Filter Modulator Version 1A Modifications
The following modifications improve the transmitted spectrum of the Version 1 PCBs, and bring them to Version 1A standard:
1. Reduce breakthrough (around 130 - 160 MHz) of the harmonics of the filter clock: Solder a 10n chip capacitor on the underside of the PCB between the +5v rail and ground near U10. The capacitor needs to be fitted between the thru-hole which feeds pin 5 of U10 and the adjacent square ground pad ON THE UNDERSIDE OF THE BOARD. You can just see it in the upper left of this photo.