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Sci.Electronics.Basics -> Receiver sensitivity and IF bandwidth??
There are 27 messages in this thread.
You are currently looking at messages 20 to 27.
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Author: billcalleyDate: 13:10 25-10-07
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On Oct 25, 12:21 am, JosephKK <joseph_barr...@sbcglobal.net> wrote:
> billcalley billcal...@yahoo.com posted to sci.electronics.design:
>
>
>
>
>
> > On Oct 23, 3:12 pm, Mark <makol...@yahoo.com> wrote:
> >> On Oct 23, 4:36 am, billcalley <billcal...@yahoo.com> wrote:
>
> >> > On Oct 22, 11:17 pm, et...@FreeNet.Carleton.CA (Michael Black)
> >> > wrote:
>
> >> > > billcalley (billcal...@yahoo.com) writes:
> >> > > > Hi All,
>
> >> > > > I keep reading that the high-gain front-end stages of
a
> >> > > > microwave
> >> > > > receiver almost completely sets the entire radio's NF and
> >> > > > sensitivity, and that the following stages (the I.F.) have
> >> > > > little effect except to amplify the signal and the noise
> >> > > > equally to a higher amplitude for the
> >> > > > radio's detector. This doesn't make complete sense to me,
> >> > > > because the I.F. would have a HUGE effect on the receiver's
> >> > > > signal-to-noise ratio, and therefore its sensitivity, if we
> >> > > > simply narrowed the IF's bandwidth down from, let's say,
1MHz
> >> > > > to 1kHz!!
>
> >> > > All an amplifier can do is amplify what's at its input.
> >> > > Whatever the signal is in reference to the noise, that ratio
> >> > > will remain at the output, even though the actual voltage level
> >> > > will be higher at the output of the amplifier compared with the
> >> > > input.
>
> >> > > To use a broad example, 1v of noise and 0.1v of signal at an
> >> > > amplifier's input will mean 10v of noise and 1v of signal at
> >> > > the the output if
> >> > > the gain is ten. You haven't altered the ratio, just made
> >> > > everything louder.
>
> >> > > It's like turning up the volume on a hearing aid to hear the
> >> > > person next to you, but which also amplifies the other sounds
> >> > > in the room that were already stronger than the person; you
> >> > > haven't actually fixed the problem because the problem was that
> >> > > the person was weaker than the surrounding sounds.
>
> >> > > So if you have a first stage that adds noise to the mix, noise
> >> > > that will help to mask the desired signal, then you've made
> >> > > things worse. Forever down the signal chain, there is nothing
> >> > > you can do to fix the problem, because once that noise is
> >> > > added, any later amplification
> >> > > amplifies it along with the desired signal. If that stage in
> >> > > the broad example generated 1v of noise, that equals the level
> >> > > of the desired signal, and thus has made the situation worse.
>
> >> > > So you want to get that signal up fast without adding any
> >> > > noise, or
> >> > > at least as little as possible. So for low level microphones,
> >> > > you'll often see a transformer to boost the signal, because it
> >> > > will introduce less noise than an active stage.
>
> >> > > A low noise first rf stage will indeed set the stage. It will
> >> > > amplify the incoming signal (and the background noise equally)
> >> > > but will
> >> > > add little of its own noise to mask the signal. If it's not
> >> > > low noise, then any incoming signal has to be above a certain
> >> > > level to stay above that noise.
>
> >> > > There is background noise picked up by the antenna along with
> >> > > the
> >> > > desired signal. That level varies with frequency, becoming
> >> > > more
> >> > > significant the higher up you go. You can't do anything about
> >> > > that, it's part of basic communication (well you can, but
> >> > > that's another story). But you can work at making sure as
> >> > > little noise as possible is added to the mix.
>
> >> > > Later stages don't matter, because the signal is stronger
> >> > > and the noise generated by later stages will not have the same
> >> > > impact. So that previous broad example, 10v of noise and 1volt
> >> > > of signal out of the first stage, the second stage will amplify
> >> > > that by ten again, so its output is 100v of noise (I said that
> >> > > was a broad example) and 10volts of signal, but if the stage
> >> > > adds 1 volt of noise that 1v is now 1/10th the level of the
> >> > > desired signal, when before it was stronger than the desired
> >> > > signal.
>
> >> > > Michael
>
> >> > Thanks Tom, Tim, Phil, and Michael for some great answers!
>
> >> > I guess I need time to digest all this. But what I still
> >> > don't
> >> > get -- just taking Michael's terrific response as a good example
> >> > -- is while I know that the receiver's front-end sets the ratio
> >> > between the input signal and the receiver's noise, and that this
> >> > S/N ratio cannot then be improved by the receiver's I.F.
> >> > *amplification* stages, why can't the receiver's I.F. *filter*
> >> > stages simply passband filter out most of that wideband input
> >> > noise to improve the receiver's SNR, which
> >> > should then improve the sensitivity of the receiver? That's the
> >> > part that still has me stumped...
>
> >> > Thanks All,
>
> >> > -Bill- Hide quoted text -
>
> >> > - Show quoted text -
>
> >> another way to think of it is that the desried input signal has a
> >> power DENSITY, i.e. power per Hz BW. and the front end ciruicts
> >> have a noise DENSITY, i.e. noise power per Hz that is determined by
> >> the
> >> noise figure. Then when the noise and signl go therough the IF
> >> filter, the IF filter sets the BW. If the signal is very narrow,
> >> the filter can be very narrow and will let in only the minimum
> >> possible
> >> amount of noise. Both the noise figure and IF bandwidth are
> >> important
> >> in determining sensitivity. But the IF BW cannot be less then the
> >> desired signal BW. And the Noise figure can't be less than 0 dB.
>
> >> I think one of the key concepts you may be missing is that even
> >> the
> >> antenna picks up noise with the signal so there is a limit to the
> >> acheivable sensitivity even if you had a "perfect" receiver. A
> >> perfect receiver would have a 0 dB noise figure. That does not
> >> mean there is NO noise, it means there is no EXTRA noise beyond
> >> that which the antenna picks up.
> >> The lowest noise floor for space communicarions is the 3degK floor.
> >> For Earth comm its room temperature. The "perfect" receiver also
> >> has a BW no wider than it needs to be to pass the desired signal
> >> but it must be wide enough to pass the signal and therefore also
> >> passes that amount of noise.
>
> >> Mark- Hide quoted text -
>
> >> - Show quoted text -
>
> > Thanks a lot guys. I think I understand all this now (at least I
> > hope I do!):
>
> > 1. If we can decrease the receiver's NF *or* bandwidth (which will
> > decrease the added noise levels), then we will improve our SNR, and
> > therefore our sensitivity.
>
> > 2. The NF dominates microwave receiver designs because that is
> > normally all we will have any control over when we are given the
> > SNR/ BW/modulation that will be used in the system.
>
> > 3. Since it is measured at a single spot frequency of 1Hz, NF
> > itself is completely independent of bandwidth.
>
> Er, not quite. It is a tradeoff bewteen bandwidth versus noise versus
> datarate. Please see Shannon's law
>
>
>
> > 4. After the receiver's high gain frontend receives the transmitted
> > RF signal-with-noise, and then adds its own frontend circuit noise,
> > the I.F. stages will only be able to, at best, maintain this same
> > signal-
> > to-noise ratio as set by the frontend. No improvement in SNR will
> > be possible, since the I.F.'s bandwidth will be "set in stone" for
> > the specific modulation in use, and cannot be less wide than the
> > modulation itself. (Therefore, when the receiver's bandwidth is
> > fixed, then the system NF is directly related to the receiver's
> > sensitivity).
>
> Except that there are dynamically programmable transmitter receiver
> pairs that adapt bandwidth and datarate to manage current noise
> environment. Space exploration vehicles like the voyager do this.
> Newer software defined radios also do things like this.
>
>
>
> > 5. I guess I will logically have to assume that the calculation for
> > receiver sensitivity, -174+NF+10log(BW)+SNRmin, must take for
> > granted that the receiver's I.F. gain will be high enough to
> > increase the received signal power enough to properly drive the
> > detector (even at the lowest RF input signal levels), since gain is
> > not part of this sensitivity equation... why it is not, I have no
> > idea!
>
> > Thanks for all of the unbelievably helpful responses!
>
> > -Bill
>
> Please note that the current IF bandwidth sets the measurement
> bandwidth for the S/N measurement. This property is called
> selectivity. As discussed for 3. and 4. above this impacts S/N for
> the total receiver.- Hide quoted text -
>
> - Show quoted text -
-------------------
ME:
> 3. Since it is measured at a single spot frequency of 1Hz, NF
> itself is completely independent of bandwidth.
JOE:
"Er, not quite. It is a tradeoff bewteen bandwidth versus noise
versus
datarate. Please see Shannon's law."
------------------
Thanks for the further info Joe. But now my head REALLY hurts!
I had no idea that a receiver's NF could change with a change in
bandwidth and/or data rate. I think I'm going to have to hit the
books yet again!!
Best regards,
-Bill
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On Oct 23, 5:30?am, billcalley <billcal...@yahoo.com> wrote:
> Hi All,
>
> I keep reading that the high-gain front-end stages of a microwave
> receiver almost completely sets the entire radio's NF and sensitivity,
> and that the following stages (the I.F.) have little effect except to
> amplify the signal and the noise equally to a higher amplitude for the
> radio's detector. This doesn't make complete sense to me, because the
> I.F. would have a HUGE effect on the receiver's signal-to-noise ratio,
> and therefore its sensitivity, if we simply narrowed the IF's
> bandwidth down from, let's say, 1MHz to 1kHz!! So, to me anyway, the
> I.F. would have a gigantic effect on the receiver's sensitivity, even
> if the front-end had infinite gain. Or am I missing something here?
>
> (BTW: I fully realize we can't just narrow-down the receiver's
> bandwidth below the bandwidth of the modulated signal, but I'm just
> asking about all this on a theoretical basis to try and understand
> "sensitivity" a bit better).
>
> Thanks,
>
> -Bill
Your quite right, reducing the bandwidth does have a huge effect.
Recievers are usually made to recieve information of some sort which
is contained in the sidebands and therefore the bandwith has to be
fixed to that width. Once youve fixed it, the recievers SN ratio is
now dependent on the front end performance.
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Author: Bill BowdenDate: 23:25 25-10-07
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On Oct 23, 3:12 pm, Mark <makol...@yahoo.com> wrote:
> On Oct 23, 4:36 am, billcalley <billcal...@yahoo.com> wrote:
>
>
>
>
>
> > On Oct 22, 11:17 pm, et...@FreeNet.Carleton.CA (Michael Black) wrote:
>
> > > billcalley (billcal...@yahoo.com) writes:
> > > > Hi All,
>
> > > > I keep reading that the high-gain front-end stages of a
microwave
> > > > receiver almost completely sets the entire radio's NF and
sensitivity,
> > > > and that the following stages (the I.F.) have little effect except
to
> > > > amplify the signal and the noise equally to a higher amplitude for
the
> > > > radio's detector. This doesn't make complete sense to me, because
the
> > > > I.F. would have a HUGE effect on the receiver's signal-to-noise
ratio,
> > > > and therefore its sensitivity, if we simply narrowed the IF's
> > > > bandwidth down from, let's say, 1MHz to 1kHz!!
>
> > > All an amplifier can do is amplify what's at its input. Whatever the
> > > signal is in reference to the noise, that ratio will remain at the
output,
> > > even though the actual voltage level will be higher at the output of
> > > the amplifier compared with the input.
>
> > > To use a broad example, 1v of noise and 0.1v of signal at an amplifier's
> > > input will mean 10v of noise and 1v of signal at the the output if
> > > the gain is ten. You haven't altered the ratio, just made everything
> > > louder.
>
> > > It's like turning up the volume on a hearing aid to hear the person
> > > next to you, but which also amplifies the other sounds in the room that
> > > were already stronger than the person; you haven't actually fixed the
problem
> > > because the problem was that the person was weaker than the surrounding
> > > sounds.
>
> > > So if you have a first stage that adds noise to the mix, noise that
> > > will help to mask the desired signal, then you've made things worse.
> > > Forever down the signal chain, there is nothing you can do to fix
> > > the problem, because once that noise is added, any later amplification
> > > amplifies it along with the desired signal. If that stage in
> > > the broad example generated 1v of noise, that equals the level of
> > > the desired signal, and thus has made the situation worse.
>
> > > So you want to get that signal up fast without adding any noise, or
> > > at least as little as possible. So for low level microphones, you'll
> > > often see a transformer to boost the signal, because it will introduce
> > > less noise than an active stage.
>
> > > A low noise first rf stage will indeed set the stage. It will
> > > amplify the incoming signal (and the background noise equally) but will
> > > add little of its own noise to mask the signal. If it's not low noise,
> > > then any incoming signal has to be above a certain level to stay
> > > above that noise.
>
> > > There is background noise picked up by the antenna along with the
> > > desired signal. That level varies with frequency, becoming more
> > > significant the higher up you go. You can't do anything about
> > > that, it's part of basic communication (well you can, but that's
> > > another story). But you can work at making sure as little noise as
> > > possible is added to the mix.
>
> > > Later stages don't matter, because the signal is stronger
> > > and the noise generated by later stages will not have the same impact.
> > > So that previous broad example, 10v of noise and 1volt of signal out
> > > of the first stage, the second stage will amplify that by ten again,
> > > so its output is 100v of noise (I said that was a broad example) and
> > > 10volts of signal, but if the stage adds 1 volt of noise
> > > that 1v is now 1/10th the level of the desired signal, when before
> > > it was stronger than the desired signal.
>
> > > Michael
>
> > Thanks Tom, Tim, Phil, and Michael for some great answers!
>
> > I guess I need time to digest all this. But what I still don't
> > get -- just taking Michael's terrific response as a good example -- is
> > while I know that the receiver's front-end sets the ratio between the
> > input signal and the receiver's noise, and that this S/N ratio cannot
> > then be improved by the receiver's I.F. *amplification* stages, why
> > can't the receiver's I.F. *filter* stages simply passband filter out
> > most of that wideband input noise to improve the receiver's SNR, which
> > should then improve the sensitivity of the receiver? That's the part
> > that still has me stumped...
>
> > Thanks All,
>
> > -Bill- Hide quoted text -
>
> > - Show quoted text -
>
> another way to think of it is that the desried input signal has a
> power DENSITY, i.e. power per Hz BW. and the front end ciruicts have
> a noise DENSITY, i.e. noise power per Hz that is determined by the
> noise figure. Then when the noise and signl go therough the IF
> filter, the IF filter sets the BW. If the signal is very narrow, the
> filter can be very narrow and will let in only the minimum possible
> amount of noise. Both the noise figure and IF bandwidth are important
> in determining sensitivity. But the IF BW cannot be less then the
> desired signal BW. And the Noise figure can't be less than 0 dB.
>
> I think one of the key concepts you may be missing is that even the
> antenna picks up noise with the signal so there is a limit to the
> acheivable sensitivity even if you had a "perfect" receiver. A
> perfect receiver would have a 0 dB noise figure. That does not mean
> there is NO noise, it means there is no EXTRA noise beyond that which
> the antenna picks up.
> The lowest noise floor for space communicarions is the 3degK floor.
> For Earth comm its room temperature. The "perfect" receiver also has a
> BW no wider than it needs to be to pass the desired signal but it must
> be wide enough to pass the signal and therefore also passes that
> amount of noise.
>
> Mark- Hide quoted text -
>
> - Show quoted text -
So, what you are saying is there is no advantage to a larger antenna
if the receiver noise figure is zero? A small antenna delivering a
small signal will be just as usable as a giant antenna delivering a
large signal since all you have to do is add receiver gain without
adding any noise?
So, the advantage of a large loop antenna for AM radio reception is
simply to provide a larger signal with the same SN ratio which causes
the receiver AGC to lower the gain and also the internal receiver
noise?
Is that about right?
-Bill
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Author: Tom BruhnsDate: 01:06 26-10-07
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On Oct 25, 8:25 pm, Bill Bowden <wrongaddr...@att.net> wrote:
> On Oct 23, 3:12 pm, Mark <makol...@yahoo.com> wrote:
>
>
>
> > On Oct 23, 4:36 am, billcalley <billcal...@yahoo.com> wrote:
>
> > > On Oct 22, 11:17 pm, et...@FreeNet.Carleton.CA (Michael Black) wrote:
>
> > > > billcalley (billcal...@yahoo.com) writes:
> > > > > Hi All,
>
> > > > > I keep reading that the high-gain front-end stages of a
microwave
> > > > > receiver almost completely sets the entire radio's NF and
sensitivity,
> > > > > and that the following stages (the I.F.) have little effect
except to
> > > > > amplify the signal and the noise equally to a higher amplitude
for the
> > > > > radio's detector. This doesn't make complete sense to me,
because the
> > > > > I.F. would have a HUGE effect on the receiver's signal-to-noise
ratio,
> > > > > and therefore its sensitivity, if we simply narrowed the IF's
> > > > > bandwidth down from, let's say, 1MHz to 1kHz!!
>
> > > > All an amplifier can do is amplify what's at its input. Whatever
the
> > > > signal is in reference to the noise, that ratio will remain at the
output,
> > > > even though the actual voltage level will be higher at the output of
> > > > the amplifier compared with the input.
>
> > > > To use a broad example, 1v of noise and 0.1v of signal at an
amplifier's
> > > > input will mean 10v of noise and 1v of signal at the the output if
> > > > the gain is ten. You haven't altered the ratio, just made
everything
> > > > louder.
>
> > > > It's like turning up the volume on a hearing aid to hear the person
> > > > next to you, but which also amplifies the other sounds in the room
that
> > > > were already stronger than the person; you haven't actually fixed
the problem
> > > > because the problem was that the person was weaker than the
surrounding
> > > > sounds.
>
> > > > So if you have a first stage that adds noise to the mix, noise that
> > > > will help to mask the desired signal, then you've made things worse.
> > > > Forever down the signal chain, there is nothing you can do to fix
> > > > the problem, because once that noise is added, any later
amplification
> > > > amplifies it along with the desired signal. If that stage in
> > > > the broad example generated 1v of noise, that equals the level of
> > > > the desired signal, and thus has made the situation worse.
>
> > > > So you want to get that signal up fast without adding any noise, or
> > > > at least as little as possible. So for low level microphones,
you'll
> > > > often see a transformer to boost the signal, because it will
introduce
> > > > less noise than an active stage.
>
> > > > A low noise first rf stage will indeed set the stage. It will
> > > > amplify the incoming signal (and the background noise equally) but
will
> > > > add little of its own noise to mask the signal. If it's not low
noise,
> > > > then any incoming signal has to be above a certain level to stay
> > > > above that noise.
>
> > > > There is background noise picked up by the antenna along with the
> > > > desired signal. That level varies with frequency, becoming more
> > > > significant the higher up you go. You can't do anything about
> > > > that, it's part of basic communication (well you can, but that's
> > > > another story). But you can work at making sure as little noise as
> > > > possible is added to the mix.
>
> > > > Later stages don't matter, because the signal is stronger
> > > > and the noise generated by later stages will not have the same
impact.
> > > > So that previous broad example, 10v of noise and 1volt of signal out
> > > > of the first stage, the second stage will amplify that by ten again,
> > > > so its output is 100v of noise (I said that was a broad example) and
> > > > 10volts of signal, but if the stage adds 1 volt of noise
> > > > that 1v is now 1/10th the level of the desired signal, when before
> > > > it was stronger than the desired signal.
>
> > > > Michael
>
> > > Thanks Tom, Tim, Phil, and Michael for some great answers!
>
> > > I guess I need time to digest all this. But what I still don't
> > > get -- just taking Michael's terrific response as a good example -- is
> > > while I know that the receiver's front-end sets the ratio between the
> > > input signal and the receiver's noise, and that this S/N ratio cannot
> > > then be improved by the receiver's I.F. *amplification* stages, why
> > > can't the receiver's I.F. *filter* stages simply passband filter out
> > > most of that wideband input noise to improve the receiver's SNR, which
> > > should then improve the sensitivity of the receiver? That's the part
> > > that still has me stumped...
>
> > > Thanks All,
>
> > > -Bill- Hide quoted text -
>
> > > - Show quoted text -
>
> > another way to think of it is that the desried input signal has a
> > power DENSITY, i.e. power per Hz BW. and the front end ciruicts have
> > a noise DENSITY, i.e. noise power per Hz that is determined by the
> > noise figure. Then when the noise and signl go therough the IF
> > filter, the IF filter sets the BW. If the signal is very narrow, the
> > filter can be very narrow and will let in only the minimum possible
> > amount of noise. Both the noise figure and IF bandwidth are important
> > in determining sensitivity. But the IF BW cannot be less then the
> > desired signal BW. And the Noise figure can't be less than 0 dB.
>
> > I think one of the key concepts you may be missing is that even the
> > antenna picks up noise with the signal so there is a limit to the
> > acheivable sensitivity even if you had a "perfect" receiver. A
> > perfect receiver would have a 0 dB noise figure. That does not mean
> > there is NO noise, it means there is no EXTRA noise beyond that which
> > the antenna picks up.
> > The lowest noise floor for space communicarions is the 3degK floor.
> > For Earth comm its room temperature. The "perfect" receiver also has
a
> > BW no wider than it needs to be to pass the desired signal but it must
> > be wide enough to pass the signal and therefore also passes that
> > amount of noise.
>
> > Mark- Hide quoted text -
>
> > - Show quoted text -
>
> So, what you are saying is there is no advantage to a larger antenna
> if the receiver noise figure is zero? A small antenna delivering a
> small signal will be just as usable as a giant antenna delivering a
> large signal since all you have to do is add receiver gain without
> adding any noise?
>
> So, the advantage of a large loop antenna for AM radio reception is
> simply to provide a larger signal with the same SN ratio which causes
> the receiver AGC to lower the gain and also the internal receiver
> noise?
>
> Is that about right?
>
> -Bill
That's not necessarily true. A small antenna (small relative to a
wavelength) in general is not an efficient antenna: it adds
resistance that's not associated with signal, and that resistance adds
noise.
Also, an antenna that is large enough to be directional may be pointed
at the source of the desired signal, and that will enhance the signal
with respect to the more evenly distributed atmospheric noise.
The nature of atmospheric noise (manmade, noise from lightening, etc.)
and galactic noise is that it's greater at lower frequencies. So for
the AM medium wave broadcast band, there's so much atmospheric noise
that it doesn't take a very efficient antenna to not add a significant
percentage more noise. So even with a small loop antenna, or a short
whip antenna on a car, you can get stations on nearly every "channel"
-- every 10kHz in North America; every 9kHz many other places. But
when you get to the FM and TV broadcast bands, the situation is
different. The atmospheric noise is much less, and the quality of
antenna matters a LOT more. Also because propagation of the higher
frequencies is much more commonly line-of-sight or nearly so, it helps
much more to get the antenna well up into the air if you're trying for
weak signals from far away.
There's a lot more to the story than this, but I hope this will
capture the essence for you.
Cheers,
Tom
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Author: MarkDate: 17:07 26-10-07
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>
> So, what you are saying is there is no advantage to a larger antenna
> if the receiver noise figure is zero? A small antenna delivering a
> small signal will be just as usable as a giant antenna delivering a
> large signal since all you have to do is add receiver gain without
> adding any noise?
>
> So, the advantage of a large loop antenna for AM radio reception is
> simply to provide a larger signal with the same SN ratio which causes
> the receiver AGC to lower the gain and also the internal receiver
> noise?
>
> Is that about right?
>
> -Bill- Hide quoted text -
>
> - Show quoted text -
No ...first no receiver has a 0 dB noise figure..
second even if you had one , a big ant is usually helpful becasue a
big antnenna will be more directional and will pick up more signal for
a given amount of noise...i.e. the antenna will have a better SNR to
give to the receiver. If the receiver has a 0 dB NF then this SNR is
the SNR you will get.
In the case of the AM loop you talked about, the receivers are usually
so poor i.e. the noise figure is so high, that in that case you are
right, bigger antenna means more signal to better overcome the noise
of the receiver. But even then, the amount of atmoshperic noise is
very high and even a rather small loop (compared to the wavelength)
will pick up enough noise that the atmospheric noise will dominate the
receiver noise.
There are different relms here.. a pocket AM radio is not the same
case as a big microwave parabolic dish pointed to space. It can help
just to keep a seperate accounting at each step of the noise and the
signal. Ultimatly it is the noise (not the gain) that limits the
sensitivity you can obtain.
Google "noise temperature" and G/T figure of merit as well as noise
figure.
Mark
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Author: Simon S AysdieDate: 19:59 26-10-07
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On Oct 22, 9:30 pm, billcalley <billcal...@yahoo.com> wrote:
> Hi All,
>
> I keep reading that the high-gain front-end stages of a microwave
> receiver almost completely sets the entire radio's NF and sensitivity,
> and that the following stages (the I.F.) have little effect except to
> amplify the signal and the noise equally to a higher amplitude for the
> radio's detector.
I think Steve already presented the Friis Noise Figure Equation for
you. That should explain the above part of the problem for you.
http://en.wikipedia.org/wiki/Friis_formula
> This doesn't make complete sense to me, because the
> I.F. would have a HUGE effect on the receiver's signal-to-noise ratio,
> and therefore its sensitivity, if we simply narrowed the IF's
> bandwidth down from, let's say, 1MHz to 1kHz!!
Yes, given white noise, the total noise power will increase as the
bandwidth is increased. But receiver composite (cascaded) bandwidth
is designed almost as a matter of tautology: "It should be wide
enough to pass the desired signal, and no wider." (Obviously there is
a bit of play in that, depending on the expected environmental
conditions.) IOW, it would make no sense to have a 1 MHz composite
receiver bandwidth if the signal bandwidth was 3 kHz.
As Lathi writes on p.531 of Signals, Systems, and Communications,
"[C]orrelation in the time domain is filtering action in the frequency
domain." That is, a filter whose bandwidth is matched to the signal
bandwidth is correlated to the signal (or vice versa).
> So, to me anyway, the
> I.F. would have a gigantic effect on the receiver's sensitivity, even
> if the front-end had infinite gain.
LOL. You'll have a big-ass signal to deal with if the gain is
infinite. TINSTAAFL!
> Or am I missing something here?
>
> (BTW: I fully realize we can't just narrow-down the receiver's
> bandwidth below the bandwidth of the modulated signal, but I'm just
> asking about all this on a theoretical basis to try and understand
> "sensitivity" a bit better).
Yes, See Friis.
Also, under low noise conditions, some modulation system's SNR gets
better as the signal bandwidth is increased. For example, FM and PM
with do 6 dB better as signal bandwidth is doubled. Most Comm Theory
texts will include that (mathematical) development.
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Author: MarkDate: 11:41 27-10-07
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>
> Also, under low noise conditions, some modulation system's SNR gets
> better as the signal bandwidth is increased. For example, FM and PM
> with do 6 dB better as signal bandwidth is doubled. Most Comm Theory
> texts will include that (mathematical) development.
this is only true above "threshold"
Mark
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