Tektronix 7000 series plugin question - 7A19 50 ohms

[Chris Wilson]

I recently got a bit of a sad Tek 7854 400 MHz analogue scope to tidy up. I have it working OK, and was using 1 M Ohm amplifier plug ins, that ran to 225 MHz. I found a 7A19 600 MHz plug in for it and tried it this morning. It works, but using the 40 mV calibration output of the scope into the 7A19 50 ohm input I see incorrect amplitude levels, and set to AC the square wave is not displayed as square. Pictures speak a thousand words, so I have zipped up some screen shots and put them at http://www.gatesgarth.com/7A19.zip  Two shots show the plug in set to DC with the waveform shown, and two show it set to AC and the resultant waveform. I have never used a 50 ohm input impedance plug in before, I guess it's really designed for RF?

I am unsure if this is due to 7A19 having a 50 ohm input, and is normal, or if the plug in is faulty? I am hoping Alan might see this, sounds his area of expertise

Thanks.

[vk6zgo]

Go back to the 1Mohm plugin,tie a 50 Ohm resistor across the input,& see if the CAL signal is now the same level as with the 7A19.

[tekfan]

Wow, nice score!

The output resistance of the calibrator is 450 ohms so when connected to a plugin with 50 ohm input resistance you form a 10:1 divider. So if you set the calibrator to 4V then you should get a square wave with an amplitude of 0.4V and so on.

Yes it is normal for the square wave to look like that when using AC coupling. The manual for the 7A19 specifies the lowest AC coupled frequency to be 1kHz, so you're right on the edge of its ratings. AC coupling basically adds a 2.2uF (for the 7A19) capacitor inline with the 50ohm input. This creates a high pass filter that limits the lower frequency bandwidth. Since the plugin is designed for measuring HF you normally don't care if you don't see below 1kHz.

You'll get the same looking square wave if you connect a 10Hz square wave to the input of your 7A26 and use AC coupling. You can see much lower frequency signals because the resistance after the AC coupling capacitor is 1Mohm.

[Chris Wilson]

Thanks for the help, I thought it would be the impedance thing, it reads the same with a 50 ohm resistor across the calibrator output with a 1 M ohm plugin! It reads spot on allowing for the division with the 7A19 plugin, see below. Much appreciated.  She's a grand old lady, must have been a hell of a beast in its day. Here she is showing a 525 MHz trace, not bad for something that looked like a health hazard 3 days ago, and is actually only rated to 400 MHz

[KTP]

Nice scope.  I have a 7104 with 7a29 plugins.  Psi helped me get it going again with the suggestion of airing it out 

Your scope has a lot more interesting buttons up top.  It looks similar to the 7104 below the display though.

Note that you can probably display a 1Ghz waveform on your scope if the input signal has enough amplitude, but it won't be WYSIWYG...at least on the vertical scale.

[Chris Wilson]

What's a sensible amplitude level to see how high it will really trigger and give a meaningful waveform? Cheers.

[KTP]

What's a sensible amplitude level to see how high it will really trigger and give a meaningful waveform? Cheers.

I don't really know what you mean.  I would guess it would trigger well into the Ghz range if your input signal has enough amplitude.  You can get some time meaning (frequency measurements) for pure sine waves well past your 400mhz bandwidth, but you may only get "meaningful" measurements of things like square waves at well below 400mhz (say, 80mhz or so).

In another thread, I mentioned building a simple fast rising edge pulse generator out of a tinylogic buffer that gives you a 4V to 5V 750ps pulse (or faster) into 50 ohms.  You could use something like this to check out the rise time of your 7A19.

The thread is here: https://www.eevblog.com/forum/product-reviews-photos-and-discussion/40mhz-square-wave-on-rigol-ds1052e-agilent-msox3024a-tek-7104/

Note I have since modified the circuit with 50 ohms of resistance in series with the source, to try and get a better waveform, and the measured pulse is now 600ps, 2.5V.

[w2aew]

Chris - looks like you got your answer.  Great scope.  I wonder if you got the waveform calculator with it?  Back in the day when that scope was new, Tektronix offered a one-day course to learn all of the ins-and-outs of the storage, measurement and waveform computational features of that scope.  Somewhere, I have my certificate from that class, probably from the mid-late 1980s.

[Chris Wilson]

Alan: I see the keyboard thing on Ebay a lot, but they are all in the US. I am not sure how useful one might be, as I am not entirely sure exactly what they do! Does it have to be used with the GPIB port, or is it "self contained"? I also got a 2901 time / mark generator from the same guy, which now looks nice as well as seemingly works well, after a fettling. I suspect i could use that to see how far it will give meaningful measurements? What else would i need to check analogue scope calibration at home Alan?

KTP : Thanks for the link to the posts, will check them out tomorrow, have my VAT (UK tax) return to do on line and pay, tonight. Seems I am working more for the government than myself right now

Thanks KTP, and Alan

[w2aew]

Alan: I see the keyboard thing on Ebay a lot, but they are all in the US. I am not sure how useful one might be, as I am not entirely sure exactly what they do! Does it have to be used with the GPIB port, or is it "self contained"? I also got a 2901 time / mark generator from the same guy, which now looks nice as well as seemingly works well, after a fettling. I suspect i could use that to see how far it will give meaningful measurements? What else would i need to check analogue scope calibration at home Alan?

Good deal on the time-mark generator.  For the most part, you can perform most basic checks on an analog scope with a time-mark generator and a good leveled sinewave generator.  For me, I'm not too picky about it with the analog scopes.  They're rarely off by more than a few %, so I don't worry much about it.  If they're off by more than that, it usually indicates a problem, not a mis-cal. 

The waveform calculator duplicates the buttons/functions that are found to the right of the CRT, and adds a couple dozen more functions - most of which you probably don't need anyway ;-).  If I recall correctly (since its been 25yrs or so), it plugged directly into the back of the scope somewhere.

[KTP]

Is a 7s11 worth getting for either the OP or my 7000 series?  Are they prone to having bad sampling head modules?

Nevermind, you need a 7T11a also.  Kind of ridiculous that this is $500 on ebay (combined) when there are several auctions for 7000 series complete mainframes with this items installed for similar money lol.

I have very little need for 14Ghz anyway 

[Chris Wilson]

Naive question again, (well, it is the beginners section....). Would my HP 3325B synthesized function generator create suitable sine waves for `scope calibration checking?

I have screen shots of what I imagine are the salient bits of the spec below. Ta!

[w2aew]

Naive question again, (well, it is the beginners section....). Would my HP 3325B synthesized function generator create suitable sine waves for `scope calibration checking?

I have screen shots of what I imagine are the salient bits of the spec below. Ta!

Well, nearly "anything" can be used to do a basic check, PROVIDED you trust or verify the test signal.  But, this is not calibration, just a rough check.  It's a good idea to be able to cross-check with another instrument.  For example, a 50% duty cycle low frequency square wave should measure 1/2Vpp on a decent AC voltmeter (provided you're well within its frequency range) - then you can use this to check the scope amplitude settings.  Be sure to use consistent load impedances when making checks like this.