SynthUSB3: 12.5MHz – 6.4GHz RF Signal Generator


The SynthUSB3 thumb drive-sized RF signal generator is designed for an extremely high quality to cost ratio.  These devices have set the bar on microwave radio frequency generators for quality, size, and price.  As always, our products are designed, manufactured, and supported in the USA!

Tune any frequency between 12.5MHz and 6.4GHz in 0.01Hz resolution.  Adjust calibrated amplitude in 0.2dB resolution up to +8dBm and across more than 50dB of range. The SynthUSB3 can also be used with an optional Breakout Board that allows connections to UART, trigger, and power.

Expect excellent performance and high reliability despite the SynthUSB3’s low cost.

Out of Stock.  Begins shipping again April 30th.

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After almost 10 successful years, the SynthUSBII gets a huge upgrade to SynthUSB3!

The Windfreak Technologies SynthUSB3 is a 12.5 MHz to 6.4 GHz software tunable RF signal generator and frequency sweeper controlled and powered by a device running Windows, Linux, or Android via its USB port. The SynthUSB3 also has nonvolatile on-board flash memory so it can be programmed to fire up by itself on any frequency, power, sweep or modulation setting (and combinations thereof) to run without a PC in the field. This makes for a highly mobile, low power, and lightweight solution for your RF signal generation needs.

Product Highlights

  • Amplitude calibrated for high amplitude accuracy
  • Great phase noise across all frequencies
  • 0.01Hz tuning capability (0.1Hz default)
  • Up to 0.2 dB amplitude resolution
  • Up to +8dBm RF power
  • Linear (or %) sweeping and list mode hopping both frequency and amplitude
  • Control with USB or UART from your embedded microcontroller
  • Trigger sweeps, hops, pulses, etc.
  • Internally or externally modulate FM, AM, or pulse
  • Windows, Linux, and Android compatibility
  • Save all settings to the device for use without a PC
  • 32 bit ARM processor onboard which is Arduino compatible
  • Designed and manufactured in the USA

For orders over 10 units discounts will apply. Request a quote here.
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Download SynthUSB3 Model

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Measure dimensions by downloading the model and installing the free edrawings viewer app..

Weight 1.0 lbs
Dimensions 7 × 5 × 2 in

Features List

  • Calibrated RF output
  • Thumb drive sized
  • Open source Labview GUI software control
  • Run hardware functions with or without a PC
  • USB or UART control via USB3 connector
  • Trigger and Modulation via USB3 connector
  • Open source USB breakout board sold separately
  • As good as 0.2dB amplitude resolution
  • 01Hz frequency resolution
  • 100uS RF lock time standard
  • 250uS per step typical sweep speed
  • Up to +8dBm output power
  • Over 40dB of power control
  • 5ppm internal reference accuracy
  • -115dBc/Hz phase noise at 1GHz @ 10KHz offset
  • Internal and External FM, AM, Pulse Modulation
  • External sweep, step, and modulation trigger
  • 500 point frequency and amplitude hop table
  • 32-bit ARM processor onboard
  • Low Power 100mA typical
  • Designed, manufactured, and supported in the USA


1. SB and UART Warning

The SynthUSB3 was designed to work with a USB 2 port, or a USB 2 cable, which can be plugged into either a USB 2 or USB3 port.  Use a USB 3 cable only when tapping into the UART and Trigger signals for 3.3V COM port control of the SynthUSB3 with your own microcontroller circuit.  Using a USB 3 cable attached to a USB 3 port on a PC may have unknown consequences as the PC is not designed to see the SynthUSB3 UART signals and vv.  See UART app note for UART usage instructions.

A USB 2 to USB 3 breakout adapter board is available for easier access to the UART and trigger, and modulation connections.

UART and Trigger signals are isolated with series 500 ohm resistors to protect the hardware.

2. Characteristics

2.1 Electrical Characteristics

Supply Voltage4.755.5VSuggested 300mA minimum
Supply Current100mA0dBm RF at 1GHz
Standby Supply Current10mARF output OFF
RF Output Frequency Range12.5-6400MHz
Calibrated Frequency Range12.56400MHz
RF Output Power Maximum8dBmSee graph
RF Output Power Minimum-50dBmSee graph
RF OFF Output Power-80dBm100% shutdown of RF section
RF Output Frequency Resolution0.01HzDefault is 0.1Hz selectable by Channel Spacing Setting
RF Output Power Resolution0.25dBBest Case – Non Monotonic
RF Output Impedance50
Internal Reference Frequency27MHz
Internal Reference Tolerance2.5ppm
Trigger-0.33.35VInternally pulled up 5V tolerant
UART-0.33.35V3.3V native, 5V tolerant
RF ConnectorNormally Polarized Female SMA

2.2 Thermal Operating Characteristics

Operating Temperature-4070°C

3. Software and Hardware

3.1 Open Source Software GUI

The included GUI is written in LabVIEW™ and source code vi’s are supplied with the purchase of hardware.  Also supplied is a Windows installer for users that do not own the LabVIEW™ development environment.  All functions of the hardware are accessible by the software.  Custom software developers can download easy-to-use API documents on the website.

SynthUSB3 LabVIEW Software

3.2 USB3 Connector Pinout

The USB3 connector is used in an unconventional way to allow access to UART and Trigger Signals.  As mentioned above, avoid connecting this device directly to a USB3 port on your PC.  Use a USB 2 extension cable instead.

USB3 Pin #Signal NameDescription
2USB D-USB 2.0 differential pair
5UART TxUART SynthUSB TX (hook to host RX)
6UART RxUART SynthUSB RX (hook to host TX)
8N/CTo Processor I/O for future use
9TriggerTrigger Input

3.3 Recommended UART and Trigger Breakout Circuit

SynthUSB3 Recommended Circuits

SynthUSB3 Circuits Illustration

4. Typical Performance

RF Output Power

The typical maximum and minimum output power of the SynthUSB3 is shown below.  This graph is of unleveled operation at the maximum and minimum RF power settings.

SynthUSB3 Power 12.5MHz-6.4GHz

Power levels are settable in 0.1dBm increments via software, but actual RF resolution and thus accuracy are dependent on amplitude.  RF power setting resolution is non-linear with finer resolution at higher output powers.  Onboard calibration is attained through a look-up table unique to each device.  Device calibration is performed at the factory and stored in onboard flash memory.  Calibration is good from 12.5MHz to 6.4GHz.

SynthUSB3 Amplitude Resolution vs Power at 100MHz

SynthUSB3 Amplitude Resolution vs Power at 5GHz

For example, in the Resolution vs Power at 100MHz chart above, resolution at +5dBm of output power is around 0.2dB, which would give a theoretical accuracy of +/- 0.1dB.  With a setting of -20dBm the output power resolution is 3dB, which gives a theoretical accuracy of +/- 1.5dB.

SynthUSB3 Calibration Performance 12.5MHz-6.4GHz

4.2 RF Output Harmonic Content

The typical SynthUSB3 harmonic distortion is shown below.  This data is taken at a leveled fundamental power of 0dBm.

If lower harmonic levels are needed, Windfreak Technologies suggest the use of low cost SMA filters from Crystek and Minicircuits.

Example: Crystek Lowpass Filter – many cutoff frequencies, 1GHz example:  CLPFL-1000, $25

SynthUSB3 Harmonic Content at 0dBm Fundamental Power

4.3 Integer Boundary Spurs

A mechanism for in-band fractional spur creation in all fractional PLL’s is the interactions between the RF VCO frequency and the internal 27MHz reference frequency. When these frequencies are not integer related, spur sidebands appear on the VCO output spectrum at an offset frequency that corresponds to the difference in frequency between an integer multiple of the reference and the VCO frequency. These spurs are attenuated when outside the loop filter which is roughly 50KHz wide.

Example if using the SynthUSB3 27MHz internal reference:  For the fundamental VCO range of 3200MHz to 6400MHz the first integer boundary happens at 27MHz X 119 = 3213MHz, the next at 27MHz X 120 = 3240MHz and every 27MHz thereafter up to 6399MHz. Below the fundamental VCO band the spacing will be affected by the RF divider.

A typical case generating 3213.04MHz would give integer boundary spurs at a 40KHz offset at around 38dBc.  A typical case generating 3213.40MHz would give integer boundary spurs at a 400KHz offset at around 52dBc.  (These were measured with the reference doubler on and PLL ICP set at 15).

4.4 Phase Noise and Jitter

SynthUSB3 250MHz Phase Noise
250MHz Phase Noise (27MHz Internal Reference with REF Doubler Enabled)
SynthUSB3 1GHz Phase Noise
1GHz Phase Noise (27MHz Internal Reference with REF Doubler Enabled)
SynthUSB3 2.5GHz Phase Noise
2.5GHz Phase Noise (27MHz Internal Reference with REF Doubler Enabled)
SynthUSB3 5GHz Phase Noise
5GHz Phase Noise (27MHz Internal Reference with REF Doubler Enabled)

5. Device Information

5.1 Mechanical Dimensions (2” X 1” X 0.5” plastic case)

SynthUSB3 Mechanical Dimensions


  • Wireless communications systems
  • RF and Microwave radios
  • Software Defined Radio (SDR)
  • Radar
  • Automated Test Equipment (ATE)
  • EMC – radiated immunity pre-compliance testing
  • Electronic Warfare (EW) and Law Enforcement
  • Quantum computing and device research
  • Plasma physics
  • Medical research and treatment


  1. Datasheet
  2. Quick Start Guide
  3. Control Software GUI Download
  4. Software GUI Users Guide
  5. Programmers API Guide (command-line tools)
  6. UART / Trigger Breakout Board Guide
  7.  SynthUSB3 Breakout Board