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Old 04-24-2011, 04:38 PM
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DaveInDenver DaveInDenver is online now
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Join Date: Jun 2006
Location: Larimer County
Posts: 8,029

Personally I like the controls of a real o'scope, so I am biased against PC scopes. FWIW, I have a US Navy surplus Tektronix 2246 Mod A. Having an external trigger is also handy. Honestly I don't use it all that much at home, having MSO and DPO scopes (our fastest is a 20GHz) at work makes it sorta seem quaint. :-)

For the price of a 100MHz PC-based 'scope I think for what you want to do with it a used analog 'scope will be better. If I was buying a new 'scope, it would be digital probably, there's not much choice anymore. Although a nice, new 20MHz analog scope is all you need for ham tinkering and surely those can't be very expensive. As a piece of ham gear there are some advantages to getting an analog over a similar DSO.

The problem is that cheap digital scopes have too low of sampling rates to effectively use their bandwidth. So if the 'scope has it's a 100MHz with a 100MS/s, that is in effect only a 50MHz scope, probably less. The bandwidth of an analog scope is it's highest useable frequency, depending on maybe the amplifier linearity. So my 2246 is a 100MHz scope and can display a 100MHz signal, albeit at -3dB from full magnitude, without distortion. It will actually go a little higher, but it's 20+ years old and it's tired so it begins to distort.

A 100MS/s DSO will be limited at 50MHz due to the Nyquist limit, but for any amount of accuracy you really need to sample 4 or more times faster than the signal under test. That means a 100MS/s scope is really likely to match the the accuracy of a 20MHz analog scope. Don't confuse sample rate and bandwidth. Also you have to be very concerned with the ADC width and memory depth. A scope can have a monster sample rate but if it does not have enough memory to store the samples, then it's do a whole lot of sampling that is going into the bit bucket. Those sorts of specs are likely buried.

Is this it?

The specs on the DSO-2250 don't seem too bad, though 250MS/s is pretty low for a 100MHz scope. How much do they hit the street for? That one above is listed at $625.

As a comparison, Agilent and Tektronix 100MHz digital scopes use 2GS/s and even their 20MHz digital scopes use 500MS/s.

The list price on a TDS2001C is around $975.

If you need it, you are welcome to borrow my 2246. I only have 20MHz probes for it, though this is probably enough for anything you're doing. I use my MFJ-269 as a source and frequency counter. Also if it would help I have an LCR meter and DMM that you can use. The one thing I do not have is a spectrum analyzer of any sort. I have a box full of parts that some day will be a spec analyzer adapter for my o'scope (another reason to get a real scope). Some day.
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