The RF board contains first mixer, diplexer, roofing XTAL filter and LO divider (by 2).
The mixer is switching type and is fed with square waveform LO signal. During the designing and testing my transceiver I faced a challenge with the birdies problem due to the mixing of the nigh order harmonics of the first and secong LO (both LOs uses square waveform). The careful shielding with feed through caps installed on the power supply lines eliminates almost all of them.
I tryed the fast CMOS switches (74CBT3384) as the mixer switching elements. They provided mixer with lower losses, but the birdies problem gets worther, so I settled on the FETs. The mixer uses the pair of the medium power UHF MOSFETs (they are Russian, KP905A). It is configuration is the half of the well known H-mode mixer.
The very unusual part of the mixer is the diplexer. During the testing I measured the IMD DR / IP3 with various diplexer schematics:
|Diplexer||IMD DR 20 kHz spacing @ 2.4 kHz BW||IP3 20 kHz spacing|
|No diplexer||85 dB||+4.5 dBm|
|50 Ohm termination||92...95 dB||+15...+19.5 dBm|
|Zero termination||102-110dB||+30...+42 dBm|
The 74AC74 IC provides square waveform LO. I used it with the 6V power supply.
After the mixer the signal passes trought the 4 XTALs roofing filter with 3.3kHz@-3dB passband. The main purpose of this filter is to attenuate the strong neaby signals and to suppress the second IF image (32 kHz above the first IF).
The schematics of the RF board is here.