Why does the value of IF for the AM receiver is 455 KHz? why

[anwer_sbh]

Why does the value of IF for the AM receiver is 455 KHz? why this value not another.

 

[youssef_zein]

because am is very old,more than 100 years old.
and you have to choose the IF not to large so you will have acceptable values for filter components and amplifier components. and not too small so you don't have mirrored signals at the mixing stage.
and you have to standartize the value, so 455 KHz is a logical choice

 

[LvW]

Re: Why does the value of IF for the AM receiver is 455 KHz?

Why does the value of IF for the AM receiver is 455 KHz? why this value not another.

The IF must be chosen so that any image frequency does not fall into the used frequency band.
This results in the equations:

F(image)=F(used)+2*F(intermediate IF)
and
F(image)>F(used)max.

Use the known AM frequency band and this will result in app. 450...480 kHz.

 

[karesz]

Hali youssef_zein,
Because Im relative old_in short will have 100 years on the bill(...
I remember, that so at 50-70 years ago they are some other IF-frequencies existing too
Much at 455Khz with such deviances(430-470KHz) & others with clear different values....
I wrote over only 1x mixed heterodyns.
K.

 

[FvM]

I think, the basic point is standardization of ceramic filters, that have been introduced in the early 70s. 470 kHz has
been mostly used by german and possibly other european products before. But they had tunable LC filters, not restricted
to an exact IF frequency.

 

[karesz]

Yes, I think too.
Then at this times it was an ITU Conference, wher are the Transmitter frequencies light retailed/corrigated too & after them it exist as IF only 455 KHz...
K.

 

[LvW]

Re: Why does the value of IF for the AM receiver is 455 KHz?

Yes, I think too.
Then at this times it was an ITU Conference, wher are the Transmitter frequencies light retailed/corrigated too & after them it exist as IF only 455 KHz...
K.

I think, a clear question deserves a clear answer!
And the question was: WHY the value of 455 kHz ?

Therefore, I will explain my former answer with a simple example:

A received AM radio signal at the lower end of the AM band of, let´s say, Fr=600 kHz will have a corresponding image frequency which is higher by twice the IF. For IF=455 kHz this leads to Fim=1510 kHz. This image frequency is at the upper end of the AM band and can easily suppressed by preselection circuitry.
For each higher receive frequency the situation becames even better.

This IF (which round the world is between 450 and 475 kHz) is a compromize between several requirements: Image rejection, preselection filter, channel separation (9 kHz), local oscillator capabilities, coordination with neighbour bands (short wave and long wave bands).
Thank you.

Added somewhat later: With respect to image frequency suppression, a higher IF would be, of course, better. But the other aspects as mentioned above lead to this compromize.

 

[karesz]

Re: Why does the value of IF for the AM receiver is 455 KHz?

Sorry DvW,
With respect, but I belief that you can not handel with:

...at the lower end of the AM band of, let´s say, Fr=600 kHz will have a corresponding image frequency which is higher by twice the IF. For IF=455 kHz this leads to Fim=1510 kHz....

Added somewhat later: With respect to image frequency suppression, a higher IF would be, of course, better. But the other aspects as mentioned above lead to this compromize.

because you will find such (only paar 100KW up to MW) transmitters below 550KHz... Their are in EU from Germany, Hungary & I think one Arabic too...
Band of MW begins at 520KHz.
K.

 

[FvM]

Additionally, a usual single ended mixer doesn't suppress the input frequency. So, besides the image frequency rejection
problem, the IF must also be clearly separated from the received bands, that means > 285 kHz (upper LW end)
and < 520 kHz (lower MW boundary).

 

[LvW]

Re: Why does the value of IF for the AM receiver is 455 KHz?

FvM: Yes, correct supplementary comment. Thanks.
Karesz: What do you mean with "cannot handle"? Is my calculation wrong? I did not fix the value of the IF. What then is your explanation instead? Have you a better one?

 

[karesz]

to LvW,
I wishd to tell /I mean(but my english...); you can not assume that the MW band begins at 600KHz, then its begin is over tenth of decads at 520 KHz...
K.

 

[LvW]

Re: Why does the value of IF for the AM receiver is 455 KHz?

to LvW,
I wishd to tell /I mean(but my english...); you can not assume that the MW band begins at 600KHz, then its begin is over tenth of decads at 520 KHz...
K.

Perhaps you misunderstood something. I did not "assume" that the MW band begins at 600 kHz. I did nothing else than to present an example for a receive frequency of 600 kHz (which is in a region "at the lower end" of the AM band starting at 520 kHz).