11
Section 6
Improving Signal to Noise:
RBW, Input Attenuation, Preamplifier and Video Average
One of the primary uses of a spectrum analyzer is searching for and measur-
ing low-level signals (such as the 3rd harmonic of an oscillator). Sensitivity
optimization of the analyzer is absolutely crucial when trying to characterize
signals of this nature. The input attenuator and bandwidth settings affect the
level of signals that can be seen on the display. The attenuator affects the level
of a signal passing through the instrument. Narrowing the resolution bandwidth
(RBW) fi lter allows you to resolve two or more signals that are close
together in frequency, and lowers the noise fl oor of the analyzer so you can
detect very small signals.
The Keysight N9340B provides a 1-dB step attenuator to allow maximum
flexibility when setting the analyzer’s dynamic range.
If after adjusting the attenuation and RBW, a signal is still near the noise,
visibility can be improved by using the video bandwidth (VBW) or averaging
functions. VBW filter smoothes small signal variations. Averaging function
simply averages the traces on screen as the sweep is made. Turning on the
internal preamplifier also reduces the effective noise floor.
Instruction
Key strokes
Connect the 100 MHz signal to RF IN
connector.
[Preset]
Zoom in on the 3rd harmonic “=
(150 kHz span).
[Frequency] [300], {MHz}, [Span],
[150], {kHz}
Turn preamp on.
[AMPTD], {preamp off on} {On}
Bring the peak of the harmonic to top
graticule line.
[AMPTD], then rotate the knob.
Reduce the resolution bandwidth to
1 kHz.
[BW/SWP] {RBW} {Man} reduce to
100 Hz
Manually reduce the attenuation.
[AMPTD], {Attenuation} {Man}, [0]
{dB}
Smooth out the noise using averaging
function.
[TRACE], {More} {Average {On}}
Figure 3. Improved signal to noise
