high side injection and low side injection in mixer

[tshah]

high side injection

Hi,

Can anybody explain me what is high side injection and low side injection?
Also, what is single side band and double side band (noise figure for mixers)?

Please explain me with some examples in sentences. Please don't refer me to read any tutorial on internet. I am tired of online tutorials and I can't understand it. If you have some good lecture notes for that or you can post your own example. It can be really helpful to me.

Thanks.\]

 

[ThomasLee]

low side injection

High side injection means your LO frequency is higher than your RF signal.
Eg. RF=1GHz, LO=1.01GHz mixes to IF=10MHz.
Low side injection is the same but LO=0.99 GHz which also mixes to 10 MHz.
However, with High side injection the IF frequency spectrum is mirrored and with low side not so it is important to know which to use.

DSB noise figure assumes that both IF frequencies contain your wanted information.
This is usually only the case for direct conversion. In this case RF=LO, the wanted RF band lies on both sides around LO and mixes down to the same baseband frequencies.
SSB noise figure applies to systems where the other first order mixing product does not contain wanted information and will be filtered.
Eg RF=1GHz, LO=1.01GHz mixes to wanted IF=10MHz but also to 2.01GHz. If you do not filter the 2.01GHz your SSB NF is 3dB higher if constant conversion gain is assumed.

 

[tshah]

high side lo

Hi thomas,

I really appreciate your reply. can you please explain me how IF spectrum is mirrored in high side injection and in low side injection why is it not mirrored?

Also, i didn't understand below:
DSB noise figure assumes that both IF frequencies contain your wanted information.

Thanks for the information.

It was really useful.

 

[ThomasLee]

high-side lo

Ok, good I could help you. Thanks for the reward points. I try to explain it with my own words and use examples again. LO is always a fixed frequency while you must consider RF not to be a single frequency but a band of frequencies.


High Side:
RF1=1.000GHz, LO=1.010GHz mixes to IF1=10MHz.
RF2=1.006GHz, LO=1.010GHz mixes to IF2=4MHz.

You see that RF2 was higher frequency than RF1 but IF2 now is lower than IF1. That's why the spectrum is mirrored.

Low Side:
Now the same RF band (6MHz wide) is downconverted with low side injection:

RF1=1.000GHz, LO=0.996GHz mixes to IF1=4MHz.
RF2=1.006GHz, LO=0.996GHz mixes to IF2=10MHz.

You see IF2 is still higher frequency than IF1 like RF2 is higher than RF1. The spectrum is not mirrored although the IF band is the same.

Hence it is important to know how your RF was modulated.






I also try to explain DSB NF. You know in a mixer there are always 2 first order mixing products: Your wanted mixing product and the image. Usually the image is filtered because it does not contain useful information. In this case SSB noise figure applies. If the image is perfectly filtered, SSB should be equal to DSB NF. If not filtered at all SSB NF is 3 dB higher.
DSB NF is only valid if you do not filter the image which you will only do if it is part of your wanted signal. To my knowledge this is only the case when LOfreq= mid of RFfreq band.

Example:

RF1=1.000GHz, LO=1.003GHz mixes to IF1=3MHz.
RF1=1.006GHz, LO=1.003GHz mixes to IF2=3MHz.

You end up with a 3MHz wide IF band (DC-3MHz) because one half of the RF band is folded onto the other. Here DSB NF must be calculated.

I hope that was all correct.

 

[tshah]

lo injection

I really appreciate your reply.

Thank you for taking time.

It was really useful.

Thanks

 

[flatulent]

low side vs. high side lo

one advantage of high side LO is that fewer IMD products usually fall into the IF. it is best to make a spread sheet to check for this in your specific situation.