2-Way RF Switch

The 2-way RF switch is a new addition to the Portsdown system. It decodes a signal sent on the symbol rate switching lines (all 3 high, which was previously unused) which is used to indicate that the LimeSDR output should be used, rather than the Filter-modulator board.

The block diagram shows a comprehensive arrangement with all the features. Many of the blocks are optional – it depends on your requirements.

Schematic

Test results

The test results for the switch (when running from 3.3v) were:

Frequency	Insertion Loss	Isolation
71 MHz	< 0.5 dB	36 dB
146 MHz	< 0.5 dB	35dB
437 MHz	0.8dB	35dB
1255 MHz	1.5dB	21dB
2405 MHz	3.5dB	19dB

The insertion loss at 2405 MHz can be reduced by 0.5 dB by trimming the centre pins of the SMA connectors to be flush with the underside of the PCB. A further improvement could probably be made by trimming the RF tracks on the top of the PCB so that they do not extend beyond the SMA centre pin.

The PCB for the RF switch is available from the BATC shop and construction details are posted on the BATC Wiki.