IF filter
45.1 MHz
RF input
0.1 - 30 MHz
IF filter
450 KHz
Synthesizer
First injection
Finj1 = 45.2 - 75.1 MHz
step = 8.75 KHz
/2
VCO 1,2,3
Fc1 = 90.4 - 150.2 MHz
step = 17.5 KHz
Fc2 = 352.4 - 369.2 MHz
step = 10 KHz
Second injection
Finj2 = 44.05 - 46.15 MHz
step = 1.25 KHz
/8
VCO 4
BW=12.1 KHz BW=4.55 KHz
0÷3 KHz
10 Hz Res
DSP
430.77 KHz
Figure 5-5. Receive Path Frequency Conversion Scheme
5-4. Synthesizer
The synthesizer is designed according the MICOM-3 frequency plan, described in Figure 5-6.
The conversion principles are as follows:
• Receive path: the analog section performs triple frequency conversion; a fourth frequency
conversion is done digitally.
Receive path bandwidth, 10 kHz, is determined by a ceramic filter at a 450 kHz frequency.
• Transmit path: the analog section performs double frequency conversion; a third frequency
conversion is done digitally.
• The conversion frequency resolution of the analog part is 1.25 kHz.
The synthesizer provides two signals to the transmission and reception paths. The first injection signal
covering 45.2 to 75.1 MHz is produced by a PLL (phase locked loop). To enable wide range coverage
while maintaining low noise performance, the 45.2 to 75.1 MHz loop uses three VCOs (Voltage
Controlled Oscillator) in three subranges. The second injection signal is also generated by a PLL IC
covering the 44 to 46 MHz range with one VCO.
In both loops, the VCO frequencies are higher than the output frequencies. The first injection uses a
division by two (U2002) and the second injection uses a division by eight (U2003). The frequency
division decreases spurious level without degrading the lock-in time of the PLL. Frequency division
improves frequency resolution too. Thus, the first injection provides a 8.75 kHz resolution and the
second injection provides a 1.25 kHz resolution.
Both loops use a single reference oscillator at 16.8 MHz. As a result, the overall radio frequency
stability and accuracy are determined by the accuracy and stability of this single reference oscillator.
The reference oscillator is a 1 ppm DTCXO (0.1 ppm OCXO for MICOM-3R). An optional 0.1 ppm
OCXO can be ordered, for enhanced frequency accuracy. The OCXO is a hybrid add-on that is
thermally isolated. It contains a 16.8 MHz crystal oscillator. The temperature of the oscillator is
5-8 Cat. No. 6886859J01
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