Softrock Lite II 06_Mixer

The RF input signal, filtered by the BPF is applied in antiphase to the inputs 1B and 2B of the Mixer U3.

The two LO signals from the Divider Stage operate the switches which connect R1 to C5 and connects R2 to C6 during the first clock cycle.

When the LO Clock changes 90 degrees later, the connections reverse: R1 now connects to C6 (Q) and R2 connects to C5 (I).

This switching sequence then repeats itself.

The resulting RF input signal is sampled over capacitors C5 and C6 as the Intermediate Frequency (IF)

On the lower bands (160m, 80m, and 40m) the dividers are clocked at the desired center frequency, which is in the pass band for the incoming RF.

On the higher bands, the situation in the Softrocl RX is a little different. The clocking frequency is at the one-third sub harmonic of the desired center frequency and is NOT within the passband of the incoming RF.

For example, consider the 20m RX:

• For 20m, the dividers are clocked at about 18.73 MHz and their output QSD clock is 18.73 MHz / 4 = 4.682 MHz
• The third harmonic of that clock frequency is 3 * 4.682 = 14.047 MHz.
• The 20m signals in the BPF's passband will be sampled at that 3rd harmonic; however, the sampling will not yield as strong an I/Q pair as does the sampling technique used in the lower bands. Hence, the higher gain OpAmps for the higher band kits.
• It is like looking at a rotating wheel with a strobe flashing once for every three revolutions of the wheel. The rotation speed of the wheel is down converted but the image is not as bright as it would be if you flashed the strobe at the rotation speed of the wheel.

If you are interested, you might want to review the "Tayloe Mixer" operation. While the Softrock mixer is not a pure Tayloe mixer, the theoretical discussion on Taylo mixers helps with understanding how this process works.