Question about bandwidth of scope?

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?

nnn wrote:
> 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?

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?

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 in digital circuitry) and the frequency.
The 12 MHz describe the roll-off frequency seen, even when you drive it
with a perfect high-frequency signal.

BTW, these specifications are ok for audio work, but be careful when
you debug fast digital logic. You might be driving blind...
Peter Alfke
==============
On Oct 31, 1:56 pm, <nnn> wrote:
> 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?

nnn,

Input impedance is the load the measurement places on the node being
measured.  In the case you mention, a 1 megohm resistor, with a 30 pF
capacitor in parallel.  If this is a 50 ohm impedance signal source,
then the 1 megohm is not affecting anything (1 megohm in parallel with
50 ohms is still very very close to 50 ohms), but 30 pF and 50 ohms is
about a 1 nanosecond time constant, so the capacitive load means the
probe itself will significantly load down signals above 100 MHz
(Z=1/2*pi*f*c = 53 ohms for 100 MHz !!!).

'Bandwidth' is where the measurement "rolls off" or is no longer
accurately able to detect the amplitude at that frequency.

'0 to 12 MHz +/- 3dB' means (to me) that the amplitude you measure can
be off by 2:1 anywhere in the 12 MHz bandwidth, which is really lousy (bad).

Typically, a scope bandwidth is stated as +/- 0.5 dB from 0 to 1 GHz
(for example), with a roll off of -3 dB at 1.5 GHz.  Basically telling
you this scope would be fine for signals with frequencies below 1 GHz,
or rise times longer than 1 nanosecond.

If you try to measure anything faster than that, the measurement will
not be accurate, and you will not see what is really there, only will
see a low pass filtered version of what is there.

Austin


nnn wrote:
> 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?
> 
> 
> 

jacko wrote:
> nnn wrote:
>> 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?
> 
> 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?
> 

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

Peter Alfke wrote:
> 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 in digital circuitry) and the frequency.
> The 12 MHz describe the roll-off frequency seen, even when you drive it
> with a perfect high-frequency signal.
> 
> BTW, these specifications are ok for audio work, but be careful when
> you debug fast digital logic. You might be driving blind...
> Peter Alfke
> ==============
> On Oct 31, 1:56 pm, <nnn> wrote:
>> 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?
> 

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

Austin Lesea <austin@xilinx.com> wrote:

>nnn,
>
>Input impedance is the load the measurement places on the node being
>measured.  In the case you mention, a 1 megohm resistor, with a 30 pF
>capacitor in parallel.  If this is a 50 ohm impedance signal source,
>then the 1 megohm is not affecting anything (1 megohm in parallel with
>50 ohms is still very very close to 50 ohms), but 30 pF and 50 ohms is
>about a 1 nanosecond time constant, so the capacitive load means the
>probe itself will significantly load down signals above 100 MHz
>(Z=1/2*pi*f*c = 53 ohms for 100 MHz !!!).

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).

-- 
Reply to nico@nctdevpuntnl (punt=.)
Bedrijven en winkels vindt U op www.adresboekje.nl

PeteS wrote:
> jacko wrote:
> > nnn wrote:
> >> 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?
> >
> > 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?
> >
>
> 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

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:

http://www.tek.com/Measurement/programs/301913X312631/

Cheers
Chris

> Huh? (in response to what Jacko wrote)
>
> x-posted to s.e.b where this is more appropriate.

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 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.
>

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?

nnn wrote:
[...]
>
>
> 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?

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 . )