Understanding the BB60C Hardware |
Residual Signals
11
Gain control is achieved in the BB60C using the front-end attenuator and preamplifier. The front end was
designed to provide good spurious-free dynamic range (SFDR) at any reference level, typically better than
50 dB.
The 14-bit ADC uses built-in dithering to further improve the linearity and decrease spurious responses
at the IF level. Spurs from the ADC are typically 70 dB below the carrier.
From the ADC, digitized IF data is handed off to an FPGA where it is packetized. The Cypress FX3
peripheral controller streams the packetized data over a USB 3.0 link to the PC, where 80 million, 14-bit
ADC samples per second are processed into a spectrum sweep or I/Q data stream.
RESIDUAL SIGNALS
3.3
A residual signal appears even when there is no signal input. The BB60C has some low level residual
signals at multiples of 10 MHz, typically not visible unless a narrow span (<10 kHz) is used. These are
typically very low (-130 dBm for a reference level of -50 dBm), except for a few frequencies where signals
may be as high as -107 dBm for a reference level of -50 dBm.
SCALLOPING LOSS
3.4
An FFT-based spectrum analyzer uses digital resolution bandwidths rather than discrete analog filters.
Moving from analog to digital introduces some new terms important to measurement accuracy, like FFT
bins, window functions, spectral leakage and scalloping loss. To sum up, an FFT produces an array of
discrete frequency bins and their associated amplitude. Real-world signals rarely line up exactly with a
single frequency bin, which can result in some ugly behavior unless a window function is used. Many
different window functions are available, with various strengths and weaknesses.
For the BB60C, swept modes default to a flat top window, which offers excellent amplitude flatness and
therefore very little scalloping loss, in exchange for a wider resolution bandwidth and longer processing
time. Most RBWs used by the BB60C are from flat top windows, so scalloping loss is negligible.
In real-time mode a Nuttall window function is used, which has a narrower bandwidth to reduce
processing time and level out impulse response. However, when a signal falls halfway between two
“bins,” the energy is split between adjacent bins such that the reported “peak” amplitude may be lower
by as much as 0.8 dB.
To get an accurate CW reading using “Marker peak”, flat top RBW shape in swept mode is recommended.
In either mode, the “channel power” utility, which integrates the power across any channel bandwidth
you specify, also eliminates this scalloping loss, giving you a full accuracy amplitude reading even in real-
time mode.