# 📡 PL Tx Subsystem (Pulse & Continuous LFM Generator)

[🏠 Project Home](../README.md)

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## Overview

The Tx subsystem implements a **pulse-based and continuous 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 Logic
        ↓
      NCO (DDS)
        ↓
   Complex Output (I/Q)

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## Operating Modes

The subsystem now supports multiple Tx modes:

### 1. Pulsed LFM (default)

- Chirp generated only during pulse window
- Phase resets at each pulse start
- Standard radar burst operation

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### 2. CW Mode (Continuous Wave)

- `tx_active = 1` continuously
- Generates a single-tone output
- Achieved by setting constant phase increment

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### 3. Continuous LFM (Workaround Implementation)

- `tx_active` forced HIGH continuously  
- A **1-cycle LOW pulse** is inserted periodically  
- This LOW→HIGH transition **resets the NCO**

Result:
- Continuous chirp
- Bounded bandwidth
- Periodic repetition of LFM

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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

### Pulsed Mode

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

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

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### Continuous LFM Mode

tx_active behavior:

1 1 1 1 1 0 1 1 1 1 ...

- 1-cycle LOW inserted at end of chirp period  
- Rising edge resets NCO  
- Defines chirp repetition interval  

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## CW / Continuous LFM Implementation Details

- CW mode bypasses FSM output
- A dedicated counter generates periodic reset pulses
- Reset timing is based on `pulse_width_cycles`

Important:

- Reset pulse is exactly **1 clock cycle**
- Ensures deterministic NCO restart
- Decoupled from PRI/FSM timing

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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:
  - Pulsed radar mode
  - Continuous wave (CW)
  - Continuous LFM (periodic chirp)

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## Design Notes

- FSM controls **timing (when to transmit)**
- NCO controls **frequency evolution**
- Continuous LFM implemented via **tx_active edge reuse**
- Minimal hardware overhead (no additional NCO logic)

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## 🔗 Related Components

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