III. Gm-C Filtering - Epublications - Université de Limoges

I.2 TV Receivers Architecture & Specifications
I.2.a TV Tuner Architecture
As said before and as in most receivers, the signal is first amplified by a LNA before
being mixed, as shown in Figure 17. The mixing stage actually consists in multiplying two
signals. Let’s consider the RF signal and the LO (which stands for Local Oscillator) one,
assuming that both are sine wave signals, mixed by a linear system:
1
cos ( ω RFt
) cos(
ωLOt
) = ( cos(
( ωRF
− ωLO
) t)
+ cos(
( ωRF
+ ωLO
) t)
) . (I.4)
2
Hence, the RF signal is actually translated to lower frequency by the LO signal. Two
new frequencies then come out:
- ωRF − ωLO
, called IF frequency, which is the useful frequency;
- ω RF + ωLO
, called image frequency which has to be rejected.
Depending on the value of the intermediate frequency (IF) fIF =fRF-fLO, receivers are
called IF, low-IF or even ZIF (which stands for Zero-IF) when the signal is directly converted
to baseband.
Figure 17. RF receiver front-end simplified architecture
As it may be seen on Figure 18, once the signal is converted to IF, the wanted channel
is filtered. It is a very selective and sharp filter so that only this channel remains among the
entire received spectrum. Then, an analog-to-digital conversion of the channel is performed
by an ADC in order to provide the signal to be demodulated.
Figure 18. TV receiver architecture
One might wonder why this tuner architecture is necessary. Indeed, a simpler concept
would be designed with an analog-to-digital converter (ADC) much closer to the antenna.
However, such an ADC would require very high sampling rate and resolution to be designed,
since two billion samples per second are needed [I.2] to convert up to 1GHz. Though this type
of architecture is emerging to handle cable TV signals [I.4], the presence of very high power
differences between channels still prevents from using it for terrestrial applications.
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