Harmonic Mixer
Understanding Harmonic Mixers
Harmonic mixers extend the frequency range of mixer-based receivers and converters by leveraging harmonics of lower-frequency LO sources. This is critically important at mmWave and sub-THz frequencies where generating fundamental LO power is difficult and expensive.
Harmonic Mixer Operation
- 2nd harmonic: f_IF = f_RF - 2 x f_LO (or 2f_LO - f_RF). LO at half the RF. Conversion loss 10-15 dB. Most common.
- 3rd harmonic: f_IF = f_RF - 3 x f_LO. LO at one-third RF. Conversion loss 18-25 dB.
- 4th and higher: f_IF = f_RF - N x f_LO. Conversion loss increases rapidly.
Anti-Parallel Diode Pair
Sub-harmonic mixers often use anti-parallel diode pairs (APDP). The anti-parallel configuration naturally suppresses the fundamental mixing product and enhances the 2nd harmonic product, providing lower conversion loss and better spurious performance for 2xLO mixing.
Frequently Asked Questions
What is a harmonic mixer?
A harmonic mixer uses a harmonic of the LO for frequency conversion, allowing a lower LO frequency. A 2nd harmonic mixer at 60 GHz only needs a 30 GHz LO. The trade-off is higher conversion loss compared to fundamental mixing.
Why use harmonic mixing instead of fundamental?
At mmWave frequencies (>50 GHz), generating sufficient LO power at the RF frequency is difficult and expensive. Harmonic mixing allows using LO sources at half or one-third the frequency, where more power is available at lower cost.
What is the conversion loss penalty?
2nd harmonic mixer: 10-15 dB conversion loss. 3rd harmonic: 18-25 dB. Fundamental mixer: 6-8 dB. The additional loss must be compensated by IF amplifier gain in the receiver noise figure budget.