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how to choose the first IF in an heterodyne receiver?

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hello,everyone.
I am now designing an wideband heterodyne receiver,sothe first IF should be high IF.I want to know what should be considered besides image rejection and half IF interference.
The spectrum analyzer in RS use 3476.4MHz as first IF,with 9kHz~3000MHz RF input.why is 3476.4MHz ? why not 3500MHz ,3600MHz ,or 3450MHz?
 

At minimum the difference between highest input frequency and I.F. must be great enough to allow some filtering attenuation by low pass filter on input with attenuation of I.F. frequency.

It is preferable to have I.F. be twice the highest R.F. frequency to avoid second harmonic distortion of mixer creating I.F. spurs. Older HP analysers used this approach. Better mixers with very good distortion performance have allowed lower I.F. on spectrum analyser.

As to exact 3476.4 MHz used by RS is likely due to their selections of processor clocks and attempt to avoid their spurs within the I.F. passband.
 

The IF requires a mixer to generate it, and a mixer to further downconvert it to a lower frequency. Both of those mixers can generate NxM spurious signals.

So you want to do a spur analysis on both of them before you set your system frequency plan in concrete. Typical problems are 1RF +/- 2 LO, 2RF+/- LO, 2RF, 3x2, etc products.

There are freeware spur analysis programs available to do so.
 

I have developed a program by matlab to calculate the mixer spur like abs(m*RF-n*LO). I take the RS spectrum analyzer as example, and find that the first IF 3476.4MHz does't show any difference with other IF as 3500MHz or 3450MHz .
Is that means the IF 3476.4MHz in RS SA is not the only choice?
 

chuckey said:
The first IF must nor be a multiple of the second(or third) IF
Frank
Click to expand...

That don't make sense. In our systems, one IF is 350M, and the other is2.6GHz. That is mixer function.
 

Sckoarn, if the second IF is 350Mhz, its harmonics fall at:- 700, 1050, 1400, 1750, 2100, 2450, 2800. You will notice that 2600 does not feature very highly!
Frank

---------- Post added at 19:07 ---------- Previous post was at 18:47 ----------
tony_lth said:
That don't make sense. In our systems, one IF is 350M, and the other is2.6GHz. That is mixer function.
Click to expand...
N X 350 = 2100, 2450, 2800 not 2600.
Frank

---------- Post added at 19:09 ---------- Previous post was at 19:07 ----------

350 X N = 2100, 2450, 2800, not 2600
Frank

---------- Post added at 19:23 ---------- Previous post was at 19:09 ----------

N X 350 = 2100, 2450, 2800, and not 2600. i have to add more words or the forum rejects my reply :)
frank

---------- Post added at 19:24 ---------- Previous post was at 19:23 ----------

I have to say at least 10 characters or the forum rejects my reply. N X 350 = 2100, 2450, 2800 and not 2600.
Frank

---------- Post added at 19:26 ---------- Previous post was at 19:24 ----------

N X 350 = 2100, 2450, 2800, and not 2600.
Frank
 

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