abot superheterodyne receiver

[mydreamhouse]

i am designing a superheterodyne receiver, but i wonder my IF VGA. (first IF :180MHZ), i found if IF VGA has third order intermodulation, and it will mix with third order distortion of LO, which will locate in bands,is this a serious problem?, who can give me some advice or refernces?

 

[flatulent]

third harmonics are large amplitude. You need to make a spread sheet to calculate mixing products so that you can easily select the optimum frequency plan.

 

[mydreamhouse]

after the first mixer, i use an IF VGA before second mixer, the third order intermodulation of IF VGA will mix with the third distortion of the second mixer, not the frequency plan

 

[mydreamhouse]

who can give me some advice?

 

[Element_115]

Filter out the harmonics as needed.
On the IF Chain put a low pass filter of ~200MHz so the 3rd Harm(524MHz)
will be attenuated enough to not be an issue.

 

[mydreamhouse]

on chip Filter? 200MHZ

 

[webdog]

Having 3:d order intermods in IF will usually not be a problem with mixing LO but it will show up in the IF as it is close to the wanted signals!

We need a lot more information abut what frequencies involved in your design if we are supposed to help!

A wide IF is *very* hard to avoid spurs from LO and RF mixing. You have to select the LO and IF carefully to minimize the problem.

 

[Element_115]

on chip Filter? 200MHZ

Didn't know it was on an IC. Many IC's do need to go off chip
to do IF Filtering.
Cheers

 

[mydreamhouse]

thanks.
what i worry about is the third order distortion of IF signal mixes that of the second LO. for instance, the IF is 200MHZ, after external LC BPF or SAW filter, an on chip IF VGA is following. due to the third order intermodulation of IF VGA, third order distortion will be appears before the second mixer. so 3IF-3LO will be in band(dual conversion superheterodyne receiver),which will degrade the sensitivity.

 

[RFDave]

So the solution is to figure out the exact frequencies that will be generated. Unless you are using a walking IF scheme, then I suspect that the 3*LO-3*IF will be out of band. You need to do the analysis to figure out if this is a real concern or not. You can also adjust the IF frequency to mitigate this problem.

The third harmonic of the LO can be suppressed by a low pass filter after the LO, as you are looking at harmonics multiplying harmonics, you might only need a few dB of additional isolation

Third, how non-linear is your IF VGA stage? Worst case, you use this requirement to set a linearity specification on the IF VGA stage.

No-one else can do this analysis for you, you need to sit down and calculate the frequencies and levels and determine if it's a problem or not.

Dave
www.keystoneradio.com