# 📡 PL Rx Subsystem (Channelizer)

[🏠 Project Home](../README.md)

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## Overview

The Rx subsystem implements a **polyphase filter bank (PFB) channelizer** followed by FFT processing, a **bypass path**, and a **multi-frame capture pipeline**.

It converts wideband ADC input into frequency-domain channels (or raw samples via bypass) and streams the result to the PS.

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## Architecture

### Channelizer Path (default)

ADC  
 ↓  
PFB Channelizer (Decimation + Filtering)  
 ↓  
FFT (512 bins)  
 ↓  
Capture (frame control)  
 ↓  
AXI4-Stream (128-bit, 4 samples/clock)  
 ↓  
DMA

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### Bypass Path (Debug / Raw Data)

ADC  
 ↓  
Bypass Path  
 ↓  
Capture (frame control)  
 ↓  
AXI4-Stream (128-bit, 4 samples/clock)  
 ↓  
DMA

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## Capture Pipeline

- Multi-frame acquisition (configurable nFrames)
- Frame size: 512 samples
- Supports asynchronous capture start (not frame-aligned)
- TLAST asserted at frame boundaries

### Behavior

- First frame may be partial
- Frames may contain ≤ 2 frame indices (expected)
- DPW spans nFrames frames but covers nFrames + 1 frame regions

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## Processing Chain (Channelizer Mode)

### ADC Input
- Sampling rate: 4096 MSPS
- Data type: **fixdt(1,16,15)** (Q1.15)

### PFB Channelizer
- Decimation: 8
- Effective bandwidth: 512 MHz

### FFT
- Size: 512
- Produces frequency bins

### Capture
- Defines frame boundaries (512 samples)
- Generates TLAST

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## Numeric Format and Scaling

### System Standardization

- End-to-end Q1.15 (**fixdt(1,16,15)**)

### Channelizer Output Scaling

- Native: **sFix25_En23**
- Quantized to: **fixdt(1,16,15)** (round + saturate)

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## Data Packing (Updated)

- 4 samples per clock
- Each sample: complex (16-bit real + 16-bit imag)
- Packed into **128-bit AXI4-Stream word**

Benefits:
- Matches datapath parallelism
- Efficient DMA transfers
- Eliminates need for serializer stage

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## AXI4-Stream Output

- Width: 128 bits
- Contains 4 complex samples per cycle
- TLAST = frame boundary

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## Debug / Validation Features

A counter-based debug mode is implemented:

- Real part → sample counter (0..511)
- Imag part → frame index

Used to validate:
- Sample continuity
- Frame boundaries
- DMA ordering and integrity

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## Key Characteristics

- Fully streaming pipeline
- Deterministic latency
- High throughput (4 samples/clock)
- Dual-mode operation (channelizer / bypass)
- Validated up to nFrames = 1024

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## 🔗 Related Components

- [🏠 Project Home](../README.md)
- [PL Tx Subsystem](pl_tx_subsystem.md)
- [PS Subsystem](ps_subsystem.md)