# 📡 PL Tx Subsystem (Pulse Generator)

[🏠 Project Home](../README.md)

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## Overview

The Tx subsystem implements a **pulse-based Linear Frequency Modulated (LFM) chirp generator** using a DDS/NCO architecture in the FPGA (PL).

The generator produces **complex baseband output**:

x[n] = exp(j·φ[n])

and operates deterministically in the PL after a trigger from the PS.

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## Architecture

TxPulseStart (PS)
        ↓
pulse_gen_ctrl (FSM)
        ↓
   tx_active
        ↓
Phase Increment Counter
        ↓
      NCO (DDS)
        ↓
   Complex Output (I/Q)

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## Chirp Generation Principle

The chirp is generated using a second-order phase accumulator:

Δφ[n] = Δφ[n−1] + step  
φ[n]  = φ[n−1] + Δφ[n]

This results in a linear frequency sweep.

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## Parameterization (PS → PL)

Inputs:

- Center frequency: Fc  
- Bandwidth: B  
- Pulse width: N (samples)

Derived internally:

f_start = Fc − B/2  
step    = B / (N − 1)

These values are converted to DDS phase increments before being written to PL registers.

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## Pulse Timing (FSM)

States:

- IDLE: waits for trigger and latches parameters  
- ACTIVE: generates pulses  
- DONE: waits for trigger reset  

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## Timing Behavior

Within each PRI:

|<------ PRI ------>|
|<-- pulse -->| idle |

- tx_active = 1 → chirp output  
- tx_active = 0 → output zero  

Chirp is reset at each pulse start.

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## Burst Trigger (PS Interaction)

- Controlled via TxPulseStart (memory-mapped register)
- Rising edge triggers burst
- PL runs autonomously afterward

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## Key Characteristics

- Deterministic timing (128 MHz)
- Efficient DDS (adder-based)
- Complex output (I/Q)
- Supports burst-mode radar operation

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## 🔗 Related Components

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