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From: G8MNY@GB7CIP.#32.GBR.EURO
To : TECH@WW

By G8MNY (Update Apr 09)
(8 Bit ASCII graphics use code page 437 or 850, Terminal Font)

They come in a few standard forms..
1:1 COAX
A terminated teed coax is the only way get the highest bandwidth into your scope. Simple nothing to adjust!
BNC T SCOPE INPUT LOAD
Input o)==============================┬─o)───────┬──┬───Scope
75/50Ω >┘ 75/50Ω coax 75/50Ω │ ≈25pF=== 1M Attenuators
Term└──┴───────┴──┴───& Amp

The scope input capacitance has the minimum effect on the signal.
Only usable on DC-RF 75/50Ω circuits, e.g. DC-200MHz.
Voltage limited to scope input/termination load rating.
Able to see low signal levels, no added calibration errors.

1:1 TEST LEAD
These are straight screened wires like a coax, with just test clips on the ends & BNC at the scope end. Simple nothing to adjust!

SCOPE INPUT LOAD
Input >─)================================o)───────┬──┬───Scope
1MΩ+ >─┘ 200pF of lead Capacitance │ ≈25pF=== 1M Attenuators
225pF e.g. 75Ω coax └───────┴──┴───& Amp

Coax capacitance per foot is about 20/30pF for 75/50Ω.

The capacitance depends on coax impedance & lead length. Only usable on DC AF & low Z circuits where the loading is not important e.g. DC-3MHz (just about OK on terminated 75Ω video!)
Voltage limited to scope input.
Able to see low signal levels.

1:1 PROBE
These have one of the 2 sizes of standard probe systems. The spring hook can changed to a long probe, just the tip or tip covers for small point or IC leg work, or be adapted to a BNC plug.
Simple nothing to adjust!

PROBE PARTS
Spring Hook ▐ Tip Probe Coax ___
_________/~~~~~~▐ _┌───────||~~~~~~│─- │~'─-┐___\\ ___┌-─'~│ -─│ ├'~~││ c_\________| ▐ ─┤_│ || │││ │ │ │ │ │││ ││ ││
\______▐ └───────||______│-─ │_.-─┘~~~~ \\~~└─-._│ ─-│___├.__││
▐ .─.__ Screw or Screw or
┌─────┐ │ │ _│ Crimp Crimp BNC
Long Probe ▐ ├─BNC_│ │ │ │
_________/~~~~~~▐ └─────┘ │_│──┘
▄▄\________| ▐ _ ▌ Earth
\______▐ /~ │Tip ▐ Clip
▐ \_ │Shroud ▌
~ ▐
Input Probe parts SCOPE INPUT LOAD
Load --┬─L┬L─)============================─33─o)───────┬──┬───Scope
1MΩ+ └┤├┘ │ 70pF Low capacity 3m lead└─────│ ≈25pF=== 1M Attenuators 100pF >───────┘ 300Ω DC, Z = 500Ω, (7pF/Ft) └───────┴──┴───& Amp

The load capacitance depends on lead length (1.2, 1.5, 2, or 3m) & the coax type which here is a very fine single strand core polly foam insulated high impedance type. There is often a tweak on the probe (tuned circuit + L shown & a series scope R) to improve HF & pulse performance.

Only usable on DC AF & low Z circuits where the loading is not important.
e.g. DC up to 6-10MHz on 75Ω terminated systems.
Voltage limited by probe & scope. Able to see low signal levels.

10:1 TRIMMED PROBE
These are the most common probe, with in built attenuator reducing input load. Must be calibrated before use on that scope's input!
Not able to see very low signal levels.

PROBE PARTS
Spring Hook ▐ Tip Probe Coax ___
_________/~~~~~~▐ _┌───────||~~~~~~~│─- │~'─-┐___\\ ___┌-─'~│ -─│ ├'~~││ c_\________| ▐ ─┤_│ || 0 Adj│││ │ │ │ │ │││ ││ ││
\______▐ └───────||_______│-─ │_.-─┘~~~~ \\~~└─-._│ ─-│___├.__││
▐

Input Probe parts
Load ┌┤├┐C Adj * SCOPE INPUT LOAD
10MΩ+ ─47─L┴9M┴)============================o)───────┬──┬───Scope
15pF >────────┘ 70pF Low capacity 3m lead │ ≈25pF=== 1M Attenuators
Z = 500Ω └───────┴──┴───& Amp

The accurate series 9MΩ gives the 10:1 DC calibration with the scope input R. The trimmer C in the probe has to be exactly 1/9 of the total capacitance of the scope & lead, then the probe's frequency & phase (pulse) response will be flat. So the probe is designed for a particular lead! Note the more complex tiny probe components are to improve bandwidth & pulse responce.

Usable DC-60MHz (3.5nS rise time). Voltage limited by probe design/10x scope input. Some older larger 10:1 probes (e.g. Tektronix) use an adjustable moving capacitance tube & have a much higher rated voltage & are useful for mains & SMPS & line output work, where the peak pulse voltages can be over 1kV @ 50kHz, which most of the smaller probes CAN NOT safely handle! See the rating curve for your probe...

Peak Volts
700┤
600┤────────┐ Typical Probe Derating Curve
500┤ ▐
400┤ ▌ This means that you can't scope HF Tx aerial!
300┤ \
200┤ '·.,__
100┤ ~~''───----....,,_______
0┼────┬────┬────┬────┬────┬────┬────┬────┬──>Hz
0 50k 100k 200k 500k 1M 2M 5M 10M

1:1/10:1 SWITCHED PROBE
These are also common, with a built in switched attenuator reducing input loading on 10:1. & the 1:1 option for seeing small signals, but note the loading changes when switching.
On 10:1 mode, it must be calibrated before use on that scope's input!

PROBE PARTS
Spring Hook ▐ Tip Probe Adj SW Coax ___
_________/~~~~~~▐ _┌───────||~~~~~~~~│─- │~'─-┐___\\ ___┌-─'~│ -─│ ├'~~││ c_\________| ▐ ─┤_│ || 0 _▄ │││ │ │ │ │ │││ ││ ││
\______▐ └───────||________│-─ │_.-─┘~~~~ \\~~└─-._│ ─-│___├.__││
▐ 1:1:10

Input Probe parts
Load ┌┤├─┐C Adj * SCOPE INPUT LOAD
10MΩ+ ─47┼9M─┼)============================o)───────┬──┬───Scope
15pF │_\_││ 55pF Low capacity 2m lead │ ≈25pF=== 1M Attenuators
or 1M >───────┘ Z = 500Ω └───────┴──┴───& Amp
+80pF

Usable up to 6-10MHz @ 1:1, & 60MHz @ 10:1. Voltage limited by probe & switch design/10x scope input or scope input @ 1:1. Due to the added switch & risk to the scope I would not use this type on any high voltage like mains or SMPSU testing!

10:1 SCOPE END TRIMMED
These are the less common, with a fixed probe capacitor, the trimmer is put at the scope end. As it is the scope input capacitance that varies scope input to scope input, this approach lets the probe maker better match the components (e.g. series Ls & Rs etc) to get a better frequency/pulse response, however the probe tip capacitance is a little bit higher!
Must be calibrated before use on that scope's input!
Not able to see very low signal levels.

PROBE PARTS
Spring Hook ▐ Tip Probe Coax ┌─────┐ BNC
_________/~~~~~~▐ _┌───────||~~~~~~│─- │~'─-┐___\\ ___┌-─'~│ -─│ ├'~~││ c_\________| ▐ ─┤_│ || │││ │ │ │ │ │││ 0 ││ ││
\______▐ └───────||______│-─ │_.-─┘~~~~ \\~~└─-._│ ─-│ Adj ├.__││
▐ └─────┘ Input Probe parts
Load ┌┤├┐Fixed C SCOPE INPUT
10MΩ+ ─47┴9M┴)=============================┬──o)───────┬──┬───Scope
15pF >──────┘ 70pF Low capacity 3m lead └─┤├┤ ≈25pF=== 1M Attenuator
Z = 500Ω *Trimmer└───────┴──┴───& Amp

The accurate series 9MΩ gives the 10:1 DC calibration with the scope input R. The fixed C in the probe has to be exactly 1/9 of the total input capacitance & the scope end *trimmer is adjusted for this.

Usable DC-100MHz where the loading is not important. Voltage limited to by probe design/10x scope input, this may be higher than the probe end trimmer type.

100:1 TRIMMED PROBE
Same as above but are less common, usualy scope end trimmed allowing for high voltage with the larger in built attenuator reducing input loading by 100, useful on sensitive large siganl DC or where higher capacity probe can't be used, if there is enough signal!
No good for low signal levels!

Input Probe parts
Load ┌┤├─┐Fixed C SCOPE INPUT LOAD
100MΩ ─47┴99M┴)==============================┬─o)───────┬──┬───Scope
+4pF >───────┘ 70pF Low capacity 3m lead === │ ≈25pF=== 1M Attenuators
Z = 500Ω Trim└──┴─────┴──┴───& Amp

The accurate series 99MΩ gives the 100:1 DC calibration with the scope input R. The high voltage C in the probe has to be exactly 1/99 of the total input capacitance for the probe to be flat.

Usable DC-60MHz. Voltage limited to by probe design/100x scope input e.g. 1.5kV Even so some are not rated for SMPSU work see derating curve above, where the peak spike voltages may be 1kV @ 50kHz!

10:1 SCOPE & PROBE TRIMMED
These are far less common, with a trimmer capacitor & presets. They give the best performance & must be calibrated before use on that scope's input!
Not able to see very low signal levels.

PROBE PARTS
Spring Hook ▐ Tip Probe Coax ┌───────┐ BNC
________/~~~~~~▐ _┌──────||~~~~~~│─- │~'─-┐__\\ __┌-─'~│ -─│ ├'~~││ c_\_______| ▐ ─┤_│ || 0 Adj│││ │ │ │ │ │││ 0 0 0 ││ ││
\______▐ └──────||______│-─ │_.-─┘~~~ \\~└─-._│ ─-│ Adj ├.__││
▐ └───────┘

Input Probe parts 3 HF
Load ┌┤├┐C trimmer 1kHz preset Rs SCOPE INPUT
10MΩ+ ─47┴9M┴)============================─R1┬─────o)───────┬──┬───Scope
16pF >──────┘ 35pF Low capacity 1.5m lead ├R2─┤├─┤ ≈25pF=== 1M Attenuator
Z = 500Ω R3 │ │ │ & Amp
├─L─┬┤├┴───────┴──┴───
└─47┘
Usable DC-300MHz where the loading is not important. Voltage limited to by probe design/10x scope input. The probe trimmer affects 1kHz & the 3 scope end presets all affect different HF frequencies allowing good calibration on square wave harmonics to over 300MHz & a pulse rise time of 1.4nS.

AP BNCs
This a surrounding locking ___ ▒▒▒▒
pin system that tells the -─│ ├'~~││
scope you have a 1:10 probe │││ ││ ││BNC
fitted, for on screen ─-│___├.__││
calibration information etc. ▒▒▒▒▄▄PIN

PROBE PARTS INTERCHANGEABILITY
Various makers often use similar parts, but not always! e.g. Tip threads can be different, & also the tip length & diameter making it incompatible to some makes of probes hooks.

Screw on leads are also made in slightly different ways & may no be compatible!

CALIBRATING A SCOPE PROBE *
Connect the probe to the scope 1kHz square wave calibrator & set the input, trigger & timebase range to show a large square wave. Then using the insulated tool provided adjust the trimmer...

TO LITTLE HF CORRECT TO MUCH HF
____ _ ______ __ │`\.___
,/' │ ,/' | | | | | | | = faint
| | | | | | | | | | Verticals
| | | | | | | | |
| `\.___│ | |______| | | ___|
│./'

With a 2 (or more) trimmer probe a higher frequency test square wave of 1MHz will be needed for the scope end trimmer(s). Adjust the trimmers in much the same way, endeavouring to get the best square "corner pulse" shape possible.

FAULTY SCOPE ATTENUATOR
If the wave shape changes (other than height) as the scope input attenuator is changed, then the scope attenuator needs recalibrating, as the input capacity (& hence the probe response) should not change if the scope is set up OK!

PROBE FAILURE
Tips: These can brake the screw in tip off as there is huge leverage with the whole probe on the end, but you can get spares with probes.

Hooks: The hook sleeve will melt if left on components while soldering! The hooks also break, (made of wire or metal strip) due to the leverage factor.
_ _________________________┌────┐
(/_/ |\\Spring\\\|▒│ │ -=Probe
~~~~~~~~~~~~~~~~~~~~~~~~~~~/└────┘ Top
Solder
I have made long lasting replacement hooked strips from scrap junior hacksaw blades cut into strips & filed out the hook shape with a needle file. A good solder (try ally solder) & key filing are needed to attached the blade metal!

Body: These fail due to broken internal wires to the socket, & also fractured PCB. The assembly can be difficult to take apart. but all the plastic covers are push fit. A broken multiplying R is usually fatal, unless you want a 1:1 probe! Be careful not to lose the red tweaker cover when sliding the hand cover off. Switched versions need extra care!

Plastic covered _ Handle pulls off >
metal probe tube │ └───----..,..
Screw _______________┌──────────── __ Tight push fit socket
on tip -<│- [ heatshrink covered PCB -[▒[■
~~~~~~~~~~~~~~~└──────────── ~~\Lead connector screw thread
Earth contact ring/│_┌───''''~~~~~
^ Keyway

N.B. Some probes use a metal plated plastic body, so are not metal at all!

Leads: These fail:-
a) broken intermittent inner of very thin (hair gauge) lose single core as it
will not stretch,
b) the push on crimped outer connectors. these have just a centre pin to make
connection to the movable inner wire. Repairing by shorting the lead by much
my uncalibrated it!

SCOPE BNC: If moulded on type little can be done. Box type these can fail due to movement of the plug/socket wires to the PCB & although fiddle are easily repaired.

PROBE PICKUP/HUM LOOPS
With different earth potentials & high capacity from mains filters to chassis it is quite normal to see unwanted signals on a sensitive range on the scope. This not helped by long resistive scope leads, so 1st try a thick bonding lead from the scope earth point to the work (NO LIVE chassis use isolation transformer).
┌───────┐
____ │ Scope │
│Work <───────────────┤Y │
│ │ │ │
│____│================│_______│
Mains earth ──┘ HUM DIFFERENCE └─────Mains Earth
SHORTED OUT

Differential mode can be useful to, where you use both scope inputs to see the difference..
┌────────┐
____ │ Scope │
│Work <───────────────┤Y1\ Diff│
│ ┌<───────────────┤Y2/ Mode│
│____│ │________│
Mains earth ──┘ HUM DIFFERENCE └─────Mains Earth

For HF noise try the ferrite ring trick on the mains or scope probe lead & see the common mode noise disappear.
┌───────┐
___________ ____ │ Scope │
│ Mains │ 2 core │Work <─────((((──────┤Y │
│ Isolation │=mains lead=│ │─┘ ==== │ │
│Transformer│ │____│ Ferrite │_______│
~~~~~~~~~~~~~ HF noisy PSU └─────Mains Earth



Also see my buls on "Oscilloscopes" & "Scope & DMM Calibrator".


Why Don't U send an interesting Bul?

73 De G8MNY @ GB7CIP

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