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Oscilloscope probes: quality recommendations?
MUFF WIGGLER Forum Index -> Music Tech DIY  
Author Oscilloscope probes: quality recommendations?
diophantine
I recently got a Tektronix 2445 (150MHz)... finally got a pair of probes for it in the mail today.

They're decent-looking things from Riko instruments. They're correct for the scope (150MHz), 1x/10x adjustable, and only $17ppd for the pair. (Actually, that should've set off the sirens in my brain, but...)

http://cgi.ebay.com/Two-New-150MHz-Oscilloscope-Probes-Probe-Tektronix -HP-/150560060219?pt=LH_DefaultDomain_0&hash=item230e14333b

Anyhow, somewhat long story... I plugged one of them in, and connected it to my shortwave radio (closest thing) and got some signals. Tried my Monotron, got some signals, but the amplitude was very tiny at the strongest settings (2mV/div on the scope, high volume on the output). MS-10 (signal, noise, etc.): nothing.

After 2 hours messing around and fearing that either (a) the scope was DOA, or (b) I'd forgotten how to use an oscilloscope, I finally decided to calibrate the probe (I know, should've been step 1). On the low-frequency calibration, there was a TON of roll-off. Turning the screw on the probe helped some, but didn't get it even close to being calibrated.

Tried the other probe, and it was quite close. Not perfect (the tops of the square waves weren't perfect, at a 10% angle or something), but at least it looked like it was supposed to be a square wave. With that probe I can see the MS-10 just fine. (Turning the screw on this one, incidentally, did nothing!)

Haven't decided yet if I am going to complain and ask for a refund, or just eat it and keep the one semi-decent probe.

Anyhow, this leads to the question: What good probes should I buy? Should I bit the bullet and buy real Tek probes on eBay (10x: P6133, 1x: ????)? Or are there any alternatives that aren't crap?

I know that for audio & LFOs and such I can just do 1x; i.e. BNC to my own connectors, and I probably will end up doing some BNC-1/4" cables (and/or some multiples with BNCs for my modular). But, I would like some proper probes for breadboarding, and other non-audio projects that may come up, so I would like some proper probes.

Any suggestions would be most welcome!!
daverj
The Tektronix probes are top quality, but pretty expensive. I've been using Probe Master probes for a while and they're pretty good for the price. They don't have the little contact on the bottom of the connector to make the scope display the shifted scale when using it as a 10:1, but that's a minor inconvenience. You just have to remember that the real scale is 10 times less than the knob says.
Big G
diophantine wrote:
Anyhow, this leads to the question: What good probes should I buy? Should I bit the bullet and buy real Tek probes on eBay (10x: P6133, 1x: ????)?


Yes

Quote:

Or are there any alternatives that aren't crap?


No.

There is no alternative to a Tektronix. The scope you have cost thousands new and is still good and will last years. Using cheap probes is like putting retread tyres on a Rolls Royce, you will never get the true performance. There are good deals on genuine probes on eBay, you just have to wait for them.
kindredlost
Is it a bad idea NOT to match the MHz of the probe to the Oscope?

For instance, would a Tektronix 6101B (1x 15MHz) probe be usable with a 150MHz scope? Assuming it is used with slower audio rate synthesizer frequencies.

I read where a probe needs to be at least the same if not higher MHz rated. Seems right in order to be able to use the full range of the scope, but I don't understand the consequences with lower frequency measurements.

ready to be schooled... hyper
Big G
kindredlost wrote:
Is it a bad idea NOT to match the MHz of the probe to the Oscope?

For instance, would a Tektronix 6101B (1x 15MHz) probe be usable with a 150MHz scope? Assuming it is used with slower audio rate synthesizer frequencies.


It is not a case of matching as the bandwidth will be reduced to whatever is the lowest. You won't get the full performance of the scope channel amplifier if you put a low pass filter in front of it.

Many scopes sold for audio servicing have a DC to 20MHz bandwidth. Where 100MHz makes a difference is looking at risetimes of digital logic signals--they will appear slewed on the screen with a 15MHz probe so you will not know what is really there.

If you put a 100MHz and a 400MHz probe on the same signal on a 400MHz scope the 400MHz channel will look out of focus. This is because it is showing more HF noise on the signal. Two octaves more.

x10 probes are more useful for general work, x1 probes are only really needed for very low level signals. x1/x10 switchable probes are available. Tektronix ones have a contact to switch the Volts/div display.

Tip: if you are making your own circuits, put a wire loop or two around the board on the 0V so that you can attach the probe ground crocodile clip. It can make quite a difference to the display.
kindredlost
Thanks for the clarity Big G.

I have two scopes but both are low bandwidth. An Owon 25MHz and a BKPrecision 20MHz. I've considered a better scope, but mainly one with a better sample rate. I really don't have the need for anything more than what is necessary to troubleshoot analog modules because I'm just building kits and not designing circuitry. If there is a digital logic signal problem then I'd be hard pressed to figure it out without detailed instruction anyway. So for my bench it seems these slow boys are adequate.

I appreciate people here at WF taking the time and effort to help us learn a bit as we go along. This is fun!
diophantine
Yes, thanks for the tips, Big G!

I'll seek out some Tek probes.

For 1x should I do a direct connection? Or do I need some circuitry in there? The probes obviously have some caps and resistors in them... just wondering if the signals I'd get with a direct audio jack to BNC connector would be bad?

Big G wrote:
Tip: if you are making your own circuits, put a wire loop or two around the board on the 0V so that you can attach the probe ground crocodile clip. It can make quite a difference to the display.

As opposed to connecting the probe ground directly to the circuit ground/0V, or a short wire coming out of ground on a breadboard?

Thanks!
kindredlost
I think he means because the ground wire of the probe is a short length you may need 0v points close to the component under test.

The majority of modules used in this forum's context are of sufficiently small (<18Vdc) voltage that the 1x probe will be safe enough. Just don't plug up to the dryer outlet.
BOOM!

Since you have a pretty nice scope why not get the switchable probes anyway? You got a scope like I was wanting but never sprung for. Nice grab! thumbs up

I have a 1/4" phone plug with wires tinned for attaching the scope probes. Works fine for everything so far. I use these with my DMM as well. I'm using a 5U modular with +-15v power and up to 5vpp or 10v max on cv/gate/trigger so the 1x probe works fine for checking signals pretty well everywhere. I do have the switchable 10:1/1:1 probes too.
Big G
diophantine wrote:
For 1x should I do a direct connection? Or do I need some circuitry in there? The probes obviously have some caps and resistors in them... just wondering if the signals I'd get with a direct audio jack to BNC connector would be bad?


A direct connection is fine. A normal scope channel input is 1Mohm impedance which becomes 10Mohm with a x10 probe, i.e. it is an attenuator.
The caps are a compensation network so that squarewares look right at the top of the frequency range. Better scopes have a calibration output, usually a 1kHz squarewave, the probe is attached to it and trimmed so the corners are neither rounded or overshooting. This is only effecting high harmonics well above the audio range.

Quote:
As opposed to connecting the probe ground directly to the circuit ground/0V, or a short wire coming out of ground on a breadboard?


No, that is what I meant. Just give yourself something decent to grip onto like a loop of heavy gauge wire. If the probe is ungrounded you cannot be sure that the display is showing you what is actually happening, you may get more noise and ringing on transients that are caused by the probe not the circuit.

If the ground clip slips off beware of it dangling and shorting your circuit.
On Tek probes the ground clip usually pulls off the probe body and you may get different lengths in the kit. The hook hood pulls off too revealing a point probe, but the bare ring around that is ground so be careful about shorting against what you are probing. There may be a variety of accessories in the kit that clip on to that ground: a plastic shield, a short ground spike and a barrel that converts the tip into a BNC plug so that it can probe a BNC socket with full screening.


diophantine
kindredlost wrote:
Since you have a pretty nice scope why not get the switchable probes anyway? You got a scope like I was wanting but never sprung for. Nice grab! thumbs up

Thanks! Super happy I scored it.
There's no Tektroniks 150MHz probes that are switchable, but I suppose I could go with a higher bandwidth probe. This list is slightly dizzying, haha:
http://www2.tek.com/cmswpt/psdetails.lotr?ct=PS&cs=psu&ci=13528&lc=EN

kindredlost wrote:
I think he means because the ground wire of the probe is a short length you may need 0v points close to the component under test.

Big G wrote:
No, that is what I meant. Just give yourself something decent to grip onto like a loop of heavy gauge wire.

Ok, perfect! Thanks for the clarification. Just wanted to make sure I wasn't missing something.

Big G wrote:
A direct connection is fine. A normal scope channel input is 1Mohm impedance which becomes 10Mohm with a x10 probe, i.e. it is an attenuator.
The caps are a compensation network so that squarewares look right at the top of the frequency range. Better scopes have a calibration output, usually a 1kHz squarewave, the probe is attached to it and trimmed so the corners are neither rounded or overshooting. This is only effecting high harmonics well above the audio range.

Excellent, that's great to know.

Thanks guys!
j9k
probe master in el cajon. good stuff i bought a set for my 2465

http://www.probemaster.com/index.php

Probe Master Inc.
215 Denny Way
El Cajon, CA. 92020

Phone (800) 772-1519
Fax (800) 854-1519

International Phone (619) 258-7412
International Fax (619) 258-7413
diophantine
Ah, cool - didn't realize that Probe Master is basically just in my backyard.
Though, seems their probes are significantly higher than the Tek ones!
I'll take a closer look at 'em, though.
daverj
diophantine wrote:
Though, seems their probes are significantly higher than the Tek ones!


????

Only if you are looking at used or counterfeit Tektronix probes. The cheapest Tektronix 1x/10x probe has a list price of $113 while the cheapest Probe Master one is $41. The absolute cheapest Tek probe is $63, while the Probe Master equivalent is $38

http://www.probemaster.com/product_info.php?cPath=1_8&products_id=34
diophantine
I'd been looking on eBay at used ones (though they look unused; full kits) that are for the model that I have. They were around $45.

Looking at ProbeMaster now, the prices are better than I thought - was only looking on my phone yesterday, and the pricing tables were jacked up in that browser.
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