Receiver sensitivity and IF bandwidth??

[billcalley]

billcalley [email protected] posted to sci.electronics.design:










Er, not quite. It is a tradeoff bewteen bandwidth versus noise versus
datarate. Please see Shannon's law




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.






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

 

[[email protected]]

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.

 

[Bill Bowden]

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

 

[Tom Bruhns]

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

 

[Steve]

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

 

[Mark]

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

 

[Simon S Aysdie]

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.

 

[JosephKK]

Steve [email protected] posted to sci.electronics.design:

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.

 

[Mark]

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