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Review: Hantek DDS 3X25. Anyone own one?

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Mechatrommer:
err, no :( only the synch out got terminated. whats the effect? i only can think of somesort of loading stuff ???

saturation:
Yes, if not properly terminated on both outputs, the values are unstable at that high frequency.  Second the sync signal is only good to 25 MHz, which its what its rated for, above it is also unstable. 

You can generate sine wave with reduced amplitude to 75 MHz, but you can only sync internally.

As you can see my outputs are no where looking like yours.

Yellow = Output
Blue = Sync out
Purple = FFT at rectangular, clean to 25 MHz, no visible harmonics at this scale.

Its phase shifted, as I wrote in this thread, but the signal is cleaner.  It also shows the importance of the right cables and terminating impedances.  I don't have the ringing you have in the other posts regarding making cables.

Enjoy!  The skills in analog electronics will last a lifetime, its worth putting effort into, it hasn't changed in ~ 100 years.



--- Quote from: Mechatrommer on March 14, 2011, 10:43:20 pm ---err, no :( only the synch out got terminated. whats the effect? i only can think of somesort of loading stuff ???

--- End quote ---

Mechatrommer:
well ok, i tried my best to make similar setting as yours. made a quick fix 50ohm for the second terminator (i cannot sacrifice another bnc connector, i only have 3 left, 2 died during soldering, gotta use the dc-bnc type) still i got ringing in synch out, maybe the 75ohm impedance connector is the culprit. and i dont understand why you say you have a clean FFT for your square signal, as i understand square consists of alot of harmonics.

redo the test with both (crude) terminated, my previous conclusion still hold, still the same graphs except the output amplitude now is halved due to termination. and i swap yellow=signal, blue=synch to comply with saturation's setting. i provided only one snapshot (lazy to repost the whole series again). c_t_12mhz.jpg = trapezia (terminated) at "not multiple" 12mhz.

mind to show your cable? :P ;D
ps: the time now is 9AM, and i think i'm retiring for today, will continue at 3-4PM... today. hava nice day ;)

saturation:
It looks better.  Your FFT on the sync output has no other spurs except expected for a 'clean' square wave, = 3rd and 5th harmonic, 75 MHz and 125 MHz.



You needn't sacrifice anything if you bought the right stuff, sorry to say.

Your device requires 50 ohms, but you have a mix of items that will cause non-ideal responses as its not terminated correctly, it was OK to get a better waveform rather than have no termination, but as you try to improve it to ideal, your chances of success will not be good.  As I see it you have a 75 ohm cable, 75 ohm BNC connectors, 50 ohm resistor of unknown composition, I hope its not carbon composite or wire wound, its not the ideal terminator.  The proper resistor is a SMT type thin or thick film.  And if you have long leads sticking out like you have in the photo, it will cause unpredictable responses.




http://books.google.com/books?id=zpTnMsiUkmwC&pg=PA2&lpg=PA2&dq=carbon+composition+resistors+parasitic+capacitance&source=bl&ots=B4EZfXzDc0&sig=geOY4nFYqO7RziVEOsQrWVwT5Ko&hl=en&ei=Nnh_TfyBMMmD0QGem72ECQ&sa=X&oi=book_result&ct=result&resnum=1&ved=0CBsQ6AEwAA#v=onepage&q=carbon%20composition%20resistors%20parasitic%20capacitance&f=false

My clean FFT is for against the sine wave, and it refers to your image here, not sure why you have so many spurs although this is FFT at the dB view:


I'd be more than happy to show you my cables, but its just generic from Jameco.com and MPJA.com.  50 and 75 ohm cables and connectors cost the same amount of money, so buying 75 ohms instead of 50 ohms well, is a path down the wrong road.

The bright side, is its all a learning experience for a better future.  When I get hold of my scope, I'll duplicate what you did in the photo with the 12 MHz triangle wave.




--- Quote from: Mechatrommer on March 15, 2011, 01:01:30 am ---well ok, i tried my best to make similar setting as yours. made a quick fix 50ohm for the second terminator (i cannot sacrifice another bnc connector, i only have 3 left, 2 died during soldering, gotta use the dc-bnc type) still i got ringing in synch out, maybe the 75ohm impedance connector is the culprit. and i dont understand why you say you have a clean FFT for your square signal, as i understand square consists of alot of harmonics.

redo the test with both (crude) terminated, my previous conclusion still hold, still the same graphs except the output amplitude now is halved due to termination. and i swap yellow=signal, blue=synch to comply with saturation's setting. i provided only one snapshot (lazy to repost the whole series again). c_t_12mhz.jpg = trapezia (terminated) at "not multiple" 12mhz.

mind to show your cable? :P ;D
ps: the time now is 9AM, and i think i'm retiring for today, will continue at 3-4PM... today. hava nice day ;)


--- End quote ---

saturation:
A 50 ohm terminator, similar to most folks use, about $2 each.  You connect these to one end of a split connector, such as the T.


T connector, about $2.50 each.  Insure they are 50 ohm types.


These are $10 each, and its straight through.  Cost more than the setup above.  There maybe some advantages to the shape of this connector at very high frequencies, but the above works for most purposes, particularly within the range fo the Rigol 1052e and the 3x25.

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