AK2401 Datasheet, PDF (32/103 Pages) Asahi Kasei Microsystems – Direct Conversion Transceiver

[AK2401]
13.5. Analog Receiving Circuit (MIXER, PGA, AAF)
RF signal that is gained by LNA is down converted to a baseband signal by the MIXER with direct
conversion method. The MIXER is operated by two local signals that have a 90 degree phase difference
and generates Ich/Qch baseband signals.
A PGA consists of a first order low-pass filter (Fc= 45 kHz or 90 kHz) that gain is changeable by register
settings. It gains the dynamic range by keeping input level constant for the ADC that is disposed in the
output stage of the PGA. The AK2401 has the AGC function that changes PGA gain automatically
according to the input signal level. Refer to “13.8.8 AGC” for details of AGC function.
An AAF consists of a third order low-pass filter (Fc=100kHz). It is an anti-aliasing filter of the following
ADC. An analog filter is composed by the PGA and AAF attenuating blocking signal that is over 100kHz
into the ADC.
13.5.1. MIXER
The polarity of I/Q demodulator is shown below (Figure 18). It is designed to have 90 degrees phase
difference between the Ich and Qch.
cosω_st I(t)
s(t)=cos(ω_LO+ω_s)t
cosω_LOt
0°
90°
-sinω_LOt
sinω_st Q(t)
Figure 18. I/Q Demodulator Polarity
The LNA has two operation modes that have different analog characteristics and power consumptions.
Normal power mode and low power mode are selected by LPMODE bit in <Address0x14>. In low power
mode, power consumption can be kept low although gain and linearity will be degraded.
017003093-E-00
32
2017/3