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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.