Friday, April 24, 2020

department of trustworthy marketing

I've been wondering if package delivery people have been getting tired of delivering to my address as of late. Maybe I should leave some milk and cookies out on a table next to the fireplace as a sacrificial offering. Perhaps next time. For now they'll just have to be satisfied with my autograph. That's actually a lie; they're not requiring acceptance signatures due to COVID-19.

Obviously I mention this because I must have received yet another thing in the mail. Another eBay buy, naturally. Nothing too fancy, but rather just fancy enough.


What we have here is a GW Instek GDS-1022, a basic, somewhat dated and low-end 25MHz Digital Storage Oscilloscope. For just shy of $100 shipped (not including $15 worth of probes), I'd argue it's a decent deal, and ticks pretty much all the boxes to suit my needs. As it turns out, these $100 purchases are somewhat of a reoccurring theme with me. You can get an impressive amount of hardware for that money. One thing I forgot to mention in that whole vacuum chamber debacle is that I received a partial refund from the seller due to the damage caused by lousy shipping prep. How much did I get back from the $260 winning bid? $100, of course.

Back to the scope. It's about as basic of a DSO as they come. It's a 2 channel, with a 25MHz bandwidth, and a claimed 250MHz sampling rate, with an abysmal 4k points recording length. The display is 5.6" with a fairly crap 320x234 resolution, which isn't surprising given that this thing is probably from the late 2000s. I didn't merely choose this device only because it was among the cheapest DSOs I could find (it was), but also because GW Instek from Taiwan is arguably a bit more reputable of a brand than some of the cheapies hailing from mainland China and sold new via eBay or Amazon which have pretty much non-existing warranties. Better to be out $100, than closer to $300.

Like far too many other software-heavy devices, the GDS-1022 shares most of it's technical specs with it's "higher-end" brethren. Meaning that this, and every other model below the top range 100MHz unit may have it's bandwidth limit set artificially via software. I can't say for certain that this is the case, but I think there's a very good chance that it is.

Things that I like:
Overall the menus are simple and intuitive. Separate volts per division and vertical offset knobs for each channel is appreciated. The unit isn't some heavy beast, but I think that thanks to it's physical depth, its quite stable when pushing buttons. The same can't be said for some other modern scopes. No frills. It does basic oscilloscope stuff but with the digital niceties such as various signal measurement capabilities and transient capture+store not found in analog units.

Now that I know it actually works, it's time for a quick teardown.


Here's the back cover removed. All it did was add some rubber feet, stickers, and a consistent look to match the front plastic. Seemed a bit unnecessary, but a nice touch that maybe adds a little bump protection. The top, bottom, and sides are all exposed chassis sheet metal.


The top/sides of the chassis is held in with 3 screws and, with a little effort, slides right off. I like the sheetmetal design and quality, and find the overall mechanical construction of the scope to be pretty good. As for the electronics, the rear side unsurprisingly contains the power supply (including LCD backlight inverter) save for a USB and calibration coax breakout board, with the rest of the electronics mounted to the front.


I'm not an expert, but the power supply seems like a pretty standard fare, with nothing too egregious going on. Sure, there might be a TO-220 or two "flapping in the breeze", but Dave Jones doesn't need to know about it. All the wires are terminated with a connector, which tends to be appreciated. The electrolytics appear to be Chemi-Con brand. Sadly, the same can't be said for the main board.




And here's the big money bits. On the digital side is an Altera Cyclone II FPGA and a Max II CPLD, plus some flash and ram. No idea what the CPLD is for. Interestingly, there is no CPU IC anywhere as far as I can tell (unless it's hiding on the other side - doubtful), so it likely has a soft core implemented in the FPGA. The analog input section is probably a partially discrete design? Most of the ICs are pretty standard op-amps save for a 4:1 buffered "video" mux. Immediately to the right of the shielding and doing some hocus-pocus to the input section are 8-channel analog mux/demux's.

Then there's the (single) ADC... It's an Analog Devices AD9288-100; a 2 channel 100MSPS part. But wait, GW Instek claims the scope has a 250MSPS sampling rate. What's going on here? They're either overclocking the chip by 250% (doubtful), or they're overclocking by 125% (a little less doubtful), then, by the magic of marketing, adding the sampling rate of the two channels together and claiming 250MSPS. The manual, unsurprisingly, does not specify. Also, from just my quick glance at the PCB, it looks like the output of each analog front-end connected directly to one of the two inputs of the ADC, so unless there was something else going on in there, I don't think they fed any channel to both inputs, and then sampled each ADC 180o out of phase from each other. It's possible, but the manual doesn't specify different sampling rates depending on how many channels are operating, unlike some other manufacturers. But lets give them the undeserved benefit of the doubt. Still, 250MSPS is pretty much only good enough for 25MHz bandwidth. The 100MHz unit with the same sampling rate would do what to a complex 100MHz signal?

Prior to purchasing this scope, I did some due diligence and tried to find whatever info I could. There was so little info it almost seemed as if they didn't exist. There was a post (I believe on the EEVblog forum) by someone who noticed this:


A 0Ω resistor next to a number presumably representing the unit's model number. In the image above, you can see it next to 1022, my scope being a GDS-1022, while the 100MHz unit is the GDS-1102. As I recall, the poster had a 1042 and attempted to solder the resistor into the 1102 slot. No change was observed, as the scope still indicated it was a -1042. Someone else mentioned that the firmware might need to have been reprogrammed to have the changes take effect. I kept this in mind when buying the unit as that would have been a cheap upgrade. But knowing what I know now, it's doubtful the "100MHz" version would be worth a shit. Still, I'm satisfied with what I have, and I think it'll serve me just fine. Time will tell.

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