UpConversion Mixer Noise

[mvj]

Hi,

In the case of a down conversion mixer, the noise from both the RF and image frequencies are down converted. This means that there is twice the noise power in the down converted signal. May I know what is the situation in case of up conversion? Does it have up converted noise only from the RF frequency or does it also have twice the noise? If so, why?

Thank you,
mvj

 

[biff44]

same as before, you have 2 bands of noise.

Lets say you upconvert 10 to 20 MHz. And the LO is 1 GHz. You would have noise at 1.010 to 1.020 GHz, AND noiise from 0.980 to 0.090 GHz.

 

[mvj]

Thank you biff44!!
The noise from 0.980 to 0.990 GHz does not overlap with the noise in the upconverted signal (i.e. 1.010 to 1.020 GHz) and it will both up and down converted to frequencies different from (1.010 to 1.1020 GHz). So can we say that in a mixer output, the upconverted signal has noise from the RF singal freq. alone where as in the downconverted signal there is noise from both RF and Image frequencies.
Of course, there is a possiblity of noise from (2.010 to 2.020 GHz) being down converted to (1.010 to 1.020 GHz). In this case there will be twice the noise in the upconverted signal. However, this would usually not be the case since the input singal is band pass filtered. Please let me know your thoughts on this.

Thank you,
mvj