Why are mixers noisy?

[Clifford Heath]

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?

 

[Phil Allison]

"Clifford Heath"

I've often heard it repeated (by radio hams) that mixers are noisy, but
never seen a good explanation of why.

** Why do radio hams say shit like that ?

Two reasons come to mind:

1. Because they have heard other hams say it and it sounded clever to them.

2. To impress fools and make themselves sound clever.


YOU need to go ask them one simple question:

What does that even mean and does it matter in a typical receiver ?

Come back when you have a sensible answer.

I won't be holding my breath.




.... Phil

 

[Robert Macy]

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?

guess...
don't mixers automatically split the tones, dropping them 6dB? Noise
stays put, thus effectively making it 'look' like noise was added.

 

[[email protected]]

I used to know how to do that calculation, now I just simulate, but I

vaguely recall, from early Garmin GPS chip days, a "passive" mixer (no

gain, diode(s) driven by an LO) has about an 11dB NF.



...Jim Thompson

--

| James E.Thompson | mens |

| Analog Innovations | et |

| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |

| San Tan Valley, AZ 85142 Skype: Contacts Only | |

| Voice480)460-2350 Fax: Available upon request | Brass Rat |

| E-mail Icon at http://www.analog-innovations.com | 1962 |



I love to cook with wine. Sometimes I even put it in the food.

noise cals in a rf system are based on nf ---

depending on the system design, the mixer tends to be one of the
first elements in the gain lineup. and as jim said, if there's a
lossey element it adds to the nf.

that's why some systems offer low noise pre-amps for certain apps.

it's all about trade off's.

 

[Joerg]

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?

The reason mixers are noisy is the cheap brush type motor in them, the
gears, and the large ice cubes people drop in there for their margarita.

Oh wait ...

 

[George Herold]

AFAICT there are two reasons: first, the conversion loss of the mixer,
which is usually about 6 dB, and second, the Johnson noise of the
switches.   (An ideal PN diode theoretically has exactly half the
Johnson noise of its small-signal resistance, but in practice you're
usually hitting it hard enough that it looks like a 300 kelvin resistor.)

If you saturate the switches, AM noise on the LO doesn't contribute much.

Cheers

Phil Hobbs
--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics, Electro-optics, Photonics, Analog Electronics

160 North State Road #203
Briarcliff Manor NY 10510

hobbs at electrooptical dot nethttp://electrooptical.net

A silly question. First is conversion loss the same as insertion
loss? And if so why does insertion loss add to the noise? I'd figure
that if I attenuate the signal by 3dB I'd also attenuate the noise by
the same amount. I was thinking that mixers are noisy because they
add signals from both the carrier and the image frequency... (I may be
using those words wrong. The output sees the noise from both sides of
the LO frequency.)

George (not an RF guy) Herold

 

[miso]

This implies if you use a Hilbert transformer and produced SSB, there is
no mixer noise. Something to meditate on when your navel is no longer a
sufficient target.


It is interesting how the better radios are the ones with more mixers,
i.e. triple conversion is praised while double conversion is lambasted
for too many images. Of course there is more to the foo than just mixer
noise.

 

[George Herold]

Yup.

There are some active mixers that mix and amplify at the same time that
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' diode ring 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, 'cause my 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?
Doesn't the noise at the input suffer the same conversion loss?
Maybe I don't know what the noise figure is. I thought it was the S/N
ratio from input to ouput. If the ratio stays the same isn't that
0dB?

George H.

 

[Jeroen]

[...]

Hi guys.. sorry for the repeat question.
(I must have a pencil stuck from ear to ear, 'cause my 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?
Doesn't the noise at the input suffer the same conversion loss?
Maybe I don't know what the noise figure is. I thought it was the S/N
ratio from input to ouput. If the ratio stays the same isn't that
0dB?

George H.

Everything coming in from the antenna is signal, by definition. The
noise is what originates from the receiver itself. Since most of that
comes from the first IF, it doesn't get attenuated by the mixer. So
since the mixer *does* reduce the signal by 6dB, that's a 6dB noise
figure right from the start.

The noise figure is S/N at the output over S/N at the input. In other
words, it's a measure of how much the receiver deteriorates the
original S/N.

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.

Jeroen Belleman

 

[Martin Riddle]

The reason mixers are noisy is the cheap brush type motor in them, the
gears, and the large ice cubes people drop in there for their
margarita.

Oh wait ...

You need one of those nice old Hamilton Beach blenders not a mixer.

Cheers

 

[George Herold]

   How do you attenuate the noise floor?

Ahh, I guess it depends on where the noise comes from.
A resistor divider will attenuate both noise and signal.
(As long as the resistors are low enough... Johnson noise of divider
R's less than input noise.)
Maybe I'm confused about what the 'noise' is.

George H.

 

[George Herold]

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?

[...]

Hi guys.. sorry for the repeat question.
(I must have a pencil stuck from ear to ear, 'cause my 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?
Doesn't the noise at the input suffer the same conversion loss?
Maybe I don't know what the noise figure is.  I thought it was the S/N
ratio from input to ouput.  If the ratio stays the same isn't that
0dB?

George H.

Everything coming in from the antenna is signal, by definition. The
noise is what originates from the receiver itself. Since most of that
comes from the first IF, it doesn't get attenuated by the mixer. So
since the mixer *does* reduce the signal by 6dB, that's a 6dB noise
figure right from the start.

ohh.. OK. I figured there must be some noise from the antenna too...
I mean you still get something from the antenna even when the signal
source is turned off.

The noise figure is S/N at the output over S/N at the input. In other
words, it's a measure of how much the receiver deteriorates the
original S/N.

I think I've got it. The 'noise' is mostly after the mixer. I was
thinking of just adding gain before the mixer. But I guess the high
frequency gain 'costs more', noise-wise, than the mixer and lower
frequency gain.

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.

 

[George Herold]

---http://en.wikipedia.org/wiki/Noise_figure

Thanks, I sorta understand noise, but some of that is stil
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.

 

[Tim Williams]

Ahh, I guess it depends on where the noise comes from.
A resistor divider will attenuate both noise and signal.
(As long as the resistors are low enough... Johnson noise of divider
R's less than input noise.)
Maybe I'm confused about what the 'noise' is.

Ah, but the low-R divider shunts the (line impedance) source, so its noise
doesn't go anywhere, anyway; the result is mostly noise from the divider
itself, at whatever impedance it has.

Tim

 

[Jeroen Belleman]

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. That's one of
the keys to low-noise design.

In my mind, all resistors have the same noise: P=kTB.

Jeroen Belleman

 

[Tauno Voipio]

Thanks, I sorta understand noise, but some of that is stil
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.

For terminating a transmission line, you need a thing looking
like a resistor of the characteristic impedance.

 

[Tauno Voipio]

Triple conversion drives me nuts with too many dead frequencies and
potential intermod mixes.

That's why you have to pay an arm and a leg for filters in the
first and second IF. The same applies for oscillator and mixer
shielding. A mixer is a very potent spewer of oscillator spurs.

 

[Jeroen Belleman]

On 18.6.13 5:54 , George Herold wrote:
[...]

Thanks, I sorta understand noise, but some of that is stil
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 resistor of
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

 

[Joerg]

You need one of those nice old Hamilton Beach blenders not a mixer.

Nah, you need this:

http://www.amazon.com/Tailgator-Gas...9&sr=8-1&keywords=gas+powered+margarita+mixer

 

[George Herold]

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, whereI can buy an opamp and lower the source impedance. (Given opamp noise andall the other caveat’s.)

(Crap... Google won't let keep using the old groups and now I'm posting viathe 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