SynthUSB3: 12.5MHz – 6.4GHz USB RF Signal Generator

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

Characteristic	Min.	Typ.	Max.	Unit	Notes
Supply Voltage	4.7	5	5.5	V	Suggested 300mA minimum
Supply Current		100		mA	0dBm RF at 1GHz
Standby Supply Current		10		mA	RF output OFF
RF Output Frequency Range	12.5	-	6400	MHz
Calibrated Frequency Range	12.5		6400	MHz
RF Output Power Maximum		8		dBm	See graph
RF Output Power Minimum		-50		dBm	See graph
RF OFF Output Power			-80	dBm	100% shutdown of RF section
RF Output Frequency Resolution	0.01			Hz	Default is 0.1Hz selectable by Channel Spacing Setting
RF Output Power Resolution	0.25			dB	Best Case – Non Monotonic
RF Output Impedance		50		Ω
Internal Reference Frequency		27		MHz
Internal Reference Tolerance		2.5		ppm
Trigger	-0.3	3.3	5	V	Internally pulled up 5V tolerant
UART	-0.3	3.3	5	V	3.3V native, 5V tolerant
RF Connector					Normally Polarized Female SMA
Weight		10		g

2.2 Thermal Operating Characteristics

Description	Min	Max	Unit	Notes
Operating Temperature	-40	70	°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 Name	Description
1	VBUS	Power
2	USB D-	USB 2.0 differential pair
3	USB D+
4	GND	Ground
5	UART Tx	UART SynthUSB TX (hook to host RX)
6	UART Rx	UART SynthUSB RX (hook to host TX)
7	GND_DRAIN	Ground
8	N/C	To Processor I/O for future use
9	Trigger	Trigger 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)