Drawbacks of Broadband LNA and Mixer

[pragash]

Many companies implements "off the shelf" components for their RF chains such us mixer chip and LNA chip. The problem with off the shelf components are usually it will be broadband.

In my opinion, a broadband LNA will amplify out of band noise in the receiver system. a broadband mixer will have poor suppression for RF, LO frequency and image frequency as well. This is my opinion. Please let me know if these are true?

please state what other drawbacks of using off the shelf broadband LNA and broadband Mixers.

 

[pancho_hideboo]

Large folding noise.

 

[pragash]

could you give me some material or explanation on folding noise? is it similar to noise folding?

 

[volker@muehlhaus]

In my opinion, a broadband LNA will amplify out of band noise in the receiver system.

As long as that noise stays outside your receiver channel, you don't care. That's what pancho_hidebo refers to: make sure noise is not mixed to your frequency range by non-linear process.

You need to have more filters to reject large out-of-band signals, so that they don't overload your receiver frontend. And the noise figure of a narrow band LNA ist often optimized to better values.

 

[pancho_hideboo]

is it similar to noise folding?

Yes.

As long as that noise stays outside your receiver channel, you don't care.

No.

Devices themselves have wide operation bandwidth for broadband mixer.
So even if you limit input noise bandwidth, output noise does not decrease.

Too wide band operation bandwidth results in large output noise.

 

[volker@muehlhaus]

That's a topic for system level design to find the appropriate gain and bandwidth for different stages. Anyway, good luck my friend!

 

[ktr]

Most of LNA's I designed or saw, sacrifices s11 for low noise figure. If LNA is wideband, its noise figure may not be optimized, and u may not want to pick up that much noise from lna stage.

 

[pragash]

hi guys,
let me put what pancho_hidebo said in a different way. receiver sensitivity is given by;

Receiver Sensitivity=-174dBm+10 log⁡(noise equivalent system BW)+F_sys (dB)+C/N ratio (dB)


10 log⁡(noise equivalent system BW) is a noise bandwidth of a system. for broadband LNA and Mixer, the noise bandwidth will be a large number and it will degrade the receiver sensitivity. Am i right in this context?

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folding noise = noise bandwidth?

 

[pancho_hideboo]

That's a topic for system level design to find the appropriate gain and bandwidth for different stages.

let me put what pancho_hidebo said in a different way. receiver sensitivity is given by;

and u may not want to pick up that much noise from lna stage.

You all misunderstand.

I don't mean noise from former stage of mixer.

I mean wide band noises from devices constituting mixer itself.

Consider k*T/C noise of switched capacitor circuits as similar simple example.

 

[pragash]

hi guys,
let me put what pancho_hidebo said in a different way. receiver sensitivity is given by;

Receiver Sensitivity=-174dBm+10 log⁡(noise equivalent system BW)+F_sys (dB)+C/N ratio (dB)


10 log⁡(noise equivalent system BW) is a noise bandwidth of a system. for broadband LNA and Mixer, the noise bandwidth will be a large number and it will degrade the receiver sensitivity. Am i right in this context?

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maybe not. Receiver always receive/operate in a single frequency at a time. so the noise BW of a brandband LNA/Mixer will be similar to noise bandwidth of a narrow band LNA/Mixer i think!

 

[pancho_hideboo]

the noise BW of a brandband LNA/Mixer will be similar to noise bandwidth of a narrow band LNA/Mixer i think!

No.
Devices consituiting Mixer have wide band width.

 

[pragash]

for a wideband LNA/Mixer, i think there is no direct effect on receiver sensitivity. however, the wideband LNA/Mixer prone to spurs/harmonics/image frequecy interference.

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No.
Devices consituiting Mixer have wide band width.

i humbly disagree. you can have wide bandwidth but when you receive, you receive on a single frequency. Noise bandwidth of a receiver system usually will be measured +/-10KHz from the receiving frequency. This noise bandwidth will be used for receiver sensitivity calculation. Assuming the response of broadband LNA/Mixer and narrowband LNA/Mixer is similar in the band of our interest, i don't see a reason why noise bandwidth of a broadband mixer/LNA will be different from a narrow band mixer LNA.

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correct me if im wrong with more information/examples. thanks.

 

[pancho_hideboo]

i humbly disagree.
you can have wide bandwidth but when you receive, you receive on a single frequency.
Noise bandwidth of a receiver system usually will be measured +/-10KHz from the receiving frequency.
......................................
correct me if im wrong with more information/examples.

Wrong.

You can not understand folding noise from devices constituting mixer.

If you have RF simulator, do noise contribution analysis.

 

[pragash]

is this folding noise is similar to noise bandwidth reduction which i have explained above?

 

[pancho_hideboo]

is this folding noise is similar to noise bandwidth reduction which i have explained above?

noise bandwidth does not reduce.

(1) RFin=10GHz, LOin=9.9GHz
(2) RFin=1.1GHz, LOin=1.0GHz

Same IFout=0.1GHz.
nearly 10 folding noise are expected for case (2).

 

[pragash]

noise bandwidth does not reduce.

this is what I also said in my statement. I stated the noise bandwidth should not be different between the wideband mixer and narrowband mixer. you disagree with the statement!.

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so you are saying that folding noise will be higher at low frequency? am I correct?

 

[volker@muehlhaus]

Wrong.
You can not understand folding noise from devices constituting mixer.
If you have RF simulator, do noise contribution analysis.

If the mixer has such a strong effect on system noise figure, why don't you increase LNA gain?

If you have RF simulator, do noise contribution analysis.

Why don't you show an example for your claimed effects?

 

[pancho_hideboo]

You are all users of prebuild mixer.
I am builder of mixer circuits.

 

[volker@muehlhaus]

I am builder of mixer circuits.

Congratulations. Show the effect on system performance in simulation, please.

 

[pragash]

hi pancho_hideboo,
finally I manage to figure out myself the noise folding. i would like to share it here for the benefit of others.

**broken link removed**

As we have seen, because of the image problem, a receive mixer down converts both desired and the image bands to IF frequency. This means folding the noise at the image frequency on top of the desired band at IF. Therefore, the total noise at IF is as follows:
The noise at desired RF band down converted to IF
The noise at image RF band down converted to IF
The noise added by the mixer noisy circuit itself.

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it is understandable that a wideband mixer will have multiple image frequencies which are

Fimage = Frf - 2LO
Fimage = Frf - 3LO
Fimage = Frf - 4LO
.....
.
.
.
Fimage = Frf -NLO

due to this narrrowband mixer which rejects other frequencies than the desired IF frequencies is preferred.