How do I calculate the conversion loss and noise figure of a passive diode mixer?
Mixer Loss and Noise
Conversion loss in a passive diode mixer comes from several mechanisms: (1) the intrinsic loss of the mixing process (the diode switching waveform is not a perfect square wave), (2) the diode series resistance (which dissipates power), (3) the balun and hybrid losses in balanced designs, and (4) mismatch loss between the mixer ports and the external impedances.
The minimum theoretical conversion loss for an ideal passive mixer is 3.92 dB (for a square-wave LO switching a resistive mixer). Practical Schottky diode mixers achieve 5-8 dB because the LO waveform is sinusoidal (not square), the diodes have finite on-resistance and parasitic capacitance, and the transformer baluns have non-zero loss.
The noise figure distinction between DSB and SSB is critical for system calculations. In a DSB system (both sidebands contain useful signal), the mixer processes both and its noise figure is referenced to the total signal power from both sidebands. In an SSB system (only one sideband contains signal), the mixer still accepts noise from both sidebands but signal from only one, effectively doubling the noise contribution. SSB NF = DSB NF + 3 dB.
Typical values:
Single diode: CL = 6-9 dB
Single balanced: CL = 5-8 dB
Double balanced: CL = 6-8 dB
NF ≈ CL + 0.5 to 1 dB (passive mixer)
SSB NF = DSB NF + 3 dB
Example: mixer with CL = 7 dB
DSB NF ≈ 7.5 dB
SSB NF ≈ 10.5 dB
Frequently Asked Questions
Why is SSB NF 3 dB worse?
Because the image frequency band contributes noise but not signal. The mixer converts noise from both RF and image frequencies to the IF, but only one sideband contains the desired signal. The noise power at the IF is doubled relative to the signal, adding 3 dB to the noise figure.
How does LO power affect conversion loss?
Conversion loss decreases as LO power increases (harder diode switching) until the optimum LO level is reached. Typical optimum: +7 dBm for Level 7 mixers, +13 dBm for Level 13, +17 dBm for Level 17. Beyond the optimum, CL increases slightly due to diode heating and nonlinear capacitance effects.
Can an active mixer have conversion gain?
Yes. Active mixers (Gilbert cell, FET ring) provide conversion gain of 5-15 dB because the mixing transistors also amplify the signal. Active mixers have NF of 8-15 dB (higher than passive mixers), but the system NF contribution is lower because subsequent stage noise is suppressed by the conversion gain.