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Review: Hantek DDS 3X25. Anyone own one?
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zibadun:

--- Quote from: PuterGeek on December 19, 2012, 06:55:18 am ---
--- Quote from: Mechatrommer on December 17, 2012, 12:32:17 am ---
--- Quote from: PuterGeek on December 17, 2012, 12:10:31 am ---What would be a good cutoff frequency do you think I should use?

--- End quote ---
100 or 200MHz?


--- Quote from: PuterGeek on December 17, 2012, 12:10:31 am ---Maybe I'll whip up a schematic to share in case someone wants to try my idea.

--- End quote ---
yes please do. i will be interested. ;)

--- End quote ---

It is going to require a Bessel filter and will need to be more than 5 poles to get good results. I will need to run a simulation to verify some things and may need to probe around with my scope too.

Not sure I will have this done before New Year because of the holidays but I am working on it.

--- End quote ---

I'd vote for 100 MHz or even 30 MHz cutoff frequency.  The "sine" wave spectrum does not look very clean to me at HF (over 5-10 mhz). May be a good low pass filter will help with this. 
PuterGeek:

--- Quote from: zibadun on December 23, 2012, 04:00:33 pm ---
I'd vote for 100 MHz or even 30 MHz cutoff frequency.  The "sine" wave spectrum does not look very clean to me at HF (over 5-10 mhz). May be a good low pass filter will help with this.

--- End quote ---

The filter will necessarily be a compromise. The maximum usable output would be about 80 MHz (40% of 200 MHz sample clock). If we only wanted sine waves we could use a 7 pole elliptical filter with quick falloff and get good results. But the elliptical filter phase delay and impulse response are horrible for arbitrary waveforms.

A Bessel filter is usable but rolls off very slowly so it requires a lot of poles (9 or more). I'll try to post some simulation output later.

p.s. I am a EE but not a filtering expert. This is based on my research on filtering AWGs.
alm:
Some AWGs use a different filter for sine output than for other signals.
PuterGeek:

--- Quote from: alm on December 23, 2012, 04:52:39 pm ---Some AWGs use a different filter for sine output than for other signals.

--- End quote ---

That would be possible but would be a more difficult mod to include inside the enclosure which was my goal. One way to do that would be to change the frequency counter input to a second output.

An alternative would be external filters which is less appealing to me.
Mechatrommer:
there's plenty of room in the ds3x25, maybe you can use relay or something and switch to change the filter instead of providing 2 separate and simultaneous output. btw, i will love to further modify my ds3x25, but i already have entirely different plan for this "arbitrary wave generator project", the only problem is "Time".
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