Reciever basic blok question

What's the FLL (Frequency Locked Loop) in an FM Radio IC such as TDA7000?
and what's the need of mute circuit?!

Thanks in advance,
Ahmad,

The mute circuit disables the audio output for situations when a station is not tuned in properly.

A frequency locked loop measures the frequency of the VCO in some way and adjusts the frequency to be some set value. Another way is to measure the received signal frequency in the IF amplifier stages and adjust the VCO frequency to be the proper value to put the received signal in the center of the IF passband. An old term for this is automatic frequency control or AFC for short.

How can the PLL be involved to make an FLL ? i think they're very similar!!
Doesn't it make a conflict?!

The difference between the FLL and the PLL is in the comparison. When you use a PLL the PFD compares the frequency and the phase, so the PLL is locked in phase also that frequency. This type of loop it is necessary if you need to demodulate M-PSK signals. The FLL locks only the frequency and not the phase, so you can use it only with frequency modulated signal. In pratical the FLL is used only for FM (analog modulation) because the consumption is lower than a PLL

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hi , i dont get how the FLL will lock to the frequency and not lock to phase , do u mean the phase error wont be constant ?!
i.e. if the frequency is the same then the phase error should be constant , can u clarify more

Well, then can the PLL with PD not PFD said to be an FLL ?

The TDA7000 chip only has a FLL inside of it. The demodulator circuit sends out an error voltage if the manual band tuning capacitor does not center the LO oscillator at the FM station's center frequency. That error voltage then slightly retunes the LO oscillator's center frequency up or down 300KHz to get the best FM station reception. As you then tune the manual band tuning capacitor (or it drifts with time or temperature), the FLL error voltage will keep the FM station tuned to its center UNTIL the FLL runs out of voltage range. At that point the FM station suddenly "breaks lock" and gets a lot of static. If you manually retune the band tuning capacitor, up or down, then at some point the FLL can reaquire the FM station and it all starts to work again.

A different type of chip could use a PLL to lock the LO oscillator irregardless of if the FM station is properly centered or not. For this to work, you are couting on the FM station to be precise in frequency, and the crystal oscillator in the PLL lock to be precise in frequency.

IF you wanted to use both a PLL and a FLL, you would need TWO independent circuits. You would use the PLL to downconvert the FM signal to a lower IF frequency--say 10.7 MHz (where there could be a +/- frequency error present). Then you would use a second circuit containing a FLL to further downconvert the IF frequency to the demodulator. In this way the two circuits, PLL and FLL, do not fight each other.

safwatonline

you write "if the frequency is the same then the phase error should be constant", but it is different to have a costant phase error and to have to PLL locked in phase. The PLL works locking the frequency and the phase, the FLL only the frequency. This is the main difference between PLL and FLL, but clearly the FLL is simply to design because there are less details. For example: in the PLL you need to have a very linear charge pump to drive the VCO by the loop filter. In the FLL usually the frequency detecto output drives the VCO.

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hi dinone,
do u mean that the FLL is a type one PLL

safwatonline

the FLL is simplier than a PLL and can be use only to demodulate frequency modulated signal. So if you need to do a FM receiver you can use a FLL, for all the other application you need to use a PLL (more complex, more consumption).

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