An exasperated engineer struggles to troubleshoot a scope with a funky frequency response
Several years ago I had a calibration lab perform
cal on a 60 MHz Tektronix scope that I had bought at the TRW swap meet. Shortly after calibration, I noticed that one
of the two channels had a low frequency rolloff at a much higher
the other channel. This anomaly was only
noticeable in AC coupling mode, of course. Further investigation determined that the rolloff was
identical on both channels except when the vertical scale on the
channel was set to 0.1, 0.2, or 0.5 volts per division, a major clue.
So I went back to the swapmeet and bought a manual
scope. In the schematic, I found that a
certain attenuator circuit was switched in only for to 0.1, 0.2, or 0.5 volts
per division. I
studied the attenuator circuit and noticed
that there were some capacitors in parallel with carbon composition or
wirewound resistors in voltage divider circuits. Also
in the circuit was a shunt trimmer
capacitor right next to the AC coupling capacitor.
Well, I tried to determine the transfer function
attenuator as a function of frequency, but I quickly got lost in the
algebra! Too many j?C’s and whatnot. I thought, “That trimmer cap must be
interacting with that AC coupling cap.” So I gritted my teeth and gingerly tweaked its capacitance with
trimmer tool while sweeping the frequency going into the scope with a
generator. I just barely touched it, and
like magic, the frequency response of the scope dropped down to where
The moral of the story is that with some decent
troubleshooting skills (and some spare time that it seems like I no
have), some intestinal fortitude, and a bit of luck, some pretty
problems (alliteration, anyone?) can be solved. Oh, and I never did use that calibration lab again!
Jim Ford is officially Senior Staff Engineer,
Broadcom Corporation. In reality, his
work involves design and layout of test circuit boards for wireless
Bluetooth, WLAN, GPS, cellular, and/or FM radios in them. His hobbies are electronics and playing bass,
keyboards, and guitar (not simultaneously!).