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 Post subject: Oscilloscope Input RC Normalizing
PostPosted: Jun Tue 01, 2010 5:02 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 3550
Location: Harviell MO USA 63945 (12 miles S of Poplar Bluff)
Part of the calibration procedure for all high-impedance attenuators in oscilloscopes includes "input RC normalizing". This is a process where you adjust the various attenuator sections of each vertical channel so that the input always presents the same input capacitance to the circuit under test. This process allows you to compensate your attenuator probe on one setting of the attenuator and have the compensation be good for all settings.

This procedure can be done with an X10 attenuator probe, but with 10X attenuation, that presents problems on the higher attenuator settings since you'd have to have a 1KHz square wave with an ungodly amplitude in order to see it on the screen. Instead, Tektronix has this series of little "Input RC Normalizers" which are nothing but a 1M ohm resistor in parallel with either a variable capacitor or a fixed and a variable capacitor where the cap combination can be adjusted to match the input capacitance of the scope. There's a hole in the side (and a Delrin plasic cover over it originally) for adjusting the normalizer. This normalizer has an effective 2X attenuation ratio.

In the past, there have been some anally-retentive Tek Service Centers who would send their RC normalizers to Beaverton to have them adjusted to exactly the input capacitance marked on them. Other Service Centers, if they had one of the LC-30 cap meters in the house, would adjust them themselves.

The whole idea would then to be able to adjust every scope with a 20pF input capacitance to exactly 20pF. A company might have five scopes all of which had 20pF inputs, Tek (or the company's own internal cal lab) could adjust them all for 20pF and technicians would never have to worry about ever compensating an attenuator probe after its initial compensation adjustment.

There're at least three problems with this. (1) After a tech is used to having to never check probe compensation, he moves the probe over to a scope (maybe a new one) that happens to have 15pF, 22pF or 30pF input capacitance and starts using an uncompensated probe or (2) techs start to get lazy and unthinking and a probe somehow gets knocked out of compensation (easy to do with an older P6006 or P6007) and the unassuming tech never checks the compensation or (3) there are scopes that just won't adjust down (or up) to their marked input capacitance because the adjust range is too small.

If you look at most of the Tektronix scopes, especially ones made since around 1975, the "wide open" attenuator setting where your're working straight into the full sensitivity of the preamp with no attenuator switched in, has a teeny, tiny adjustment capacitor, usually one with a thin brass screw going in and out of a Teflon insert. This adjustment doesn't have much over 1 or 2pF of adjustment range, if that, so you can't expect to "pull" the capacitance very far. If the cal tech's RC Normalizer was calibrated and is off by a pF or two, he may never be able to adjust that scope to match the normalizer's setting.

The proper way to use an RC Normalizer is to remove that little plastic protective cover and throw it away. Connect it to the input of the scope being calibrated with the scope set at it's "wide open" setting (usually 5mV/DIV) with a 1KHz square wave connected to the Normalizer and adjust the normalizer for a properly-compensated square wave. Move it down and check the other channel(s) and adjust that teensy tiny cap if necessary. On some models, you may find that it seems to have no affect at all! You may have to go back and forth between/among the channels at their wide-open settings, adjusting the teensy tiny caps and/or the normalizer until all channels look the same and are compensated. THEN go back and adjust each channel as described in the cal procedure for full attenuator compensation, never again touching the normalizer adjustment until the next scope comes along.

This way, you at least try to make sure that all the vertical preamp inputs on the scope have the same RC characteristics, but not the inputs for the entire fleet of scopes.

The addition of an adaptor to take a UHF input connector to a BNC scope probe connector adds to the capacitance. You may have to use an RC Normalizer with a higher capacitance, but maybe not. The one you're using may adjust to what you need just fine.

You can make your own normalizer using a 1% 1M ohm resistor and a ceramic variable capacitor in parallel inside a small aluminum case. A cast aluminum case with the ends drilled and tapped for 3/8-32 to accept BNC male and female connectors would be best, of course. I've made one using a variable cap that would adjust from 9pF to 47pF, which will cover almost anything you will run across. If you can't find an aluminum box if you're in the U.K. or Australia, you could always use one made from aluminium instead. :)

The bottom line is, don't treat an Input RC Normalizer as some kind of RC Standard. It's a NORMALIZER, not a standard, and as such, it's an adjustment tool.

_________________
Dean, Electronics Curmudgeon
Contributing editor emeritus in Poptronics magazine, R.I.P.


Last edited by Dean Huster on Jun Wed 02, 2010 3:03 am, edited 4 times in total.

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 Post subject:
PostPosted: Jun Tue 01, 2010 5:05 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 1974
Location: Hinsdale, IL, USA
Dean,

This is great info. Recently I needed to calibrate a 1A7A (in a 547) and didn't know what an "RC standard or normalizer" was. Your info confirmed my assumptions.

I ended up dedicating a pair of probes to that plug-in for differential mode operation. I aligned the 1A7A input the best I could to minimize the difference of the two probes. Now I can get decent CM rejection.

Hopefully your information will help others understand these "RC normalizers"

Thanks

_________________
Chuck D. KB9UMF


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 Post subject:
PostPosted: Jun Tue 01, 2010 9:11 pm 
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Joined: Feb Thu 08, 2007 12:36 am
Posts: 2139
Location: Hillsboro, Oregon
Thanks, Dean.

I did all my BNC-input 20pF plugins the same. The 1A4 dictated the normalizer setting due to its small range. The CA procedure says use a hard extender, but I found that for best results I had to do it in the instrument. You can do all except the last couple B ranges, where a mechanical part gets in the way.

I discovered that the B channel of UHF-equipped CA's could not reach this normalizer setting, even without a BNC adaptor, so I had to do them with a larger setting. I made all my UHF-input 20pF plugins the same.

The 1A1 at 15pF and the Z at 24 are their own little worlds. Early 1A1's with the concentric rotary input switches are supposed to be 12pF but I'll believe that when I see it. In the 1A1's 21-jewel attenuator, lead and component dress is critical.

Last come the 47's. I'll try the 1A7 as a reference because it's already set toward the low end of its range. The others are early types with the large ceramic trimmers - they can adjust to nearly anything.

Dave Wise


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 Post subject: Re: Oscilloscope Input RC Normalizing
PostPosted: May Sun 22, 2016 2:37 am 
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Joined: May Sun 22, 2016 2:27 am
Posts: 1
Nearly six years later...
I registered just to thank Mr. Huster for his post.

Dean - thanks for the education!


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