THEORY OF OPERATION: Frequency Generator (RF) Section
16
The mixer has two inputs (RF and first LO) and one output (IF). The first LO sig-
nal is generated at a frequency of 109.65 MHz above the received frequency. For
example, if the RX is 851 MHz, the first LO frequency will be 960.65 MHz. The
first LO signal is supplied by the main VCO (see “Main VCO Synthesizer” on
page 19).
In the back-end of the receiver, the signal goes through the 3-pole crystal filter.
This filter provides a narrow bandpass selectivity (21 kHz) for a 25kHz channel
centered at 109.65 MHz, providing rejection to adjacent channels. The signal is
then fed into the iZIF IC input (pin D1), which mixes it down to baseband I and Q.
This information is sent to the ADDAG IC for digitizing prior to sending it to the
DSP.
Frequency Generator (RF) Section
This section contains the following main components in the RF board:
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Low Voltage (3 Volt) Fractional-N (LV Frac-N) synthesizer
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Crystal-Based Reference Oscillator Circuit
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Discrete voltage controlled oscillator (VCO) circuit
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Second local oscillator (LO)
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DSP phase locked loop (PLL)
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Host system clock synthesizer
All frequencies in the iO1000 originate from the 16.8 MHz reference frequency
provided by the Low Voltage Fractional-N synthesizer and the crystal-based refer-
ence oscillator circuit. The UM5 crystal generates the 16.8 MHz signal, which is
DC voltage-warped (or tuned) and temperature-compensated using the internal
D/A converter in by the Low Voltage Fractional-N ASIC. See Figure 9 on page 17.