Date: Tue, 30 Aug 2005 06:45:13 -0500 (CDT) Subject: people think I work at Fry's (no pictures, sorry!) X-UID: 85 The subject of this email comes from a funny incident at Fry's yesterday evening. I wore a white dress shirt to work today, replete with khaki Dockers and white running shoes. So I look very nerdy. While I was looking at some heat shrink tubing, a man came up to me asking about USB headers. I told him that I do not work at Fry's. He said that a sales associate had looked over at the electronics section and pointed at me standing there, then told him to go to me. After his explanation, I said that I had fooled him with my white shirt! Then I said that I would help him anyway. He gave a rambling description but I figured out what he wanted and pointed him over to the aisle next to the discrete components. This was probably less weird than the time a woman came up to me at Walmart with the trap tube from her sink. In that case, I was also able to answer her question and advise her to go to Home Depot instead. -------------------------------------------------------------------------- Here are the oscilloscopes I considered with the specifications that seemed most important from my point of view. I didn't consider analogue, only digital storage models. I wanted interoperability with a PC, preferably running Linux. The products are listed in order of increasing price which also happens to correspond to signal capture performance level (not too surprising). Bandwidth Sample Rate Communications Linux OK? Bitscope 100 MHz 12.5 MS/s USB, Ethernet Yes (25 MS/s) Instek 820C 150 MHz 100 MS/s RS-232, USB Yes EZ Digital DS-1100 100 MHz 100 MS/s RS-232, USB No (LG Precision) (200 MS/s) TDS 1012 100 MHz 1 GS/s RS-232, GPIB No Tektronix The Bitscope is interesting. It is a mixed signal oscilloscope with one analogue and eight logic channels. It has excellent communications options. It is a completely documented and open design, very Linux and hacking friendly. They even have instructions on how to mod the board to overclock it. The major limitation is the sample rate. It has a single ADC (analogue to digital converter) that has a nominal 25 MS/s (million samples per second) rate. It can go to 40 MS/s but at the cost of noise (sort of like setting a digital camera with a high ISO - the ADC doesn't have enough time to accurately approximate the analogue signal so the picture is noisy). The key observation about the Bitscope is that it has only one ADC. This means that if both analogue channels are used, the sample rate is only half the ADC rate. So that means 12.5 MS/s per each channel. Taking the Nyquist rule of thumb of around 3 samples per wave period (instead of 2), this means the Bitscope has a real time capability of around 4 MHz on each analogue channel. So where does the 100 MHz bandwidth number come from? That is from sub sampling, or "equivalent time sampling". It basically means that a periodic signal is repetitively sampled over many periods to build up a time averaged view of it. The Bitscope is really an eight channel logic analyzer with a DSO (digital storage oscilloscope) bolted on. The Tektronix has extremely high performance at 1 GS/s. None of the off-brands come anywhere near this sample rate at the price point. ADCs this fast aren't available off the shelf (I believe). US companies like Tektronix and Agilent/HP have a significant performance edge over their Asian competitors. I believe this is due to the many generations of military product development, a market demanding the highest possible performance. Interesting Trivia - Jean Auel worked for Tektronix before she wrote _The Clan of the Cave Bear_. That said, the Korean DS-1100 (EZ Digital/LG Precision) has 200 MS/s which is respectable. Eventually, Asian design and manufacturing will figure out this technology, miniaturize it and then companies like Tektronix will be in trouble. It seems crazy to say that now. But Kodak was in big trouble for a while given competition from Canon, Nikon and Sony. And Bausch and Lomb no longer makes optics - pretty much all optics comes from Japan and Germany now (with Chinese manufacturing). The Tektronix communications module includes RS-232 and GPIB (IEEE-488 instrumentation standard, I think this is a bus, not just an electrical interfacing specification) and costs $350! So if you purchase a TDS 1012 DSO for $1150 from Fry's (out of stock in the metroplex), you'll need to add $350 for a total of $1500 for something you can connect to a PC. It's just way too much. The base scope is ok given the high performance. But the necessary options are exhorbitant. The resale value on this class of Tektronix equipment is very high. There just isn't that much of a discount on the DSOs. You might pay around 50% to 60% the new price for equipment that is six or seven years old. In the end, I chose the Taiwanese Instek 820C. It has 100 MS/s on each analogue channel and is twice the size and weight of the Tektronix, while the DS-1100 is almost three times. For the same price as a base Tektronix 60 MHz DSO, it has both serial and USB communications as well as a color LCD. What sold me was the attitude of the company with their technology. They give away software for the scope and have extensive programming documentation. The DS-1100 has higher performance but is a closed design. So it seems that as scope performance increases, the products become less documented and more proprietary. Everyone I know who has a scope has Tektronix. But they all have analogue and none bought new. I don't know anyone with a digital scope as the price is so high. I've gone with the off-brand. It should be interesting to others to see how well this works out. There are always quality limitations when you go cheap. But I've also seen how this can be the right choice. My milling machine was a cheap rebranded mainland Chinese model. The more expensive and precise products like the American made Sherline are simply too small to take on most of the projects I've done with my mill. The tradeoff is quality. I remember brushing off casting sand from the milling column. So even though I've had to deal with many quality issues, fundamentally the brute power in trade was worth it for me. What made me feel comfortable about the off-brands is that it's very obvious where they saved money. They don't try to compete directly with the Tektronix. Basically, they match or exceed the Tek models in every quality except for two: ADC sample rate; size and weight. I think I can live with something big and heavy. Whether the lower sample rate is ok is unknown.