Choice of Number of mixers and IF frequencies

[darcyrandall2004]

Hello,

When designing a receiver how does one decide how many mixers are necessary and what IF frequencies and how many to use?

Why do designs tend to utilise 455kHz, 10.7MHz, 21.4MHz etc as the IF frequencies?

Thankyou

 

[rfndmw]

455KHz for AM and 10.7 MHz for FM et. are used because of the fact that they are the set communication standards....and as to the no. of mixers used in a system that is a function of the kind of mixer u r using and the frequency of operation etc. to get the required IF...

 

[darcyrandall2004]

Okay but then why do some designs show the input signal first mixed with a local oscillator to produce a smaller frequency and then again this frequency mixed with another oscillator to produce an even lower frequency and then the demodulation is performed on this frequency? Why not just use one local oscillator to produce the final low frequency from which the intelligence is extracted?

Thankyou

 

[honeydewsan]

Because it depends on the selectivity required. It is easier to achieve higher selectivity at lower frequency.

 

[biff44]

You ask a very general question. IF frequency and the number of mixers is determined by your technical requirements:
Spurious output level
Input frquency range
Desired instantaneou receive bandwidth
Dynamic range
suppression of jammers/interference
system tolerance of known spurs
system tolerance of image frequencies
cost
dynamic range
ADC clock speed
choice of I/Q or single ended IF
choice of fixed IF or zero IF
availability of filters
number of bits in the ADC
complexity of the digital processing
available LO tuning speed
LO broadband phase noise/spurious
tuning range of LO
available clock frequencies
receiver input environmental noise floor
group delay flatness/parabolic requirement/phse linearity
a few other specific system concerns that slip my mind

Rich