Wednesday, October 21, 2009

N2PK VNA - RF-IV sensor PCB

I've completed the PCB for the RF-IV sensor. Now I have to come up with an enclosure. In addition, I have to install a mini-DIN connector on the VNA's rear panel with power and control signal for the RF-IV sensor.

The PCB is very simple and easy to work with. A thing to note is the slightly awkward placement of the RF connectors - it is needed to achieve certain level of port-to-port isolation. There are two transformers, one (on the left) takes the Current sample, the other (on the right) the Voltage sample - this part of the circuit is almost the same as in Larry's N8LP coupler for the LP-100. The DUT is connected to the RF DDS of the VNA thru the current transformer. The RF switches (Peregrine PE4220 ) are controlled by the software and switch the signal path of the samples to the RF DET input of the VNA. While the VNA takes a sample of the current (I), the voltage (V) transformer is terminated with 50 ohm load and vice versa. There is an on-board 3.3V voltage regulator supplying power to the RF IC switches. Only one detector is used in the VNA in this configuration which improves the stability and accuracy of the measurements as both -the I and the V samples are measured by the same detector alternatively at a high frequency. The control signal to switch between the samples is generated by the software and in this case is just looped thru the VNA.

Winding and installing the transformers is a bit tricky because of their construction and small size. One of the winding is done with very fine (AWG #36) wire. In addition, the current transformer (shown on the picture) has a grounded electrostatic shield between the primary and the secondary. Both transformers are attached to the board with tin-plated brass strips bent into U-shape. The strips provide RF screening and stress-relief at the same time.

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