Understanding Mixer's Conversion loss

conversion loss

Hi,

can anyone tell me what is the difference of conversion loss manupulation of active and passive mixer.

Pasive mixer conversion loss is directly used as insertion loss (or Noise Figure)


But active mixer gives you gain but its conversion loss is also mentioned (which is high than the passive mixer) so how to use that conversion loss of active mixer for noise figure calculations.

Best Regards.

passive mixer conversion loss

waseemsh said:

Hi,

can anyone tell me what is the difference of conversion loss manupulation of active and passive mixer.

Pasive mixer conversion loss is directly used as insertion loss (or Noise Figure)


But active mixer gives you gain but its conversion loss is also mentioned (which is high than the passive mixer) so how to use that conversion loss of active mixer for noise figure calculations.

Best Regards.

Click to expand...

I think conversion loss is just the conversion gain. And the relationship between conversion gain and NF is same with that in LNA design.

Active mixers usually include an amplifier, either before or after the converting mixer. Passive mixers alone have a conversion loss, typically 6-10 dB. Active mixers can have a conversion gain due to amplifier gain.
To know mixer noise figure, in the passive mixers it equals to conversion loss, if both RF sidebands (above and below LO frequency) are converted to IF. If one sideband is suppressed by a RF filter, however, the noise figure is ~3 dB higher.
With active mixers, the noise figure depends on where the amplifier is located. If before the mixer, then amplifier noise figure determines the system noise figure. Otherwise, one must use the Friis' formula to include gains and losses and noise figures of individual components.

jiripolivka said:

If one sideband is suppressed by a RF filter, however, the noise figure is ~3 dB higher.

Click to expand...

Could you please explain why?

Both signal and noise at the input side are filtered by the RF filter. So the NF is not affected, correct? Note NF=(Sout/Sin)*(Nin/Nout). So halfing the Sout and Nin does not change NF.

By rejecting "half of the RF signal" the noise output remains the same. Then you simply get only one-half of possible RF signal down-converted. This reflects in a 3-dB NF increase.
You can read a book on receivers where more details are given.
Receivers with no RF filters are often used in mm-wave radiometers which detect extremely low levels of noise from natural sources. Such receivers have the "DSB" noise figures (double-sideband) and their effective bandwidth is twice the IF bandwidth.
Receivers with a RF filter are used to process communication signals; a RF filter rejects unwanted signals from "image" band, etc. Their NF is "SSB", single-sideband, and is always higher than DSB by 3 dB. Mixer front-end receivers with RF filters cannot have their NF lower than 6 dB. Only by using a RF preamplifier, their NF is then determined by this low-noise preamplifier.

jiripolivka, can I ask you something. You stated above that active mixers might include an amplifier. If we use a single diode or Transistor as a mixer and biased, so that it is active. It is actually working in its active region and is capable to amplify. So is it correct to say that the diode itself will amplify the signal too?
Thats why you get conversion gain (say + dB)

I do not think so.
By biasing a mixer diode (or a diode pair), it does not make it "active". DC biasing only sets diode operation point closer to the I/V curve "knee" or the point of an optimum mixer conversion loss.
By "active" we usually mean that the mixer "block" comprises an amplifier which gives it a gain, so the overall conversion loss becomes negative (or a gain).
Most often the down converters have a LNA before the mixer, some MMICs have a LO amplifier or frequency multiplier, but the LO amplifiers only optimize mixer pumping; the conversion loss remains a loss.
From the basics: any mixer is a non-linear element to which we introduce a high-level LO signal, and a RF signal. Due to diode or transistor non-linearity, combination frequencies are generated of which we usually only select one. Others are suppressed by filters- all this causes a loss.

The 3 db NF degradation due to filtering of image is not 100% true.

For an active mixer there is a noise termination effect. Just as desired RF has an optimum noise figure termination, the image noise can be minimized with an optimum termination impedance.

Another concern is the local oscillator sideband noise level in degrading noise figure. The amount this effects a mixer NF is dependent on mixer topology and frequency separation between RF and LO.

jiripolivka said:

I do not think so.
By biasing a mixer diode (or a diode pair), it does not make it "active". DC biasing only sets diode operation point closer to the I/V curve "knee" or the point of an optimum mixer conversion loss.

Click to expand...

I agree by the fact that the word "active" can have many definitions. But you stated that biasing fixes the operating point. At maximum nonlinearity point orI/V curve "knee" or in active region of diode (as it is defined by Sedra) amplification will take place. However, in a single diode parasitic losses due to its series resistance , junction capacitance are too high thats why usually we have conversion loss instead of gain.

But if the conversion loss of this biased diode is compared with that of unbiased diode it will be less in magnitude that means little bit gain will be there. This gain is because of maximum amplification at knee point of diode, even though diode will produce harmonics as it satisfies Id ~ Is Exp (vt/nKT), its current equation.

As we can agree on a resonable definition what "active" means, we can go on.

In a diode mixer, LO power is rectified to establish an operating point on I/V curve, close to "knee" where nonlinearity is optimum for mixing process.
By DC bias we do the same thing , so less LO power is needed.
A diode mixer with P/N junction diodes is a passive device.
If tunnel or similar diodes are used which exhibit a "negative resistance", then a mixer with such elements is an active device and does exhibit a conversion gain.
If a transistor is used in a mixer, it can be adjusted as a signal amplifier and can achieve the conversion gain, so it can be an active device.

Good passive mixers are often preferred as they are wideband, well matched and with a reasonable LO power input they can be "linear" and process high-level signals. A pre- or post-amplifier is then used to achieve conversion gain, improvement of receiver noise figure or increase of output power (in upconverters).

Rarely one can have all advantages in one device only.