SOLVED - TekTronix TDS2024 / Square Wave problems - Page 1
Forum section: electronics / testgear

author: uknerd
author_rank: Contributor
timestamp: January 03, 2015, 11:00:55 pm
content: Heya AllHappy new year.Long time viewer, first time poster on forumDAVE - Thankyou for all your hard workI have a bit of a problem I think its prob attenuation on probe cables.But......I am currently building a z80 single board computer from scratchI have built multiple clocksclock one - 4093 (as per z80 nop tester circuit) with a 62 and a 68K resistor - works fine as square waveclock two - 4mhz 2 pin crystal on 74ls00 - the circuit shows the right mhz, but the signal is similar to the wave shape shown in daves rigol firmware fix videoclock three 4mhz 4 pin oscilator - exactly the same as clock 2Can i ask (as a relative newbie into electronics) what i could be doing wrong to get the signal to show wronglymany thanksNerd

author: tautech
author_rank: Super Contributor
timestamp: January 03, 2015, 11:33:01 pm
content: Welcome to the forum.2 possibilities that amount to the same thing.1x probe?All connections for measurements affect the DUT in some manner.The trick is to be aware of this and when a truer waveform is required use different methods as to not load the DUT's signal.It is to do with probe tip capacitance mainly.There are some useful pdf's on the Tek site that you should download.

author: uknerd
author_rank: Contributor
timestamp: January 04, 2015, 01:46:20 am
content: thankyou for the warm welcomeI have to say the result is the same if the probe is on 1 or 10xthe probe markings are: -P6100 100Mhz x10/x1 600/pkcat 1when i measure against the prob comp tags on the front of the unit it works finep.s. dave i saw in one of your videos #187 that its not worthy of a review - booooooo! hehehehee

author: Howardlong
author_rank: Super Contributor
timestamp: January 04, 2015, 02:26:55 am
content: FWIW I've had a TDS2024B for ten years, and it was my only scope for most of those, and helped me develop a number of items, a few orbiting the planet, but also tens of thousands down here on mother earth. While it's far from the best scope these days, it has done me proud, and let me change career to the one I always wanted to do albeit about 30 years late, and for that reason it's been worth the £1,800 I paid for it all those years ago. But knowing what I know now I wouldn't buy another one!By the way what frequency is the 4093 running at when you do your test? Not sure what a z80 nop tester is, but I do know what a Z80 is, I've designed many things based on that device but not recently.As Tautech suggests, probe tip capacitance becomes far more prevalent than the 10Mohm resistive element, but I'd have thought at only 4MHz you should get a fairly reasonable square wave.Any chance of a screen shot or two? I know those TDS2024's are dog slow doing that!

author: uknerd
author_rank: Contributor
timestamp: January 04, 2015, 11:57:53 pm
content: i believe the 4093 is meant to run about 28Hz (yes Hz)Pics as requested

author: uknerd
author_rank: Contributor
timestamp: January 04, 2015, 11:59:58 pm
content: pic descriptionsfull size render - built in probe check mechanism single shot capturefull size render(1) - 4093255 - from teh mhz oscillator (not single shot)256 - probe into 4093254 - prob into oscillatormany thanksnerd

author: Andy Watson
author_rank: Super Contributor
timestamp: January 05, 2015, 12:15:06 am
content: What is the last trace showing? In your original post you claim to be looking at a 4MHz clock (?), are you trying to display this with the scope timebase set to 25milliseconds ?

author: uknerd
author_rank: Contributor
timestamp: January 05, 2015, 12:19:22 am
content: Hi AndySorry i really really am a newbie to electronicsi understand principles and am trying to self-teach scopes etc...The pattern remains the same regardless of my time base,The last trade is the oscillator (4mhz/)

author: uknerd
author_rank: Contributor
timestamp: January 05, 2015, 12:25:14 am
content: another playch1 - 500mVtime: 500us49.991Hzits meant to be 4Mhzsimilar pattern

author: Andy Watson
author_rank: Super Contributor
timestamp: January 05, 2015, 12:32:44 am
content: Quote from: uknerd on January 05, 2015, 12:19:22 amCrank-up the timebase to 0.5microseconds, or faster, to see something sensible on a 4MHz clock. (I don't know how fast the scope will go). A 4 MHz waveform will be changing every 125 nanoseconds. Once you've found the waveform you might like to experiment with the bandwidth limit - as it is set at the moment (20MHz), it will make a 4 MHz waveform look "rounded".

author: uknerd
author_rank: Contributor
timestamp: January 05, 2015, 12:38:55 am
content: ahso this could be a limit of my scopemy scope tells me its 200Mhz 2GBsMy time base options are: -50s25s10s5s2,5s1s500ms250ms100ms - looks spkey @ 49.873hz being read50ms - saw tooth @ 49.919hz being read25ms10ms - looks like a sine wave5ms2.5ms1ms500us250us100us50us25us10us5us2.5us1us500ns250ns100ns50ns25ns10ns5ns2.5ns1nsthey all read around the 49-50hz rangedc couplingbw limit 200mhz offcoarse volts/div1x probe - also as per physical probe)invert offthis is similar no matter which oscilator i usethankyounerd

author: uknerd
author_rank: Contributor
timestamp: January 05, 2015, 12:41:28 am
content: sorry im being caught out as a newbie and asking a dumb question

author: Andy Watson
author_rank: Super Contributor
timestamp: January 05, 2015, 12:58:34 am
content: Quote from: uknerd on January 05, 2015, 12:38:55 amprobably aliasingdon't worry, it'll be back to bite you when you least expect itSet the timebase to 100nS and then tweak the trigger level to get a stable waveform.

author: uknerd
author_rank: Contributor
timestamp: January 05, 2015, 01:10:21 am
content: had a quick playstill shows 50~ hz

author: Howardlong
author_rank: Super Contributor
timestamp: January 05, 2015, 02:18:01 am
content: Here are some steps for you, I've dusted off my TDS 2024B.Firstly, let's make sure the probe is good.o Plug in the probe to CH 1.o Set probe to 10Xo Clip the probe to the PROBE COMP (upper metal bit)o Press DEFAULT SETUPo After a few seconds, the scope will be set to 500us timebase and 1V/div.o You will likely see something but it might well not be triggered (ie, not steady)o Adjust the TRIGGER LEVEL up (clockwise) and you will see the yellow arrow at the right side of the display move upo The display should also miraculously become steady (if it wasn't already)o Adjust the CH1 VERTICAL POSITION (the smaller knob above the CH1 probe input) so the full waveform is on the screen.o Adjust the HORIZONTAL SEC/DIV (aka timebase) one notch clockwise to widen the waveformNote: the HORIZONTAL SEC/DIV setting is shown in the middle of the screen, second line from bottom in white, e.g. "M 250us"If you don't have nice, square corners, you need to adjust your probe's compensation, there's usually a little round hole either on the probe itself or on the BNC connector end. You'll need a trimmer tool to do it, or a very small screwdriver at a pinch, but take care because the trimmer is quite delicate and a metal screwdriver will have some affect too. Adjust for the squarest square wave you can, ie the smallest amount of overshoot or undershoot.Regarding the TRIGGER LEVEL, as long as the yellow arrow at the right hand side is between the upper and lower extremes of the input waveform, the scope should trigger and hopefully you'll get a nice steady waveform.If at any point you get lost in the scope, maybe one setting too many, just press DEFAULT SETUP and start again. It's your get out of jail free card.Now hook up your 4MHz oscillator. The screen should be a flicker of yellow (at least it does on mine). The speed of the 4MHz is too high for the current timebase. Rotate the HORIZONTAL SEC/DIV to "M 100ns". You should get a square wave, but most likely with some ringing. This ringing is largely due to the inductance of the leads to the probe in combination with the probe's capacitance.If you're still not getting anything, check the current to your circuit. The 5V canned oscillator I'm using here draws 1.7mA, but there will be some difference between different manufacturers and parts, only the datasheet for your particular device will tell you, but probably anything up to about 5 or 10mA is about right. Make sure there's 5V between pins 2 and 4. Make sure you have some decoupling on there, it's good practice. a 100nF directly across pins 2 and 4. In this scenario it probably won't make much difference, but get into the practice, it'll save you a lot of headache one day.One final thing, it is possible that your canned oscillator has an Enable or Output Enable pin usually on pin 1. The correct state of this will be on the datasheet, either to ground or +5V. (It is also common practice to use 10k resistors for static pullups, rather than tying them straight to +5V, largely a hangover from the TTL days of 20+ years ago).

author: Howardlong
author_rank: Super Contributor
timestamp: January 05, 2015, 02:48:36 am
content: By the way the 50Hz you're picking up is just what's in the ether from your mains wiring. Which makes me question whether sticking the probe tip into the breadboard is making any connection at all. Maybe use a short length of solid core wire, or a resistor lead, and clip to that instead?

author: Howardlong
author_rank: Super Contributor
timestamp: January 05, 2015, 03:43:24 am
content: One further thought looking at one of those pics, are you using a properly regulated power supply? The +5V should be rock solid on the scope (ie, flat). If you're not, the consequences are not always very nice...

author: uknerd
author_rank: Contributor
timestamp: January 05, 2015, 06:54:17 pm
content: I will give these all a try tonightI can confirm the psu is regulated and good on the scopeUsing the probe check all looks goodThankyou for all the helpSent from my iPhone using Tapatalk

author: uknerd
author_rank: Contributor
timestamp: January 05, 2015, 11:42:30 pm
content: Well what can i sayTHANKYOUi owe you all a pintturned out was the trimpot on the probeThankyou once againI can now read 1.84 and 4mhz crystals correctlyThankyou once again for everyone's help