vnasim — Multi-VNA SCPI Communications Simulator

Early development — this project has had very little real-world testing so far. Command coverage for each instrument model is based on driver source code analysis, not validated against physical hardware. Expect missing commands, incorrect responses, and rough edges. Unrecognised commands are logged to vnasim_unhandled.log to help identify gaps.

A TCP-based simulator that emulates multiple VNA instruments over the network. Any SCPI client connects to the simulator exactly as it would to real hardware and receives realistic responses — no physical instruments required.

Quick Start

pip install -e ".[dev]"
python -m vnasim config.yaml

The simulator starts one TCP listener per instrument defined in config.yaml. Connect using a VISA resource string:

TCPIP::localhost::5025::SOCKET

with the appropriate driver type selected.

Supported Instruments

Model	config.yaml type	Default Port
Siglent SNA5000A 2-port	sna5000	5025
Siglent SNA5000A 4-port	sna5000	5026
Keysight E5071B 2-port	e5071b	5027
Keysight E5071C 4-port	e5071b	5028
Keysight E5080B 2-port	e5080	5029
Keysight E5080B 4-port	e5080	5030
Copper Mountain S2VNA	copper_mountain	5031
R&S ZNB8 4-port	rs_znb	5032
Anritsu MS46522B	anritsu_shockline	5033

Configuration

Edit config.yaml to choose which instruments to simulate:

instruments:
  - name: "My SNA5012A"
    model: sna5000
    port: 5025
    num_ports: 2
    idn: "Siglent Technologies,SNA5012A,SNA5XXXX00001,2.3.1.3.1r1"

Each entry needs:

• model — one of the types listed above
• port — TCP port to listen on
• num_ports — number of VNA ports (2 or 4)
• idn — the *IDN? response string (controls auto-detection by the client application)

Proxy / Emulator Mode

Add mode: proxy and a backend: block to forward data operations to a real VNA. The client thinks it's talking to one instrument type while vnasim translates and delegates to a different backend:

  - name: "E5071B → SNA5000 Proxy"
    model: e5071b
    port: 5035
    num_ports: 2
    idn: "Agilent Technologies,E5071B,MY00000001,A.09.00"
    mode: proxy
    backend:
      host: "192.168.1.100"
      port: 5025
      dialect: sna5000

This lets you validate drivers against real measurement data from a different physical instrument. The backend can be a real VNA or another vnasim synthetic instance.

How It Works

SCPI Client  -->  TCP socket  -->  SCPI Parser  -->  VNA Model  -->  Synthetic Data
                  (per instrument)   (tree-based,     (state machine,   (duplexer
                                      short-form       per-channel)      S-params)
                                      matching)

• SCPI Parser — tree-based command router with IEEE 488.2 short-form matching. SENS, SENSE, SENSe all match. Handles numeric suffixes for channel/trace indices.
• VNA Models — each model maintains per-channel state (frequency, points, IFBW, power, averaging, calibration coefficients, etc.) and registers its SCPI command tree. Models inherit from each other where command sets overlap.
• Synthetic Data — generates a realistic 3-port duplexer response (bandpass transmission, passivity-based reflection, TX-RX isolation).

Unhandled Command Logging

When the simulator receives a command it doesn't recognise, it logs a WARNING with the preceding 5 commands and the following 5 commands for context. These are written to both the console and vnasim_unhandled.log (truncated on each restart).

This makes it straightforward to identify and implement missing commands.

Tests

python -m pytest tests/ -v

Adding a New VNA Model

1. Create src/vnasim/models/my_vna.py subclassing an existing model (or VNAModel)
2. Override _build_tree() to register the instrument's SCPI command paths
3. Add the model to MODEL_REGISTRY in models/__init__.py
4. Add an entry to config.yaml

The SCPI parser, TCP server, and synthetic data generation are fully reusable across models.

Architecture

src/vnasim/
    __main__.py            CLI entry point
    server.py              asyncio TCP server (one port per instrument)
    config.py              YAML config loader
    scpi/
        parser.py          Tree-based SCPI command router
        types.py           ParsedCommand and Unhandled types
    models/
        base.py            Abstract VNAModel interface
        common.py          Shared state, handlers, core registration
        mixins.py          SCPI registration mixins per dialect group
        proxy.py           ProxyVNAModel for emulator/pass-through mode
        sna5000.py         Siglent SNA5000A
        keysight_ena.py    Keysight E5071B/C
        keysight_e5080.py  Keysight E5080A/B
        copper_mountain.py Copper Mountain S2VNA/S4VNA
        rs_znb.py          R&S ZNB/ZNA/ZVA
        anritsu_shockline.py Anritsu MS46xxx
    backend/
        client.py          TCP client for real backend VNA
        translator.py      SCPI command translators per backend dialect
    data/
        synthetic.py       S-parameter data generation

Each model composes CommonVNAModel with the registration mixins it needs — no inheritance between instrument models. Proxy models extend ProxyVNAModel instead, delegating data operations to a real backend VNA via a translator.