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Why are mixers noisy?

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George Herold

Jan 1, 1970
0
On 18.6.13 5:54 , George Herold wrote: > On Jun 17, 6:10 pm, John Fields <[email protected]> wrote: >> On Mon, 17 Jun 2013 13:26:41 -0700 (PDT), George Herold >> >> >> >> >> >> <[email protected]> wrote: >>>On Jun 17, 2:48 pm, Tim Wescott <[email protected]> wrote: >>>> On Mon, 17 Jun 2013 09:38:11 -0700, Jeff Liebermann wrote: >>>>> On Mon, 17 Jun 2013 16:11:55 +1000, Clifford Heath <[email protected]> >>>>> wrote: >> >>>>>> I've often heard it repeated (by radio hams) that mixers are noisy, but>>>>>> never seen a good explanation of why. I'm not talking about injecting >>>>>> noise either directly or via jitter/phase-noise, I mean the mixer>>>>>> itself. Where does the noise come from? How can the noise be minimised? >> >>>>>> Anyone care to expound? >> >>>>> I think you may have misunderstood what they said. I believe they were >>>>> referring to the mixer Noise Figure, not that it was contributing any >>>>> additional noise. >> >>>>> In an HF receiver, it's very common for the 1st mixer to be the first >>>>> device on the RF path, just after the front end band pass or low pass >>>>> filter. This makes it the key part in determining the system noise >>>>> figure and determining the receiver sensitivity. An RF stage in the >>>>> front end of an HF receiver is a waste of effort because the high >>>>> atmospheric (lightning) noise levels will dominate the overall >>>>> sensitivity figure by covering up any weak signals. The effect >>>>> decreases with frequency, so that by the time you get to about 30MHz, >>>>> the atmospheric noise is sufficiently low that an RF amplifier will be >>>>> useful. Ignoring atmospherics, it's mostly the mixer that determines >>>>> the major performance numbers (sensitivity, 3rd order intercept, dynamic >>>>> range,spurious responses, etc) on the receiver data sheet. >> >>>>> Phil Hobbs covered the double balanced mixer, which is excellent for >>>>> dynamic range, but has a 6dB conversion loss, which translated directly >>>>> into a sensitivity loss. Including internal losses, a good mixer has a >>>>> conversion loss and NF (noise figure) of about 6.5dB, which may be the >>>>> "noise" that the hams were discussing. It's not really noise, but noise >>>>> figure. >> >>>>> There are other types of mixers such a D flip flop, active FET mixers, >>>>> DGMOS FET mixers, image reject mixers, single diode mixers, rusty bolt, >>>>> etc. Any non-linear device can act as a mixer. Active devices can add >>>>> gain, but also add noise, just like an amplifier stage.. Of course, some >>>>> are better (or worse) than others depending on whatyou are trying to >>>>> accomplish. >> >>>>> Understanding Mixers - Terms Defined, and Measuring Performance >>>>> <http://www.minicircuits.com/app/AN00-009.pdf> >> >>>>> Mixer Noise Figure (with some controversy): >>>>> <http://www.microwaves101.com/encyclopedia/mixer_NF.cfm> >> >>>>> Basics of RFMixers in Radio Receivers / Mixer Tutorial / Frequency >>>>> Conversion >>>>> <
> (16:31 min) >> >>>> Yup.>> >>>> There are some active mixers that mix and amplify at the same timethat >>>> can have lower noise figures. But that comes at the cost of circuit >>>> complexity, a need for careful treatment to get it to work right, exotic >>>> components, or loss of dynamic range. >> >>>> A plain ol' diodering mixer that magically contributed no noise or >>>> losses beyond the switching losses would have a 6dB noise figure. >> >>> Hi guys.. sorry for the repeat question. >>> (I must have a pencil stuck from ear to ear, 'causemy brain ain't >>> workin'.) >>> So let's stick with the 'plain' diode ring mixer. It's got 6dB >>> (voltage goes down by 2) of conversion loss. No problem. But why is >>> this a 6dB noise figure? >> >> ---http://en.wikipedia.org/wiki/Noise_figure >> --- >> >> -- >> JF- Hide quoted text - >> >> - Show quoted text - > > Thanks, I sorta understand noise, but some of that isstil > confusing. > I guess if I knew how to measure the noise factor I'd understand it > better. > > (Seems what we need is a way to terminate the signals without a > resistor, suck 'em into an active device.) > > George H.Sorry to disappoint you, but but... As long as you have something with theratio of voltage and current like a resistor, it will have at least the thermal noise of an equivalent resistor, due to the thermodynamics' law of entropy. For terminating a transmission line, you need a thing looking like aresistor of the characteristic impedance. -- Tauno Voipio

Well at low frequencies, and ~k ohm (or greater) impedance levels, and withactive devices you can play some games to make a lower noise resistor. But you have to give up something else as far as I've been able to determine.(like less dynamic range, in the one example I know of.)
(I can't recall the right words to google... or I'd provide a link.)

(Oh try "artificial resistor")

George H.
 
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George Herold

Jan 1, 1970
0
On 2013-06-18 10:38, Tauno Voipio wrote: > On 18.6.13 5:54 , George Herold wrote: >[...] >> >> Thanks, I sorta understand noise, but some of that isstil >> confusing. >> I guess if I knew how to measure the noise factor I'd understand it >> better. >> >> (Seems what we need is a way to terminate the signals without a >> resistor, suck 'em into an active device.) >> >> George H. > > > Sorry to disappoint you, but but... > > As long as you have something with the ratio of voltage and current > like a resistor, it will have at least the thermal noise of an > equivalent resistor, due to the thermodynamics' law of entropy. That isn't true! It's quite possible, using active devices, to make a resistive impedance with less noise than a resistorof that value. I've done it. It works. > > For terminating a transmission line, you need a thing looking > like a resistor of the characteristic impedance. Yes. Jeroen Belleman

I tried something like this,
http://www.google.com/patents/US4352982

George H.
 
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John S

Jan 1, 1970
0
On 2013-06-18 04:42, George Herold wrote: > On Jun 17, 5:57 pm, Jeroen <[email protected]> wrote: [...] >> >> By the way, putting attenuators on a noise source does not necessarily >> reduce the noise level. I have one where it would actually *increase* >> the noise. I use it for amplifiers with noise figures well below 3dB. > > Well OK if you've got big value resistors. > (Just give me enough power and these 1% milli-ohm resistors and I'll > have that excess noise down to almost nothing. :^) > > George H. Mmmh. Attenuators are usually impedance matched. What you are actually doing by choosing very high or very low resistor values is to intentionally mis-match the corresponding noise sources, so that their available noise power is mostly reflected and doesn't end up in your signal.

Thanks Jeroen, I guess I'm mostly living in the low frequency world, where I can buy an opamp and lower the source impedance. (Given opamp noise and all the other caveat’s.)

(Crap... Google won't let keep using the old groups and now I'm posting via the new groops... which looks like $H!t. Sorry.)

George H.


That's one of the keys to low-noise design. In my mind, all resistors have the same noise: P=kTB. Jeroen Belleman

Hi, George -

Take a look at Eternal September for groups. It's free and it works for me.

John S
 
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Jeroen

Jan 1, 1970
0
Thanks Jeroen, I guess I'm mostly living in the low frequency world,
where I can buy an opamp and lower the source impedance. (Given
opamp noise and all the other caveat’s.)

(Crap... Google won't let keep using the old groups and now I'm
posting via the new groops... which looks like $H!t. Sorry.)

George H.

Wow George, you'll need to lose Google or soon no one will want to
talk to you. :)

It was quite enlightening to me when I realized that impedance
matching and reflection are *not* exclusively RF concepts. They
are valid from DC to daylight and beyond.

Jeroen Belleman
 
G

George Herold

Jan 1, 1970
0
On Tuesday, June 18, 2013 1:50:15 PM UTC-4, Jeroen wrote:
Wow George, you'll need to lose Google or soon no one will want to

talk to you. :)



It was quite enlightening to me when I realized that impedance
matching and reflection are *not* exclusively RF concepts. They
are valid from DC to daylight and beyond.

Jeroen Belleman

Oh, that's interesting I never thought about power reflections at/near DC.
Well, there's the battery power matching physics problem... I've never used it in practice. Mostly at DC I want no source impedance. :^)
George H.
 
J

josephkk

Jan 1, 1970
0
Levers and gears were invented before transformers.

Pity there's no decent mechanical equivalent to a buck-boost switcher.
Cars still have to use bunches of gears.

No small part of that is the properties of mechanical energy storage;
springs and moving masses.

?-)
 
-------------

You do. I mean, you do attenuate it as far as the noise that was *inputted*. But the "attenuator" creates its own -174 dBm/Hz noise. Because of that, a 3 dB attenuator has a 3 dB noise figure.



You can't distinguish between the white noise that was inputted and the white noise that is internally generated in the "attenuator," which is how the mixer behaves. They are random and uncorrelated.

ding ding ding...that was correct

however of course you can cool the attenuator (or mixer) and then it will attenuate without adding so much of it's own noise...

now for extra credit

how can an active LNA stage that is physically at 290K look like a good 50 Ohm load and at the same time have a noise figure better than 3 dB?

Mark
 
S

Simon S Aysdie

Jan 1, 1970
0
On Monday, July 22, 2013 10:18:15 PM UTC-4, Simon S Aysdie wrote:


ding ding ding...that was correct however of course you can cool the attenuator (or mixer) and then it will attenuate without adding so much of it's own noise...

now for extra credit how can an active LNA stage that is physically at 290K look like a good 50 Ohm load and at the same time have a noise figure better than 3 dB? Mark
 

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