SynthHD (v2): 10MHz – 15GHz Dual Channel Microwave RF Signal Generator

Specifications

1. USB and UART Warning

The SynthHD v2 was designed to work with, and ships with, a USB 2 cable. Use a USB 3 cable only when tapping into the UART signals for 3.3V COM port control of the SynthHD 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 SynthHD UART signals and vv. See UART app note for UART usage instructions.

2. Characteristics

2.1 Electrical Characteristics

Note 1: Phase tuning speed, phase resolution and carrier frequency are inter-related. Phase tuning speed slows as RF carrier frequency and Channel Spacing settings decrease. Smaller Channel Spacing will have higher phase and frequency resolutions but slower phase tuning speed. Going below 100MHz carrier with smaller Channel Spacing than 100Hz may be prohibitively slow and/or erratic.

2.2 Thermal Operating Characteristics

3. Typical Performance

3.1 RF Output Power

The typical output power (per channel) of the SynthHD is shown below. This graph is of raw unleveled operation at both the maximum and minimum gain settings of the output variable gain section. Gain is set via dBm power commands allowing output power levels anywhere in the range between the minimum and maximum levels shown below. RF port power and frequency can be set independent of each other. Power levels are settable in 0.01dBm increments. On board calibration is attained through a look up table for each channel. Device calibration is performed at the factory and stored in onboard flash memory. Calibration is good from 10MHz to 20GHz. Operation from 20GHz to 15GHz is uncalibrated and unspecified. All parts of the signal chain have high quality voltage regulation, and the D/A’s driving the VGAs have a 1% voltage reference controlling their outputs.

3.2 RF Output Harmonic Content

The typical SynthHD harmonic distortion is shown below for the second and third harmonics. 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

3.3 RF Port to Port Isolation

Port to port isolation is shown below with both channels at 0dBm output power. One trace is taken with a 3MHz offset between channels. The other trace is taken with a 20KHz offset between channels. The 20KHz offset places each signal within each other’s loop bandwidth and the leakage modulates each other’s VCO control voltages where loop gain is high. Offsets inside the loop bandwidth will have worse isolation. The below data is taken with both PLL_ICP settings at 15.

Note: The plot above shows the conducted port at 5.0 GHz. The terminated port is set at 5.001 GHz. The spur location is at a 2MHz offset since the SynthHD uses an RF divide by 2 circuit to achieve 5GHz. As the RF frequency decreases, the isolation spurs move out, until they are eventually outside of the loop bandwidth and thus significantly attenuated.

3.4 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, internal 10MHz or arbitrary external 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 30KHz wide. By having two selectable internal reference frequencies of 10MHz and 27MHz the problem is eliminated by switching reference frequencies when working around a boundary.

Example if using the SynthHD 27MHz internal reference: For the fundamental VCO range of 3400MHz to 6800MHz the first integer boundary happens at 27MHz X 126 = 3402MHz, the next at 27MHz X 127 = 3429MHz and every 27MHz thereafter up to 6777MHz. Above and below the fundamental VCO band the spacing will be affected by the RF doubler or RF divider respectively. If the desired VCO operating frequency is 3402.01MHz this would give spurs 10KHz on either side of the carrier that may be unacceptable. In this case, using the 10MHz reference would be suggested since its closest integer boundary is at 3400MHz. Spurs 2MHz away will be attenuated to satisfactory levels by the loop filter.

3.5 Phase Noise and Jitter

3.6 Intermodulation Distortion after an External Wilkinson Combiner

It’s possible to lock both channels in both frequency and amplitude for easy tuning during IMD testing of IP3 in passive and active components. The plot below shows two tones combined with a YL-70 0.5-2.0 GHz KL combiner that has roughly 20dB of isolation between ports. The tones are centered at 1GHz and separated by 1MHz. This method will allow for IP3 testing below roughly +40dBm.

3.7 Performance over Temperature

The SynthHD has an algorithm to reduce amplitude drift over temperature. The user can specify 4 different control settings in the firmware.

1. No temperature compensation
2. Compensation only during a frequency or amplitude setting
3. #1 plus periodic adjustments every 1 second
4. #1 plus periodic adjustments every 10 seconds (factory default)

Temperature compensation 2 and 3 are turned off during active modulation. Frequency sweeps and hops are only compensated for each step unless the setting is 0.
All subsequent temperature plots are based on an internal temperature measurement with temperature compensation turned ON. When the SynthHD has one RF channel turned on in a lab environment with moderate air flow at 25°C the internal temperature will usually be around 40°C.

4. Device Information

4.1 Mechanical Dimensions

4.2 Product Rendering