Frequency converter oscillator question

[kurtulmehtap]

I was wondering why frequency converters use 2 mixing stages (and 2 different oscillators). Why not use the same oscillator for both the mixing stages. Is it the phase noise or the step size issue?
Thank you in advance.

 

[davenn]

I was wondering why frequency converters use 2 mixing stages (and 2 different oscillators). Why not use the same oscillator for both the mixing stages. Is it the phase noise or the step size issue?
Thank you in advance.

because for a 2 stage converter(mixer) you need 2 different oscillator frequencies ... otherwise you are not going to get your up or down conversion

eg the first Osc may convert the incoming signal say ~ 430MHz down to a first IF of 10.7MHz using Osc 1. The a second much lower freq Osc may convert that 10.7 MHz IF down to say 455 kHz

 

[kurtulmehtap]

For this case you are correct. But for cases like a 8-8.4 Ghz to 700 Mhz converter, you can have a 3.65-3.85 Ghz oscillator and use it in both mixing stages.

 

[FvM]

But for cases like a 8-8.4 Ghz to 700 Mhz converter, you can have a 3.65-3.85 Ghz oscillator and use it in both mixing stages.

Sounds like a purely hypothetical design that doesn't make much sense in terms of converter performance.
Main problem you need to get rid of is the possible second IF mirror frequencies. The imagined design is far from being optimal in this regard.

 

[vfone]

You want to use for the first LO the 2nd harmonic of 3.65GHz oscillator [8GHz - (3.65GHz*2) = 700MHz].
So how can you use further the fundamental LO on 3.65GHz for other conversion, when the IF is already on 700MHz?

 

[kurtulmehtap]

For 8 ghz input and 3.65 first LO we will have. 4.35 Ghz IF and products located at 7.3, 3.65 and 0.7 Ghz and some spurs. After filtering we have the 4.35 Ghz IF. Mixing it again with the LO will give you the 700 Mhz result and products at 4.35, 3.65 Ghz.

 

[chuckey]

Its late at night but have you not done a (8 -3.6) followed by another -3.6, which I think is close to (8-7.2?). So why not 8 - 2 X 3.6?

Oscillator frequency is ever so importent with respect to IF images and "birdies" (fixed hetrodynes within the pass band). The whole IF/Osc. systems are designed to optimise these numbers. So should be approached carefully

Frank

 

[FvM]

I keep my comment in post #4. Having a second IF at 4.3 GHz can be used to improve selection and image suppression. But it's not so much better than implementing the selection on the HF side. But there are many better dual superhet implementations with different LO frequencies.