Service Manual

MM-E 2072-09333-00
2-7
2-1.1.3.3 Frequency Conversion Subsystem
The Micom-Z frequency conversion scheme is described in Figure 2-4.
1.6 MHz
30 MHz
DSP processing
DSM
Audio
45.1 MHz
First IF amplifier
1.05 MHz
Second IF amplifier
0.1 MHz
30 MHz
D
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P
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Audio
20 kHz
Third IF amplifier
450 kHz
Second IF amplifier
45.1 MHz
First IF amplifier
A. Transmit Path B. Receive Path
Figure 2-4. Frequency Conversion Scheme
The conversion is implemented as follows:
• Transmit path: the analog section performs double frequency conversion, as shown in Figure 2-4.
• Receive path: the analog section performs triple frequency conversion, as shown in Figure 2-4.
Two synthesizers provide two local oscillator (LO) signals for the transmit and receive paths:
• The first LO signal covers the 45.2 to 75.1 MHz range in three subranges, and is generated by a
PLL (phase locked loop) using three VCOs (voltage controlled oscillators). Refer to Figure 2-5.
• The second LO signal is generated by a PLL covering the 44 to 46 MHz range with one VCO.
Refer to Figure 2-6.
In both synthesizers, the VCO frequencies are higher than the LO frequencies: to achieve the final
frequencies, the first synthesizer divides its output frequency by 2, and the second synthesizer divides
its output frequency by 8. Using frequency division decreases spurious levels without degrading the
lock-in time of the PLL, and also improves the frequency resolution.
Thus, the first synthesizer provides a resolution of 8.75 kHz, and the second synthesizer provides a
resolution of 1.25 kHz. The additional processing needed to achieve an overall frequency resolution of
10 Hz is performed by the DSP.
Both synthesizers use a single reference frequency, generated by a 16.8 MHz oscillator. As a result,
the overall radio frequency stability and accuracy are determined by the accuracy and stability of this
single reference oscillator.
The standard reference oscillator is a digitally temperature-compensated crystal oscillator (DTCXO).
An optional ovenized crystal oscillator (OCXO) with higher accuracy and stability can be ordered, for
enhanced frequency accuracy.