[SOLVED] AM Mixers?? Why what the use?

Hi all,
I would like to know whether there is any application requiring AM Mixer. From what I learnt about AM Receivers, there is no necessity to shift the RF signal to any IF level as the modulation basically depends on the amplitude modulation and its carried out by the envelope detector stages. In that case, why would people design AM Mixers ?

Any suggestions please..

(Or they use it only in the AM Transmitters to do the upconversion ?? )

Dan

Are your pointing to synchronous AM detection where an in-phase locked carrier is multiplied with the IF AM signal?

Regarding AM receiver setup. Making a fixed-frequency filter is easier than making a tunable filter. Therefore the signal to be received is converted (with a mixer) to an IF frequency, amplified and demodulated.

You may know that IF filters can have many resonators (6..8 is not uncommon in good equipment) to approximate a brick-wall pass band curve. Making such an arrangement tunable over the whole band of interest is practically impossible.

There are receivers without IF stages, these may convert directly to zero Hz (direct conversion receiver).

analog_curious said:

Hi all,
I would like to know whether there is any application requiring AM Mixer. From what I learnt about AM Receivers, there is no necessity to shift the RF signal to any IF level as the modulation basically depends on the amplitude modulation and its carried out by the envelope detector stages. In that case, why would people design AM Mixers ?

Any suggestions please..

(Or they use it only in the AM Transmitters to do the upconversion ?? )
Dan

Click to expand...

Greetings Dan

AM mixers have been used since the days of valve receivers. Have you never heard of the 455, 456 kHz etc IF freq's in an AM receiver ?
Even in modern receivers they still down convert to those IF freq's before recovering the audio

Dave

Thanks to you both for the replies.
I am not sure whether I understood your opinions completely..expecially the one from the user WimRFP.
Its definitely due to the fact that I dont know much about these things at the moment.

I got cleared with few things though and so I would point it out here. If I understood anything wrong, please feel free to correct me as I would prefer that.

1. Not necessary for a tunable filter..: Hmmm...I dont know..May be...Its because in AM as soon as the bandpass filter next to antenna tunes to a particular AM station (say 3 MHz, manually tuned here by the way), then the carrier gets demodulated mostly by envelope detection in which the R and C (resistor and capacitor used in envelope detector) are designed to have a low pass filter response until say 15 kHz or even until 20 kHz in order to cover the entire audio frequency ranges. So what exactly this AM mixer will change this flow in a more efficient way? I dont understand it

2.455 kHz: Yeah certainly, when I started to study about AM receivers, I came across this number a couple of times. But seriously what could be the advantage of it besides an official spectrum allocated by the Radio standards organisation..? Because in FM, IF frequency makes more sense to me. If you dont have an IF stage in FM receivers,then you end up in requiring a super high(absolutely impossible in an IC level to realise) quality factor for the RF filter to extract the information.(i.e I meant the inability to design inductors with such high Q factors). So you use a mixer then, convert it down to 10.7 MHz, there you go..! you could easily make a fixed bandpass filter and place it next to mixer in order to carry out the further processing steps in a FM receiver stage.

Could you have comments/suggestions/extra information on these points mentioned...

Dan

Regarding tunable filters: The problem is not in the detector, but in the filters.

Imagine what you need for (as an example) an 8 kHz (-6 dB) wide filter with <3 dB pass band ripple and >60 dB attenuation at 7 kHz from the center (so the shape factor is 1.75). The filter should be tunable over the full short wave band (3 MHz to 30 MHz).

You may start with designing a fixed frequency filter (say at 10 MHz) and find out how many resonators you need and what the Q-factor should be. This will for sure answer the question regarding the use of mixers in receivers.

analog_curious said:

Thanks to you both for the replies.

2.455 kHz: Yeah certainly, when I started to study about AM receivers, I came across this number a couple of times. But seriously what could be the advantage of it besides an official spectrum allocated by the Radio standards organisation..? Because in FM, IF frequency makes more sense to me. If you dont have an IF stage in FM receivers,then you end up in requiring a super high(absolutely impossible in an IC level to realise) quality factor for the RF filter to extract the information.(i.e I meant the inability to design inductors with such high Q factors). So you use a mixer then, convert it down to 10.7 MHz, there you go..! you could easily make a fixed bandpass filter and place it next to mixer in order to carry out the further processing steps in a FM receiver stage.
Could you have comments/suggestions/extra information on these points mentioned...

Dan

Click to expand...

Hi again Dan

Ahhhh but the 10.7 IF is only the first IF, you will find in most cases that it is followed by a second IF and that is usually 455kHz
the the audio is detected/demodulated from that 455 kHz IF stage

I was going to post an example image of a circuit using the MC3361 but the upload feature isnt working at present .... will try again later



Dave

Thanks again to both WinRFP and Dave for guiding me to know more about AM Radio Receivers and in common about Radio Receivers

I have done some literature search and finally realized that I tried confusing myself between synchronous AM Detection(Oscillator is tuned automatically to the incoming carrier frequency through a PLL) and asynchronous AM Detection(through a tuneable RF filter next to antenna and an envelope detector stage to demodulate).
Anyways the difference is clear now and also the article i referred helped me to know the advantages and disadvantages of either way of AM detection.
(https://www.radio-electronics.com/i...plitude-modulation-detection-demodulation.php)
Dan