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1st Mar 2015, 2:26 pm | #1 |
Octode
Join Date: Jan 2012
Location: Wimbledon, London, UK.
Posts: 1,464
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'Scope Probes
Hi,
I had the luck to acquire a couple of Tektronix 'scopes recently, one being a 457A with 250 MHz bandwidth. I thought that I might try to source a couple of probes that would be compatible with this bandwidth, rather than the standard, cheapish type that go up to 60 MHz. Looking through the various manuals in my possession, I found (more or less) what models of probes were supplied with several of the 400 series of Tektronix 'scopes. When I then tried looking for sources of these probes, they were not only pretty rare, but also eye-wateringly expensive. A nicety on the 400 series of 'scopes is the probe scale-factor switching arrangement, where either of two back-lights can illuminate the "volts/div" switches on the vertical channels. These are chosen by shorting (or not) an extra ring-contact on the input BNC to ground via a low resistance. This works with switchable (x10/x1) probes and fixed x10 probes that Tektronix produce for these 'scopes. The situation that I found is twofold: Tektronix probes with the scale-factor switching are rare and expensive. There are probes from the Far East which claim 100 MHz bandwidth which are not expensive, but don't have the scale-factor switching. While my interests are mainly with the audio spectrum, a 'scope with a wide bandwidth is a nice thing to have (parasitic oscillation at RF can and does happen in audio circuits). Apart from the obvious explanations of quality and of the kudos which goes with the Tektronix brand (rightfully so, in my opinion), why are these probes so disparate in cost? There is, I suppose, the possibility of the bandwidth of the Far Eastern probes being more imagined than true, which would not be the case with Tektronix-made equipment. Any thoughts? Colin. |
1st Mar 2015, 3:14 pm | #2 |
Octode
Join Date: Nov 2005
Location: Ayrshire, UK.
Posts: 1,096
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Re: 'Scope Probes
Hi Colin
You are correct about the cost of 'proper' 'scope probes. At the University they had some 1Gig 'scopes and the cost of the probes was eye watering. Each probe represented a significant proportion of my annual salary. Tim
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1st Mar 2015, 3:33 pm | #3 |
Moderator
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Re: 'Scope Probes
Probes also tend to get broken, so that further reduces availability on the second-hand market, and thus the price would go up.
Look at the scope input BNC connector and nearby you should see the input resistance and capacitance printed. You need to find a scope probe which is specified as suitable for this impedance. Usually it's the capacitance compensation range that's the problem. On a wideband scope the input capacitance will be as low as they can make it, and most lower bandwidth probes have capacitance adjusters which don't go so low. Beware of switchable x1/x10 probes. They inevitably ruin the bandwidth on x10. x1 probes are needed so rarely that I buy only fixed x10 ones. Once you check the capacitance compensation range, you can then use HP probes or Tek probes, whichever you come across. Thre don't seem to be any other makers of high performance probes. David
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1st Mar 2015, 3:43 pm | #4 |
Octode
Join Date: Nov 2005
Location: Ayrshire, UK.
Posts: 1,096
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Re: 'Scope Probes
Could I also recommend the P6109 probe. Although these are 150MHz probes we used to use them with 2465 'scopes at work and never found them lacking. I can confirm that they do have the range switching 'pin' on the BNC connector.
There are a variety of them offered on eBay at various prices mostly from the USA. I'm sure they will be available from other auction sites as well. Also, have you tried asking the folk at Stewarts of Reading. They can be quite helpful but of course they are a business and prices reflect that. Hope this helps. I do agree that if you have a good 'scope you really want/need the proper probes to go with it. TimR
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1st Mar 2015, 3:59 pm | #5 | |
Octode
Join Date: Jan 2012
Location: Wimbledon, London, UK.
Posts: 1,464
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Re: 'Scope Probes
Quote:
I got my info from two sources; the Tektronix manuals and this website: http://www.barrytech.com/tektronix/p...tekprobes.html which is most informative. Colin. |
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1st Mar 2015, 4:01 pm | #6 |
Heptode
Join Date: Jul 2012
Location: Bromley, London, UK.
Posts: 702
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Re: 'Scope Probes
The blue-bodied Coline M12 or M20 x10 probes are rated to 250MHz and sometimes turn up cheaper than HP or Tek ones. They were also own-branded by several companies including RS. They did do a version with the 'pip' on the BNC plug to change the scale factor but these are much rarer. I use both a Coline M20 and a 300MHz Agilent N2863B probe with my Tek475 and both compensate nicely.
Mike |
1st Mar 2015, 4:47 pm | #7 |
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Re: 'Scope Probes
It looks like they may have sed similar attenuators across a family, so not all are as low a C as they could have done, but as Mike says, just find probes which will compensate the capacitance you have.
My GHz scopes are all 50 Ohm inputs and need active probes, and you can just imagine the prices of those! Stewarts and Telford Electronics may be your best bet in the UK, else it's a case of buying from the US. David
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1st Mar 2015, 5:11 pm | #8 |
Octode
Join Date: Nov 2005
Location: Ayrshire, UK.
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Re: 'Scope Probes
I've found this web site to quite informative:
http://www.reprise.com/host/tektroni...age_probes.asp It's part of a larger site by two ex-Tek' guys. Tim
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2nd Mar 2015, 1:01 am | #9 |
Nonode
Join Date: Dec 2012
Location: Kirk Michael, Isle of Man
Posts: 2,346
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Re: 'Scope Probes
I also have a 475A acquired thro this site. Not used it much yet, and I find that auto switching with their Xs 10 probes a bit of a pain. The switch contact seems poor, with the light swopping around as contact improves or fails. I may just use a pair of fixed Xs 10 HP probes I have instead. After 35 years of scope use, I am simply used to the automatic Xs 10 computation in my head, not exactly difficult.
It has quite a few little niggles I have yet to sort out, so I tend to use my HP1703A instead. I have an HP 1725A, and if I can fix that, it will replace the Tek. Problem is it MAY have a faulty tube, so I keep putting off looking into it. Trouble is I have so many working scopes around, there is no pressure to look into the 1725A. Les. |
2nd Mar 2015, 1:19 pm | #10 |
Nonode
Join Date: Jan 2009
Location: Bristol, UK.
Posts: 2,059
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Re: 'Scope Probes
I have both Tektronix (x10 and X100) and cheap RS probes (X10 and switched X1/X10). The only obvious difference, other than the scale switching, is a more ripple after a fast pulse with the cheap ones. For the cheap RS probes, the switched ones are a little worse that the un-switched ones but not hugely so.
The bandwidth is probably not that relevant, work out the effect of tip capacitance at, say, 100MHz and ask yourself what effect that will have on the circuit you are probing. I think its a waste of money buying anything over 100MHz for a X10 10M probe If I need to see the fidelity of a pulse, or I am looking at more than 100V, then I will use one of my tek probes other wise the cheap ones are fine. dc |
2nd Mar 2015, 2:12 pm | #11 |
Octode
Join Date: Jan 2012
Location: Wimbledon, London, UK.
Posts: 1,464
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Re: 'Scope Probes
I thought this might be an interesting topic; thanks to all who have contributed so far!
Maybe I'm being foolish, but where companies with good reputations, such as Tektronix are concerned, I tend to believe what they write in their manuals, etc. This leads me to think that if Tek charge £100 for a probe, then that's pretty much the proper price and anyone offering something with a stated similar specification, but a fraction of the price is probably being "economic with the truth". (I am thinking of the sub-£10 offerings from the Far East, here). It also seems to me that a probe which is obsolete can command a ridiculous price when it only has a handful of components, whereas a complete oscilloscope with hundreds, if not thousands of components can be had for a song once it has become obsolete, even though it would have cost thousands of pounds when new. I don't think that the possibility of easy breakage of probes, leading to limited availability, is the whole story. A further oddity is the enormous range of probes that have been made. Is it so difficult to design a probe that does the job well in all cases? (given the bandwidth variations). Various contributors have made suggestions that bandwidth is not such a big issue, but that beggars the question as to why wide-bandwidth probes were made and supplied with wide-bandwidth oscilloscopes in the first place. Another thing that bothers me is concerned with compensation. I was always under the impression that the leading edge of a square-wave was significant in terms of the higher frequencies (rise-time, anyone?), but some of the literature I have read on probe compensation suggests that it is low-frequency compensation, despite observing the leading-edge of the square-wave. Even the cheaper probes that I own (60MHz bandwidth) will optimise the square-wave leading edge, so I don't think "compensation" is the whole story, either. Colin. |
2nd Mar 2015, 2:26 pm | #12 |
Nonode
Join Date: Jan 2010
Location: Edinburgh, UK.
Posts: 2,008
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Re: 'Scope Probes
Here is a link to a DIY DC-1Ghz probe that may be of some interest.
http://emcesd.com/1ghzprob.htm Al
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2nd Mar 2015, 2:59 pm | #13 |
Moderator
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Re: 'Scope Probes
Ha! Doug Smith.
Doug was the editor of QEX for several years, and he was also the guy behind the 'Kachina' DSP-based transceiver. His articles on software defined radio in QEX (of course!) are worth tracking down and reading. Probes like that were available from Sealectro, and I'd love to rip bits off whoever nicked mine. Note that these probes are made for 50 Ohm coax and a scope/analyser with 50 Ohm . termination. Once you get to reasonably high frequencies the probe ground inductance becomes an issue. Very short ground spikes or even special probe sockets enter the fray. David
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2nd Mar 2015, 10:14 pm | #14 |
Dekatron
Join Date: Sep 2010
Location: Cheltenham, Gloucestershire, UK.
Posts: 3,077
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Re: 'Scope Probes
If you don't mind buying old/used probes then maybe consider the HP 10073A (B) (C) probe. I've got one of these here and I use it a lot because it is small and is just a x10 probe.
It also has the slip ring surround that makes it compatible with my old Tek 465scope in that it can light the correct scaling lamp for x10 operation. It is supposed to be a 500MHz probe but I can't really test this on my scopes as they are all <= 100MHz models. But I think you could buy one of these probes on ebay for a low price if you are patient. About ten years ago I bought a used Tek TDS 2012 and this came with a couple of Tek P2220 probes (200MHz?). These also have (had) the outer slip ring and I recall (but can't be sure) that it also worked with the Tek 465 lamps. They also have a x1 to x10 slide switch along the body of the probe that is annoyingly easy to switch by accident. eg by just picking it up you can alter this switch by accident. Also, the outer slip ring at the BNC connector isn't very securely fitted and I've removed it as it comes loose after a while. These probes feel quite cheap and flimsy compared to the HP 10073A but apart from the above issues I quite like using these probes as they are light and the probe hood/clip works well. I also have some old RS branded scope probes and some really old Tek probes but these usually stay in a drawer as 'goto' spares when I can't find one of the above probes.
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2nd Mar 2015, 10:26 pm | #15 |
Dekatron
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Re: 'Scope Probes
I found a link to the Hp 10073A scope probe datasheet and it doesn't show the slip ring.
So maybe mine is a special? Or does the 'probe readout' feature mentioned in the datasheet refer to this outer slip connector? The HP 10073A has the 'probe readout' feature but some other models on the datasheet do not... See below for the plastic surround and slip connector. http://www.keysight.com/upload/cmc_u...0070-92001.pdf
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2nd Mar 2015, 10:45 pm | #16 |
Dekatron
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Location: Staffordshire Moorlands, UK.
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Re: 'Scope Probes
It's horses for courses isn't it. I only use my scope for general fault-finding and alignment on tv and audio stuff, so a cheap probe is good enough. I'm through ordering expensive probes. You only have to trip over the cable, snap the tip off (where it cant be unscrewed) or get it caught on something and that's alot of money down the drain. Of course if you're recalibrating mobile phone transmitters or designing a life support system you might want to invest.
Chinese probes you directly import I'd steer clear of as there's no guarantee they're even safe to use on live equipment. I think my last probe came from Rapid.
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2nd Mar 2015, 10:57 pm | #17 | |
Dekatron
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Location: Cheltenham, Gloucestershire, UK.
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Re: 'Scope Probes
Quote:
I've also had experience of cheap probes becoming intermittent after a while and this can be very annoying so I tend to go for the branded ones as they are usually more robust. But I've never bought any of the branded probes as new. All of mine were either free with the (used) scope or were used ones bought cheaply at a radio rally. If good performance is required for a 200MHz scope then the HP10073A is a very nice scope probe if it can be found cheaply. It also has the slip ring (x10 scaling) connector to suit the old tek scopes like the 400 series. I had a look on ebay and they seem to fetch about £50 incl postage. I wouldn't want to pay this much for one... I can't remember what I paid for mine but it was probably about £15 at a rally many years ago.
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Regards, Jeremy G0HZU Last edited by G0HZU_JMR; 2nd Mar 2015 at 11:06 pm. |
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2nd Mar 2015, 11:27 pm | #18 |
Dekatron
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Location: Seaford, East Sussex, UK.
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Re: 'Scope Probes
From my experience the cheap probes from China work pretty well and do work to 100MHz. However, they are easily damaged whilst a 'proper' Tek probe will survive pretty rough treatment.
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3rd Mar 2015, 11:38 am | #19 |
Octode
Join Date: Jan 2012
Location: Wimbledon, London, UK.
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Re: 'Scope Probes
Yes Jeremy, I believe that the "probe readout" mentioned on the datasheet does indeed refer to the little peg which contacts the slip-ring on the 'scope input. Does anyone know of a HP 'scope that had this feature, or did HP make these probes with the feature simply to try to get some of Tek's customers?
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3rd Mar 2015, 4:30 pm | #20 |
Heptode
Join Date: Jul 2012
Location: Bromley, London, UK.
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Re: 'Scope Probes
I'm pretty sure that HP did the same as Tektronix with the x10 slip-ring switching method but don't know any specific models of oscilloscope. Radio Wrangler can answer this far better than I. IIRC, Tek went further with this method of probe switching on some 'scopes, where the resistance between ground and the BNC 'pip' was checked to identify what probe was connected and its attenuation. So the 'scope could detect, say, x1, x10 and x100 probes.
In response to one of the other points, my Coline M12 x10 probe paperwork does say that the adjuster in the probe body adjusts the L.F. response for best displayed square wave (leading edge) from the 'scope's calibrator. But there is another H.F. tweaker under the BNC box shroud for best pulse shape when driven from a fast rise time (<1ns) pulse generator. Mike |