Matching Oscilloscope and Probes

[gibsonfs]

Hi All,

New to the forum but I've been lurking around the blog for a while. Relatively new to electronics as well, but it's looking like there's a few knowledgable folk around here that I can learn a lot from.

I recently acquired an old Telequipment D43 oscilloscope from an acquaintance after mentioning that one day I would like to have a crack at designing some tube amps. He figured it would be next to indestructable for my purposes and offered it to me. Unfortunately the scope didn't come with any probes, and I've been trying to hunt some down.

The datasheet mentions input impedance of 1M shunted by 40pF. From my knowledge of probes I believe that I probably want passive probes with the above RC for 1x and 10x with 9M resistance and compensation capacitor somewhere around 4 to 5pF. Unfortunately it seems that probes with these kinds of specs are a little bit difficult to come across.

Can anyone provide any advice on existing probes that might fit these specs or whether it will be possible to get away with using mismatched probes without too much trouble? It is also perfectly likely that my calculations are off too.

Regards,

Gibson.

[w2aew]

10x probes are specified by what scope capacitance they can compensate for, not what capacitance is included in the probe. The actual value will depend on the probe construction, cable length, and scope input. 1x probes are not compensated.  This old scope is only about 15MHz, even less at some vertical settings. Compensation will help flatten the response, but for a scope of this age, it's response probably isn't that flat anymore anyway.  If you want to find one, look for one that has a BW of 20MHz or so if you want to find one that comes close to the right compensation range. I think this scope has UHF connectors on the vertical inputs, not BNC, so it's unlikely that you find a probe that will have this connector on it.

For some background, you may want to view my video on probes:

[notsob]

Had a quick search for this scope, looks like it has UHF connectors whereas most probe leads these days have BNC  connectors -  so a solution is to get UHF to BNC adaptors and some leads from ebay

here's a ref to the scope showing adaptors being used
http://www.ebay.co.uk/itm/Telequipment-D43-Oscilloscope-working-condition-/250863324354

PLEASE CHECK to see if these are the correct type and sex adaptors

UHF to BNC adaptor 
http://www.ebay.com/itm/UHF-male-plug-BNC-female-adapter-connector-/380261197224?pt=US_Radio_Comm_Coaxial_Cables_Connectors&hash=item58895565a8

20MHz scope 1x/10x probes
http://dx.com/p/20mhz-oscilloscope-probe-with-x1-x10-switch-max-600v-34219

[PA4TIM]

Or make your own probes to start with until you one. Look for High Z probe, these are esy to make. Normal 10X compensated probes are also not hard for 15MHz

Allmost every probe will do for this scope. You can buy probes from 10 dollar upto thousants of dollars. But the scope will never be more as 15MHz.
For audio no problem.

For tube amps, most China el cheapo probes are not rated high enough in Voltage.

[shebu18]

Hello, so i've got a Philips PM 3211, 15Mhz analog oscilloscope. It did not come with probes so i need to by some probes. As i understand from the video, if i use 10x than the signal will be attenuated 10 times, so from 10V inpus i will get a 1V shown on the screen. I do no think that the scope has a selector for 10 or 1x.
The probes i have seen on ebay are 1x-6mhz, 10x-100mhz, so if i use it uncompensated i can only use the probe for a maximum of 6mhz? THis would mean i need to search for a probe that has on the uncompensated input a minimum of 15mhz. Is that right?

[ciikucli]

I judy got a BK2160A 60MHZ oscilloscope for $40.
http://www.bkprecision.com/downloads/manuals/en/2160A_manual.pdf
 Am I right in assuming that I can get 100Mhz leads, and that it has BNU connectors?
Would this work?
http://dx.com/p/100mhz-oscilloscope-probe-32422?item=2

Is there a more common site like digikey or mouser that sells them for comparable prices. I understand the difference between x10 and x1 probes though.

Thank you

[PA4TIM]

10x is compensated, a good high frequency probe is only 10X and has up to three adjutable compensation capacitors.
1x is not compensated and that will allways be low frequency. You see in specs of 1x/10x probes they are full bandwidth only in the 10X position.

Deal extrem sell cheap probes but a good high voltage probe or tube amps will be very expensive.
Find some old Philips or Tek probes that are rated for higher voltage, i have a few Philips 10X probes that are rated 1000 and 1500V.

Other thing is, i the scope does 10V/div and you have 8 divisions 800Vtt using a 10X is the max you can see. If 5V/div is the max you need a 100X probe for tube equipment.

[shebu18]

So if I am on the 1V div, the probe on 1x and measure a 10V rectangular wave i get 10divisions, if i use the same settings but on 10xprobe i will get only 1 division high?

[PA4TIM]

Correct. But also remember, you need for a squarwave about ten times the bandwidth for the scope.
So a 15 MHz scope can show a 1,5 MHz squarewave as it is. A 15MHz square will show as a sinewave.
Above you see a picture from a 50 MHz scarewave ( 1 ns risetime) on a 100 MHz scope coupled through a 50 ohm input ( the way to get max bndwitdh at high frequencys)

[shebu18]

So how can i get over that problem with the compensation? How can i have 10 div if i measure 10v and have 10xcomp?

[vk6zgo]

So if I am on the 1V div, the probe on 1x and measure a 10V rectangular wave i get 10divisions, if i use the same settings but on 10xprobe i will get only 1 division high?

The compensation adjustment is nothing to do with this problem,it is merely for adjusting the frequency response of the probe +'scope system

If you want a  high impedance for looking at points in circuits which may be more than the normal 1Mohm input Z of the 'scope,the loss of sensitivity is the price you pay.

The voltage presented to the 'scope input is actually that at the measurement point,divided by 10,so you have to multiply the displayed waveform's voltage by the same factor,hence,"X10",so if you measure the 10V waveform,it will be displayed as 1V,but you can multiply by 10 in your head,on paper,or with a calculator to get the correct figure.

After a while,you will look at your probe,see it is X10,& read the voltage off the screen,as 10X what is really displayed.
It is not at all difficult,thousands of people do it  every day.

Some 'scopes & their probes automatically change the volts/division to match the probe type.(but only with probes of their own manufacture.)
This is normally on those analog 'scopes which have a readout on the screen,& with most,if not,all digital 'scopes.

[gibsonfs]

Thanks guys,

My apologies for not posting the information I have on the D43 earlier, as there is some general info here with some data sheets available here.

Regardless, what I'm getting from this is that due to the age of the scope I shouldn't be expecting perfect frequency response so obtaining correctly matched probes shouldn't be a priority overall, especially since it is likely to be difficult. Instead, making sure that probe bandwidth exceeds the maximum bandwidth of the scope is best. I can deal with this. I had figured that the scope may have limited utility due to its age/lack of features anyway, but it should be good enough.

At this stage, working with vacuum tubes is a distant target, though hopping on the bandwagon and playing with nixies is not unlikely in the more immediate future. notsob has linked some 600V rated probes, while PA4TIM has advised in getting something a little better. Using some kind of vague logic for using a probe that is rated for twice the expected input voltage, I expect something around the 400-600V mark would probably work out okay. Are branded probes just likely to be of a greater quality that makes it worthwhile to not buy chinese if possible?

The scope itself has a 1x/10x Y-gain control, as well as additional attenuators for up to 50V/div on the Y-axis so I would think that there is no need for any 100x probes.

Yes, it has UHF connectors on the input modules but also already has one UHF-BNC adapter attached, so thankfully I only need to grab one more of those

Thanks,

Gibson

[PA4TIM]

40 pF will not be hard to compensate, most probes can do that. My old teks are alo around that value and I can amost use all my probes on that. Possible 200 or more MHz probes will have a to small comensation span.

Compensation is only needed for 10X probes. You hook te scope to the calibrator and turn the capacitor, or probe head or whatever it is made, until you see a nice squarewave with flat horizontal tops.

If your scope has. 50V/div input it can handle a least 300 to 400 VDC direct and probably more. Look in the manual. But remember, the scope cabinet is the return ground so if you measure over a resistor and you clip the groundclip of the probe under the resistor and the probetip above it and the groundclip is at 200V and the tip at 210V, you see a 10V trace but if you grab the chassis of the amplifier and touch the chassis of the scope the same time your one hand is at ground level, the other at 200V. That hurts 

You do not have to take twice the voltage for a probe. But look at the specs. Is it is 100VDC +AC they mean it can handle 100V peak. This can be a 80VDC voltage with a 1KHz 40Vpp AC voltage riding on it so the voltage goes 1000 times a second from 60V to 100V. So always look how much Volt peak-peak a ACrms voltage is. Example:
If you measure a 100VAC sinewave, that is 100Vrms, the signal your scope sees and the probe must handle is 2 X (squar root 2) X 100V  so around 280Vpeak-peak for the above meantioned 100V probe this is almost three times to much. You need a 300V probe.

You use 10X probes because the do not load ( influence) that what you are meauring. This is important at higher frequencys. ( the MHz regions) not so much around 50 Hz or audio.

The 10X on your scope is most times for the horizontal gain ( a sort of zoom on your timebase) are you sure it is for Y ?

With a 10X pobe and 8 divisions you can see 8 x 50 = 400V x 10 = 4000V peak-peak. Most 10X probe will not do that. A good one is rated 500VDC. I have a 10X 1500VDC probe but often they are 100X at those values. My 100X is 2KV  ( or 700Vrms)