Oscilloscope bandwith question

I just want to make sure I got this right, but if I want to analyze a signal that is rated at some frequency, I need a scope that has a BW rated at twice that freq ?

I'm thinking about getting a scope but would like to do some rf stuff, the FM band for now, but maybe hit 433Mhz and or 900Mhz band. But that would mean I would a scope that >1Ghz ?

I gues 2.4Ghz is out of the question because I was looking at some scopes and I think the cheapeat for 2.4Ghz was like $20K !

I guess I'll stick with my led blinking happily

Though you can, you generally don't use a scope for RF stuff in the VHF-UHF band. Spectrum analyzers, RF voltmeters, communications test sets, etc are generally what's used to test radios. A scope will show a sinewave as being -3db lower than it actually is when at the rated BW of the scope, so it really depends on how accurate you want to be. A scope with a BW apx 3-5x the signal frequency is a good rule of thumb.

The bandwidth rating on an oscilloscope is the frequency at which the sine wave response has dropped -3dB or 0.707 of the correct value. So you can use an oscilloscope up to it's rated bandwidth but it's accuracy suffers near the limit.

And yes, oscilloscope costs tend to rise rapidly with their frequency response. 1GHz and above tend to be very pricey.

I don't see how a time domain waveform is much use for RF, surely you want frequency domain i.e. a spectrum analyser.

Well-designed scopes such as those made by Tektronix, Hewlett-Packard and others tend to maintain flat bandwidth all the way up until they approach their rated bandwidth. In other words, a typical 100MHz Tektronix 465 will not show any appreciable amplitude drop at 90 or 95 MHz and then start to drop slowly. Most will actually have a bandwidth well-over their specification: a typical 465 will have an actual 3dB point at 130MHz. Most still will have a useable display (although you can't measure amplitude) at 1.5 to 2 times their bandwidth. The triggering circuits usually have a bandwidth much higher than that of the vertical system.

Dean

I like to look at RF with my scope. I want to see if the output is sign wave or flat toping. I cannot afford a scope that can see my third or fifth harmonics.
So:
I use a demodulation probe on AM and some times on FM signals, with a slow scope.
A spectrum analyzer is a way of seeing how much energy is available at each harmonic. You can get a lot of information that way. With out seeing the 'RF' directly.