API Functions | Signal Hound
rbwType The possible values for rbwType are BB_NATIVE_RBW and
BB_NON_NATIVE_RBW. This choice determines which bandwidth table is
used and how the data is processed. BB_NATIVE_RBW is default and
unchangeable for real-time operation.
rejection The possible values for rejection are BB_NO_SPUR_REJECT and
BB_SPUR_REJECT.
Description
The resolution bandwidth, or RBW, represents the bandwidth of spectral energy represented in each
frequency bin. For example, with an RBW of 10 kHz, the amplitude value for each bin would represent
the total energy from 5 kHz below to 5 kHz above the bin’s center. For standard bandwidths, the API
uses the 3 dB points to define the RBW.
The video bandwidth, or VBW, is applied after the signal has been converted to frequency domain as
power, voltage, or log units. It is implemented as a simple rectangular window, averaging the amplitude
readings for each frequency bin over several overlapping FFTs. A signal whose amplitude is modulated at
a much higher frequency than the VBW will be shown as an average, whereas amplitude modulation at
a lower frequency will be shown as a minimum and maximum value.
Native RBWs represent the bandwidths from a single power-of-2 FFT using our sample rate of 80 MSPS
and a high dynamic range window function. Each RBW is half of the previous. Using native RBWs can
give you the lowest possible bandwidth for any given sweep time, and minimizes processing power.
However, scalloping losses of up to 0.8 dB, occurring when a signal falls in between two bins, can cause
problems for some types of measurements.
Non-native RBWs use the traditional 1-3-10 sequence. As of version 1.0.7, non-native bandwidths are
not restricted to the 1-3-10 sequence but can be arbitrary. Programmatically, non-native RBW’s are
achieved by creating variable sized bandwidth flattop windows.
sweepTime applies to regular sweep mode and real-time mode. If in sweep mode, sweepTime is the
amount of time the device will spend collecting data before processing. Increasing this value is useful for
capturing signals of interest or viewing a more consistent view of the spectrum. Increasing sweepTime
has a very large impact on the amount of resources used by the API due to the increase of data needing
to be stored and the amount of signal processing performed. For this reason, increasing sweepTime also
decreases the rate at which you can acquire sweeps.
In real-time, sweepTime refers to how long data is accumulated before returning a sweep. Ensure you
are capable of retrieving as many sweeps that will be produced by changing this value. For instance,
changing sweepTime to 32ms in real-time mode will return approximately 31 sweeps per second
(1000/32).
Rejection can be used to optimize certain aspects of the signal. Default is BB_NO_SPUR_REJECT, and
should be used in most cases. If you have a steady CW or slowly changing signal, and need to minimize
image and spurious responses from the device, use BB_SPUR_REJECT. If you have a signal between 300
MHz and 3 GHz, need the lowest possible phase noise, and do not need any image rejection,
BB_BYPASS_RF can be used to rewire the front end for lowest phase noise.
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