I found a review for the 150MHz version.
http://welecw2000a.sourceforge.net/docs/Hardware/GW_Instek_GDS-1152A.pdfFirst thing to notice is, the sampling rate is 500MSa/s when using both channels, so the manual is a little deceiving, since I would think it's a really important thing to mention, and they didn't mention it at all.
The ADC configuration is the interesting part.
It uses the same ADC (AD9288), except the 100MSPS version, but only 4 of them (8 ADCs, since they are duals) instead of 5 in Rigol. Which one is better depends on what theory you believe in. If you believe speed grade is just marketing and doesn't matter, Rigol is better. If you believe speed grade matters, Instek is better (25% overclock vs 125% overclock).
The author, who sounds like he knows what he is talking about (he is the developer of an alternative open source firmware for Welec scope), thinks highly of the hardware design, especially the frontend. It would be great if we can compare it to Rigol's. Even a component listing can help. From a very superficial glance, I see quite a few aluminum polymer caps on the Instek, and all electrolytic on the Rigol.
I also read an informative thread on a Chinese (yes, I understand Chinese
) forum about this issue.
http://www.ourdev.cn/bbs/bbs_content.jsp?bbs_sn=3428440&bbs_page_no=1&bbs_id=3043Most of what they talk about is the same as what we talked about here (whether speed grade matters, if it's ethical, lot's of angry posts, etc).
A few interesting notes -
1. Only 9 chips are sanded - the 5 overclocked ADCs, and 2 AD8370 (variable gain amplifier). The other 2 no one knows. DAC maybe? (for voltage offset, the Instek has it)
2. In a post, search for "?750??", someone tried running a 100MHz rated part similar to the AD9288 (AD9283BRS100) at 100MHz, 120MHz, and 150MHz, measuring a 1MHz active crystal oscillator with a 10:1 resistive divider without compensation. He noted that 100MHz and 120MHz graphs looked "normal", but 150MHz graph has significantly more noise, even on the unconnected second input (green). Unfortunately he didn't write much about the testing methodology.
Someone can conceivably do the same test with the chip in use, if you have time and an FPGA board lying around, and don't mind making a small PCB. The chips themselves don't cost that much.
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My impression is the Instek's build quality is higher, but of course the Rigol has the 100MHz hack. I also have higher confidence in Taiwanese build quality vs China. How much is that worth is up to you.
They are both somewhat dishonest in my opinion - overclocking and sanding chips, and deceiving manual and specs. I guess it's lesser of the 2 evils, since we don't really have any other sensible choice at this price point.
I'm not sure if the 10ns/div limit on the long memory for the Instek really matters. 10ns/div means you can see 1 period / 2 divs on a 100MHz signal. Maybe it just means the long memory doesn't work with equivalent time sampling? The bandwidth is not even that high, and I doubt I will be working with that kind of high speed analog signals anyways. For digital stuff a logic analyzer would be more suitable. That said, I haven't got a reply from them yet. Maybe I should write them in Chinese, too.
By the way, Sparkfun's very interesting new product -
http://www.sparkfun.com/commerce/product_info.php?products_id=9857$50 logic analyzer with open source software + firmware.