Could someone explain why this instrument only have 20Mhz arbitrary wave output, when its DAC is running at 1230Mhz, please?
I feel confused. 
20MHz is how fast it can repeat whole Arb memory (it also depends how many Arb memory points waveform have)
Exactly. If you fill up the memory, it can repeat itself 20 million times per second, but what you have on that memory can have higher frequency components in it.
You can also fill only a small part of the memory, say 1/2 of the memory, then it will repeat at 40MHz, if I am not mistaken. Is that right, rf-loop?
My quess is: No. I do not know exactly how just this Siglent SDG2000X works in details. I do not have it (yet).
I think user can select only max 20MHz repeating frequency for stored arbitrary waveform.
It is "True Arb" technology and if it works so that it do not "jump over" memory points lets think.
( this is not Siglent or any other named generator. Just basic thinking:
If need handle every single Arb memory points in memory and Arb repeating frequency is 20MHz. Hmm... if Arb memory read speed is 100Mpoints/s how many memory points we can read. 20MHz cycle time is 50ns. We can read only 5 memory points in one turn.. (5x10ns=50ns) If we can read arb memory points using 1Gpoints/s then with 20MHz repeating frequency we can read 50 memory points.)
I do not know at all what is exactly maximum speed what Siglent SDG2kX can process Arb memory and how it is arranged for amounts of waveform points etc. I hjave understood they use "True Arb" for imply that it do not jump over any single memory points (?)
Siglent web side:
? High-performance sampling system with 1.2GSa/s sampling rate and 16-bit vertical resolution. No detail in your waveforms will be lost
? Innovative TrueArb technology, based on a point-by-point architecture, supports any 8pts~8Mpts Arb waveform with a sampling rate in range of 1?Sa/s~75MSa/s
This is one "good to read" paper
http://www.alliedelec.com/images/Products/mkt/pb/Tektronix/Pdf/understanding_signalgeneration.pdfI hope Siglent do better material for show exactly how it works and what are limits exactly. Whole principle exactly with all information with numbers how it works and how it do not work. It is very extremely different what is example in SDG800, 1000 and 5000 generators.
Example with every SDG1000 can produce 50MHz sine out using Arb, There Arb max repeating frequency is 5MHz. There you can do 10 cycle sine to arb memory and run it using 5MHz and out come 50MHz. But if there is short details in arb memory it may (randomly!) jump over it because this model have fixed arb memory lenght and fixed clock. Missing details is one disadvantage what user need know. When it have 125MSa/s it mean it have 8ns step interval. It have 16kpoint Arb memory. If handle every Arb memory points without missing points then how much time it need. 8ns x16000 = 128000ns = 128us. Whole Arb memory every single points can then handle only if repeating frequency is under 7.8 kHz ! If go over this Arb frequency it need start skip over memory points and due to fixed clock in every turn different memory points are skipped. (depending frequency. There can also find special frequencies where skipping and waveform iare "in synch". This is disadvantage in all fixed clock fixed memory lenght "simple principle Arb generators" independent of manufacturer name.)
SDG2000X is very very different and it have more advanced principle. It have variable Arb memory lenght and variable Arb memory "stepping" speed and then this Arb repeating frequency what have limit value max 20MHz. And this is not only in Siglent, it is and have been long time, in many bit more advanced generators. But perhaps with different details combination.
All is adjusted so that there is "No detail in your waveforms will be lost." And it is perhaps much much more important than maximum frequency what it can produce.
It need think for what we use and need arbitrary waveform function.
But, as told, Siglent need write better specification so that we can read exactly what it do and what it do nnot do. How it works, exactly. This is important so that buyer do not disappoint. If you promise gold and buyer find brass all are disappointed and angry. But you have his money and you are still happy, you win. If you do it fast enough you can win many.... But, try repeat it... no way. On the chinese street corners this happends thousends of times every day and you can change street corner and repeat, you can change city, and repeat... but here, international markets you can not run to ther corner, other city... so, this is different.
If you promise brass and buyer find he also get brass he is satisfied. If you promise gold and buyer get gold, agen he is satisfied. In both cases you can continue and continue win. If one is not satisfied he tell it to thousends, ten thousends, to whope world of peoples. You have just one shot... If one is satisfied perhaps he do not talk anything.. Good enough deep and open information about product, it is ONLY way to do and continue. Please learn how HP build they brand... start reading example HP journals from history. How much they tell and explain lot of things.. well explained some new equipment working principles including also very deep details. Why they do not afraid copycats. They show all schematics, they show even part of design calculations and theories and things how they have developed something. Why they do not afraid copycats.
Everyone can copy schematic, everyone can solder and do printed circuits, all can do.
What can not copy? Lets assume that price is still limit. Answer is: Quality. This can not copy (so that price is cheap enough for copycat business)
Who is first chinese company what really realise this and do it. I'm waiting.
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