Re: Receiver sensitivity and IF bandwidth??


"billcalley" <billcalley@xxxxxxxxx> wrote in message
news:1193337552.634629.107060@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
On Oct 25, 12:21 am, JosephKK <joseph_barr...@xxxxxxxxxxxxx> wrote:
billcalley billcal...@xxxxxxxxx posted to sci.electronics.design:





On Oct 23, 3:12 pm, Mark <makol...@xxxxxxxxx> wrote:
On Oct 23, 4:36 am, billcalley <billcal...@xxxxxxxxx> wrote:

On Oct 22, 11:17 pm, et...@xxxxxxxxxxxxxxxxxxx (Michael Black)
wrote:

billcalley (billcal...@xxxxxxxxx) 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




Bill:
Hold the headache.
Noise Figure doesn't change with bandwidth, nor is it related to channel
capacity and Shannon's law. On the other hand, total output noise power
does change with bandwidth, just as SNR and sensitivity both change with
bandwidth. But you already understood that. Noise Figure is a
figure-of-merit for a receiver, and it is independent of the actual
bandwidth of the receive path (precisely so that you can make a single
useful measurement for a receiver with switched IF bandwidths). I refer you
back to the (corrected) sensitivity equation where their independence is
clearly seen.

Steve


.

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