§7.3 FET mixers
Advantage: conversion gain, higher dynamic range. (1dB compression、
3rd order intercept) Disadvantage:
① Higher noise figure than diode mixer, because of larger current. The proper comparison should include the cascade effect of adjacent stages considering the conversion gain. ② small bandwidth than diode mixer. ③ may be self-exciting.
conversion gain.
④ Minimum LC often occurs for LO power between 5 and 13dBm, which means nonlinear analysis is required.
4. Noise figure
① Noise is generated by diode or transistor elements, thermal sources due to resistive losses. ② Diode mixers generally achieve lower noise figures than transistor mixers. ③ The power of an SSB signal is one-half that of a DSB signal.
For resistive mixers:
FDSB LC
F 1 Te T0
Te
nT 2
( LC
1)
n≤2,
Diode ideality factor , T=T0
§7.2 Diode mixers—resistive mixers
Analysis method: ① Small-signal approximation
1. Frequency Conversion
Transmitter up-conversion Receiver down-conversion
2. Image Frequency
① For down-conversion f RF1 f L0 f IF fRF2 fL0 fIF
They are indistinguishable at the IF stage, unless to reselect the signal and stop the noise or interference.
② f L0 f RF f IF noise of LO
③
f2L0 fRF fIM
f IM f L0 f L0 f RF
Two times mixing
f2Lo is the two times harmonies of LO, coming from inside or outside of mixer.
In receiver:
① LO RF port, radiated from the antenna,
Solved by BPF or amplifier.
② LO IF port, solved by filter.
In transmitter:
LO, IF RF Solved by filter
F Si / N0 S0 / N0
FSSB 2FDSB
SiDSB 2SiSSB
SODSB 4SOSSB
FDSB
SiDSB / N0 SODSB / N0
2SiSSB / N0 4SOSSB / N0
1 2 FLeabharlann SBFor SSB signal:
①
② Image reject mixers
5. Isolation
3. Conversion loss
LC
10lg
PRF PIF
① It comes from resistive losses and frequency conversion
process.
② Diode mixers have conversion losses several dB.
③ Transistor mixers have lower conversion loss and may have
Microwave and RF Design of Wireless Systems
Chapter 7
Mixers
Dr. Zhang Yonghong
A mixer is a three-port device, that uses a nonlinear or time-varying element to achieve frequency conversion.
Filter must be used to select the desired frequency.
§7.1 Mixer characteristics §7.2 Diode mixers §7.3 FET mixers
§7.1 Mixer Characteristics
1. Frequency Conversion 2. Image Frequency 3.Conversion Loss 4. Noise Figure 5. Isolation
Assume the signal power presented to the diode is small enough that a Taylor series approximation can be made for the diode current in terms of the diode voltage. Useful for many rectifier, detector and mixer. ② Large-signal: using a fully nonlinear analysis. ③ Switch model.