U N D E R S T A N D I N G T H E B B 6 0 A H A R D W A R E
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.
9 Understanding the BB60A Hardware
Front End Architecture 9.1
The AC-coupled RF input is first
attenuated. It then passes through
a band select filter to reject the
image and out-of-band responses,
after which it is amplified or
attenuated before mixing (a gain
setting of 1 or less attenuates the
RF).
At the mixer, a local oscillator is
injected high-side to produce the
intermediate frequency (IF). This
IF is either ~2.35 GHz, or ~1.27
GHz, depending on band. The IF
passes through a SAW filter with
approx. 60 MHz of bandwidth,
and is then amplified or attenuated (a gain of 0 attenuates the IF). The SAW filters were selected to have
good rejection 280 MHz from center.
Inside the IF-to-bits subassembly, the IF is mixed to 140 MHz (the image 280 MHz away having been
filtered out in the previous step), filtered to a 20 MHz bandwidth, then digitized at 80 MSPS. Additional
gain may be applied in the IF-to-bits subassembly when gain is set to 3.
Spurious Signals 9.2
A spurious signal appears as a function of an input signal. These include signals from intermodulation,
image responses, local oscillator spurs, and ADC aliasing.
Spurious signals from intermodulation can be controlled by limiting the amount of power into a mixer or
amplifier. Decreasing the power by 10 dB will often reduce intermodulation products by 20 or 30 dB.
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