Re: Receiver sensitivity and IF bandwidth??

Steve sjburke1@xxxxxxxxxxx posted to sci.electronics.design:


"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

I do not remember that one. Please post for us, with the proper
explanation. There is a clear implication of such dependency in
Shannon's law though.

.

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