Reducing Spurious And Residual Responses; If-To-Bits Receiver; If And Rbw Selection - Test Equipment Plus Signal Hound USB-SA44B User Manual

T H E O R Y O F O P E R A T I O N
Spans using 6.5 KHz - 250 KHz RBW or VBW: the Signal Hound operates by
⇒
rapidly stepping LO frequencies in 200 KHz steps, covering above and below the
center frequency. These are processed with small FFTs and masked together to
reject image responses and produce the displayed trace.
o For RF frequencies below 25 MHz, image rejection requires additional
steps and the trace will be slower.
5 MHz RBW: Rapidly sweeps across the spectrum, to find a strong signal quickly.
⇒
Certain RF frequencies may produce spurious responses in this mode, and
amplitude accuracy is not guaranteed.
5.1.2

Reducing Spurious and Residual Responses

Certain RF frequencies may produce small spurious and/or residual responses. To
verify a displayed signal, center it and step the
span down to 10 KHz or less. If it disappears, it
was likely a mixing artifact or a harmonic from a
system clock. To avoid known residual
responses at multiples of the primary system
clocks, a secondary clock frequency is selected
for spans 100 KHz or below.
The mixers can typically operate with up to +0 dBm input, but keeping the input level
–25 dBm or lower will greatly improve linearity.
5.1.3

IF-to-Bits Receiver

The IF-to-bits receiver has three gain ranges and several selectable bit rates. The gain
range will be automatically selected based on attenuator settings and reference level to
avoid IF ADC compression, which will greatly distort the data. The software should
warn you if compression is occurring. If this happens, change reference level,
attenuator, and/or input level settings.
5.1.4

IF and RBW Selection

The I/Q data comes in over USB and is processed using an FFT with a Flat Top
window. The software controls the bit rates and the size of the FFT based on your
selected RBW. The available RBWs are a function of the span, since very large RBWs
with a small span would result in a trace with only a few data points and a blocky
appearance, and very small RBWs with a large span would result in a large data set that
would be difficult to manage and process.
The resolution bandwidths are not the traditional analog 1,3,10 KHz filters. The
bandwidths are a function of the size of the FFT (which is a power of 2) .
The RBW listed on the GUI is calculated from the bit rate divisor and FFT size. This
number is the approximate RBW and is listed in the lower left region of the GUI. For
Measurement tip: Linearity
of a mixer improves at lower
signal levels. For accurate
harmonics measurements, you
should have less than –25 dBm
into the mixer!
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