This new revisions builds up on the schematic resources of the Dual Source.
The biggest changes in this design are a more powerful microcontroller and an external clock input.
The PIC18F67J50 with its high count of additional pins replaces the old microcontroller with the GPIO expander.
Moreover, a new I2C external EEPROM allows the storage of user configuration and calibration data.
In order to allow an synchronization of the microwave signals an extra SMA input allows the user to feed in an external clock.
A subsequent RF-switch allows to switch between the Local Oscillator (19.2 MHz) and the external input (< 100 MHz).
Fig. 1. - Rendered 3D Model from KiCad.
Schematic. Click for PDF.
The new layout includes more space for the RF-traces and an ground shielding around the PCB outline and the two PLLs.
Mounting holes for an aluminium case are also provided. Additionally, a 50 Ω matching at the output of the SMA connector for 6 GHz is attained by structuring
the underneath ground layer. The geometry of this plane has been verified by a field simulation with CST Microwave Studio®.
Fig. 2. - Board Layout from KiCad.
Power Supply Hierarchy
The basic power supply system is derived from the components of the Single Source
Here two synchronized step-down switching regulators create two basic voltage rails with 3.6 V and 5.0 V respectively.
To assure a stable noise free supply for both PLLs two seperate ultra-low noise LDOs are used in combination with several ferrite beads.
The digital logic is provided by a common 3.3 V switching regulator.
The internal reference clock for the PLL requires an input of 1.8 V which is supplied by a respective LDO.
Fig. 3. - Functional Diagram of the Power Supply Hierarchy.
New RF Power Detector Circuit
Except of the aforementioned changes, the old RF Power Detector Circuit has been replaced due to problems with its small BGA package and measurement gain.
The new Power Detector LTC5532 features a higher precision and controllable gain within a frequency range of 300 MHz to 7 GHz.
In order to measure the RF power at the output a splitter has to be designed.
Due to the wide bandwidth a resistive tap is the best choice here.
The coupling factor of this structure can easily be calculated by using th internal impedance of the detector IC:
The designed circuit aims for a coupling factor of 20dB, which is equivalent to (here ).
The value of this coupling factor is highly depended on
the parasitics and accuracy of the chosen Resistor and its geometry in the layout. Luckily the LTC5532 allows an adjustable
measurement gain. An additional output pin of this IC (