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Any mixer can be used as up- or down-converter. In a mixer, a non-linear element is "pumped" by a local-oscillator signal, and generates harmonics. If another signal is added (most often with a lower power than LO is), harmonic combinations of both signal frequencies are generated.
Then we use various filters to utilize some of the generated signals while others are suppressed. A mixer then has a "conversion loss" which indicates how well we transferred one signal at a chosen frequency into another one at another frequency.
If e.g. an IF frequency of 1 MHz is upconverted to 3 MHz, we can use a local oscillator running at 2 or 4 MHz. In receivers, we use mostly down converters, then they can convert either one or two side bands to the IF.
Their noise figure depends if the "image" filter is used (single-sideband conversion ), or not (DSB, double-sideband conversion). Noise figure of a DSB mixer is one-half of that of a SSB mixer, but DSB mixers are rather used in receivers processing noise while SSB mixers are used to process human-generated signals.'
Similar rules are valid also if a receiver utilizes an upconverter (some HF receivers do). Among communication engineers there still is a confusion about how the noise is processed in mixers.
Radio astronomers who process noise in their receivers mostly use DSB mixers, and there is no confusion about their noise performance.
---------- Post added at 19:20 ---------- Previous post was at 19:16 ----------
One good reference is : www.agilent.com/find/eesof, another, www.deetc.jsel.ipl.pt, chapter 12. A bad reference just appeared in Microwaves &RF, Jan.2012, pp.86-89. Mr.Monzello simply never understood the Friis' formula for noise in receivers.
i thought dsb mixers have around 3db higher noise figure than ssb mixer? Correct or not?
Regarding to my previous post, i thought about the meaning of noise figure in an upconverter which is feeding for example some antenna or pa. I found no reason to specify the noise figure for an upconverter in transmitter.
Can you please check your links, specially the second one doesn't work.
I agree with you about Mr. Monzello and his relationship to the Friis-formula.
i thought dsb mixers have around 3db higher noise figure than ssb mixer? Correct or not?
Regarding to my previous post, i thought about the meaning of noise figure in an upconverter which is feeding for example some antenna or pa. I found no reason to specify the noise figure for an upconverter in transmitter.
Can you please check your links, specially the second one doesn't work.
I agree with you about Mr. Monzello and his relationship to the Friis-formula.
No, the reverse is true: SSB mixers reject the image response and this makes them to have the NF higher by 3 dB than DSB mixers in which both sidebands are converted to IF.
The second link I can only read "www.deetc.jsel.ipl.pt", sorry if it does not work. Please check any recent textbook on microwave receivers, where the Friis' formula is discussed.
Noise figure of any mixer does not depend on the frequency plan, only on filtering unwanted bands. Upconverters as well as down converters are usually passive and only translate one frequency to another, with the aid of a local oscillator. You can define and measure the NF in any mixer the same way. Utilization of harmonics reduces conversion loss and selecting or rejecting the image response by filters decides their NF which is proportional to conversion loss.
Usually in transmitters there is no need to define noise figure as their noise power level is typically many orders below the desired RF power level, so it makes no technical sense. Instead, linearity and intermodulation, power supply and other noise count. Noise figure relates to thermal noise.
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