05 Local Oscillator; Dds And Pll System; First Local Oscillator For The Main Band Receiver - Kenwood TS-990S In-Depth Manual

05 LOCAL OSCILLATOR

DDS and PLL System

The local oscillator circuit in the TS‑990S is configured to give best‑suited characteristics to signals to be processed by
the VCO division type and the DDS (Direct Digital Synthesizer) direct system (switchable) for the main band receiver,
DDS system for the sub band receiver, and PLL system for the transmission block.

First Local Oscillator for the Main Band Receiver

The first local oscillator for the main band receiver is not a DDS direct system but a conventional VCO/PLL system
(excluding the 10 MHz, 28 MHz, and 50 MHz bands).
Generally, the VCO/PLL system is good for C/N characteristics on distant carrier frequencies and superior in spurious
response by the effect of the filter in the PLL loop, while it tends to be disadvantageous for characteristics of the
adjacent carrier to noise ratio.
In the TS‑990S main band receiver, down converting all bands enables it to lower the local oscillator frequencies.
Consequently, the first local oscillator for the main band receiver newly employs the division type that oscillates the
conventional‑type VCO/PLL output signals using a higher frequency than the desired frequency and then divides the
output signals.
Theoretically, the C/N characteristics of the VCO/PLL output signals are improved by the effect of division at the ratio of
20*log (division ratio) (1/10 division maximum: 20*log (1/10) = ‑20 dB).
DDS IS AD 9951
Fig. 45 Block Diagram of the First Local Oscillator for the Main Band Receiver
With the VCO division type local oscillator employed, low spuriousness, which is an advantage of the VCO/PLL system
on the adjacent frequencies, and fine C/N characteristics on distant carrier frequencies are achieved.
Fig. 46 C/N Characteristics of the First Local Oscillator for the Main Band Receiving Circuit
28
Block Diagram of First
Local Oscillator for
Main Band RX Circuit
PLL DDS PLL
AMP
82.78 ~ 115.48 ~
1.8/3.5/5M ... N=10
92.48MHz 128.992MHz
14/18/21/24M ... N=4
(Other than the
96.48 ~ 127.48 ~
107.48MHz 138.992MHz above, DDS Direct)
Divider
1/N
AMP
7M ... N=8