
WSA5000 Functional Overview
multiple SAW filters. SAW filter selection depends on the frequency of the input signal.
The output of the SAW filter feeds the I/Q mixer.
The three signal processing paths are further classified into different modes of operation
for the capture engine as shown in
. The radio modes ZIF, SH, SHN and HDR
support tuning the center frequency from 50MHz to the maximum frequency supported by
the particular product model (ex. 8GHz, 18GHz, 20GHz, and 27GHz for WSA5000-x08,
-x18, -x20, and -x27, respectively, where x is a model number variant).
The ZIF, SH and SHN radio modes support a tuning resolution of 10Hz. Digital
frequency shifting can then be used to enhance the tuning resolution to the nearest 1Hz
(±0.23Hz) The frequency shifting technology used is an embedded Numerically
Controlled Oscillator (NCO) (a Direct Digital Synthesizer or DDS) as described in the
).
The HDR radio mode supports a tuning resolution of 10Hz. No further fine tuning is
available.
The remaining two radio modes, DD and IQIN, support 50MHz IBW direct digitization of
the baseband from the external RF IN or I and Q IN ports, respectively. Hence, neither
of these modes support frequency tuning of the radio although the DSP's frequency shift
mode may be applied.
Direct-Conversion Receiver Technology
Direct-conversion (or ZIF) receivers are ideal for signal analysis of wideband waveforms,
such as 4G/LTE, Wi-Fi and Bluetooth. With that benefit comes the drawback of both IQ
and DC offsets which are inherent to direct-conversion technology.
DC Offset Correction
The WSA5000's WB ADC sampling rate is 125 MSa/s, intermediate frequency (IF) is 0
and the entire IF bandwidth is 125MHz. The analog filter results in an amplitude roll-off at
approxi50MHz around the center frequency Fc, as illustrated in
Direct-conversion receivers have a DC offset at the center of the band. The offset is
primarily compensated for in real-time in the receiver hardware but there always is some
residual offset that (depending on the application and bandwidth of interest) might need
to be compensated for in software. Several options such as calibration or dynamic offset
compensation in software have been described in the open literature.
DC
Offset
Fc-50 MHz
Fc+50 MHz
125 MHz
Fc
Analog filter
Figure 3:
DC Offset with Amplitude Roll-Off at +50MHz
ThinkRF WSA5000 Wireless Signal Analyzer Programmer's Guide
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