Spectral purity


Make your rig compliant by removing unwanted spurs

Your µBITx v3 or v4 board is almost certainly not compliant on bands above 18MHz as supplied.  It has been shown that most boards radiate spurs that may only only be 25dB down on your intended frequency of operation.  These spurs appear to be generated in the 45MHz mixer stage.

A simple fix has been found to these issues.  This involves replacing L5 and L7 with 680nH surface mount 1210 inductors as illustrated in the photo above (do not use 1206 surface mount inductors as they are lossy).   The 1210 inductors are type EPCOS TDK MURATA.  They can be found on ebay in an assorted pack of surface mount inductors or in a pack of just this value.

v5 boards include other modifications that should eliminate spurs.

Make your rig (mostly) compliant by removing unwanted harmonics

While the Low Pass Filters in the output stage of your µBITx work fine at suppressing harmonics, issues with the layout of this part of the circuit board on v3 and v4 boards and the choice of switching relays will have reduced their effectiveness.   For most operation on the low bands, your output is likely to be non-compliant with US and international requirements for out of band emissions.

The good news is that a fix has been uncovered for v3 and v4 boards  that works for most bands/modes and is reasonably straight forward.  v5 boards do not require modification.

The fix involves replacing the existing four relays near the antenna connection with Axicom D2n 12v relays (model V23105-A5403-A201).  They cost around US$3 each and have different internal configuration of the switch and coil components, which lowers coupling between the RF line and the coil.

Removing these relays involves heating each relay lead with a soldering iron and using soldering wick to remove the solder.  Use of solder flux can help with removing the solder.  When all of the pins are free of solder you can replace the relay.  Take your time and be careful not to damage your board.

Note that it is possible that on some bands/modes your rig may be non-compliant even after doing this fix.   However, this fix is by the far the most straightforward of the mods available for removing harmonics and is, therefore, recommended.