G8MNY > TECH 12.06.25 09:30z 337 Lines 15944 Bytes #174 (0) @ WW BID : 34605_GB7CIP Subj: Scope Probes Path: ED1ZAC<ED1ZAC<GB7CIP Sent: 250612/0927Z @:GB7CIP.#32.GBR.EURO #:34605 [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)==============================ÂÄo)ÄÄÄÄÄÄÄÂÄÄÂÄÄÄScope 75/50ohm >Ù 75/50ohm coax 75/50R ³ ÷25pF=== 1M Attenuators TermÀÄÄÁÄÄÄÄÄÄÄÁÄÄÁÄÄÄ& Amp The scope input capacitance has the minimum effect on the signal. Only usable on DC-RF 75/50ohm 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 >Ä)================================o)ÄÄÄÄÄÄÄÂÄÄÂÄÄÄScope 1M >ÄÙ 200pF of lead Capacitance ³ ÷25pF=== 1M Attenuators 225pF e.g. 75ohm coax ÀÄÄÄÄÄÄÄÁÄÄÁÄÄÄ& Amp Coax capacitance per foot is about 20/30pF for 75/50ohm. 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 75ohm 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 Þ Tip Probe Coax ___ _________/~~~~~~Þ _ÚÄÄÄÄÄÄÄ||~~~~~~³Ä- ³~'Ä-¿___\\ ___Ú-Ä'~³ -ij Ã'~~³³ c_\________| Þ Ä´_³ || ³³³ ³ ³ ³ ³ ³³³ ³³ ³³ \______Þ ÀÄÄÄÄÄÄÄ||______³-Ä ³_.-ÄÙ~~~~ \\~~ÀÄ-._³ Ä-³___Ã.__³³ Þ .Ä.__ Screw or Screw or ÚÄÄÄÄÄ¿ ³ ³ _³ Crimp Crimp BNC Long Probe Þ ÃÄBNC_³ ³ ³ ³ _________/~~~~~~Þ ÀÄÄÄÄÄÙ ³_³ÄÄÙ ÜÜ\________| Þ _ Ý Earth \______Þ /~ ³Tip Þ Clip Þ \_ ³Shroud Ý ~ Þ Input Probe parts SCOPE INPUT LOAD Load --ÂÄLÂLÄ)============================Ä33RÄo)ÄÄÄÄÄÄÄÂÄÄÂÄÄÄScope 1M+ À´ÃÙ ³ 70pF Low capacity 3m leadÀÄÄÄÄÄij ÷25pF=== 1M Attenuators 100pF >ÄÄÄÄÄÄÄÙ 300R DC, Z = 150R, (7pF/Ft) ÀÄÄÄÄÄÄÄÁÄÄÁÄÄÄ& 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 75ohm 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 Þ Tip Probe Coax ___ _________/~~~~~~Þ _ÚÄÄÄÄÄÄÄ||~~~~~~~³Ä- ³~'Ä-¿___\\ ___Ú-Ä'~³ -ij Ã'~~³³ c_\________| Þ Ä´_³ || 0 Adj³³³ ³ ³ ³ ³ ³³³ ³³ ³³ \______Þ ÀÄÄÄÄÄÄÄ||_______³-Ä ³_.-ÄÙ~~~~ \\~~ÀÄ-._³ Ä-³___Ã.__³³ Þ Input Probe parts Load ڴÿC Adj * SCOPE INPUT LOAD 10Mê+ Ä47RÄLÁ9MÁ)============================o)ÄÄÄÄÄÄÄÂÄÄÂÄÄÄScope 15pF >ÄÄÄÄÄÄÄÄÄÙ 70pF Low capacity 3m lead ³ ÷25pF=== 1M Attenuators Z = 150R ÀÄÄÄÄÄÄÄÁÄÄÁÄÄÄ& 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´ 600´ÄÄÄÄÄÄÄÄ¿ Typical Probe Derating Curve 500´ Þ 400´ Ý This means that you can't scope HF Tx aerial! 300´ \ 200´ 'ú.,__ 100´ ~~''ÄÄÄ----....,,_______ 0ÅÄÄÄÄÂÄÄÄÄÂÄÄÄÄÂÄÄÄÄÂÄÄÄÄÂÄÄÄÄÂÄÄÄÄÂÄÄÄÄÂÄÄ>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 Þ Tip Probe Adj SW Coax ___ _________/~~~~~~Þ _ÚÄÄÄÄÄÄÄ||~~~~~~~~³Ä- ³~'Ä-¿___\\ ___Ú-Ä'~³ -ij Ã'~~³³ c_\________| Þ Ä´_³ || 0 _Ü ³³³ ³ ³ ³ ³ ³³³ ³³ ³³ \______Þ ÀÄÄÄÄÄÄÄ||________³-Ä ³_.-ÄÙ~~~~ \\~~ÀÄ-._³ Ä-³___Ã.__³³ Þ 1:1:10 Input Probe parts Load Ú´ÃÄ¿C Adj * SCOPE INPUT LOAD 10Mê+ Ä47RÅ9MÄÅ)============================o)ÄÄÄÄÄÄÄÂÄÄÂÄÄÄScope 15pF ³_\_³³ 55pF Low capacity 2m lead ³ ÷25pF=== 1M Attenuators or 1M >ÄÄÄÄÄÄÄÄÙ Z = 150R ÀÄÄÄÄÄÄÄÁÄÄÁÄÄÄ& 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 Þ Tip Probe Coax ÚÄÄÄÄÄ¿ BNC _________/~~~~~~Þ _ÚÄÄÄÄÄÄÄ||~~~~~~³Ä- ³~'Ä-¿___\\ ___Ú-Ä'~³ -ij Ã'~~³³ c_\________| Þ Ä´_³ || ³³³ ³ ³ ³ ³ ³³³ 0 ³³ ³³ \______Þ ÀÄÄÄÄÄÄÄ||______³-Ä ³_.-ÄÙ~~~~ \\~~ÀÄ-._³ Ä-³ Adj Ã.__³³ Þ ÀÄÄÄÄÄÙ Input Probe parts Load ڴÿFixed C SCOPE INPUT 10Mê+ Ä47RÁ9MÁ)=============================ÂÄÄo)ÄÄÄÄÄÄÄÂÄÄÂÄÄÄScope 15pF >ÄÄÄÄÄÄÙ 70pF Low capacity 3m lead ÀĴô ÷25pF=== 1M Attenuator Z = 150ê *TrimmerÀÄÄÄÄÄÄÄÁÄÄÁÄÄÄ& Amp The accurate series 9Mê 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 Ú´ÃÄ¿Fixed C SCOPE INPUT LOAD 100Mê Ä47RÁ99MÁ)==============================ÂÄo)ÄÄÄÄÄÄÄÂÄÄÂÄÄÄScope +4pF >ÄÄÄÄÄÄÄÄÙ 70pF Low capacity 3m lead === ³ ÷25pF=== 1M Attenuators Z = 150R TrimÀÄÄÁÄÄÄÄÄÁÄÄÁÄÄÄ& 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 Þ Tip Probe Coax ÚÄÄÄÄÄÄÄ¿ BNC ________/~~~~~~Þ _ÚÄÄÄÄÄÄ||~~~~~~³Ä- ³~'Ä-¿__\\ __Ú-Ä'~³ -ij Ã'~~³³ c_\_______| Þ Ä´_³ || 0 Adj³³³ ³ ³ ³ ³ ³³³ 0 0 0 ³³ ³³ \______Þ ÀÄÄÄÄÄÄ||______³-Ä ³_.-ÄÙ~~~ \\~ÀÄ-._³ Ä-³ Adj Ã.__³³ Þ ÀÄÄÄÄÄÄÄÙ Input Probe parts 3 HF Load ڴÿC trimmer 1kHz preset Rs SCOPE INPUT 10Mê+ Ä47RÁ9MÁ)============================ÄR1ÂÄÄÄÄÄo)ÄÄÄÄÄÄÄÂÄÄÂÄÄÄScope 16pF >ÄÄÄÄÄÄÄÙ 35pF Low capacity 1.5m lead ÃR2Ä´ÃÄ´ ÷25pF=== 1M Attenuator Z = 150R R3 ³ ³ ³ & Amp ÃÄLÄ´ÃÁÄÄÄÄÄÄÄÁÄÄÁÄÄÄ À47RÙ 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 Ã'~~³³ scope you have a 1:10 probe ³³³ ³³ ³³BNC fitted, for on screen Ä-³___Ã.__³³ calibration information etc. ±±±±ÜÜPIN 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 ____ _ ______ __ ³`\.___ ,/' ³ ,/' | | | | | | | = faint | | | | | | | | | | Verticals | | | | | | | | | | `\.___³ | |______| | | ___| ³./' 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. _ _________________________ÚÄÄÄÄ¿ (/_/ |\\Spring\\\|±³ ³ -=Probe ~~~~~~~~~~~~~~~~~~~~~~~~~~~/ÀÄÄÄÄÙ 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 ³ ÀÄÄÄ----..,.. Screw _______________ÚÄÄÄÄÄÄÄÄÄÄÄÄ __ Tight push fit socket on tip -<³- [ heatshrink covered PCB -[±[þ ~~~~~~~~~~~~~~~ÀÄÄÄÄÄÄÄÄÄÄÄÄ ~~\Lead connector screw thread Earth contact ring/³_ÚÄÄÄ''''~~~~~ ^ 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? _____ ÚÄÄÄ¿ 10:1===========µScope³ ³HF ³ / ÀÄÄÄÄÄÙ ³SigÃÄ330RÄÁÄ>=============== ³Gen³ ³ bad lead ÀÄÄÄÁÄÄÄÄÄÄÄÄÄÙ 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). ÚÄÄÄÄÄÄÄ¿ ____ ³ Scope ³ ³Work <ÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ´Y ³ ³ ³ ³ ³ ³____³================³_______³ Mains earth ÄÄÙ HUM DIFFERENCE ÀÄÄÄÄÄMains Earth SHORTED OUT Differential mode can be useful to, where you use both scope inputs to see the difference.. ÚÄÄÄÄÄÄÄÄ¿ ____ ³ Scope ³ ³Work <ÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ´Y1\ Diff³ ³ Ú<ÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ´Y2/ Mode³ ³____³ ³________³ Mains earth ÄÄÙ HUM DIFFERENCE ÀÄÄÄÄÄMains Earth For HF noise try the ferrite ring trick on the mains or scope probe lead & see the common mode noise disappear. ÚÄÄÄÄÄÄÄ¿ ___________ ____ ³ Scope ³ ³ Mains ³ 2 core ³Work <ÄÄÄÄÄ((((ÄÄÄÄÄÄ´Y ³ ³ Isolation ³=mains lead=³ ³ÄÙ ==== ³ ³ ³Transformer³ ³____³ Ferrite ³_______³ ~~~~~~~~~~~~~ HF noisy PSU ÀÄÄÄÄÄMains 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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