Technique For Measuring Noise - HP 3580A Operating And Service Manual

Section III
BASIC OPERATING PROCEDURES
Model 3580A
0 Hz and the negative frequencies on the left-hand side of
0 Hz are blanked. Set the FREQUENCY dial to 20.0 kHz.
When the trace is updated by a new sweep, the 20 kHz
frequency component will appear in the center of the
display.
m. Set the BANDWIDTH switch to 300 Hz. This will
make the 20 kHz component wider because the analyzer's
response to a CW signal is an ainplitude vs. frequency plot
of the IF Filter.
n. Release the BLANK STORE button. The permanent-
ly stored trace will reappear on the display. Even though
the sweep parameters have been changed, the stored trace
appears exactly as it did when the STORE button was
initially pressed.
o. Set the FREQ. SPAN/DN to 5 KHz and allow IO
seconds for the display to be updated.
p. Release the STORE button. The previously stored
trace will disappear and a series of dots will appear on the
current trace. The dots will be cleared when the display is
updated by a new sweep.
3-207. Technique For Measuring Noise.
3-208. The 3580A uses peak detection on the swept spec-
trum. Therefore, the noise displayed is peak noise and can
be several dB higher than average noise. Average noise
measurements can be made if the following technique is
used:
a. Use display smoothing.
b. Ignoring the adjust warning light, decrease Sweep
Time/Div until the displayed noise level no longer
3-30
decreases. The spectrum shape of the noise should be
gradually changing, not abrupt, allowing the spectrum
analyzer to follow it well.
3-209. Average Detection Error. The video detector is an
average responding full wave detector. This type of detector
has an inherent error when detecting noise. In the 3580A,
the error occurs in both the linear and log modes of opera-
tion. To correct for this error, multiply the displayed read-
ing by 1.128 to get therms value.
3-210. Log Conversion Error.
In the logmode of opera-
tion, an additional correction must be made to compensate
for log conversion error. Add 1.5 dB to the corrected
display reading.
NOTES
1. Only "Gently" varying noise spectra can be
accurately measured using this technique.
Accurate measurement of both discrete lines
and noise levels in the same spectrum is not
generally possible.
2. To calculate the equivalent noise bandwidth,
multiply the 3 dB bandwidth by 1.12. Remem-
ber that the 3 dB bandwidth has a tolerance of
±
15% and therefore should be measured if
accurate results are desired.
3. The recorder Y Axis output is linear and
continuous. Noise measurements can be made
by connecting a true rms reading voltmeter to
this output. See Paragraph 3-168 for operating
information concerning the Y Axis output. The
use of an X-Y recorder may also prove benefi-
cial in making noise measurements.