EEVblog #208 – Tektronix 2225 Oscilloscope Teardown and Calibration - Page 1
Forum section: electronics / general

author: Lightages
author_rank: Unknown
timestamp: October 20, 2011, 08:00:12 pm
content: Dave:As you know I enjoy your work and have benefited from the knowledge on these forums. The video blog entry on the Tek 2225 adjustments has a small error. When you are adjusting the vertical and/or horizontal gain, you should not use the edge of the displayed trace as the reference, but rather the middle. The trace can vary in width depending on the quality of the scope and the quality of focus that can be obtained.When making adjustments or measuring values from the trace on the screen it can actually be useful to de-focus the trace sometimes so that you can see the reticule splitting the trace more exactly.I think I know what happened during these adjustments. You were talking through the whole procedure and could not have had your tongue at the correct angle.That scope is in very nice condition.

author: Chet T16
author_rank: Frequent Contributor
timestamp: December 07, 2011, 05:11:56 pm
content: When calibrating the scope why is the signal fed in with a 50ohm terminator?

author: wkb
author_rank: Super Contributor
timestamp: December 07, 2011, 06:28:16 pm
content: Quote from: Chet T16 on December 07, 2011, 05:11:56 pmProbably the signal generator used has an 50ohm matched output. Termination of the 50ohm cable used with thecharacteristic impedance ensures the signal is not distorted.

author: Unknown
author_rank: Unknown
timestamp: December 07, 2011, 06:37:46 pm
content: Exactly. Pulse generators usually require a terminated transmission line for best fidelity, since reflections will distort the pulse. In the case of sine waves, you might get standing waves in an unterminated transmission line, the result is that the amplitude at the scope input depends on the cable length and wave length.

author: don.r
author_rank: Frequent Contributor
timestamp: December 07, 2011, 07:00:58 pm
content: Just like to say to Dave that my 2225 has the same bodge resistor on the back (but mine seems to be done much more professionally). The 45 degree trim "pot" (its a cap actually) under the tube is completely vertical on mine so maybe yours got tilted by an over-enthusiastic calibrator. Mine was last calibrated in 1996 but still appears to be within spec!

author: Chet T16
author_rank: Frequent Contributor
timestamp: December 07, 2011, 07:42:08 pm
content: Quote from: alm on December 07, 2011, 06:37:46 pmOk, thats understandable but does that not mean you would need to match the impedance of any signal you were trying to display on the scope?

author: don.r
author_rank: Frequent Contributor
timestamp: December 07, 2011, 08:06:48 pm
content: Quote from: Chet T16 on December 07, 2011, 07:42:08 pmNormally, you are tapping into a circuit which is impedance matched already so its not an issue.

author: Chet T16
author_rank: Frequent Contributor
timestamp: December 07, 2011, 09:01:08 pm
content: OK, thanks!Just opened up my 2225 and i have the same resistorMy scope is made in the UK too, or at least the PCB says it under where Daves has a sticker. Also, mine has no cap on the CA3102 and both of them are socketed

author: Unknown
author_rank: Unknown
timestamp: December 07, 2011, 11:39:51 pm
content: Actually reflections from scope inputs are an issue for the front-end designers (no easy transmission line solution at 1 Mohm), and it's one of the reasons why no-one (?) made a 1 Mohm front-end beyond 500 MHz (the other being capacitive loading). It's why bandwidth tests are performed with as much attenuation as possible between the signal generator and the scope, since this attenuates the reflections from the scope. I wouldn't expect this to be much of an issue at 100MHz though.