If you want lots of memory, the used Teks (except the old high-end stuff, but that's still quite expensive and less nice to use) probably aren't the place to look, Tektronix always seemed to choose sample rate over memory (the TDS200 samples at 1GS/s, when the competition still did equivalent time sampling at 100MS/s, but has only 2.5kpoints memory). Even the current TDS3000C series is only 10k (is just TDS3k + some communication features like USB and software changes like Wavealert), you need to move up to the MSO/DPO series for more. I tend to use high bandwidth more than lots of memory, although ideally you want both. The memory is what lets you use that fast sample rate at anything but the fastest sweep speeds. I upgraded from the TDS220 to the TDS3052 for probably a little over $2k used (delivered to my home and plenty of time to test), it does 500MHz/5GS/s, but only 10kpoints memory.
Just yesterday I was troubleshooting a 50MHz pulse generator, and looking at the timing between internal 50MHz square(ish) waves and the amount of ringing. 100MHz would have made them all sinusoidal. There was a 4-40pF trimmer capacitor connected to the net I was probing, so a ~10pF loading from a passive probe would have a significant influence, and I used a FET probe (got it used, obviously). I also used the B trigger to get to the fourth pulse, and used advanced measurements and statistics (got the advanced analysis option via a hack). I wouldn't have been able to do this all with my old scope.
Features not found in my TDS220 that I use often:
- Extra bandwidth and memory (obviously).
- More measurements (eg. duty cycle). Statistics for measurements. Indicators which points it uses for measurements (let me see that it was measuring a pulse off-screen when I used the delay feature). Many of those come with the advanced analysis option.
- B trigger, delayed sweep/trigger. Lets my trigger on a certain event after the first trigger, possibly on another channel.
- Better UI for things like zoom (which is quite clumsy on the TDS220 in my opinion).
- Ability to set the inputs to 50ohms.
- Ability to use active probes.
- Variable persistence (DPO stuff).
- Much better screen, I like colors to distinguish traces.
- Ability to store four waveforms instead of two.
I'm not suggesting that you buy a TDS3000 series, just highlighting some (IMO) interesting features. One issue with Chinese manufacturers is accessories, support and documentation. Will they have software that supports the scopes for current OS versions ten years from now? Do they have good user, programming and calibration documentation? What about specialty probes? Even the standard passive probes might suck. The fastest passive probe from the Instek website, GTP-250A-2, has a fairly high input capacitance (17pF), and the specifications are quite limited. No mention of rise time, transient response and aberrations, and these are fairly hard to test. Companies like Probemaster have made some real junk, these might be no better. At least the Tek/Agilent/Lecroy people know how to design probes, and have the advantage of selling lots of probes (economics of scale). What if you want to use an active probe? Or differential? Or current? You can sometimes use those from a different brand, but most of the recent ones are designed to get power from the scope, so you need an expensive external power supply to use them with other brands. Plus most of the probes on Ebay seem to be from Tektronix or Agilent/HP.
Can't help you with the Chinese brands, the above reasons have kept me to Agilent/Lecroy/Tektronix. Agilent and Lecroy might have some interesting offerings, I've heard positive things about the current Lecroy scopes, but no idea about pricing. I just clicked on the GDS-1062A link on the website Saturation posted, all images and tabs appeared broken, not a good first impression
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. And the description about the 250MHz probe included 'Connect this sentence to the previous sentence.'
If I were to upgrade, I'd want more bandwidth in the process, not spending all that money for another 100MHz scope.
That's my $0.02
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