A question about bandwidth

[peterpops]

Hi!

I have a question about bandwidths.
The circuit I'm using is an analog multiplexer, ADG732BSUZ, and it has a typical "-3dB bandwidth of 18MHz"

My understanding is that this means that if I have a 18MHz signal of 1V inputed to the switch (ADG732BSUZ) the output will be around 0.5V, is that correct?

Now to the question, in the figure below I've marked in red a period of a signal, lets say 10MHz... however the green interval is a much faster change from LOW to HIGH voltage than 10MHz. So which one is it that is accounted for in the datasheet's 18MHz?

The reason I ask is that when I'm sending a signal through the multiplexer at 1, 2, 4, 6, 8, and 12MHz the output voltage is much lower than anticipated from the datasheet. And the higher frequency the more distorted the signal gets.

The signal is outputed by a signal/arb generator and measured by an oscilliscope



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My understanding is that this means that if I have a 18MHz signal of 1V inputed to the switch (ADG732BSUZ) the output will be around 0.5V, is that correct?

Or should the output be 0.708...V?

 

[barry]

1) The -3dB spec is the minimum performance you can expect from the part, but it very well might have a -3db BW of 25MHz.

2) A 10 MHz square-wave has all the odd harmonics, which accounts for the fast rise time. If your bandwidth is less than, say, 5 times the square-wave frequency, you will distort the waveform (as you are seeing) because you are filtering out some of the energy of the signal.

RULE OF THUMB: Bandwidth of an edge=.35/rise time.

For example, if you had a 10 MHz square-wave with a 10nS rise time, you would need a bandwidth of 35MHz to pass the signal accurately

 

[Audioguru]

Each halfwave of 10MHz is 50ns long. If the rise time and fall time are each 10ns then the waveform is a clipped triangle wave, not a square wave.
The bandwidth should be at least 10 times the fundamental frequency for a square wave so the rise time should be 5ns or less.

 

[chuckey]

The -3 dB point is half POWER or .7071 voltage level.
Frank

 

[THINKER_KAM_MAN]

just a suggestion

to grantee that your information frequency will get through your
channel of communication the basic rule is that the channel band width
should be at least twice the information frequency.
so the bandwidth of your amp should be >24Mhz
the result if the band width requirements are not met, is attenuation of your signal and desegregation in the rise and fall times and bad noise
immunity. but you may be able to recover the signal by using line receivers.