Therefore, since the signal from V3 is not present at the outputs of A1 and A2, the attenuation from V3 at 1 MHz to the mixer outputs is infinite. The 1 MHz inputs that each mixer receives are totally suppressed at the mixer outputs. The reason is that the balanced mixers A1 and A2 each deliver two outputs, one at the sum frequency of 1 MHz plus 1 MHz which equals 2 MHz and the other at the difference frequency of 1 MHz minus 1 MHz which equals zero Hz (hence the name zero frequency IF). However, the Bode plotter does not respond to this. Please consider this zero-frequency IF circuit from my Living Analog blog entitled, Zero frequency IF.įor the input V3, at the same frequency as the local oscillator represented by V1 and V2 at 1 MHz, the signal output at the junction of R3 and R4 replicates the signal input.

This is great, but there is a SPICE booby trap waiting for the unwary user of the Bode plotter.