[sdiy] Old 20Mhz scope high on LSD!!

[Roy J. Tellason]

On Tuesday 28 December 2004 08:41 pm, TIm Daugard wrote:
> I have a Y input to my scope. I have an RF signal source. When my wife lets
> me back into my work shed, I will connect the two and see at what frequency
> the scope stops deflecting. This should at least give me an indication of
> the speed I can run for horizontal.

Up to a point,  yeah.  And assuming that the generator output is linear across 
the board.  :-)

> Thinking RF - The 3db point of my scope is 5Mhz (I know, old - but still
> useable for audio and some RF) I should be able to upgrade the signal path.

The -3dB point is one thing,  the response beyond that point also matters.  
Now,  I understand that the Tek sales dude I had a conversation with on this 
some years back was biased as he was trying to sell me a scope,  but their 
stuff does taper off gradually after that,  while the design of some other 
scopes has a response curve that's darn near vertical after that point.

I have a book around here someplace ("Basic Television",  I think?  Bernard 
Grob comes to mind,  though I could be wrong about this) which contains an 
*excellent* chapter on the design of video amplifiers.  The object there 
being to get some fairly wide bandwidth,  or to at least extend the bandwidth 
of what you've got to be usable over some specified range.  It made for some 
interesting reading with regard to this issue,  and how a good solid design 
made a lot of difference to begin with.

I know that the -3dB point might be at one point in the frequency response, 
the -6dB point is another issue -- in a poorly design amplifier it's gonna be 
awfully close to that first point,  while in a better design it's gonna be a 
whole lot further out...

> As far as creating a digital front end for a scope, the problem would be
> the ADC and the memory following it. If you assume 50 nS memory ICs, you
> are limited to 20 MHz bandwidth. If you interleave memories, you can get to
> 40 MHz.

Having passed on the idea of getting a storage scope and just going for the 
best Tek I could afford,  in a non-storage scope,  I don't think I'm gonna go 
there.  :-)

> . . . and of the subject, what does everyone prefer? ac or AC, dc or DC.
> Don't answer with the official SI unit is . . . I know what it is. I'm
> asking preferences.

It's an abbreviation,  so I use AC, DC.