The spurs can be conquered by a relatively straight forward modification. The circuit diagram for the mod is shown above and the photo below:
The filter is a 45MHz 15Khz type filter identical to the existing roofing filter in the µBITx and replacing R27 in the original circuit. Warren WA8TOD encapsulated the filter, transformer, and capacitor in shrink wrap and connected the ground of the filter to the nearby junction of R13 that is grounded.
Warren’s implementation of the filter uses a single transformer on the output of the crystal filter. A capacitor is also needed between the low impedance transformer output and the center tap of the mixer transformer. He soldered a 0.1 uF SMD, one side to the output pad of R27 and the other side served as a connection for the transformer low impedance output. No doubt a through-hole component could also be deployed.
The filter input is attached directly to the other R27 pad. Warren elected to not use a second transformer because the source is much closer to the filter impedance than it is to 50 ohms.
In Warren’s case the transformer does not dramatically reduce the spurs versus the bare filter. The primary effect, and one that is desirable, is that a much more reasonable level of audio drive is available. The radio is still susceptible to intermodulation distortion because the levels are increased closer to the stock configuration, giving only slightly reduced power output.
Other implementations of the anti-spur filter
A number of others have now implemented the anti-spurs 45 MHz filter and demonstrated that it reduces spurs. uBITx.net believes this modification should be included in the production of the µBITx immediately, in order to clean up the output on SSB.
Glenn VK3PE has made his own board to mount the components for the filter mod on, and Nick VK4PP has designed a board (which will be for sale or included as a freebie with his LPF board). Photos of their board designs follow:
