參數(shù)資料
型號: ATA5728P6-TKQY
廠商: ATMEL CORP
元件分類: 通信及網(wǎng)絡
英文描述: SPECIALTY TELECOM CIRCUIT, PDSO20
封裝: SSOP-20
文件頁數(shù): 42/46頁
文件大?。?/td> 776K
代理商: ATA5728P6-TKQY
5
9106E–RKE–07/08
ATA5723/ATA5724/ATA5728
3.
RF Front-end
The RF front-end of the receiver is a low-IF heterodyne configuration that converts the input sig-
nal into about 1 MHz IF signal with a typical image rejection of 30 dB. According to Figure Figure
1-2 on page 3 the front-end consists of an LNA (Low Noise Amplifier), LO (Local Oscillator), I/Q
mixer, polyphase low-pass filter and an IF amplifier.
The PLL generates the drive frequency f
LO for the mixer using a fully integrated synthesizer with
integrated low noise LC-VCO (Voltage Controlled Oscillator) and PLL-loop filter. The XTO (crys-
tal oscillator) generates the reference frequency f
REF =fXTO/2 (868 MHz and 433 MHz versions)
or f
REF =fXTO/3 (315 MHz version). The integrated LC-VCO generates two or four times the
mixer drive frequency f
VCO. The I/Q signals for the mixer are generated with a divide by two or
four circuit (f
LO =fVCO/2 for 868 MHz version, fLO =fVCO/4 for 433 MHz and 315 MHz versions).
f
VCO is divided by a factor of 128 or 64 and feeds into a phase frequency detector and is com-
pared with f
REF. The output of the phase frequency detector is fed into an integrated loop filter
and thereby generates the control voltage for the VCO. If f
LO is determined, fXTO can be calcu-
lated using the following formula:
f
REF =fLO/128 for 868 MHz band, fREF =fLO/64 for 433 MHz bands, fREF =fLO/64 for 315 MHz
bands.
The XTO is a two-pin oscillator that operates at the series resonance of the quartz crystal with
high current but low voltage signal, so that there is only a small voltage at the crystal oscillator
frequency at pins XTAL1 and XTAL2. According to Figure 3-1, the crystal should be connected
to GND with two capacitors C
L1 and CL2 from XTAL1 and XTAL2 respectively. The value of
these capacitors are recommended by the crystal supplier. Due to an inductive impedance at
steady state oscillation and some PCB parasitics, a lower value of C
L1 and CL2 is normally
necessary.
The value of C
Lx should be optimized for the individual board layout to achieve the exact value of
f
XTO and hence of fLO. (The best way is to use a crystal with known load resonance frequency to
find the right value for this capacitor.) When designing the system in terms of receiving band-
width and local oscillator accuracy, the accuracy of the crystal and the XTO must be considered.
Figure 3-1.
XTO Peripherals
The nominal frequency f
LO is determined by the RF input frequency fRF and the IF frequency fIF
using the following formula (low-side injection):
f
LO = fRF – fIF
XTAL2
TEST2
TEST3
XTAL1
DVCC
VS
CL2
CL1
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