A simple question on radio tuner

Thread Starter

AlwaysNumber1

Joined Dec 4, 2016
52
Hello everyone !
Have already asked a similar question today but would like to change it a little bit:

1. Can you please explain in your own words how radio tuner works (of AM)
2. Can we assume that the bandpass filter, shown below, is of radio tuner ?
 

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crutschow

Joined Mar 14, 2008
23,783
Most AM radios are superheterodyne.
That means the input RF signal is mixed (multiplied) with an oscillator frequency which gives sum and difference frequencies.
The difference frequency is called the IF (Intermediate Frequency, which is typically 455kHz for an AM radio) and that is what all the bandpass filters in the IF amplifier chain are set to.
So, to tune the radio to a specific station, the oscillator (mixer) frequency is adjusted so that the difference frequency of the desired station's broadcast frequency is equal to the IF frequency.

The advantage of this, is that all the filters and amplification can be done at a fixed frequency.

In some sensitive radios there may be an RF amplifier before the mixer but that is not necessary for local AM stations.
 

Thread Starter

AlwaysNumber1

Joined Dec 4, 2016
52
Most AM radios are superheterodyne.
That means the input RF signal is mixed (multiplied) with an oscillator frequency which gives sum and difference frequencies.
The difference frequency is called the IF (Intermediate Frequency, which is typically 455kHz for an AM radio) and that is what all the bandpass filters in the IF amplifier chain are set to.
So, to tune the radio to a specific station, the oscillator (mixer) frequency is adjusted so that the difference frequency of the desired station's broadcast frequency is equal to the IF frequency.

The advantage of this, is that all the filters and amplification can be done at a fixed frequency.

In some sensitive radios there may be an RF amplifier before the mixer but that is not necessary for local AM stations.
Thanks a lot for such explanation !

Could you please also answer the questions above ?
 

Dodgydave

Joined Jun 22, 2012
8,586
The Superhetrodyne system uses a "Local Oscillator" to produce the IF frequency down to usually 470Khz, expensive superhets like amateur radios use a Dual conversation system taking the First IF down to 455Khz for better filtering and selection of stations.
As said normally AM radios will use only one local osc and down convert to 470khz..

Double-conversion_superheterodyne_receiver_block_diagram.svg.png
 

mrred128

Joined Feb 15, 2016
6
I feel sorry for the OP, as some people need to bring their heads out of the sand.

The simple am radio receiver has a parallel tuned circuit tuned to the station at hand. A single diode rectifier is used as if you were making a power supply and then (optional) a capacitor / resistor is used to filter off the inaudible tones.

It is that simple and that's why crystal radios do not need power. Their power comes from the RF itself.

Look here http://makearadio.com/crystal/images/cs09-schematic.jpg
 
I feel sorry for the OP, as some people need to bring their heads out of the sand.

The simple am radio receiver has a parallel tuned circuit tuned to the station at hand. A single diode rectifier is used as if you were making a power supply and then (optional) a capacitor / resistor is used to filter off the inaudible tones.

It is that simple and that's why crystal radios do not need power. Their power comes from the RF itself.

Look here http://makearadio.com/crystal/images/cs09-schematic.jpg
Hello, A simple am radio broadcast receiver such as a crystal set gets it's tiny amount of power from a large antenna and therefore must be very sensitive
 

mrred128

Joined Feb 15, 2016
6
That depends. I currently live in Montreal and the stations here are in the mega-watts. In the town where I gre up, the local AM station was only 300 watts and my crystal radio worked well.

I made one in jr high with a toilet paper tube for the coil base. I remember my antenna was only 20ft long and it was strung in the rafters of our basement. If your earphone is high enough impedance and you use a real germanium diode, there will be enough juice.

Later ones I made used a ferrite loopstick and did work better.
 
A long time ago America had a tube set named the all American five. It's a good example of a simple 5 tube set. The tuning capacitor is a two gang unit. The small unit controlled the local osilator and the large one controled the lc tank antenna. Both of these circuits were coupled into a pentagrid converter tube a 12SA7 which has two grids. The control grid connects to the oscillator and the injector grid connects to the loop lc varible tank antenna. The plate which is the output has a composite of four signals ; the incoming tuned station, the oscillator freqency, the incoming tuned station plus the oscillator frequency and the incoming frequency minus the oscillator frequency. Because the plate output is connected to the first inermediate transformer which is nothing but a tuned lc network and stays fixed at 455 kilocycles it only passes the oscillator frequency PLUS the incoming frequency that equals 455 kiloherts. For example if you tune to a station that broadcasts a carrier frequency of 1000 kilohertz, the oscillator must be tuned to 1455 kilohertz to pass through the IF stages. On a properly tracking radio the oscillator will range from 1005 kilohertz for an incoming of 550 kilohertz to 2055 kilohertz for an incoming of 1650 kilohertz. In essence the radio is nothing more than a 455 kilohertz radio regardless of what it's tuned to. ...The next stage is nothing more than a tuned amplifier tube number 12SK7 a pentrode with one input grid that connects to the secondary of the first IF transformer. The plate output goes to the second 455 IF transformer primary. The secondary goes to the diode plates of the detector/ first audio amp 12SQ7. This tube has a diode circuit and audio amplifing circuit all in one tube. After exiting the diode detector circuit it is filtered by a small capacitor which has a low reactance to the 455 kilohertz and a high reactance to the speech or music, from there it goes to the volume control which has two paths, one goes to the automatic volume control to prevent over driving the converter and IF tube during a strong station. The other path goes to the audio power output tube 50L6 to drive the speaker. The last mention is the power rectifier 35Z5.
 

BR-549

Joined Sep 22, 2013
4,938
"1. Can you please explain in your own words how radio tuner works (of AM)"

The words "radio tuner" usually refers to a resonant circuit that is externally adjustable for frequency selection. They can be series or parallel and come in different configurations. They may also be ganged or connected together to allow multiple circuits to be tuned with one adjustment. I assume you understand resonance and the effect of capacitance and inductance.

There are different ways to tune an AM signal. A duration of time is similar to frequency. Using digital switches.....we can electrically mimic a radio tuner without resonant circuits. A time slice tuner. Actually in this case we don't tune the signal.....we sample it.

Once the station is selected with the tuner......the AM signal has to be "detected". This separates the audio from the carrier.

"2. Can we assume that the bandpass filter, shown below, is of radio tuner ?"

Not a tuner per se. It's not adjustable. However it could be providing a tuned function. If you make the cap or/and coil externally adjustable......one could tune with it. There can be many other resonant tuned circuits in an AM radio. These are adjustable too.....but just once. They are not radio tuners.....but are adjustable tuned circuits.
 
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