LO filters

Testing on the 3 prototype boards has shown that, due to the square wave output of the ADF4351, it will need filtering between the output of the ADF4351 module and the modulator board LO input when used on 146 and 437 MHz. This filter only needs to be a simple low pass filter to remove the third harmonic and no filtering is required on 23cms.

The effect of running without a high pass filter on 146 and 437 is an imbalance in the modulator resulting in reduced MER and RF performance as seen on the screen shot below - it does not affect the spectrum transmitted and is OK to put on air.

Performance of the Portsdown modulator board with and without filtering on the LO input is shown here.

The project team have designed 2 simple single band filters which can be hand built very easily and is working on a design for a multi band PCB automatically switched to the correct band by the Portsdown software.

NOTE: These are simple low pass filters and not suitable to be used on the output of the modulator filter board - see this wiki page for filters designed for transmitter and receiver filtering.