Tektronix 067-0532-10 Calibration Fixture Manual page 8

Operating Instructions--Type 067-0532-01
Bandwidth Measurement
The Constant Amplitude Signal Generator provides a low distortion
sinewave for measuring oscilloscope bandwidth. Bandwidth is de-
fined as the frequency at which the voltage (or current) is -3dB
of the response at a reference frequency. Since voltage is common-
ly displayed on an oscilloscope it is convenient to convert the -3dB
amplitude to voltage amplitude.
E
1
-3dB = 20 LOG —
Ep
Where E, is the reference voltage, 1, and E, is the voltage at the -3dB bandwidth
3 1
1
E,
For voltage (or current), the bandwidth is the frequency at which
the amplitude is 70.7% of the reference amplitude.
Assuming that the oscilloscope response has, essentially, a smooth roll-
off characteristic, then the reference frequency should be about 1/20
(or less) of the bandwidth limit frequency. (This insures that the re-
ference amplitude point is close to the center of the response charac-
teristic. The Constant Amplitude Signal Generator reference frequency
is 3 MHz, or about 1/20 of 65 MHz, the lowest of the variable frequency.
Switch FREQUENCY RANGE to 65-500 MHz.
O A P W N F
Reference -3dB (0.707)
Amplitude Amplitude
4 DIV 2.82 DIV
5 DIV 3.53 DIV
6 DIV 4.24 DIV
7 DIV 4,94 DIV
8 DIV 5.65 DIV