VP310

PRELIMINARY DATA

14

1.6.2 Direct conversion

Figure 12 below shows a direct conversion system that mixes the L-band input to the tuner

directly down to I and Q baseband channels at zero intermediate frequency.

The RF AGC amp and tracking filter provide the required tuner noise figure and limit the total

power reaching the SL1925. These elements also give isolation between the SL1925 local

oscillator and the L-band tuner input. This is an important factor since both signals are at the

same frequency.

The baseband filter is an anti-alias filter. This replaces the filtering normally carried out with a

SAW filter in conventional single conversion tuners.

It is important to note that all the channel filtering needed to isolate low Baud rate signals is

contained within the VP310. The low pass filter before VP310 is designed not to filter channels,

but to minimise any aliasing due to sampling. To illustrate this, let the sampling frequency be 90

MHz and the maximum symbol rate be 45 MBaud. The bandwidth of the 45 MBaud QPSK signal,

with 0.35 roll-off, is about 60 MHz. If the channel has been mapped precisely to base-band, the

pass-band of the low pass filter should extend up to 30 MHz. However, it is preferable to make

this bandwidth larger by about 5 MHz, partly to reduce the in-band phase distortion introduced by

the filter and partly to reduce the loss of signal due to LNB offset. The filter must attenuate

signals beyond 60 MHz by about 30 dB, as these signal will alias to the useful frequency range

with 90 MHz sampling.

Although the system is designed for 45 MBaud, if the actual symbol rate is much lower, say 1

MBaud, then VP310 will automatically introduce all the digital filtering needed to isolate the 1

MBaud signal.

Direct

Conversion

Tuner

SL1925

Low pass

Filter

Channel

Decoder

VP310

Synthesiser

SP5655/

SP5769

I²C bus control

Transport

stream O/P

I²C control

I I/P

Q I/P

RF I/P

AGC control

Tank

AGC

AMP

SL1914

I

Q

Figure 12. Direct Conversion System Diagram.