TX4915 View Datasheet(PDF) - Unspecified

Part Name

Description

Manufacturer

TX4915

VHF/UHF TRANSMITTER

Unspecified 

the wireless IC company

TX4915

Functional Description

The TX4915 is a low-cost AM/ASK VHF/UHF transmitter designed for applications operating within the

frequency range of 100MHz to 960MHz. In particular, it is intended for 315/433MHz band systems remote

keyless entry systems, and 868/915MHz ISM Application. It can also be used as a local oscillator signal

source. The integrated VCO, phase detector, prescaler, and reference oscillator require only the addition of

an external crystal to provide a complete phase-locked loop. In addition to the standard power-on mode, the

chip also includes an autonatic lock-detect feature that disables the transmitter output when the PLL is out-

of-lock.

The TX4915 Functional Blocks

A PLL consists of a reference oscillator, a phase detector, a loop filter, a voltage control oscilator (VCO),

and a programmable divider in the feedback path. The TX4915 includes all of these internally, except for the

loop filter and the reference oscillator’s crystal and two feedback capacitors.

Reference crystal oscillator

Pins 1 (OSCIN) and 2 (OSCOUT) provide connections to a transistor that is used as a Colpitts type oscilla-

tor. The Colpitts configuration is a low parts count topology with realiable performance and reasonable

phase noise. Alternatively, an external signal could be injected into the Pins 1 (OSCIN). The drive level

should, in either case, be around 500mVpp. This level prevents overdriving the device and keeps the phase

noise and reference spurs to a minimum.

Voltage Controlled Oscillator

VCO is a tuned differential amplifier with the bases and collectors cross-coupled to provide positive feed-

back and a 360° phase shift. The tuned circuit is located in the collectors, and is comprised of internal varac-

tors and external inductors. The designer selects the inductors for the desired frequency of operation. These

inductor also provide DC bias for the VCO.

Prescaler

Using a series of flip-flops divide the VCO frequency by either 64 or 128, depending upon the logic level

present at the SW pin. A high logic level will select the 64 divider. A low logic level will select the 128 divi-

dor. This divided signal is then fed into the phase detector where it is compared with the reference frequency.

Phase dector and Charge pump

Phase dector is implemented using flip-flops in “digital tri-state comparator” topology. It compares the

phase of the reference oscillator to the phase of the VCO. The charge pump consists of two transistors, one

for charging the loop filter and the other for discharging the loop filter. When both inputs of the flip-flops are

identical, the signals are both frequency and phase locked. If they are different, they will provide signals to

the charge pump which will either charge or discharge the loop filter, or enter into a high impedance state.

Lock-detect Protection

The Lock-detect circuitry connect to the output of the phase dector ciruits and is used to disable the transmit-

ter when the VCO is not phase-locked to the reference oscillator. This is necessary to avoid unwanted out-

of-band transmission and to provide compliance with regulatory limits during an unlocked condition. Pin 15

(LD) is used to set the threshold of the lock-detect circuit. A shunt capacitor is used to set an RC time con-

stant with an on-chip series 1KΩ resistor. The time constant should be approximately 15 times the reference

period.

Page 5

March 2000

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