AK2401 Datasheet, PDF (30/103 Pages) Asahi Kasei Microsystems – Direct Conversion Transceiver

[AK2401]
13.4. Analog Receiving Circuit (LNA)
This is a Low Noise Amplifier (LNA) that gains receiving RF signal while keeping noise low. An impedance
matching circuit is necessary for the input/output of the LNA.
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 linearity will be degraded.
It also has normal gain mode and low gain mode to prevent characteristics degradation in high input
environment. These modes are selected by LNA_LGMODE bit <Address0x20>. In low gain mode,
although linearity and noise will be degraded, gain level can be kept low. In addition, the LNA has
automatic gain control (AGC) function that switches operation mode automatically according to input
signal level. Refer to “13.8.8. AGC” for details of the AGC function.
An equivalent circuit at the LNA Input is shown in Figure 16. An AC coupling capacitor (C1) is needed
since the LNAIN pin is DC biased internally. The LNACONT pin should be connected to the ground via a
source inductor (LS). C2 and LG are matching elements for impedance conversion.
Constant of the matching elements can be changed according to the following sequence and expressions
if using the AK2401 in a different frequency condition that is shown in “16. Recommended External
Circuits”.
Z_in
= sL_S
+ sL_G
+
1
sC₂
+
L_Sg_m
C₂
∴Re : ^LSgm = 50[Ω]
C₂
∴Im
: ω₀L_S
+ ω₀L_G -
1
ω₀C₂
= 0[Ω]
ω₀: Central Angle Frequency
⋯(1)
⋯(2)
by (1)
C₂
=
L_Sg_m
50
⋯(3)
by (2)
1
L_G = _ω02C2 - L_S
⋯(4)
Determine the source inductor (LS) value first. If the value of LS increases, gain will be decreased. If the
value of LS decreases, gain will be increased. Refer to constants written in “16. Recommended External
Circuits” for the default value. C2 value can be calculated by determining LS value by expression (3).
Adjust C2 value by measuring S11 to have real part as 50Ω. LG value can be calculated by determining
LS and C2 by expression (4). Adjust LG value by measuring S11 to have the imaginary part as 0Ω. These
values should be determined on a sufficient evaluation since there are some influences by stray
capacitances of the printing board and components.
g_m indicates transfer conductance of the internal transistor. The g_m value will be decreased when
changing the operation mode to low power mode from normal power mode.
Use a protection diode to limit input amplitude as written in “7. Absolute Maximum Ratings” if the input
amplitude at the LNAIN pin exceeds 2.4 Vpp. Connect a LD as needed. It is an inductor that cancels the
difference of input impedance caused by a stray capacity of the diode.
017003093-E-00
30
2017/3