The T03DSP High Performance Transceiver with DSP IF processing

The IF Board

The IF board contains RX blocks: first IF amplifier, 2 XTALs filter, the second RX mixer, secong IF amplifier. And TX blocks: first TX mixer, first IF TX amplifier.

The design of the first IF amplifier is taken from the CDG2000 project. The only change is the replacement of the 4 low power JFETs (J310) with the one high power Russian JFET (KP903A). It provides a stable 18 dB gain with the 50 Ohm input impedance, very low noise figure (2-3 dB) and high dynamic range (IMD DR >100dB @ 2.4 kHz BW). The very important part is feedback transformer.

The two XTALs filter removes the IF amplifier noise in the second IF image. Without it the noise figure should degrade by 3 dB theoretically.

The RX/TX mixer is made from the dual CD4052 analog switch (one half is used for RX and anohter for TX). It is fed with the square LO waveform and provides adequate performance.

The second IF amplifier is a very important part of this design. It should provide a very low IMD and noise figure. I used one of the best available audio OP amplifier AD797. The variable resistor on the IF board output sets the overal RX analog gain.

The transmitt part of the board uses usual design. The attenuator at the IF amplifier output provides better maching with the XTAL filter on the RF board.

The schematics of the IF board is here.