SynthUSB3: RF Signal Generator for 12.5MHz

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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.

Specifications

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

CharacteristicMin.Typ.Max.UnitNotes 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 Impedance50Ω 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 Weight10g

2.2 Thermal Operating Characteristics

DescriptionMinMaxUnitNotes 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.

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 1VBUSPower 2USB D-USB 2.0 differential pair 3USB D+ 4GNDGround 5UART TxUART SynthUSB TX (hook to host RX) 6UART RxUART SynthUSB RX (hook to host TX) 7GND_DRAINGround 8N/CTo Processor I/O for future use 9TriggerTrigger Input

3.3 Recommended UART and Trigger Breakout Circuit

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.

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.

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.

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

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

5. Device Information

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