How to calibrate TEK 465B 'scope?

[Eric R Snow]

Greetings All,
A couple years ago I installed the meter kit in my TEK 465B
oscilloscope. I have all the manuals for the 'scope but not anything
against which to test it. It seems to be OK but calibration would be
great so can anybody point me to a good place to buy the proper signal
generator? I'm just a rank amateur but the thing has beenh helpful. A
good example is how I tuned up my neighbor's generator after some hack
had messed up all the adjustments. I looked at the output from a 9
volt AC wall wart that I first plugged into the mains. I noted the
wave form (not a perfect sine wave, but bent over) and the frequency.
The frequency was correct for the scale I used so at least I know
that's working correctly. I then plugged the wall wart into his
generator and used both an optical tach and the 'scope to measure
frequency. Along with a DVM I adjusted the carb and governor so that
the generator put out 60 cycles and 120 volts with at least a 300 watt
load. Unloaded the frequency went up to about 66 cycles and the
voltage went up to about 130 volts. With a full load the frequency is
pretty steady and the voltage drops to 115 volts. He now loads it up
with a little space heater before he plugs in his fridge and some
lights. There was no way to keep the voltage above 105 volts and the
frequency above 52 Hz when the thing was loaded if the unloaded
frequency was 60 and the voltage 120.
ERS

 

[Bob Masta]

Greetings All,
A couple years ago I installed the meter kit in my TEK 465B
oscilloscope. I have all the manuals for the 'scope but not anything
against which to test it. It seems to be OK but calibration would be
great so can anybody point me to a good place to buy the proper signal
generator?

You can download my Daqarta software and use its free sound card
signal generator. (You don't have to buy the software: When the
30-day/30-session trial period expires, only the signal *input* stops
working. The signal generator keeps working, along with all the
spectrum and waveform analysis displays.)

Since sound cards have crystal-controlled sample rate clocks,
the signal generator accuracy and stability will be excellent.
However, note that the output is limited to frequencies (typically
20 kHz or less). That's fine for calibrating the slower sweep
ranges, but not much use for the fast end. With (say) a 10 kHz
square wave output, you'd have 100 usec per cycle, or 50 usec
per phase. With the scope on 5 usec/div, this will just fill a 10-div
trace. Still, it's a good start, and it's free.

Note that sound cards don't come with any sort of level
calibration. Daqarta can be calibrated for your card, and
has some features to help automate this. But somewhere
along the line you will need to provide a reference level to
tie into the calibration. This gets into a chicken-and-egg
problem unless you can borrow a known-good generator
to get you going.

Another sound card issue is that they don't pass DC,
so you can't (for example) just measure a battery with
your DVM and then adjust Daqarta to read the same.
And most inexpensive DVMs lack a sensitive AC Volts
range, or you could just read the sound card output
and adjust the calibration to match the meter. One
workaround I have come up with is to measure a
battery with the DVM (since they always have sensitive
DC ranges), then use the sound card to read the peak
voltage that the AC coupling passes. You have to keep
shorting the input and repeating until you get a good
bounce-free spike capture, then assume the peak is
the same as the battery voltage.

If you are more adventuresome, you can use the
DVM to measure a high AC voltage, say from an
isolation transformer on the 120V mains, and divide it down
to a volt or so for the sound card to read. It's easy to
get a known divider ratio, so you should be able to get
a fairly accurate reference.

Best regards,



Bob Masta

D A Q A R T A
Data AcQuisition And Real-Time Analysis
www.daqarta.com
Scope, Spectrum, Spectrogram, Signal Generator
Science with your sound card!