# Question about scope ac-dc diagram

Discussion in 'Electronic Basics' started by [email protected], Oct 8, 2006.

1. ### Guest

I would like to ask about the ac coupling in a scope, what sort
of capacitor do I need to use?

2. ### Homer J SimpsonGuest

Why do you need to know? What purpose do you intend it to serve?

3. ### john jardineGuest

0.047uF, 400V, non-polarised.

4. ### Guest

Hello,

So why this value then? (I want to understand a bit
what I am doing)

5. ### Phil AllisonGuest

** The standard input resistance for a scope is 1 megohm - so using a
coupling cap of 0.047 uF passes AC signals with frequencies down to around
20 Hz with little loss. The - 3dB frequency is at 3.5 Hz.

A smaller cap value would offer no advantages while a much larger one would
take an annoyingly long time to charge up and settle when a high DC voltage
was applied.

The maximum DC voltage that can be displayed on a scope screen is generally
around 300 volts - without the use of a 10:1 or 100:1 divider probe. A
plastic film cap with a 400 or 600 volt DC rating does the job OK.

........ Phil

6. ### Guest

Great stuff, now after this coupling capacitor one design from
veleman scopes uses another capacitor of 2.2nF in parallel
to 100R resistor. What could be the reason for that?

7. ### Phil AllisonGuest

** Need to see the whole schematic to say for sure.

But a 100 ohm resistor would limit charging current flow in the coupling cap
to a safe value whenever the scope's input is *grossly* over driven with AC
or DC voltage causing input device protection diodes ( to supply) to
conduct.

....... Phil

8. ### Guest

Ok, so 100R protects against some unforseen circumstances
(maybe against failure of the 47nF coupling)
Then, the 2.2n excludes the 100R protection for f > 1MHz, correct?

Now, the next stage there is a 3 stage resistor divider.
91K, then into 10K and to signal ground. (signal ground
is raised, and is 2.5v compared to real ground.)
So in total this is 1M as you said.
Now the not so clear thing is that in parallel with the
910K we have a varicap 22pF, then in parallel with the
91K we have 150pF and a varicap 30pF, then in parallel
with the 10K we have 2.2nF in series with 3R3. Please can
you explain the purpose of all the capacitors, and the 3R3.
The only clue I can give you is that in the calibration process
I am supposed to connect the input to test point 1, and then
adjust CV1 (22pF) untill a test square wave is flat on top and
bottom, and does not have rounded or pointy edges.
Then I do the same with CV2 (30pF) for test square wave 2.
If you carefully follow my text you can draw the schematic.

9. ### john jardineGuest

Give us a break Joakent!. It feels like we're being asked to analyse a
whole 'scope circuit with the proviso we are spoon fed only one component
at a time.
Scan the Y amplifier circuit, put it up someplace where we can all take a
look.

As the input signal moves into the heavy circuitry, a huge number of valid
component arrangements become available to the original designer. Hence
individual components can only be examined in context of the particular
circuit block of which they form an essential part.

The 2n2 and 100R, I've no idea why they are there. If you'd said "100K" and
2n2, then they most surely would have been overvoltage protection
components for the first FET.

All those cap's strung across the resistor string are perfectly normal and
quite essential to allow a flat frequency response from the range switch. If
this capacitor 'frequency compensating network' wasn't there, the frequency
response of the scope would be from DC to about 3kHz.
Notice if you multiply the component values of each C-R shunt section
(trimmers notionally at half their range), you'll find they give pretty much
the same 'time constant' number. If you wanted to extend the ranges out then
it's a case of adding extra resistors to the chain and then adding shunt
cap's that calculate out to give this same value.
They're a kind of balancing act, used in a precise, controlled manner to
swamp out numerous, havoc causing stray capacitances, that would otherwise
act with the range resistors, to form unwanted RC low pass filters.

[ NB: When you tweek the trimmers while looking at the 'squareness' of a low
frequency square wave, you're actually setting and seeing the amount of
multi-MHz , harmonic components of that square wave, allowed through into
the Y amplifier. A neat trick!]

Attenuator design and input circuitry for scopes and suchlike is often
overlooked but is a fascinating subject in it's own right and if done badly
can break a product.
If you've queries ... Post the circuit somewhere!!.
john

10. ### Homer J SimpsonGuest

Which is why I asked, "Why do you need to know? What purpose do you intend
it to serve?"

11. ### Phil AllisonGuest

** Not the latter, as the cap may be bypassed any time with the DC/AC
coupling switch.

** The 100 ohm resistor feeds the input attenuator's parallel capacitance of

With no cap in parallel, response would to fall by about 1 dB at 20 Mhz.

** Two words - " stray capacitance". The purpose of those capacitors is
to *swamp* the effect of all the stray capacitances of the resistors, PCB
tracks and wiring. Although only a few pF each, such strays ruin the
desired flat response of the attenuator at high frequencies.

Note how the capacitors impedances at any given frequency are in similar (
ie 10:1 ) ratios to the resistors and so ( when precisely trimmed ) create a
matching capacitive voltage divider in parallel with the resistive one.

All input signals go to the top of this divider chain and so see a load of 1
megohm in parallel with about 30 pF regardless of the attenuation at the
input of the scope's electronics.

BTW 1

At 10 MHz, the input load impedance is about 500 ohms due to the effect of
30 pF.

BTW 2

Kindly post the schem of the input circuity - what you have chosen NOT to
describe holds answers to you questions.

BTW 3

Do not refer to a " trimmer capacitor" as a " varicap" cos that is a whole
nother beast.

........ Phil

12. ### Guest

How do you calculate 30pF? The first capacitor in the chain is 22pF,
the other capacitors on on the next stage of the divided so they add
in series, i.e. 22pF will be slightly reduced even.

Can you give an example of a stary capacitance, i.e. draw it in the
diagram,
and show how e.g. the 22pF capacitor act to cancel it?

Hence what?
O.k. , so trimmer, as opposed to potentiometer-like, is that what
you mean?

13. ### Phil AllisonGuest

** BEWARE:

** Plus **strays** = " about 30 pF " - ASSHOLE !!

** Huh ????

Better go learn that capacitance exists between all conductors.

Something an ASD fucked, code scribbling moron like YOU never learns.

PLUS - What fucking diagram ??

The one YOU have so far refused to post ????

Fat chick porn exceeds your Lilliputian intellectual ability - ASSHOLE

**** the HELL OFF.

........ Phil

14. ### Guest

100K? that would be almost like scope design suicide
(and I am not a designer, yet)
How did you calculate 3KHz?
Can you explain the importance of the time constant concept?
Can you show this in a diagram, i.e. introduce a capacitor called
'stray'
and show how our capacitors cancel it out?
Can you explain where these harmonics are coming from,
and why and how to we need to compensate for them?
Very true!
I trust my problems will keep you busy for a while ...
p.s. I will post the diagram, I need to setup a yahoo
account, hopefully I will manage it today.

15. ### Phil AllisonGuest

** Piss this pig arrogant Google moron off.

......... Phil

16. ### PuckdropperGuest

@individual.net:

*plonk*

Puckdropper

17. ### Alan BGuest

On 11 Oct 2006 16:39:48 GMT, in message
<452d1e54\$0\$97261\$>, Puckdropper
<> scribed:

Well now that's playing fast and loose with your entertainment dollar!

18. ### JoeBloeGuest

As fucked up, retarded and stupid as Phil-baby is, I still think it
far more retarded to announce one's filter file edits.

You know... as if anyone gives a shit. Grow the **** up.

19. ### Guest

Well?
I thought you wanted to help?