Sensitivity; Dynamic Range; Amplitude Measurements (Linear Mode) - HP 3580A Operating And Service Manual

Section III
GENERAL OPERATING INFORMATION
Model 3580A
3-41. Sensitivity.
3-42. Sensitivity
is
a figure of merit that defines the
analyzer's ability to detect or respond to a given input level.
There are three types of sensitivity that are of interest when
operating the 3580A:
a. Maximum Sensitivity
b. Full Scale Sensitivity
c. Display Sensitivity
3-43. Maximum Sl!nsitivity. Maximum Sensitivity refers to
the smallest signal that can be detected by the analyzer.
The maximum sensitivity of the analyzer is limited by its
own internally generated noise and is commonly defined as
the point where the signal level is equal to the noise level.
This is sometimes called "tangential sensitivity".
3-44. Nyquist's Noise Equation 1 reveals two important
things about noise that apply to the 3580A:
a. Noise is proportional to the square root of band·
width. . .
Noise level decreases and sensitivity increases as
the BANDWIDTH setting is narrowed.
b.
Noise is proportional to the square root of input
resistance... The 3580A has a high (l Megohm) input
resistance. This means that noise is largely dependent on
the source resistance placed at the INPUT terminals. Signal
sources having low output resistances will produce a lower
noise level than those having high output resistances.
3-45. Noise level is also dependent on the tuned frequency
of the instrument. Semiconductors in the input stages of
the instrument exhibit surface noise which has a 1 /f
frequency spectrum. This surface noise is predominate at
frequencies below 1 kHz. When the 3580A is tuned below
1 kHz, the noise level increases and sensitivity decreases.
3-46. Figure 3-9 is a family of curves showing the specified
noise levels vs. frequency for the 300 Hz, 30 Hz and 1 Hz
BANDWIDTH settings. Typically, if the source resistance is
less than 10 kilohms, the noise levels will be below those
indicated by the curves.
3-47. Full Scale Sensitivity. Full scale sensitivity defines
the input level that will produce full scale deflection on any
given range. For absolute measurements, full scale sensiti-
vity ranges from 20 V rms to 0.1 µV rms in the Linear
mode and from
+
30 dBV/dBm to -140 dBV/dBm in the
Log (10 dB) mode. With the amplitude VERNIER control
set fully counterclockwise, full scale sensitivity ranges from
approximately 100 V rms to 0.2 µVrms in the Linear mode
and from
+
40 dBV/dBm to - 130 dBV/dBm in the Log
mode.
3-48. Display Sensitivity. Display Sensitivity or "scale
calibration" expresses the analyzer's response in units per
- 1 2 0 r - - - - - t - - - - - - + - - - - t - - - - - - - l
BW• 300 Hz
- 1 6 0 r - - - - - r - - - - - - - + - - - - t - - - - - - - I
10
100 lk
FREQUENCY
I Ok
Figure 3-9. Noise vs. Frequency.
IOOk
35SOA~B~3565
vertical division. For absolute measurements in the Linear
mode, display sensitivity ranges from 2 V per division to
10 nV per division. For absolute or relative logarithmic
measurements, display sensitivity is 10 dB per division in
the Log 10 dB mode and 1 dB per division in the Log 1 dB
mode.
3-49. Dynamic Range.
3-50. The dynamic range of a spectrum analyzer defines its
ability to detect large and small signals and display them
simultaneously. For operating purposes, dynamic range can
be expressed as the ratio of the largest to smallest signals
that can be simultaneously displayed on the CRT. In both
the linear and Log modes, the largest signal that can be
displayed (full scale sensitivity) is determined by the
INPUT SENSITIVITY, amplitude VERNIER and AMPLI-
TUDE REF LEVEL control settings. The smallest signal
that can be displayed is determined by the display range or
by the internal noise floor (maximum sensitivity). In the
Linear mode the smallest signal that can be displayed is
approximately 1 % of full scale. Thus, the dynamic range is
approximately 40 dB as long as the internal noise floor is
more than 40 dB below full scale. With the AMPLITUDE
REF LEVEL switch in the NORMAL position, the display
range in the Log 10 dB mode is greater than 80 dB. The
dynamic range is, therefore, at least 80 dB as long as the
noise floor is more than 80 dB below full scale. In the Log
1 dB mode, the display sensitivity is increased to 1 dB per
division and the dynamic range, determined by the display
range, is 10 dB.
3-51. Amplitude Measurements (Linear Mode).
3-52. Figure 3-10 is a simplified block diagram showing a
portion of the 3580A amplitude section in the Linear
mode. The INPUT SENSITIVITY switch and amplitude
VERNIER potentiometer control the input attenuation and
gain of the Input Circuits and establish the maximum input
level as outlined in Paragraph 3-40. In addition, the INPUT
SENSITNITY switch operates in conjunction with the
AMPLITUDE REF LEVEL switch to establish the full-scale
sensitivity and measurement range.
1
En = (4 kTBR)
1 / 2
.
Where En
=
noise level; k
=
Boltzmann's constant; T
=
temperature (° K); B
=
bandwidth (Hz); R
=
input resistance.
3-8