ELECRAFT K3, Руководство пользователя

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board. T he KREF3 obtains its DC and low-frequency I/O signals via an 8-pin connector on the RF board, but its
RF outputs are fed to the RF board (and sub receiver, if applicable) via coax cable assemblies.
KSYN3
Low phas e noi s e i s key t o bot h r e cei ve r and t r ans mi t t e r pe r f or ma nce. I n t he K3’ s s ynt hes i zer
module (KSYN3),
we start with a clean, wide-range voltage-controlled oscillator (VCO). T he VCO frequency is placed near the
desired band of operation using 128 carefully-selected L-C combinations, which keep the ratio of fixed
capacitance to tunable 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-
s t abi l i t y r ef er e nce i npu t . The PLL’ s r ef e r ence i nput i s obt ai ned f r om a
direct-digital-synthesis (DDS) IC, which is tunable in about 0.2-Hz steps. The reference for the DDS itself is the
49.380-MHz signal from the KREF3 module.
To keep the synthesizer
’ s out put s i gnal vi r t ual l y s pur
-free, the DDS is followed by a 4-pole crystal filter. This
eliminates both directly-occurring spurs and the 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.