PS20060

Data Sheet

An AGC IF amplifier is included which provides an output signal to a digital demodulator.

1.1 RF Converter

The PS20060 contains three input stages to cover the VHF1, VHF3 and UHF frequency bands. The inputs would

normally be driven by front end amplifiers and tracking filters. All three inputs are differential, however, the VHF1

and VHF3 inputs would normally be driven single ended. These inputs therefore can share a common input

reference pin. The UHF input should be driven with a differential signal. The inputs are all high impedance. The

differential converter IF output is then passed through an external interstage filter. This can be tuned for 36 MHz for

DVB-T applications but can also be used at 44 MHz and 57 MHz to be compatible with other TV standards. The

recommended filter circuit is shown in figure 11. The design of this filter provides an impedance transformation as

well as rejection of adjacent channels. A 0.5 dB Chebychev filter with 10 MHz bandwidth is recommended. This

gives a flat response across the pass band and takes into account normal component tolerances.

1.2 SAW Driver Amplifier

The output of the interstage filter then passes to the SAW filter drive amplifier. This provides further amplification

and interfaces to the SAW filter. Two SAW filter drive outputs are provided for hybrid analog and digital applications.

Both output stages are identical, however, the digital output (SD, SDB) should always be used for digital

applications, as the pin out of the device has been optimized to give the best isolation performance in this

configuration. Output selection is programmable however it should be noted that the unselected output is not

powered off but operates at a lower power level which means that a signal will still be present on the output, but

with reduced output drive capability.

The differential outputs will drive a balanced SAW filter with a tuning inductor to resonate with the SAW filter input

capacitance. The SAW filter can also be driven without the tuning inductor but this requires the addition of 360 ohm

resistors to ground on the SAW driver outputs to increase the output drive capability. This will increase total current

consumption by approximately 14 mA.

1.3 AGC Detector and ADC

The PS20060 contains a broadband AGC detector circuit which provides an output to provide gain control for the RF

frontend gain stages. The detector input signal is derived from the signal level in the SAW driver amplifier. The

composite signal at this point is the wanted signal plus adjacent channels (N +/- 1, N +/- 2, N +/- 3). The AGC

detector threshold point at which the agc output becomes active can be programmed to one of eight levels via the

attack current is fixed, however, the decay current can be programmed to two levels. The agc output can only drive

a high impedance e.g., a dual gate FET. If RF gain control uses a PiN diode then a simple buffer circuit will be

required.

less than 4 volts.

The agc output level can also be monitored by an on chip 3 bit ADC. Although the ADC is 3 bits, only 5 levels are

available. Alternatively the ADC can be programmed to measure the voltage on an external pin (ADC Pin 4).

1.4 IF AGC Amplifier

The AGC amplifier amplifies the output of the SAW filter for the digital channel and provides a differential output to

the demodulator. The analog gain control signal is normally derived from the demodulator. At least 41 dB of gain

control is provided.

The AGC amplifier can be powered down independently of the rest of the device if not required. This mode could be

used in analog applications to reduce overall power consumption.

Data Sheet 291359 issue 1

Plessey Semiconductors Ltd.

Design & Technology Centre, Delta 500, Delta Business Park, Great Western Way, Swindon, UK SN5 7XE

Tel: +44 1793 518000

Fax: +44 1793 518030

Web: www.plesseysemi.com

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