24-08-2013, 08:55 PM
(24-08-2013, 02:58 PM)pwdrive Wrote: I'm hoping Santa will bring me a 'scope for Xmas . . . . but I need at least 30 MHz bandwidth.
I guess that you've specified that freq. because that's the 'top freq.' where your interests mainly lie, i.e. the HF end of the 10m. AmRad band. However, when it comes to 'oscilloscope bandwidths', I fear that you must tread carefully: the specs. for 'scope b/widths can be very misleading, especially with "b/width = y MHz" and what you understand by that statement. Usually, (but not always) that statement will mean that the response of the vert. amp. will be -3dB at y MHz. But not always: sometimes the stated B/W is for low-level signals; a much lower B/W 'becoming available' for large signals. And the B/W of the vert. amp. is not necessarily the same as the 'real' B/W, i.e. {input socket to display screen} B/W. And, of course, if you require a flat response up to 30 MHz, then you'll want a 'scope with a stated 'bandwidth' that is a lot more - something like 50 MHz absolute minimum; 100 MHz would be better, and what I would choose.
When a B/W is stated as being x dB at y MHz, the tendency is to assume that the response is smooth and flat up to just before y MHz and then rolling off. Unfortunately, many 'scopes don't behave like that. They start to roll off many MHz before y MHz and then eventually reach x dB at y MHz - which, if you need a flat response to even approaching y MHz, that you are not going to get.
And then on top of all that, (just to add to the fun), what about the 'scope probe? For a 50 MHz (true) B/W 'scope, you'll need a probe with a real flat freq. response to about 250 MHz (according to Tektronix). And if you have a 2-channel 'scope, you'll need two 'scope probes . . . expensive 'scope probes.
Al. / Aug. 24, 2013 //
