SWRA004A
Low-Band Mixer
The purpose of the mixer in a down-conversion receiver is to translate incoming signals from one frequency to another. The low-band mixer in the TRF1500 is a three port high-side injected circuit. The mixer takes two known input signals, a radio frequency (RF) signal and a local oscillator (LO) signal and mixes them together to create a sum and difference intermediate frequency (IF). High-side injection means the LO is higher in frequency than the RF by the IF frequency. The output of the mixer is the IF and contains the difference and the sum of the RF and LO signals.
The difference of the RF and LO signals is the desired IF frequency in a down-conversion receiver. The undesired signal, the sum of the RF and LO frequencies, can be attenuated by using a low pass filter. The low-band mixer section of the TRF1500 is a Gilbert cell design with open collector outputs. The Gilbert cell structure was used for its robust isolation and harmonic suppression characteristics.
The TRF1500 mixer typically achieves a noise figure of 7.5 dB with an input third order intercept point of 3.5 dBm. Stand-alone mixer performance can be ascertained by reconfiguring the evaluation board as noted on the datasheet.
Low-Band Mixer RF Input
Figure 7 details the mixer RF input configuration. The signal from the LNA passes through the external image-reject SAW filter and back into the device’s low-band mixer input terminal (MIX_IN_LOW_BAND). Minimal mixer input impedance matching is required. A high-pass shunt-L (L11) and series-C (C13) network are used for impedance matching the SAW filter output to the mixer RF input. The shunt inductor presents a short at the IF frequency. This configuration minimizes the IF leakage and prevents unwanted interfering signals at, or near, the IF frequency from degrading the mixer’s noise figure performance.
Figure 7. Low-Band Mixer RF Input Configuration
22 | TRF1500 Integrated Dual-Band RF Receiver User’s Guide |
