select TS in tuner
tuner selects a different TS by changing output frequency of local oscillator of tuner.
input RF signal substructed by local oscillator output signal, the result is intermediate frequency(IF) signal and its frequency is 38MHz. IF signal is modulated signal(video signal + carrier signal).
local oscillator's output frequency should be stable to ensure stable IF signal. the IF signal must be stable otherwise bad TV video viewing experience will happen.
there is a feedback from the IF signal to local oscillator of tuner. if IF signal is not equal to 38MHz, it means local oscillator's output frequency was not the correct, namely a small difference between actual freq and correct one; a feedback signal will be applied on local oscillator. so the LO will output correct frequency signal.
RF input will pass through high frequency amplifier in tuner for the RF signal is weak.
two types of tuner(as I known)
1. variable capacitance of special diode
there is a type of tuner which changes the output frequency of LO by changing the reverse voltage on a special diode. the capacitance of such diode is changed by different reverse voltage on the diode.
different output frequencies of LO of tuner correspond to different TS.
2. freq scaler applied to LO's output
there is a another type of tuner which changes output frequency of LO by connecting a frequency scaler after LO's output. changing the frequency scaling factor, the final frequency of LO will change correspondingly. for example:
Foo: the original frequency of LO's output
the frequency scaler factor: 4
the final frequency Ff = Foo/4
so for this type tuner, selecting different TS by different frequency scaling factor.
attaching a function diagram of TDA18250 from NXP
picture from: http://blog.csdn.net/jianchenglee/article/details/12655453
notice:
the bandwidth of video signal is 6 MHz