filter.
Mini-Circuits and other folks make nice surface-mount lowpass filters, but they are most all in the GHz range. I want maybe 25 MHz. You'd think there would be a market for packaged MHz-range lowpsss filters.
It's worth pushing the DAC rate as high as possible to simplify the lowpass filter. Stay far away from Nyquist.
Wouldn’t your nu- hertz package create a filter out of a few 0603 parts far cheaper than a bought in filter?
Sure, but that will take a bunch of parts on the board, times four. The filters will be bigger than the DACs.
I'm updating an old VME board design, a good seller full of obsolete parts. It used an Analog Devices DDS chip, and the filter has four parts, LCLC, which isn't too awful. But some little ceramic LTCC thingie would be great.
Some people make ferrite-bead sorts of things, 3-terminal emi filters with caps inside. I'll look into those. Might get lucky.
That kind of circuit cries out for finite impulse response low pass filter.
You feed the digital signal through a shift register and hang sampling resistors on each tap, and sum the currents fed through the resistors. You do have to watch out for truncation error - Gibb's oscillations - and use a Hamming window when you calculate the value for each sampling resistor.
The neat thing about it is that it is essentially frequency independent
- the cut -off frequency scales with the clock rate.
It's sort of bulky - my 32-stage example need two or three E-96 precision resistors per tap to get the precision you need, but in surface mount that's tolerable.
Shorter shift registers don't cut off as sharply but can still do much better than analog parts.
It's interesting that there is a class of people who want to do totally impractical expensive things on circuit boards. People with no common sense. The name for such people is "fired."
Also, a DDS lowpass filter can have ghasty passband response. What matters is stopband rejection. All the classic filter responses try to optimize passband flatness.
The jitter of a DDS at low frequencies is domnated by the number of MSB bits that we pick from the phase accumulator. It's usually better to synthesize a clean octave and divide down as needed.
It's depressing that there is a class of people who suffer from "not invented here" and complain that anything that they didn't think of is impractical and expensive.
If cobbled together by the likes of John Larkin. The sort of people who can get ghastly jitter out of an ECL-to-TTL converter chip.
So John doesn't know what he is talking about.
And doubles down on being ill-informed.
I've asked for pricing on KR 2434-20.
Have something to add? Share your thoughts — no account required.
Ask the community — no account required