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.