Receiver Theory Of Operation - Codan Cascade Operation Manual

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Cascade Transceiver
R E C E IV E R T H E ORY O F O PER AT IO N
The CASCADE Receiver is a standard superheterodyne architecture. It can demodulate Analog FM
and Digital C4FM (P25 Phase I) modulation and is composed of four main sections:
• RF Front End
• IF Filtering and Amplification
• Analog RF to Digital Conversion
• LO Synthesis
The RF Front End includes all the receiver circuitry from the antenna input to the mixer. This includes
a bandpass filter, an LNA and a second bandpass filter used for image rejection.
The first bandpass filter is wide band over the entire switching range of the receiver. It blocks any
strong out-of-band interfering signals from entering the receiver. The LNA provides the first stage
amplification and increases the sensitivity of the receiver. The image-reject filter also helps to block
the out-of-band interfering signal as well as eliminate any image frequency response of the receiver.
The IF Filtering and Amplification stage includes a mixer, crystal filters, an LNA and some
automatic gain control circuitry (AGC).
The AGC Circuitry protects the components from being overdriven by a high-level signal. The mixer
down-converts the incoming RF signals from the front end to a lower intermediate frequency (IF)
which is then filtered by highly selective crystal filters and amplified again by the IF LNA. This
decreases the noise floor and increases the sensitivity of the receiver. The crystal filters also help
provide excellent in-band, off-channel filtering.
The Analog RF to Digital Conversion is handled by an integrated circuit. The IF frequency is
downmixed to another lower 2nd IF frequency which is directly sampled with a sigma-delta ADC and
converted to a digital baseband signal. This digital signal then passes through two FIR filters before it
is sent off to the single board computer on the digital PCB for more signal processing.
Two stages of LO Syntheses happen in this receive chain.
The 1st LO is a programmable frac-n synthesizer. This LO feeds directly into the mixer in the "IF"
section and is used in the RF-to-IF down conversion process. The LO's ability to be programed to
any required frequency over the entire receiver band is what allows this receiver to be software
programmable to any receiver channel by the user without having to do any manual tuning.
The 2nd LO is generated in the "clock distribution" section of the RF PCB. It feeds directly into the
integrated circuit that handles the digital downmixed conversion discussed above. It is used in the
process of converting the main IF frequency to the second lower IF frequency.
Cascade System Modules Operation Guide
OG-CASC-SYS-MOD-1-0-0P