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-   -   oscilloscopes. USB+computer vs the real thing (http://www.risingsun4x4club.org/forum2/showthread.php?t=15576)

leiniesred 04-24-2011 11:57 AM

oscilloscopes. USB+computer vs the real thing
 
I've been learning about hf stuff by workin' on a tech special 30 year old hybrid radio.

Right now, I have it so mal-adjusted that I don't think I can solve the alignment problems without an oscilloscope.

What are your thoughts on the usb+a computer oscilloscopes vs a traditional stand-alone scope?

The USB unit might be a lot easier to store and I'm leaning that way.
Also, the USB ones will (probably) give me a built in frequency counter. A tool essential to getting my kenwood back on the air. Something Like the Hantek DSO-2250?


Thanks,

Leiniesred

DaveInDenver 04-24-2011 04:38 PM

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?

http://www.testequipmentdepot.com/pr...s/dso_2250.htm

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.

http://www2.tek.com/cmswpt/psdetails...ci=17517&lc=EN

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.

DaveInDenver 04-26-2011 12:24 PM

Instek OS-622G 20Mhz O-scope - $175 (Nederland, CO)

For sale is an Instek Oscilloscope with two x10/x1 probes. The unit works fine, the one picture is displaying the calibration signal. I bought the scope about three months ago from an electronics shop for a project. I have no need for an o-scope in the future.

Respond via email or call 303 258 0143.

Also, I drive weekfly from Nederland and Littleton, so it's not difficult to meet up West of Denver. Other delivery options will need to be negotiated.

DaveInDenver 04-26-2011 12:25 PM

Oscilloscope HAMEG HM205-2 - $155 (Centennial/Littleton)

Oscilloscope 20 MHz Digital Storage HAMEG HM205-2 . General purpose 2 Channel with two 150MHz Oscilloscope Probes with Accessory Kit Tektronix HP and one BNC Test Lead set and power cord.

Perfect operating condition. Some minor dents and dings on the case.
54 page operatorís manual (file .pdf format)
http://www.opweb.de/model.php?id=351

Call Cliff at 303 517 2466.

Located in Centennial near South Colorado Blvd. & Dry Creek Road
Firm price, cash only.

leiniesred 04-26-2011 06:06 PM

Dave: Will I be able to tune/adjust 10m stuff with a 20 Mhz 'scope? Should I hold out for a 30Mhz scope so I can measure stuff around 28-29Mhz?

Can I accurately calculate frequency by counting divisions by hand with a conventional low-buck oscilloscope or will I still be shopping for a frequency counter?

thanks for your input, Dave.
-Stephen

DaveInDenver 04-26-2011 09:18 PM

Well, Stephen, that's the thing. Maybe 10m, 12m and 15m will not be doable. Or maybe they will, mostly. This is the advantage of an analog scope. That 20MHz spec is where the scope can guarantee a particular magnitude, but at 30MHz most scopes will still produce a waveform, just one that's no longer valid in magnitude.

This might be just fine depending on the alignment procedure. Sometimes you can use frequency counters or beat matching to augment. There is actually very little in your radio, all radios, that is at RF. Things are mixed down to IF quickly because it's easier and cheaper to work in KHz. It's likely that with a 20MHz scope you can do 90% of what you need, even on VHF radios. And you can adapt, set your trigger to 1/2 of the frequency. I mean, a 15MHz trigger might just be every other rising edge of a 30MHz signal, right? You're not building spaceships here, the accuracy is more than sufficient for a boat anchor.

For a digital scope, the sampling rate becomes your limitation. So for a 29MHz signal you need at least about 150MS/s to achieve any sort of accuracy.

My $0.02 is that a used analog scope should fall in the ~$150 range and I would try to talk both of these 20MHz guys down. But I'd probably watch for a 50~100MHz scope, then there's no question. Test equipment is like radios, you can always find a use for it. :-)

leiniesred 04-27-2011 12:26 PM

Thanks for the input Dave. In looking at the design/testing/repair documentation, I'm finding that a frequency counter is more important than the scope. RFvoltmeters seem to be more commonly used than an oscilloscope in testing/tuning these radios! (I can use either to neutralize my final and driver tubes.

DaveInDenver 04-27-2011 06:27 PM

Quote:

Originally Posted by leiniesred (Post 182009)
Thanks for the input Dave. In looking at the design/testing/repair documentation, I'm finding that a frequency counter is more important than the scope. RFvoltmeters seem to be more commonly used than an oscilloscope in testing/tuning these radios! (I can use either to neutralize my final and driver tubes.

And you can build your own RF volt meter, cheap. Cap, diode, current limiting resistor and well insulated probes for tubes. Tickles a bit if you hit plate voltage. As in knock you off your chair and make you pee your pants sorta tickle.


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