Question about bandwidth of scope?

• posted

Hi, I have seen a scope which has

1 Mohm 30 pF input impedance

and frequency response

0 to 12MHz (+/- 3dB)

My question:

What is the difference between input impedance and frequency response?

• posted

input impedance has to be driven, and so will prsent power transfer to the voltage mesurement equipment, and the 3db response of the line by wobble matching the graticule scale, will be the bandwidth. the line driver has its own input impedance, which can only be driven by limited power.

ok?

• posted

These two have nothing to do with each other. The 1 megohm is a dc load and means that you can measure dc voltages with very little loading. The 30 pF is in parallel, and loads down any ac input. How much it loads down is determined by the driving impedance (nowadays often a few

100 Ohms or even less > Hi,
• posted

Huh? (in response to what Jacko wrote)

x-posted to s.e.b where this is more appropriate.

The input impedance of the measurement device specifies the load it presents to the signal being measured. It is important to know for a number of reasons.

The +/-3dB response is specified as that is how bandwidth is normally specified (-3dB is a half power point). In this case, it specifies that the amplitude response of the scope is within 3dB between DC and 12MHz.

I would suggest a google search for some basics (although s.e.b. can be an appropriate forum too).

Cheers

PeteS

• posted

A 12MHz scope isn't good even for low speed digital logic (with the way low speed is nowadays specified ;)

The general rule of thumb is the scope should have at least 3 times the bandwidth of the measured signal, so 4MHz tops in this case - that's low speed nowadays.

Cheers

PeteS

• posted

In addition to this: If you use a 1:10 probe, the signal gets attenuated 10 times, but also the input capacitance of the oscilloscope gets attenuated 10 times. So instead of 30pf, your circuit will see a load of 3pf which decreases the load on the circuit under test 10 times. The downside is that you'll need to calibrate the capacitor inside probe. Most oscilloscopes provide a 1kHz square wave output to do this calibration (you have enough pointers to use Google to learn more on this subject).

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• posted

And the OP can also look at the ultimate scope reference for newbies, "The XYZs of Oscilloscopes" from the folks at Tektronics. He'll have to fill out a quick questionaire before downloading:

Cheers Chris

• posted

Yes I know, my isp doesn't carry sci.electronics so I thought I'd try here. I'll use Google posting next time to sci.electronics.

The point is this, if 12MHz is the 3dB cutoff, then we get an input impedance of

1/(2pi*f*c)= 1/(2*3.14*12M*30p)=442ohms

where are these 442ohms are they between the plus and minus? and where is the 30pF are they between the plus and minus? what is so special about 442ohms?

• posted

nnn wrote: [...]

Yes, at 12MHz, the 30pf is looking like 442 ohms between the plus and minus. Yes, the 30p is between the plus and minus. It actually consists of distributed strays and real capacitance along the long path from the input socket to the first amplifier transistor. Yes ... They are both the same thing. 30pf acts like a 422ohms resistor up at

12.00MHz.

Nothing remotely special about that 442ohms. You just can as validly say the input looks like 1/4Mohm at 20kHz. It's simply an indication of how quickly the loading effects increase when frequencies goes up and there's stray capacitance about. Thus the reason people pay out good money for those 10:1 divider probes, or the very expensive FET probes.

(why oh why is it always "-3dB" when we're really interested in when the trace has shrunk to 70%. Is it only the immortal industry gods who are allowed to posess those special dB calibrated scopes? and then in deference, us mere mortals pretend we have them as well . )

• posted

I think the 12MHz is the 3 dB point. At this point the input voltage will only register on the scope screen as 50% of it's actual value.

3 dB must be with respect to a certain load, I suppose 50ohms or perhaps 75ohms i'm not sure, which doesn't make sense, as at 12MHz 30pF=463ohms
• posted

(1) The frequency response of a scope input has nothing to do with the input impedance. The frequency response of a scope is measured by driving the input with an ideal 50 Ohm signal generator. The input impedance matters little, and with most very high bandwidth scopes the input impedance is often 50 Ohms. The -3dB bandwidth is measured in this way. That is the response of the scope by itself. No probe involved. This is the ideal conditions - scope alone.

(2) The input impedance of the front end of the scope is reactive, and has both a resistive component and capacitance component. Those are the figures you quote. The idea is to match/trim a probe to this input impedance.

(3) When you attach a probe (say 10:1), you adjust/trim the series capacitance of the probe so that it maintains the 10:1 ratio across all frequencies. The Voltage divider is formed by the series impedance of the probe and the front end impedance of the scope. That is how the probe input impedance ends up at 10M Ohms, while the scope is 1M. However all probes are designed to work/trim against a fixed range of input impedance. So for a scope with a 15pf input, a probe that can trim against a 30-60pf input impedance would not trim right. They tell you what the input impedance is so you can select a probe that will match.

(4) The 12MHz bandwidth you quote is very low. For any digital work today you need at least a 100MHz scope. Most 10:1 probes today easily go to

100MHz or even 500MHz. In your case the most limiting factor is the scope. 12MHz is 20-30 years out of date. You cannot troubleshoot modern circuitry with 5nS edge times using a scope with 12MHz bandwidth. The 5nS edge rates represent about 200MHz bandwidth. Your signals could be ringing everywhere causing massive errors in the logic and you would never even see it.

You can by a 100MHz scope on Ebay for about \$100.

Regards, Chris.

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