updated doc: Tx subsystem

docs/pl_tx_subsystem.md

@@ -1,4 +1,4 @@
-# 📡 PL Tx Subsystem (Pulse Generator)
+# 📡 PL Tx Subsystem (Pulse & Continuous LFM Generator)
 
 [🏠 Project Home](../README.md)
 
@@ -6,7 +6,7 @@
 
 ## Overview
 
-The Tx subsystem implements a **pulse-based Linear Frequency Modulated (LFM) chirp generator** using a DDS/NCO architecture in the FPGA (PL).
+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**:
 
@@ -24,7 +24,7 @@ pulse_gen_ctrl (FSM)
         ↓
    tx_active
         ↓
-Phase Increment Counter
+Phase Increment Logic
         ↓
       NCO (DDS)
         ↓
@@ -32,6 +32,39 @@ Phase Increment Counter
 
 ---
 
+## 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
+
+---
+
+### 2. CW Mode (Continuous Wave)
+
+- `tx_active = 1` continuously
+- Generates a single-tone output
+- Achieved by setting constant phase increment
+
+---
+
+### 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
+
+---
+
 ## Chirp Generation Principle
 
 The chirp is generated using a second-order phase accumulator:
@@ -72,7 +105,7 @@ States:
 
 ## Timing Behavior
 
-Within each PRI:
+### Pulsed Mode
 
 |<------ PRI ------>|
 |<-- pulse -->| idle |
@@ -80,7 +113,31 @@ Within each PRI:
 - tx_active = 1 → chirp output  
 - tx_active = 0 → output zero  
 
-Chirp is reset at each pulse start.
+---
+
+### 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  
+
+---
+
+## 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
 
 ---
 
@@ -97,7 +154,19 @@ Chirp is reset at each pulse start.
 - Deterministic timing (128 MHz)
 - Efficient DDS (adder-based)
 - Complex output (I/Q)
-- Supports burst-mode radar operation
+- Supports:
+  - Pulsed radar mode
+  - Continuous wave (CW)
+  - Continuous LFM (periodic chirp)
+
+---
+
+## 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)
 
 ---