How does a "modern" radio work?

[sakishrist]

Hello there,

I was searching the net for information on how radios work. The thing is, everywhere I searched I found stuff about the old radios like this one:
**broken link removed**
I read somewhere that on the old ones either the capacitor or the inductor is variable. But how exactly does a modern radio like this work (how does it change the frequencies):
**broken link removed**

Thanks in advance!

 

[bychon]

A fast oscillator makes a frequency which is divided by digital chips.

 

[sakishrist]

Could you please explain this a bit farther or post a link to a site?

Anyways, thanks for the reply!

 

[Nigel Goodwin]

Google for 'Phase Locked Loop'

 

[MikeMl]

The principal difference between the Grundig and the Sangean is how the local oscillator is configured; the Grundig uses a free-running tunable oscillator while the Sangean uses a Phase-Locked-Loop where the local oscillator frequency is derived off a crystal oscillator. Othewise, they are both SuperHeterodyne receivers.

 

[mdanh2002]

So in terms of sensitivity and selectivity, which radio is better, analog or digital?

 

[Nigel Goodwin]

So in terms of sensitivity and selectivity, which radio is better, analog or digital?

Neither, it depends on facrtors other than that.

 

[sakishrist]

I could not really understand what the pll does. Isn't a VCO enough for the frequency "selection"?

 

[MikeMl]

I could not really understand what the pll does. Isn't a VCO enough for the frequency "selection"?

A free-running VCO would not be as stable as the LC tuned oscillator in the Grundig. Phase Locking the VCO to an digitally divided crystal oscillator makes it as stable as the crystal oscillator is... It is cheaper to build a digital PLL than to use a quality variable capacitor.

 

[sakishrist]

So the main job of the pll is to stabilize the frequency, right?

 

[Ubergeek63]

So the main job of the pll is to stabilize the frequency, right?

it also sets the frequency. FM intermediate frequency is 10.7MHz. when you mix two frequencies you get the sum and difference as well as the two originals out.

you pick a station, say 99.1, and add 10.7MHz to it, that would be 109.8MHz, and send it to the mixer with the incoming signal. on the other side of the mixer you put a 10.7MHz filter, these days there are simple cheap 3 terminal ceramic jobs that do fine.

to understand PLL draw a square wave on graph paper skipping 4 blocks before each transition, this is your radio station. now draw another one under it following the graph grid, this is your reference oscillator from the PLL. Those 2 waves are in phase. now if you imagine the first one changing frequency a little bit you would see each transition drifting a little farther from the reference one. A PLL changes the reference oscillator a little at a time to make the two match and it could lock the 4X signal to the incoming since only really concerned with the phase (the edges of the lower frequency matching up with the ones of the higher)

in the case of a VCO the PLL is changing the command voltage to the VCO. in the case of DDS you would load a register for the channel divider in the same way but instead of adding an error term to the command voltage you add an error term to the channel command. if you are indeed looking at an FM signal then error term IS your audio. you can also phase lock to an AM signal in which case you would put another mixer down to I think an AM detector on the IF but I do not think anyone puts anything there.

it is also possible to set the PLL output frequency to the station frequency for direct conversion. then there is no mixer at all. just an RF amp and a PLL. again the error signal is the audio output. As i understand it there is a problem with direct conversion AM receivers.

 

[sakishrist]

So the PLL does both, select the desired frequency and extract the audio from the modulated signal, right?

Now then, what is it that actually changes the frequency whenever I press the "next" button? is it the divider in the PLL? If so how does it do it?

Thanks in advance

 

[Nigel Goodwin]

So the PLL does both, select the desired frequency and extract the audio from the modulated signal, right?

No, it doesn't extract the audio - though an extra PLL can be used for demodulation (but it's nothing to do with the tuning PLL).

Now then, what is it that actually changes the frequency whenever I press the "next" button? is it the divider in the PLL? If so how does it do it?

It alters the division ratio.

 

[Ubergeek63]

So the PLL does both, select the desired frequency and extract the audio from the modulated signal, right?

Now then, what is it that actually changes the frequency whenever I press the "next" button? is it the divider in the PLL? If so how does it do it?

Thanks in advance

when you change the channel you are changing the divider (they call it a fractional divider- a reloadable counter that triggers a /2 to get a 50% duty cycle) in the PLL. if you are trying to lock onto a 10MHz carrier with a 100KHz channel spacing you need to be able to set the counter such that you get the 100KHz steps. If you feed it 2GHz you would put in a 100 for a reload and the divide by 2 gives you a 50% duty cycle at 10MHz with a 100KHz channel spacing.

now that is oversimplified .... the fact is that it is a phase locked loop and not a frequency locked loop ... what that basically means it that you can lock onto harmonics as well but i figured basic theory would be easier to understand and well a lot easier to explain.

dan

 

[Ubergeek63]

So the PLL does both, select the desired frequency and extract the audio from the modulated signal, right?

Now then, what is it that actually changes the frequency whenever I press the "next" button? is it the divider in the PLL? If so how does it do it?

Thanks in advance

BTW the error signal IS the audio in an FM signal. they might do other things as well but you can use the PLL error signal as audio.

 

[MikeMl]

BTW the error signal IS the audio in an FM signal. they might do other things as well but you can use the PLL error signal as audio.

Yeah, but most FM entertainment radios use a conventional IF followed by a "ratio detector" or "discriminator" type of FM detector. The PLL is used only for tuning the predefined 200KHz channels (..., 88.9, 90.1, 90.3, ... MHz). AM broadcast in the US is on 10KHz channels (..., 990, 1000, 1010, ... KHz). The PLL is used for tuning accuracy, and is cheaper than the expensive coils and ball-bearing variable tuning capacitor that it replaces.

 

[Nigel Goodwin]

BTW the error signal IS the audio in an FM signal. they might do other things as well but you can use the PLL error signal as audio.

Not the tuning PLL signal you can't - you use a separate demodulator after the IF, which 'can' be a PLL type.

 

[Ubergeek63]

Not the tuning PLL signal you can't - you use a separate demodulator after the IF, which 'can' be a PLL type.

yes you can and it is actually becoming quite common these days: Direct-conversion receiver - Wikipedia, the free encyclopedia. of course you would not see it since most things are designed to be thrown out and you would not know what was inside the chip even if you did get one back to your shop.

 

[Nigel Goodwin]

yes you can and it is actually becoming quite common these days: Direct-conversion receiver - Wikipedia, the free encyclopedia. of course you would not see it since most things are designed to be thrown out and you would not know what was inside the chip even if you did get one back to your shop.

Which says nothing about using the tuning PLL to demodulate the signal.

Direct conversion is used mainly for SSB and CW, not usually for AM or FM.

 

[Ubergeek63]

Which says nothing about using the tuning PLL to demodulate the signal.

Direct conversion is used mainly for SSB and CW, not usually for AM or FM.

OY! FM is frequency modulation ... the error signal in a PLL is used to change the VCO to "match" the carrier frequency ... In other words the the error signal IS the demoulated audio if you are talking mono... stereo obviously requires an addition step