How does a radio tuner work?
Discussion
I’ve been reading up on the basic principles, and I’m OK on the idea that (to simplify) an alternating current in the transmitting aerial causes an alternating current to be picked up by the receiving aerial.
When it comes to tuning the receiver to select the desired frequency, again to simplify. You have an induction coil which will allow current to pass providing it’s alternating below a particular frequency. You have a capacitor which will allow current above a particular frequency. Anything too high for the induction coil and too low to get through the capacitor ends up going through the earpiece or audio amplifier because it can’t go anywhere else.
If there was simply one frequency at a time coming down the aerial I could understand it (vaguely). But the whole point of tuning is that you are picking up loads of frequencies and just want one. How can you have signals alternating at different rates in the same component? Can you have electrons going both ways at once?
When it comes to tuning the receiver to select the desired frequency, again to simplify. You have an induction coil which will allow current to pass providing it’s alternating below a particular frequency. You have a capacitor which will allow current above a particular frequency. Anything too high for the induction coil and too low to get through the capacitor ends up going through the earpiece or audio amplifier because it can’t go anywhere else.
If there was simply one frequency at a time coming down the aerial I could understand it (vaguely). But the whole point of tuning is that you are picking up loads of frequencies and just want one. How can you have signals alternating at different rates in the same component? Can you have electrons going both ways at once?
A theoretical perfect 'tuned circuit' effectively lets a single frequency pass and rejects all other frequencies. (obviously in reality this is impossible to make)
Mechanical analogy:
Consider an upturned round wastepaper bin, put onto the turntable of a record player. We have cut a single hole into that bin, just big enough to pass through a tennis ball.
We turn on the record player, the bin revolves, and every so often (depending on how fast the bin is spinning) the hole is facing us. Now, we fire a load of tennis balls at that bin with a serving machine. The only balls that pass through the hole are those that arrive exactly when the hole is facing the firing machine. In effect, even if we fire balls at the bin at random moments in time, the only ones that get in, and those matching the rotating frequency of the bin. We have built a band pass filter.
This is same with a tuned electronic circuit. Using inductors and resistors and capacitors, the circuit oscillates at a specific frequency. Hence it only increases in energy if electrons arrive at the matching frequency on average.
Take a look at old analogue tuners:
You can see the big adjustable air cored capacitor (all those metal plates) By turning the tunning knob, the different amount of overlap of those plates results in a difference capacitance, and hence a different tuned frequency of the LC filter, and hence the ability to reject energy outside of that frequency, but let energy at that frequency pass into the subsequent amplifier stage etc.
These days of course, all this is replaced with digital filters, where a large frequency band is accepted and descretised into the digital domain, and then mathematical software band pass filters are used to pull the correct signal out. (google "software defined radio" for more info)
Mechanical analogy:
Consider an upturned round wastepaper bin, put onto the turntable of a record player. We have cut a single hole into that bin, just big enough to pass through a tennis ball.
We turn on the record player, the bin revolves, and every so often (depending on how fast the bin is spinning) the hole is facing us. Now, we fire a load of tennis balls at that bin with a serving machine. The only balls that pass through the hole are those that arrive exactly when the hole is facing the firing machine. In effect, even if we fire balls at the bin at random moments in time, the only ones that get in, and those matching the rotating frequency of the bin. We have built a band pass filter.
This is same with a tuned electronic circuit. Using inductors and resistors and capacitors, the circuit oscillates at a specific frequency. Hence it only increases in energy if electrons arrive at the matching frequency on average.
Take a look at old analogue tuners:
You can see the big adjustable air cored capacitor (all those metal plates) By turning the tunning knob, the different amount of overlap of those plates results in a difference capacitance, and hence a different tuned frequency of the LC filter, and hence the ability to reject energy outside of that frequency, but let energy at that frequency pass into the subsequent amplifier stage etc.
These days of course, all this is replaced with digital filters, where a large frequency band is accepted and descretised into the digital domain, and then mathematical software band pass filters are used to pull the correct signal out. (google "software defined radio" for more info)
Max_Torque said:
We turn on the record player, the bin revolves, and every so often (depending on how fast the bin is spinning) the hole is facing us. Now, we fire a load of tennis balls at that bin with a serving machine. The only balls that pass through the hole are those that arrive exactly when the hole is facing the firing machine. In effect, even if we fire balls at the bin at random moments in time, the only ones that get in, and those matching the rotating frequency of the bin. We have built a band pass filter.
This is same with a tuned electronic circuit. Using inductors and resistors and capacitors, the circuit oscillates at a specific frequency. Hence it only increases in energy if electrons arrive at the matching frequency on average.
So does the tuning set up the frequency of the circuit, then the circuit will only pick up signals that match the frequency of the circuit? That would make sense, it was explained to me as if the tuning circuit somehow selected the required signal to go one way and the other frequencies to go the other.This is same with a tuned electronic circuit. Using inductors and resistors and capacitors, the circuit oscillates at a specific frequency. Hence it only increases in energy if electrons arrive at the matching frequency on average.
Dr Jekyll said:
<snip>
But the whole point of tuning is that you are picking up loads of frequencies and just want one. How can you have signals alternating at different rates in the same component? Can you have electrons going both ways at once?
It's all about resonance.But the whole point of tuning is that you are picking up loads of frequencies and just want one. How can you have signals alternating at different rates in the same component? Can you have electrons going both ways at once?
Think of an elastic band - you hold it at various tensions, and the resonance frequency changes.
A tuned circuit operates on the same principle.
The tuning capacitor in the radio receiver is constantly changing the resonant frequency of the circuit, to match the frequency of the station you wish to receive.
Of course there are radio waves of differing frequencies all around as, however aerials are tuned for a specific range of frequencies.
This is why the 'old' TV UHF aerials were so small, because they were tuned to a shorter wavelength than an FM aerial.
The aerial is tuned to resonate across a range of frequencies, in the same way as your radio receiver.
This is quite a simplified explanation, as there are other intricacies to radio reception such as intermediate frequency etc. but that's the gist of it.
It is physics principle called superposition. https://en.wikipedia.org/wiki/Superposition_princi...
All the incoming waves acting on a particle (etc) add and don't interfere with each other. In linear systems anyway. It gets really complicated very quickly with non-linear ones.
The filtering method is usually called TRF (tuned radio frequency) and was popular with early AM radio receivers that worked on comparatively low frequencies and weren't too picky about bandwidth, interference and being able to separate one station from another. It doesn't work for more advanced techniques such as frequency modulation.
A more useful method uses an intermediate frequency (IF). When you mix 2 frequencies in a non-linear device (called, erm, a mixer) which we can call F1 and F2, you get an output which is a mix of both: F1+F2, F2+F1 (which are the same), F1-F2, F2-F1. so if you have a receiver that tunes 88-108 MHz, we can mix the received signal with an oscillator that itself is tuned to 88-108MHz and theoretically produce an audio frequency signal. This is called direct conversion. In reality, it is better to convert to a true intermediate frequency with a fixed filter, and then afterwards convert it to an audio signal in a separate stage. You get more precise channel selection.
All the incoming waves acting on a particle (etc) add and don't interfere with each other. In linear systems anyway. It gets really complicated very quickly with non-linear ones.
The filtering method is usually called TRF (tuned radio frequency) and was popular with early AM radio receivers that worked on comparatively low frequencies and weren't too picky about bandwidth, interference and being able to separate one station from another. It doesn't work for more advanced techniques such as frequency modulation.
A more useful method uses an intermediate frequency (IF). When you mix 2 frequencies in a non-linear device (called, erm, a mixer) which we can call F1 and F2, you get an output which is a mix of both: F1+F2, F2+F1 (which are the same), F1-F2, F2-F1. so if you have a receiver that tunes 88-108 MHz, we can mix the received signal with an oscillator that itself is tuned to 88-108MHz and theoretically produce an audio frequency signal. This is called direct conversion. In reality, it is better to convert to a true intermediate frequency with a fixed filter, and then afterwards convert it to an audio signal in a separate stage. You get more precise channel selection.
1 over 2 pi square root L C.... That's the resonant frequency.
From that you can find the Q factor.
Like resonance in a mechanical system or elastic band as described above when a tuned electrical circuit hits resonance you get a large jump in amplitude, this is how a tuned circuit selects narrow frequency bands.
The 2 pi bit should push home the reality that we're talking about a cyclical waveform and in ac theory territory, the realm of imaginary numbers. That's not really relevant but while you're thinking you might as well have a go at that.
As mentioned above these days, unfortunately, most radios are digital and direct converted in DSP... Mr Marconi would be spinning in his grave.
From that you can find the Q factor.
Like resonance in a mechanical system or elastic band as described above when a tuned electrical circuit hits resonance you get a large jump in amplitude, this is how a tuned circuit selects narrow frequency bands.
The 2 pi bit should push home the reality that we're talking about a cyclical waveform and in ac theory territory, the realm of imaginary numbers. That's not really relevant but while you're thinking you might as well have a go at that.
As mentioned above these days, unfortunately, most radios are digital and direct converted in DSP... Mr Marconi would be spinning in his grave.
Those big Marshall valve amplifiers of the 1960's (the ones that went up to 11) were notorious for picking up stray radio transmissions.
There is a Jimi Hendrix live recording (I believe it is Machine Gun at the Isle of Wight festival) where he starts receiving the walkie talkie messages of the security guards. Being a genius, he manages to weave them into the performance.
It was explained to me that a bit of wiring within the circuitry could become 'radiophonic' (if that's the word) if it were of a length that harmonised with a radio signal. The drummer in my band was an electronic engineering student and actually built one of our amps. I can recall him struggling with this on one occasion as it kept on getting Radio 4.
Of course, I know nothing about electronics, and this just my very possibly wrong understanding.
There is a Jimi Hendrix live recording (I believe it is Machine Gun at the Isle of Wight festival) where he starts receiving the walkie talkie messages of the security guards. Being a genius, he manages to weave them into the performance.
It was explained to me that a bit of wiring within the circuitry could become 'radiophonic' (if that's the word) if it were of a length that harmonised with a radio signal. The drummer in my band was an electronic engineering student and actually built one of our amps. I can recall him struggling with this on one occasion as it kept on getting Radio 4.
Of course, I know nothing about electronics, and this just my very possibly wrong understanding.
I think the answer is "sort of". In those days, electrical equipment had pretty much no shielding, and any signal induced in an unshielded wire leading to (or inside) an amplifier will have that signal amplified. Normally, a radio frequency signal would be way outside human hearing (200kHz for Radio 4, human limit for a young person is approx 20kHz) and the frequency response would cause the signal to be filtered out.
The thing is though, guitar amplifiers are not designed to be nice linear amps, but instead are intended to add distortion - Marshall made their name out of amps that generate really nice distortion for electric guitars. The problem there, is that they effectively become a radio receiver, rectifying the signal (like a cat's whisker or diode). The wires don't need to be resonant, but it does make the problem worse. To resonate on Radio 4 Long Wave though, it would need to be at least a quarter of 1500m long, and unshielded. That's one hell of an amplifier cable.
LC (inductor capacitor) resonance. Inductors and capacitors are pretty much the opposite of each other. A capacitor won't pass a direct current, but as you increase the frequency of the signal the AC resistance (which we call impedance) reduces to the point where it looks like a plain wire at high frequency. On the other hand, an inductor looks like a wire at DC, and at high frequencies will block everything. At one specific frequency though, the impedance of both is exactly the same. This is the resonant frequency.
The thing is though, guitar amplifiers are not designed to be nice linear amps, but instead are intended to add distortion - Marshall made their name out of amps that generate really nice distortion for electric guitars. The problem there, is that they effectively become a radio receiver, rectifying the signal (like a cat's whisker or diode). The wires don't need to be resonant, but it does make the problem worse. To resonate on Radio 4 Long Wave though, it would need to be at least a quarter of 1500m long, and unshielded. That's one hell of an amplifier cable.
LC (inductor capacitor) resonance. Inductors and capacitors are pretty much the opposite of each other. A capacitor won't pass a direct current, but as you increase the frequency of the signal the AC resistance (which we call impedance) reduces to the point where it looks like a plain wire at high frequency. On the other hand, an inductor looks like a wire at DC, and at high frequencies will block everything. At one specific frequency though, the impedance of both is exactly the same. This is the resonant frequency.
I'm a avionics tech on aircraft, a large part of my training was on comms.
Look up superheterodyne radios. There are some good descriptions out there.
The filters you describe above are used to filter out anything outside the useable frequency range of your radio. Uhf for example is 225-400 MHz, you then mix the incoming RF radio signal (carrier wave) with a local oscillator to isolate the required frequency out into what is called an intermediate Frequency (IF), this is then passed through further filters, a demodulator and then amplifiers to boost it for headphones or speakers.
It's about 20 years since I did it. When I was taught it was a mixture of really old avionics stuff think Nimrods (effectively first ever jet airliner) valves etc and some more modern stuff like tornado etc where valves were replaced with microchips and capacitors etc.
If I have time at work today I will draw up a real basic example of a superheterodyne tx and rx if the op can't find a suitable answer on YouTube.
Look up superheterodyne radios. There are some good descriptions out there.
The filters you describe above are used to filter out anything outside the useable frequency range of your radio. Uhf for example is 225-400 MHz, you then mix the incoming RF radio signal (carrier wave) with a local oscillator to isolate the required frequency out into what is called an intermediate Frequency (IF), this is then passed through further filters, a demodulator and then amplifiers to boost it for headphones or speakers.
It's about 20 years since I did it. When I was taught it was a mixture of really old avionics stuff think Nimrods (effectively first ever jet airliner) valves etc and some more modern stuff like tornado etc where valves were replaced with microchips and capacitors etc.
If I have time at work today I will draw up a real basic example of a superheterodyne tx and rx if the op can't find a suitable answer on YouTube.
Edited by Markbarry1977 on Wednesday 28th June 07:01
Twenty years nothing! It was the best part of 50 years ago that I had a Marshall! God, I feel old sometimes.
I am sure I remember him trying to stop this amplifier thinking it was a radio. Of course we had medium wave in those days, too, if that helps. It might even have been short wave BBC World Service. Getting closer?
There were also stories about people picking up radio on their fillings, but should that go in the urban myths thread?
I am sure I remember him trying to stop this amplifier thinking it was a radio. Of course we had medium wave in those days, too, if that helps. It might even have been short wave BBC World Service. Getting closer?
There were also stories about people picking up radio on their fillings, but should that go in the urban myths thread?
You may or may not be interested to know that I am typing this about a mile and a half from Marconi's first factory, and a lot less than that from the research establishment his firm set up in 1936. Lifting my eyes I can see through the window the top of the radio mast erected on the site - the only remaining tower from the Chain Home Radar system that helped win the Battle Of Britain.
https://en.wikipedia.org/wiki/Marconi_Research_Cen...
I have also seen his funerary monument in Santa Croce in Florence. Couldn't hear any whirring noises as he was buried somewhere else.
Local hero.
https://en.wikipedia.org/wiki/Marconi_Research_Cen...
I have also seen his funerary monument in Santa Croce in Florence. Couldn't hear any whirring noises as he was buried somewhere else.
Local hero.
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