VNA Hardware Trigger Wiring

Hardware trigger synchronization (Tier 2) achieves ±10-50µs timing between VNAs—100x tighter than software coordination. This guide shows you how to wire the trigger lines between devices.

When You Need This

Use Case	Tier 1 (Software)	Tier 2 (Hardware Trigger)
Antenna comparison	✓	✓
SWR/impedance	✓	✓
Signal level measurements	✓	✓
Transmission path timing	Limited	✓
Differential S21	Limited	✓
Multi-VNA transfer functions	✗	✓

For phase-aligned measurements, you’ll also need external clock (Tier 3).

Parts Needed

• Jumper wires (Dupont or similar)
• Soldering iron and solder (fine tip recommended)
• NanoVNA-H with custom firmware supporting trigger commands

Caution

This modification requires soldering to small test points. If you’re not comfortable with fine soldering, consider having an experienced friend help.

Pin Locations

The trigger system uses two GPIO pins on the STM32F303:

Pin	Function	Location	Description
PA0	TRIG_IN	See board diagram	External trigger input (rising edge)
PB14	TRIG_OUT	See board diagram	Pulse output at sweep start

NanoVNA-H Rev 3.4 Board

┌─────────────────────────────────────────┐
│                                         │
│   ┌───────────────────────┐             │
│   │                       │             │
│   │       LCD             │             │
│   │                       │             │
│   └───────────────────────┘             │
│                                         │
│                                         │
│   [PA0]●                      ●[PB14]   │
│   TRIG_IN                     TRIG_OUT  │
│                                         │
│   ┌──┐                         ┌──┐     │
│   │P1│                         │P2│     │
│   └──┘                         └──┘     │
│  Port 1                       Port 2    │
└─────────────────────────────────────────┘

Note

Pin locations vary by board revision. On some boards, these pins are on test points. On others, you may need to solder directly to the MCU. Consult your board’s schematic.

Wiring Steps

1. Identify the leader VNA

   Choose one VNA as the “leader”—it will generate the trigger pulse that starts all sweeps. The others are “followers.”

2. Solder wires to pins

   On the leader VNA:

   • Solder a wire to PB14 (TRIG_OUT)

   On each follower VNA:

   • Solder a wire to PA0 (TRIG_IN)

   Use thin wire (30 AWG or similar) to avoid stressing the pads.

3. Connect the trigger bus

   Wire all followers’ TRIG_IN lines to the leader’s TRIG_OUT:

   Leader VNA                    Follower VNA #1
   ┌─────────┐                  ┌─────────┐
   │  PB14   │──────────────────│   PA0   │
   │(TRIG_OUT)│                  │(TRIG_IN) │
   └─────────┘         │        └─────────┘
                       │
                       │        Follower VNA #2
                       │        ┌─────────┐
                       └────────│   PA0   │
                                │(TRIG_IN) │
                                └─────────┘

   For more than 2 followers, daisy-chain or use a splitter.

4. Verify ground reference

   All VNAs share ground via USB (same host computer). No additional ground wire needed.

   If using VNAs on different computers, add a common ground wire.

5. Test trigger signaling

   Use a multimeter or oscilloscope to verify:

   • PB14 can be toggled (use raw_command("trig_out pulse"))
   • PA0 sees the pulse on followers

Firmware Requirements

Stock NanoVNA firmware doesn’t include trigger commands. You need custom firmware with:

Command	Description
trig_mode {software|hardware|leader|follower}	Set trigger mode
trig_out {on|off|pulse}	Manual trigger control
trig_status	Read trigger state

Check capability with detect_capabilities():

{
  "device_id": "0001234567",
  "capabilities": {
    "hardware_trigger": true,
    "external_clock": false
  },
  "tier": 2
}

Tip

See the NanoVNA-H community firmware for trigger-enabled builds, or build your own from the ChibiOS-based source.

Using hardware_sync_sweep

Once wiring is complete and firmware supports triggers:

Say: “Run a hardware-synced sweep from 144 to 148 MHz with device 0001234567 as leader”

The assistant calls:

hardware_sync_sweep(
  leader_device_id="0001234567",
  follower_device_ids=["0009876543"],
  start_hz=144000000,
  stop_hz=148000000,
  points=101
)

This automatically:

1. Sets leader to “leader” mode
2. Sets followers to “follower” mode
3. Runs parallel scans (hardware synchronized)
4. Restores “software” mode

Testing the Trigger Line

Manual test via raw_command

# On leader - generate pulse
raw_command(device_id="0001234567", command="trig_out pulse")

# On follower - check status
raw_command(device_id="0009876543", command="trig_status")

Expected output: trigger_count should increment.

Oscilloscope verification

• Pulse width: ~10µs
• Rising edge: < 1µs
• Voltage: 3.3V logic level

Troubleshooting

Trigger not detected

1. Verify wiring continuity with multimeter
2. Check firmware has trig_mode command: raw_command("help")
3. Ensure ground is shared between VNAs
4. Try shorter trigger wire (< 30cm recommended)

Erratic triggering

1. Add 100Ω series resistor on TRIG_OUT to reduce ringing
2. Keep trigger wire away from RF paths
3. Use shielded wire for long runs (> 30cm)
4. Check for EMI from nearby equipment

Sweeps still not synchronized

1. Verify both VNAs have same sweep settings (start, stop, points)
2. Check trigger mode is correctly set before sweep
3. Ensure leader starts sweep before followers timeout

Capability shows false

1. Firmware may not support triggers
2. Flash custom firmware with trigger support
3. Run detect_capabilities() after firmware update

Electrical Specifications

Parameter	Value
TRIG_OUT pulse width	10µs
TRIG_IN threshold	1.6V (TTL-compatible)
Maximum trigger cable length	1m (unshielded), 5m (shielded)
Edge-to-sweep latency	< 50µs

Next Steps

• External Clock Modification — Add Tier 3 phase coherence
• Multi-VNA Coordination — Understand all synchronization tiers
• Tool Reference — Hardware trigger tools