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TS-590S CONTENTS 19
3 LOCAL OSCILLATOR
In the first local oscillator, instead of the conventional PLL/VCO system, a 14-bit DDS (direct digital
synthesizer) is adopted to provide the output signal directly to the mixer. During the down
conversion, the oscillator frequency is lower than that in up-conversion operation, and, therefore, the
output has far better C/N (carrier-to-noise ratio) characteristics that contribute to superior reciprocal
mixing
*1
characteristics.
*1
Reciprocal mixing: First, the signal sourced from a signal generator (SG) at a frequency far from the receive frequency is
injected. Then, the output level of the signal generator is changed to measure the level at which the signal is detected as
noise. The higher the value, the lower the noise that is produced due to adjacent undesired signals occurring and therefore
quieter reception is possible.
Figure 3-1 Comparison: C/N Characteristics
Figure 3-1 is a graph to compare the measurement results of TS-590S and of TS-480S at 14.200 MHz.
For example, at the 10 kHz point, if the receive frequency is 14.200 MHz, an unmodulated carrier
signal of 14.210 MHz is supplied from a signal generator to the antenna terminal of the transceiver.
In the TS-480S, if the output level from the signal generator reaches -45 dBm, the noise level starts to
rise. However, in the TS-590S, you can see the noise level goes up if the signal sourced from the
signal generator is raised to -25 dBm.
Figure 3-2 shows a graph of C/N characteristics plotting the data measured at the TS-590S’s first
local oscillator.
Figure 3-2 C/N Characteristics of the First Local Oscillator
C/N characteristics of the first local oscillator
when receiving 14.2 MHz
1 kHz -116.5 dBc/ Hz
10 kHz -134.4 dBc/ Hz
100 kHz -142.5 dBc/ Hz