Add FixedAttenuator class and udev rules for RF test bench

FixedAttenuator supports --attenuator fixed:XX for non-programmable
inline SMA pads (set_db is a no-op returning the fixed value).
Udev rules grant non-root USB access for NanoVNA and HMC472A.

tools/rf_testbench.py

@@ -219,6 +219,30 @@ def _make_mock_skywalker(verbose=False):
     return sw
 
 
+class FixedAttenuator:
+    """Stand-in for a fixed (non-programmable) inline attenuator.
+
+    Reports the declared fixed attenuation for every set_db() call.
+    Used when testing with a fixed pad instead of the HMC472A.
+    """
+
+    def __init__(self, fixed_db: float = 20.0):
+        self._fixed_db = fixed_db
+
+    def status(self) -> dict:
+        return {"attenuation_db": self._fixed_db, "step": 0,
+                "version": "fixed-pad", "note": "non-programmable"}
+
+    def set_db(self, attenuation_db: float) -> dict:
+        # Can't change a fixed pad — just return what it is
+        return self.status()
+
+    def config(self) -> dict:
+        return {"db_min": self._fixed_db, "db_max": self._fixed_db,
+                "db_step": 0, "version": "fixed-pad",
+                "hostname": f"fixed-{self._fixed_db:.1f}dB"}
+
+
 class MockHMC472A:
     """Mock attenuator for testing without hardware."""
 
@@ -745,7 +769,8 @@ hardware setup:
                         help="Path loss calibration CSV (NanoVNA S21 sweep)")
     parser.add_argument("--attenuator", type=str, default="auto",
                         help="HMC472A connection: 'auto' (USB then HTTP), "
-                             "/dev/ttyACMx (USB serial), or http://host (REST) "
+                             "/dev/ttyACMx (USB serial), http://host (REST), "
+                             "or 'fixed:20' for a non-programmable pad "
                              "(default: auto)")
     parser.add_argument("--nanovna", choices=["auto", "manual"],
                         default="auto",
@@ -794,11 +819,22 @@ hardware setup:
 
 
 def _connect_attenuator(target: str):
-    """Connect to HMC472A via auto-detect, USB serial, or HTTP REST.
+    """Connect to HMC472A via auto-detect, USB serial, HTTP REST, or fixed pad.
 
     Args:
-        target: "auto", a serial port path (/dev/ttyACM*), or an HTTP URL.
+        target: "auto", "fixed:XX" (dB), /dev/ttyACM* (USB), or http://... (REST)
     """
+    # Fixed attenuator mode (non-programmable inline pad)
+    if target.startswith("fixed:"):
+        try:
+            fixed_db = float(target.split(":", 1)[1])
+        except ValueError:
+            print(f"HMC472A: invalid fixed value '{target}' (use fixed:20)")
+            sys.exit(1)
+        atten = FixedAttenuator(fixed_db)
+        print(f"HMC472A: fixed {fixed_db:.1f} dB pad (non-programmable)")
+        return atten
+
     # Auto-detect: try USB serial first, fall back to HTTP
     if target == "auto":
         port = detect_hmc472a_serial()

udev/99-rf-testbench.rules

@@ -0,0 +1,24 @@
+# RF Test Bench udev rules
+# Install: sudo cp udev/99-rf-testbench.rules /etc/udev/rules.d/ && sudo udevadm control --reload-rules
+#
+# Provides non-root access and stable /dev symlinks for:
+#   /dev/skywalker1  - Genpix SkyWalker-1 DVB-S receiver (USB bulk device, not serial)
+#   /dev/nanovna     - NanoVNA-H vector network analyzer (ttyACM)
+#   /dev/attenuator  - HMC472A digital attenuator on ESP32-S3 (ttyACM)
+
+# --- Genpix SkyWalker-1 (09c0:0203) ---
+# Custom firmware: Product="SkyWalker-1 Custom", Serial="0001"
+# Stock firmware:  Product="Genpix SkyWalker-1", Serial="00857"
+# Kernel dvb_usb_gp8psk driver blacklisted in /etc/modprobe.d/blacklist-gp8psk.conf
+SUBSYSTEM=="usb", ATTR{idVendor}=="09c0", ATTR{idProduct}=="0203", MODE="0666", SYMLINK+="skywalker1"
+
+# Cypress FX2 bare/unprogrammed (04b4:8613) - for recovery/development
+SUBSYSTEM=="usb", ATTR{idVendor}=="04b4", ATTR{idProduct}=="8613", MODE="0666"
+
+# --- NanoVNA-H (0483:5740) ---
+# Match on model to avoid hitting other STM32 CDC devices
+SUBSYSTEM=="tty", ATTRS{idVendor}=="0483", ATTRS{idProduct}=="5740", ATTRS{product}=="NanoVNA-H", MODE="0666", SYMLINK+="nanovna"
+
+# --- HMC472A attenuator on ESP32-S3 native USB CDC ---
+# Espressif VID=303a, PID=1001 (USB JTAG/serial), match on product string
+SUBSYSTEM=="tty", ATTRS{idVendor}=="303a", ATTRS{product}=="hmc472a-attenuator", MODE="0666", SYMLINK+="attenuator"