u2740b ETC-unknow, u2740b Datasheet - Page 3

• Manufacturer Part Number: u2740b
• Description: Am/fm Transmitter
• Manufacturer: ETC-unknow

TELEFUNKEN Semiconductors
Rev. A3, 24-Feb-97
Circuit Description
The transmitter PLL U2740B contains the complete RF
part for a radio control system. The IC consists of a VCO,
a complete PLL circuit, a crystal oscillator, a power
amplifier and a power-up module. An integrated switch
can be used to change the load capacitance of the crystal.
VCO
The VCO is a voltage-controlled current source. The
frequency is determined by the external LC-tank. The
frequency is changed via a Varicap diode.
PLL
The complete PLL consists of a prescaler, a digital
phase/frequency detector (PFD) with charge-pump
output. The output frequency is locked to 128 times the
frequency of the XTO. The PFD, however, operates at a
frequency four times lower than that of the XTO. A lock-
detect output indicates that the PLL is locked.
VCO
Preliminary Information
Figure 4. Power output stage of U2740B
0.7 V
Q2
Crystal Oscillator (XTO)
It is a single-pin crystal oscillator, operating at the series
resonant frequency of the crystal. Depending on the type
of crystal used, this oscillator takes 3 to 20 ms until
settling after setting CS to V
be used to change the load capacitance. Thus, the output
frequency is FSK modulated.
Power Amplifier (PA)
As figure 4 shows, the differential PA switches its output
current between the two power output pins (ANT1,
ANT2). The output current is seven times the current
flowing into the AM_in Pin. The achievable output power
is about 1.5 mW.
Power-up
When CS = 0 V, the circuit is in standby mode with a
power–down supply current of type. IS,off = 0.1 A. With
CS = V
Iout = 3.5 mA
S
, the circuit is in power-up mode.
7
Q1
1
0.5 mA
4.6 k
S
. The integrated switch can
0.7 V
U2740B
ANT1
PAGND
ANT2
VS
AM_in
3 (10)