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.
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.
The holy grail of HF receiver design is dynamic range. At the low end, there's noise floor and minimum detectable signal as defined by temperature, detection bandwidth, and Boltzmann's Constant. At the upper end, it's the point where something loses linearity (1dB gain compression) and starts to mangle the modulation envelope. Properly specifying the mixer is a key part of maximizing the dynamic range. The best receiver is where all the various stages overload at exactly the same signal level (with no AGC involvement).
You can put active devices in a mixer and get some free gain, but there's a problem. Amplifiers want to be linear, while mixers needs to be non-linear. Finding a common operating point that works well for both amplifying and mixing is not easy. At best, you get a compromise.
Triple conversion drives me nuts with too many dead frequencies and potential intermod mixes.
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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.
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.
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. :^)
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
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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.
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.
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.
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.
Commercial Telemetry for Aerospace is triple conversion. Plenty of shielding and careful selection of the IF frequencies prevent the problem. First IF at 300 MHz, and the second at 70 MHz was pretty much standard. We used either SMA or MCA connectors between modules, depending on the age of the equipment.
Cheap designs were RF to 70 MHz and had plenty of birdies. They could get away with that in cheap C-band sat gear because of the 20 MHz channel spacing and the use of alternating V & H polarization, plus the capture effect of FM.
a noise source does not necessarily >> reduce the noise level. I have one w here it would actually *increase* >> the noise. I use it for amplifiers wit h noise figures well below 3dB. > > Well OK if you've got big value resisto rs. > (Just give me enough power and these 1% milli-ohm resistors and I'll
tenuators are usually impedance matched. What you are actually doing by cho osing very high or very low resistor values is to intentionally mis-match t he corresponding noise sources, so that their available noise power is most ly 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
On Tuesday, June 18, 2013 4:38:15 AM UTC-4, Tauno Voipio wrote:
wrote: >> On Mon, 17 Jun 2013 13:26:41 -070
0 (PDT), George Herold >> >> >> >> >> >> wrote: >>> On Jun 17, 2:48 pm, Tim Wescott wrote: >>>> On Mon , 17 Jun 2013 09:38:11 -0700, Jeff Liebermann wrote: >>>>> On Mon, 17 Jun 2
013 16:11:55 +1000, Clifford Heath >>>>> wrote: >> >>>
tmospheric (lightning) noise levels will dominate the overall >>>>> sensiti vity figure by covering up any weak signals. The effect >>>>> decreases wit h frequency, so that by the time you get to about 30MHz, >>>>> the atmosphe ric noise is sufficiently low that an RF amplifier will be >>>>> useful. Ig noring 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 c overed the double balanced mixer, which is excellent for >>>>> dynamic rang e, but has a 6dB conversion loss, which translated directly >>>>> into a se nsitivity loss. Including internal losses, a good mixer has a >>>>> convers ion loss and NF (noise figure) of about 6.5dB, which may be the >>>>> "nois e" that the hams were discussing. It's not really noise, but noise >>>>> fi gure. >> >>>>> There are other types of mixers such a D flip flop, active F ET mixers, >>>>> DGMOS FET mixers, image reject mixers, single diode mixers , rusty bolt, >>>>> etc. Any non-linear device can act as a mixer. Active d evices can add >>>>> gain, but also add noise, just like an amplifier stage . Of course, some >>>>> are better (or worse) than others depending on what you are trying to >>>>> accomplish. >> >>>>> Understanding Mixers - Terms Defined, and Measuring Performance >>>>> >> >>>>> Mixer Noise Figure (with some controversy): >>>>> >> >>>>> Basics of RF Mixers in Radio Receivers / Mixer Tutorial / Frequency >>>>> Conversion >>
Well at low frequencies, and ~k ohm (or greater) impedance levels, and with active devices you can play some games to make a lower noise resistor. Bu t 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.)
d 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.) >> >> G eorge 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 the rmodynamics' law of entropy. That isn't true! It's quite possible, using ac tive 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 impe dance. Yes. Jeroen Belleman
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.
can buy an opamp and lower the source impedance. (Given opamp noise and all the other caveat?s.)
the new groops... which looks like $H!t. Sorry.)
same noise: P=kTB. Jeroen Belleman
Hi, George -
Take a look at Eternal September for groups. It's free and it works for me.
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.
can buy an opamp and lower the source impedance. (Given opamp noise and all the other caveat?s.)
new groops... which looks like $H!t. Sorry.)
Google is increasingly evil.
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Precision electronic instrumentation
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wrote: >> On Mon, 17 Jun 2013 13:26:41 -0700 (PDT), George Herold >> >> >> >> >> >> wrote: >>> On Jun
17, 2:48 pm, Tim Wescott wrote: >>>> On Mon, 17 Jun 2013
09:38:11 -0700, Jeff Liebermann wrote: >>>>> On Mon, 17 Jun 2013 16:11:55 +1000, Clifford Heath >>>>> 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? >>
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
Hobbs covered the double balanced mixer, which is excellent for >>>>> dynamic range, but has a 6dB conversion loss, which
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 what you are trying to >>>>> accomplish. >> >>>>> Understanding Mixers - Terms Defined, and Measuring Performance >>>>>
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
a 6dB noise figure? >> >> ---
formatting link
>> --- >>
understand noise, but some of that is stil >
it > better. > > (Seems what we need is a way to terminate the signals without a
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
active 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.)
A good RF front end can look like a very cold 50 ohm resistor at its input. Equivalent temperatures get down into the 50K range over modest fractional bandwidths. What you give us is, mostly, that it needs a power supply, where a real resistor doesn't.
--
John Larkin Highland Technology Inc
www.highlandtechnology.com jlarkin at highlandtechnology dot com
Precision electronic instrumentation
Picosecond-resolution Digital Delay and Pulse generators
Custom timing and laser controllers
Photonics and fiberoptic TTL data links
VME analog, thermocouple, LVDT, synchro, tachometer
Multichannel arbitrary waveform generators
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