Automatic Probe Attenuation Setting, just a resistor? (tektronix tds320) - Page 1
Forum section: electronics / testgear

author: dteck
author_rank: Contributor
timestamp: July 21, 2014, 03:46:12 pm
content: Hi everyone I just picked up my first scope a used TDS 320 off ebay.I'm still learning my way around it but I had a question about the probe attenuation. It recommends the p6109b which can go for almost as much as I paid for the scope itself. These probes have a pin that contacts a ring around the BNC on the scope that tells it the probe is a 10x. I picked up one of those probes and it works great (minus the missing grabber) but I also picked up some cheap 100mhz Chinese probes. The cheap ones obviously don't have the pin contact for automatic attenuation setting. But I pulled out a meter and got 11k ohms of resistance from the pin to ground. And measuring the connector ring on the scope I got 5 volts.My assumption is that there is just a resistor in series with the pin going to ground. But I wanted to ask if there is more to this circuit than I think. I did do a test where I put a resistor across the ground and the attenuation ring and it did set the automatic attenuation level on the scope. but being relatively new to this hobby and completely new to scopes I thought I would ask people more experienced than I if there is some important thing I'm missing.Thanks.

author: David Hess
author_rank: Super Contributor
timestamp: July 21, 2014, 05:11:32 pm
content: It is just a resistor going to the BNC ground. Offhand I do not have the table showing which resistances indicate which attenuation factors but the list is floating around out there.Texas makes some inexpensive probes which support the readout function and work reasonably well:http://www.ebay.com/itm/One-new-250MHZ-Oscilloscope-clip-probe-w-readout-pin-/280107318452?pt=LH_DefaultDomain_0&hash=item4137b27cb4

author: alex.forencich
author_rank: Frequent Contributor
timestamp: July 21, 2014, 05:16:01 pm
content: Yep, that pin is just a resistor. The are tables out there of what resistor values correspond to what probes. Active probes can have an I2C interfaces on them, but they will have a lot more than 1 extra pin. Not a whole lot you can do with one pin. Also, you can usually override the attenuation setting in the scope if your probe does not have an ID pin.

author: dteck
author_rank: Contributor
timestamp: July 21, 2014, 08:13:50 pm
content: Quote from: David Hess on July 21, 2014, 05:11:32 pmQuote from: alex.forencich on July 21, 2014, 05:16:01 pmawesome thanks guys. I was thinking of just 3d printing up a plastic jacket like the ones on the p6109b and having it hold a resistor against the connector and the ring.

author: dteck
author_rank: Contributor
timestamp: July 22, 2014, 03:45:57 pm
content: So Just a quick update I did print out some adapters for the cheap probes and they work great.Its just a plastic ring with a cavity for holding a through hole resistor. It then uses a friction fit to hold one lead of the resistor against the metal grip on the BNC, also keeping the plastic jacket in place.Then there is an opening for the second lead of the resistor to come out and contact the ring.I put the 3d model (STL) and the sketchup file (SKP) up on the thingiversehttp://www.thingiverse.com/thing:403067Ill also add them to my post here in case any one wants them.

author: babysitter
author_rank: Frequent Contributor
timestamp: July 22, 2014, 04:52:07 pm
content: Great solution, one of the best 3d printer usages I have seen yet.(Might a spring-loaded pogo stick also fit?)

author: dteck
author_rank: Contributor
timestamp: July 22, 2014, 05:10:02 pm
content: Quote from: babysitter on July 22, 2014, 04:52:07 pmThanksI don't see why a pogo wouldn't work. They were actually my first thought since that would be closer to the actual readout ends. but I don't have any pogos on hand. I also thought about using an SMD resistor so you wouldn't need the bulge but again its what I had on hand.Plus this keeps the barrier as low as possible. Just a 3d printer and a 1/4w resistor.

author: George_Race
author_rank: Regular Contributor
timestamp: August 03, 2014, 07:18:06 pm
content: dtek, what a great idea!Any chance that you are "printing" and selling these probe adaptor rings?I would sure like at least 4 of them, just the ring with out the resistors.George

author: G7PSK
author_rank: Super Contributor
timestamp: August 03, 2014, 09:35:01 pm
content: I Did much the same for my scope, I used smd resistors and pogo pins insulated with shrink sleeve. The 100 X probe is just a dead short the 10 X is 10 K and 1 X is open

author: George_Race
author_rank: Regular Contributor
timestamp: August 03, 2014, 11:13:34 pm
content: G7PSK:Never heard of Pogo Pins before this.I see a lot of them on eBay.What length do I buy to do what you did with yours?Can you give me a bit more detail on how the resistor is held and connected inside of the shrink tubing?Thanks,George

author: G7PSK
author_rank: Super Contributor
timestamp: August 04, 2014, 09:00:11 am
content: George. I used 16mm pogo pins, the body was insulated with shrink sleeve leaving the pin that pushes in free, I then cut 2 notches in the sleeve at the top end and soldered two 20K smd resistors (0805 is what I had). The BNC had to have a grove cut into it in order to bring the pogo pin within the annular ring on my scope (Iwatsu) the ring dia may vary with scopes this is something I don't know. I then glued the pogo pins in place using epoxy once set I soldered the free ends of the resistors to the BNC outer. The reason I used 2 resistors is I found that they supported the end of the pogo pins better, I then put a bit more epoxy on the pins the heat from soldering tends to weaken the epoxy or at least the 5 minute variety that I used after that I put a piece of shrink sleeve over the whole lot. On the switched 10X probes I put a dab of super glue on the switch before switching to the 10 X so that there would be no chance of accidental switching. I could not think of any easy way to make it so the resitors were switched in or out according to the probe setting.

author: George_Race
author_rank: Regular Contributor
timestamp: August 04, 2014, 06:03:09 pm
content: Thanks, that explanation really helps.George - WB8BGY

author: calin
author_rank: Regular Contributor
timestamp: August 05, 2014, 04:16:06 am
content: On another thread we were talking about some UNI-T 300Mhz probes ... (this onehttps://www.eevblog.com/forum/reviews/cheap-china-p6xxx-probes-good-bad/) and while looking around I found a nice table that some guy did and measured actually the resistances you need.That thread with the tables here :http://www.electronics-related.com/sci.electronics.design/thread/116046/readout-pin-on-oscilloscope-probes.php

author: dteck
author_rank: Contributor
timestamp: August 12, 2014, 12:09:52 am
content: Quote from: George_Race on August 03, 2014, 07:18:06 pmHey sorry for the late response I'm moving soon so things have been a bit crazy around here. But I could probably print a few of these if your still interested.Maybe do $2 USD a piece plus whatever shipping to you would be.

author: George_Race
author_rank: Regular Contributor
timestamp: August 16, 2014, 01:26:46 am
content: Found on eBay, a place in Illinois that has HP 250 MHz P6106A probes for $10 each. They are used but just like brand new.Got a couple so for now I am all set.Thanks,George

author: electric$
author_rank: Newbie
timestamp: September 20, 2014, 11:29:40 pm
content: Hello dtek, I have just joined the eev blog in search of info on this very scope. I am totally new to the electronics world and have just purchased a TDS320 from Ebay as well. Hope to see it delivered by Tuesday or so. Anyway, I was wondering if you were satisfied with your purchase/choice of scope and if you had put up any videos of it in use?I really like the adapter collars you designed and made via a 3d printer, awesome use of todays technology to say the least.I hope to make alot of new friends in this forum and gain some knowledge in the field to help pass time in my new retirement status. All help appreciated greatly.

author: tggzzz
author_rank: Super Contributor
timestamp: September 21, 2014, 08:53:11 am
content: Quote from: dteck on July 22, 2014, 03:45:57 pmGood job, and a neat way of avoiding having to mentally shift the decimal point!Now I suggest doing something similar for theotherend of the probe - so that you can get amuch better trace on your scopewith high-speed digital signals. That's vital if you are interested in the signal integrity of "digital" signals and/or want to see glitches and relative phases. ("digital" since all signals are analogue, even if they are interpreted as binary).Objective: get rid of the 150nH inductance in the 6" ground strap plus croc(k)-clip. With a 20pF scope tip capacitance, when hit with a sharp voltage transition (e.g. TTL/CMOS) you will see significant ringing at 70-80MHz, which will be clearly visible on a 100MHz scope as a 20% overshoot. See the end ofhttp://www.dfad.com.au/links/THE%20SECRET%20WORLD%20OF%20PROBES%20OCt09.pdffor examples.Usually probes come with a short wire spring that slips over the end of the ground sleeve and contacts a nearby ground plane - which is fine if there is an accessible ground at exactly the right distance from the point being probed. But professional high speed probes use a "bayonet", e.g. see the attached picture of an old HP probe. That example is mostly metal, but that isn't important - I have a 1.5GHz Z0 probe where the bayonet support is mostly plastic.Consider connecting the swivelling bayonet to the probe ground using thin phosphor bronze sheet bent into the right shape (sheet = lower inductance, phosphor bronze = springy). I find pogo-pins are too fragile for the bayonet: use a nail or maybe a sewing needle.I suspect UV-cured resin might be more rigid and finer resolution than typical 3D printer plastic, but I haven't tried it.

author: dteck
author_rank: Contributor
timestamp: September 22, 2014, 05:20:25 am
content: Quote from: electric$ on September 20, 2014, 11:29:40 pmWell you might get more info on the scope from more senior members I am also still fairly new to electronics and very new to scopes. This is my first scope and was just an ebay find.But so far I've been pretty happy with it. It helped me find out that a piezo driver circuit I was trying to build was not even close to hitting the 2.4mhz I needed.It has also been useful in making sure surface mount components are working correctly on boards I've made. (I wasn't sure I had the correct footprint for some 16mhz crystals)The “Autoset” button will be your best friend at first. It guesses at the best time divisions for you and its usually pretty good.Here is a copy of the user manualhttp://www10.ujaen.es/sites/default/files/users/ingele/manualtds310.pdfhere is a copy of the service manualhttp://www.mdc.umn.edu/mdc/files/electrical/Elec%20Manuals/Tektronix%20TDS%20320%20Two%20Channel%20Oscilloscope.pdf