Mirror frequency.

[EngIntoHW]

A bandpass signal with a bandwidth Bs is transmitted using a given frequency fc.

The modulated signal is received by the following receiver:

**broken link removed**
(*LNA = Low-Noise Amplifier).
(LO generates A*cos(ωH*t) where ωH = ωC + ωIF).

They said in the notebook that this receiver is not good since it doesnt filter the mirror frequency fr

**broken link removed**

What I dont get is where does this mirror frequency come from?

I'd be happy if you could explain it to me.

Thank you.

 

[ericgibbs]

hi,
The LO/Mixer produces the sum and difference frequencies of the IF frequency.
Look here:
Image frequency - Wikipedia, the free encyclopedia

 

[EngIntoHW]

hi,
The LO/Mixer produces the sum and difference frequencies of the IF frequency.
Look here:
Image frequency - Wikipedia, the free encyclopedia

Thanks Eric.

Genreally, we would tune the frequency of the LO's output signal to fc + fIF.
Therefore, the input frequencies of the mixer are fc (frequency of input signal) and fc+fIF (frequency of LO's output signal).

Thereupon, the mixer's output would contain 2 frequqencies: fIF and 2fc + fIF.
Is 2fc + fIF the mirror frequency that we would want to filter?

Moreover, they said in the link you attached that:

Sensitivity to the image frequency can be minimised only by (a) a tunable filter that precedes the mixer...

How can we filter the mirror frequency before it was created?

 

[audioguru]

Everybody else says it is an image frequency, not a "mirror" frequency.

 

[EngIntoHW]

Everybody else says it is an image frequency, not a "mirror" frequency.

Yeah you're right.
However, you can call it in both terms:

The unwanted frequency is called the image of the wanted frequency, alternatively the mirror frequency

*Wikipedia.

I dont understand how is the image frequency eliminated before it is created by the mixer.

 

[ericgibbs]

Yeah you're right.
However, you can call it in both terms:
*Wikipedia.

I dont understand how is the image frequency eliminated before it is created by the mixer.

hi,
Its not eliminated.
Consider that Ithe ntermediate Frequency [IF] transformers are tuned to one of the LO/Mixer frequency outputs , example 465kHz, this would ,mean the other LO/Mixer freq would not be within the bandwidth/freq response of the IF amplifiers..
OK.?

EDIT:
Look at this
**broken link removed**

 

[EngIntoHW]

Hi Eric.

Thank you very much for the link.
I'll firstly read the tutorial in the link and will reply to you once I finished reading it.

Thanks again.

 

[EngIntoHW]

Hi again Eric.

The link you provided me with was so great.
I really appreciate your help.