Filter Modulator Version 1A Modifications
The following modifications improve the transmitted spectrum of the Version 1 PCBs, and bring them to Version 1A standard.
Spectrum before modification (1255MHz 333ks 50 Mhz span)
Spectrum after modification
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. The capacitor is visible in the upper left of this photo.
2. Reduce filter clock breakthrough on the I and Q signals:
Solder 300 pf capacitors (chip or discrete) between the downstream ends of R15 and R16 and ground on the top side of the board. These capacitors form simple low pass filters with R15 and R16 to reduce clock breakthrough from the switched-capacitor filters. There is a convenient (square) ground pad just "north" of R15 and R16. A short wire link is required if using a chip capacitor adjacent to R16. Both are visible in this photo.
3. Reduce the level of the drive to the IQ modulator at higher SRs (1000 KS and above).
The original design slightly overdrove the IQ modulator at higher SRs, resulting in higher than necessary "shoulder" levels and differing power output depending on SR selection. This modification reduces the drive amplitude, but also affects the modulator balance and may require some "tweaking" of resistor values.
First, R03 and R04 (both 2400 ohms) need to be removed and replaced with 3300 ohm resistors. This reduces the I and Q levels to within the modulator specification but upsets the balance. Then R05 and R06 need changing from 4700 to 3830 ohms to restore the balance. You may find that a parallel resistor on R05 or R07 (for the I channel) or R06 or R08 (for the Q channel) may be required to align the quadrature balance point with that set for the lower (<1000K) SRs using RV1 and RV2.