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Old 12th January 2007, 04:43 PM   #21
gootee is offline gootee  United States
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Quote:
Originally posted by nhuwar


My mistake Inever said I know everything I just thought I did

Don't sweat it. I think we've all probably been THERE, a few times!
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Old 12th January 2007, 05:22 PM   #22
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Quote:
If I want to check that my opamp-based amp does not ocillate, what MHz should I "check up to" ?
Iíve seen op amp oscillation well into the tens of MHz region, but you donít necessarily need a scope to go that high...

If you bought a 20MHz scope and probed an oscillating op amp, you could zoom in enough to see a large blur across the whole screen and thatís enough to tell you it's oscillating ie you donít need to see each individual cycle to know what's going on.

Having typed that, having ample bandwidth is always nice, but I'm spoilt by having 6GHz, 20GS/s scopes at work!

Nice one,
David.
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Old 12th January 2007, 08:44 PM   #23
gootee is offline gootee  United States
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Quote:
Originally posted by m8o
Somewhat OT, but related...

Question for those in the know, for the non-electronics-diy-newbz like me ...

If I want to check that my opamp-based amp does not ocillate, what MHz should I "check up to" ?

I ask, as this will define the ocilliscope bandwidth I should be looking for (and partially, how much $$$ I will need to spend). I have a -OLD- Heathkit scope my dad made when he took their self-study at-home courses. I'd like something better tho...

If that is too broad of a question to answer given all the variables that may go into it, is there a "frame of reference" that can be made based on the open loop gain frequency response of a opamp/device then? something like "check up to 1/2 the bandwidth" or "check up to twice the bandwidth", etc.?

thanx all

There may well be a rule-of-thumb, for that. I don't know. An opamp that is capable of a very fast slew rate, or that has a very high frequency capability, might be more prone to oscillation problems. But it seems like they can all oscillate very well, given the right conditions.

The first test I use is to try touching each opamp. If it's excessively warm (or blisters your finger!), and "shouldn't be", it might be oscillating pretty good. Higher frequency signals have proportionately-more energy, all other things being equal. So they will often generate a lot of heat.

With a scope, even if the bandwidth and trigger "bandwidth" are pretty far below the oscillation frequency, you can often still see the oscillation. If it's a low-amplitude oscillation, your signal's trace will look "thick". Then you can often crank the timebase up to its fastest speed and use a more-sensitive vertical attenuator setting to try to get a better idea of what you're dealing with. If it's oscillating with a higher amplitude, it will be more obvious.

- Tom Gootee

http://www.fullnet.com/~tomg/index.html

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Old 12th January 2007, 08:51 PM   #24
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This question has been explored on this forum many times in the past. I reccomend that you go back and do a search and you'll learn alot more.

There is only one answer to this question for a general purpose scope. Good used Tektronix 465B 100 mhz scope. Don'rt buy anything less than 100 mhz!!! I got mine for 75 bucks on E-Bay.

ALSO!

Don't buy a Digital Scope unless you can get a top of the line model(new or used). The inexpensive Digital Scope Meters cannot properly display all types of waveforms due to slow refresh rates of the display.

Bandwidth more than 20 mhz may be needed to check an oscillation in an amp/preamp!


Mark

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My Scope collection missing the TEK 465B and 7603 that are at work.
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Old 13th January 2007, 12:05 PM   #25
AndrewT is online now AndrewT  Scotland
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Hi Mark,
select the next best one after the 465 from that scrap heap and post it over to me.
I'll buy you a pint (of whisky - without the "e").
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Old 13th January 2007, 02:14 PM   #26
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Scrap heap??? Come on Andrew... each one of those scopes and modules is a work of art in its own right!!

Mark
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Old 20th January 2007, 04:54 PM   #27
m8o is offline m8o  United States
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To all those in the know...

Continuing this extraodinarily useful thread to us newbz (as I've been away from school, DIY, and associated tools used in both for a few decades now) ... What about probes?

Thanx everyone for the input so far. I settled on a decent looking (I have to see once it's delivered) Tek 465B with DM44 multimeter that I won for a good price on eBay. But no probes. As the seller wants something like 60$ for the 465B manual that he's selling in another auction (but not convering the DM44 ; forget about it either way), I found some scanned 465B manuals, so I know the probe is P6105. The only scanned manuals I found including the DM44 were for the 475.

Now for the questions:

1) Can I assume the 475+DM44 will work fine for my 465B+DM44 in regards to just the DM44 part? I ask, because I understand after 465B/466 the scope's circuits changed a good deal. So my worry is that affects the DB44 part somehow.

2) Can I depend on the probes stated in that 475+DM44 document for the DM44 section's current probe and temperature would work for my 465B+DM44, as far as the DM44 is concerned ?

3) What other probes would work for me -- say high voltage probes if I do work with Tubes in the future -- for the 465B, or what probes will work that may have come with later models?

Is there a cross-reference anywhere? It's not on Tektronix's site as far as I could find; at least not one that covers the 465B in it if it exists?

4) And finally, can I 'trust' non-original-Tektronics , aftermarket, probes I find being sold new on eBay? Like this:
http://cgi.ebay.com/For-HP-Tektronix...QQcmdZViewItem

Thanx very much again!
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Old 20th January 2007, 04:59 PM   #28
wlai is offline wlai  United States
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Ah-ha, so you were the one who out bid on the 465 with the DMM :-) Enjoy
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Old 20th January 2007, 05:01 PM   #29
m8o is offline m8o  United States
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Lol ... I'm sure I will. Thanx. (are you the seller? )

edit: nevermind ... I just re-read and you said "out bid". sorry ; there's one like it with a "buy it now" for $90 @ an eBay store.
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Old 20th January 2007, 08:32 PM   #30
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Try this HP1707B. The seller made an error in his description that an astute buyer will probably be able to take advantage of- it is a 75 MHz scope, not a 1 MHz BW scope as he states in the description. If you're lucky, none of the potential bidders will realize the error and it will sell cheap. Has service/operations manual, too!

http://cgi.ebay.com/ws/eBayISAPI.dll...ME:L:DBS:US:11

This scope is solid state except for the CRT, so it is relatively small and light weight.

I have the military version of this scope (AN/USM-338) and it is very nice! Delayed sweep! If you don't know what that is or why it is useful, look it up. I can tell you it is very handy for looking at rising or falling edges of digital waveforms, for example. You can see photos of the guts of my scope here:
http://mark.rehorst.com/AN_USM_338/index.htm

I_F
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