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G8MNY  > TECHNI   25.11.21 11:20l 337 Lines 15907 Bytes #999 (0) @ WW
BID : 54030_GB7CIP
Read: GUEST
Subj: Scope Probes
Path: PI8DRE<PI8CDR<DB0RES<ON0AR<IW0QNL<IZ3LSV<IW8PGT<LU4ECL<GB7CIP
Sent: 211125/0958Z @:GB7CIP.#32.GBR.EURO #:54030 [Caterham Surrey GBR]
From: G8MNY@GB7CIP.#32.GBR.EURO
To  : TECH@WW

By G8MNY                                      (Update Jul 20)
(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
75/50 >      75/50 coax         75/50  25pF=== 1M  Attenuators
                                   Term& Amp

The scope input capacitance has the minimum effect on the signal.
Only usable on DC-RF 75/50 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
1M+   >    200pF of lead Capacitance     25pF=== 1M  Attenuators
225pF           e.g. 75 coax              & Amp

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

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 75 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               ___
  _________/~~~~~~    _||~~~~~~- ~'-___\\ ___-'~ -ij   '~~
c_\________|        Ĵ_       ||                              
           \______     ||______- _.-~~~~ \\~~-._ -___.__
                              ..__    Screw or             Screw or
                    Ŀ      _    Crimp                 Crimp      BNC
 Long Probe        BNC_       
  _________/~~~~~~     _
\________|          _         Earth
           \______  /~ Tip     Clip
                    \_ Shroud  
                       ~        
Input  Probe parts                                SCOPE INPUT LOAD
Load   --LL)============================33o)Scope
1M+         70pF Low capacity 3m leadij 25pF=== 1M  Attenuators
100pF  > 300 DC, Z = 150, (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 75 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               ___
  _________/~~~~~~   _||~~~~~~~- ~'-___\\ ___-'~ -ij   '~~
c_\________|       Ĵ_       ||  0 Adj                        
           \______    ||_______- _.-~~~~ \\~~-._ -___.__
                  

Input  Probe parts
Load       ڴÿC Adj *                        SCOPE INPUT LOAD
10M+ 47L9M)============================o)Scope
15pF  >  70pF Low capacity 3m lead   25pF=== 1M  Attenuators
                      Z = 150               & Amp

The accurate series 9M 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
 600Ŀ   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               ___
  _________/~~~~~~   _||~~~~~~~~- ~'-___\\ ___-'~ -ij   '~~
c_\________|       Ĵ_       || 0  _                          
           \______    ||________- _.-~~~~ \\~~-._ -___.__
                                  1:1:10

Input  Probe parts
Load     ڴĿC Adj *                        SCOPE INPUT LOAD
10M+ 479M)============================o)Scope
15pF     _\_  55pF Low capacity 2m lead   25pF=== 1M  Attenuators
or 1M >        Z = 150             & 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             Ŀ  BNC
  _________/~~~~~~   _||~~~~~~- ~'-___\\ ___-'~ -ij     '~~
c_\________|       Ĵ_       ||                           0    
           \______    ||______- _.-~~~~ \\~~-._ - Adj .__
                                                                  
Input  Probe parts
Load     ڴÿFixed C                             SCOPE INPUT
10M+ 479M)=============================o)Scope
15pF  >  70pF Low capacity 3m lead  Ĵô 25pF=== 1M  Attenuator
                     Z = 150          *Trimmer& Amp

The accurate series 9M 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     ڴĿFixed C                           SCOPE INPUT LOAD
100M 4799M)==============================o)Scope
+4pF  > 70pF Low capacity 3m lead   ===  25pF=== 1M  Attenuators
                      Z =  150          Trim& Amp

The accurate series 99M 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            Ŀ  BNC
  ________/~~~~~~   _||~~~~~~- ~'-__\\ __-'~ -ij       '~~
c_\_______|       Ĵ_      || 0 Adj                  0 0 0   
          \______    ||______- _.-~~~ \\~-._ -  Adj  .__
                                                              

Input  Probe parts                        3 HF
Load     ڴÿC trimmer 1kHz            preset Rs   SCOPE INPUT
10M+ 479M)============================R1o)Scope
16pF  > 35pF Low capacity 1.5m lead   R2ĴĴ 25pF=== 1M  Attenuator
                    Z = 150                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     -ij   '~~
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.
 _  _________________________Ŀ
(/_/           |\\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!

FAULTING LEADS
Broken intermittent inner of very thin (hair gauge) lose single core as it will
not stretch, or poor 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 too much my uncalibrated it! But with some
patience, these can often be cut back a few cm & reterminated, but which end?
                            _____
Ŀ       10:1===========Scope
HF        /              
Sig330R>===============
Gen           bad lead

This test circuit will show up which end has is the break, by not dropping the
scope trace level when the centre is connected.

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).
                                  Ŀ
             ____                  Scope 
            Work <ĴY      
                                       
            ____================_______
Mains earth     HUM DIFFERENCE     Mains Earth
                    SHORTED OUT

Differential mode can be useful to, where you use both scope inputs to see the
difference..
                                  Ŀ
             ____                   Scope 
            Work <ĴY1\ Diff
                <ĴY2/ 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.
                                               Ŀ
 ___________              ____                  Scope 
   Mains      2 core   Work <((((ĴY      
 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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