board. The KREF3 obtains its DC and low-frequency I/O signals via an 8-pin connector on the RF board, but its
RF outputs are fedto the RF board(and sub receiver, if applicable) via coax cable assemblies.
KSYN3
Low phase noise is key to both receiver and transmitter performance. In the K3’s synthesizermodule (KSYN3),
we start with a clean, wide-range voltage-controlled oscillator (VCO). The VCO frequency isplaced near the
desired band of operation using 128 carefully-selected L-C combinations, which keep the ratio of fixed
capacitance totunable capacitance (varactor diodes) as high as possible.
The VCO is held exactly on frequency by a phase-locked-loop IC(PLL), which samples the VCO output
continuously and compares it to its high-stability reference input. The PLL’s reference input is obtained from a
direct-digital-synthesis (DDS) IC, which istunable in about 0.2-Hz steps. The reference for the DDS itself is the
49.380-MHz signal fromthe KREF3 module.
To keep the synthesizer’s output signal virtually spur-free, the DDS is followed by a 4-pole crystal filter. This
eliminates both directly-occurring spurs andthe Nyquist sampling spurs that normally accompany a DDS-driven
PLL system.
The combination of all of these noise-minimization techniques results in very low phase noise and negligible
discrete spur content.
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