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UAA3201T

UHF/VHF remote control receiver

Philips Electronics 

Philips Semiconductors

UHF/VHF remote control receiver

Product specification

UAA3201T

FUNCTIONAL DESCRIPTION

The RF signal is fed directly into the mixer stage where it

is mixed down to nominal 500 kHz IF by the integrated

oscillator controlled by an external SAWR (see Fig.1). The

IF signal is then passed to the IF amplifier which increases

the level. A 5th-order elliptic low-pass filter acts as main

IF filtering. The output voltage of that filter is demodulated

by a limiter that rectifies the incoming IF signal. The

demodulated signal passes two RC filter stages and is

then limited by a data comparator which makes it available

at the data output.

Mixer

The mixer is a single balanced emitter coupled pair with

internally set bias current. The optimum impedance is

320 Ω at 430 MHz. Capacitor C5 (see Fig.9) is used to

transform a 50 Ω generator impedance to the optimum

value.

Oscillator

The oscillator consists of a transistor in common base

configuration and a tank circuit including the SAWR.

Resistor R2 (see Fig.9) is used to control the bias current

through the transistor. Resistor R3 is required to reduce

unwanted responses of the tank circuit.

IF amplifier

The IF amplifier is a differential input, single-ended output

emitter coupled pair. It is used to decouple the first and the

second IF filter and to provide some additional gain in

order to reduce the influence of the noise of the limiter on

the total noise figure.

IF filters

The first IF filter is an RC filter formed by internal resistors

and an external capacitor C7 (see Fig.1).

The second IF filter is an external elliptic filter. The source

impedance is 1.4 kΩ and the load is high-impedance. The

bandwidth of the IF filter in the application and test circuit

(see Fig.9) is 800 kHz due to the centre frequency spread

of the SAWR. It may be reduced when SAWRs with less

tolerances are used or temperature range requirements

are lower. A smaller bandwidth of the filter will yield a

higher sensitivity of the receiver. As the RF signal is mixed

down to a low IF signal there is no image rejection

possible.

Limiter

The limiting amplifier consists of three DC coupled

amplifier stages with a total gain of 60 dB. A Received

Signal Strength Indicator (RSSI) signal is generated by

rectifying the IF signal. The limiter has a lower frequency

limit of 100 kHz which can be controlled by capacitors C12

and C19. The upper frequency limit is 3 MHz.

Comparator

The 2 × IF component in the RSSI signal is removed by the

first order low-pass capacitor C17. After passing a buffer

stage the signal is split into two paths, leading via

RC filters to the inputs of a voltage comparator. The time

constant of one path (C14) is compared to the bit duration.

Consequently the potential at the negative comparator

input represents the average magnitude of the RSSI

signal. The second path with a short time constant (C13)

allows the signal at the positive comparator input to follow

the RSSI signal instantaneously. This results in a variable

comparator threshold, depending on the strength of the

incoming signal. Hence the comparator output is switched

on, when the RSSI signal exceeds its average value, i.e.

when an ASK ‘on’ signal is received.

The low-pass filter capacitor C13 rejects the unwanted

2 × IF component and reduces the noise bandwidth of the

data filter.

The resistor R1 is used to set the current of an internal

source. This current is drawn from the positive comparator

input, thereby applying an offset and driving the output into

the ‘off’ state during the absence of an input signal. This

offset can be increased by lowering the value of R1

yielding a higher noise immunity at the expense of reduced

sensitivity.

Band gap reference

The band gap reference controls the biasing of the whole

circuit. In this block currents are generated that are

constant over the temperature range and currents that are

proportional to the absolute temperature.

The current consumption of the receiver rises with

increasing temperature, because the blocks with the

highest current consumption are biased by currents that

are proportional to the absolute temperature.

2000 Apr 18

4

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