Bandwidth vs. Noise,,,Take 2
 

Okay, let me restate, since I seem to be mis-understood,

The I.F. is stagger tuned, with swamping resistors to kill the Q. It is
running at 60mhz with a 6db bandwidth of 8mhz. It is part of a 'short pulse'
marine radar unit. The pulses were like maybe .06microseconds at about 1khz
rep rate.

The transmitter now squirts out 'long pulses' of about .8microseconds. I
want to raise the value of the swamp resistors, and 'un-stagger' the tuning
to get a 1.5mhz bandwidth at the 6db point.

So the original question was "If I reduce the bandwidth from 8mhz to 1.5mhz,
and everything else remains the same, what will happen to the signal to
noise ratio in terms of db snr?

I have done similar mods before, and the units work better than ever. Just
getting the swamp resistors up in value helps receiver gain more than the
numbers would apparently indicate, as I can see small craft at 24 miles, when
I could hardly see them at 12 miles before mods.

Why not just use the narrow IF that is already in the radar for the
wider pulse width?

73
Gary K4FMX

On Fri, 06 Feb 2004 21:16:04 -0000, (gudmundur)
wrote:

Okay, let me restate, since I seem to be mis-understood,

The I.F. is stagger tuned, with swamping resistors to kill the Q. It is
running at 60mhz with a 6db bandwidth of 8mhz. It is part of a 'short pulse'
marine radar unit. The pulses were like maybe .06microseconds at about 1khz
rep rate.

The transmitter now squirts out 'long pulses' of about .8microseconds. I
want to raise the value of the swamp resistors, and 'un-stagger' the tuning
to get a 1.5mhz bandwidth at the 6db point.

So the original question was "If I reduce the bandwidth from 8mhz to 1.5mhz,
and everything else remains the same, what will happen to the signal to
noise ratio in terms of db snr?

I have done similar mods before, and the units work better than ever. Just
getting the swamp resistors up in value helps receiver gain more than the
numbers would apparently indicate, as I can see small craft at 24 miles, when
I could hardly see them at 12 miles before mods.

Why not just use the narrow IF that is already in the radar for the
wider pulse width?

73
Gary K4FMX

On Fri, 06 Feb 2004 21:16:04 -0000, (gudmundur)
wrote:

Okay, let me restate, since I seem to be mis-understood,

The I.F. is stagger tuned, with swamping resistors to kill the Q. It is
running at 60mhz with a 6db bandwidth of 8mhz. It is part of a 'short pulse'
marine radar unit. The pulses were like maybe .06microseconds at about 1khz
rep rate.

The transmitter now squirts out 'long pulses' of about .8microseconds. I
want to raise the value of the swamp resistors, and 'un-stagger' the tuning
to get a 1.5mhz bandwidth at the 6db point.

So the original question was "If I reduce the bandwidth from 8mhz to 1.5mhz,
and everything else remains the same, what will happen to the signal to
noise ratio in terms of db snr?

I have done similar mods before, and the units work better than ever. Just
getting the swamp resistors up in value helps receiver gain more than the
numbers would apparently indicate, as I can see small craft at 24 miles, when
I could hardly see them at 12 miles before mods.

Because that is just it..... It is a cheap radar with only AN 8 MHZ FACTORY
SUPPLIED I.F. BANDWIDTH which is typical of these units. The newer ones
have at least 2 bandwidths, and perhaps 3 on the really expensive units.
I would not re-invent the wheel, I just want an answer in db snr.

My answer is this, The screen full of echos looks real good after I do my
voodoo. Now, can someone just answer the question?

In article ,
says...

Why not just use the narrow IF that is already in the radar for the
wider pulse width?

73
Gary K4FMX

On Fri, 06 Feb 2004 21:16:04 -0000, (gudmundur)
wrote:

Okay, let me restate, since I seem to be mis-understood,

The I.F. is stagger tuned, with swamping resistors to kill the Q. It is
running at 60mhz with a 6db bandwidth of 8mhz. It is part of a 'short pulse'
marine radar unit. The pulses were like maybe .06microseconds at about 1khz
rep rate.

The transmitter now squirts out 'long pulses' of about .8microseconds. I
want to raise the value of the swamp resistors, and 'un-stagger' the tuning
to get a 1.5mhz bandwidth at the 6db point.

So the original question was "If I reduce the bandwidth from 8mhz to 1.5mhz,
and everything else remains the same, what will happen to the signal to
noise ratio in terms of db snr?

I have done similar mods before, and the units work better than ever. Just
getting the swamp resistors up in value helps receiver gain more than the
numbers would apparently indicate, as I can see small craft at 24 miles, when
I could hardly see them at 12 miles before mods.

Because that is just it..... It is a cheap radar with only AN 8 MHZ FACTORY
SUPPLIED I.F. BANDWIDTH which is typical of these units. The newer ones
have at least 2 bandwidths, and perhaps 3 on the really expensive units.
I would not re-invent the wheel, I just want an answer in db snr.

My answer is this, The screen full of echos looks real good after I do my
voodoo. Now, can someone just answer the question?

In article ,
says...

Why not just use the narrow IF that is already in the radar for the
wider pulse width?

73
Gary K4FMX

On Fri, 06 Feb 2004 21:16:04 -0000, (gudmundur)
wrote:

Okay, let me restate, since I seem to be mis-understood,

The I.F. is stagger tuned, with swamping resistors to kill the Q. It is
running at 60mhz with a 6db bandwidth of 8mhz. It is part of a 'short pulse'
marine radar unit. The pulses were like maybe .06microseconds at about 1khz
rep rate.

The transmitter now squirts out 'long pulses' of about .8microseconds. I
want to raise the value of the swamp resistors, and 'un-stagger' the tuning
to get a 1.5mhz bandwidth at the 6db point.

So the original question was "If I reduce the bandwidth from 8mhz to 1.5mhz,
and everything else remains the same, what will happen to the signal to
noise ratio in terms of db snr?

I have done similar mods before, and the units work better than ever. Just
getting the swamp resistors up in value helps receiver gain more than the
numbers would apparently indicate, as I can see small craft at 24 miles, when
I could hardly see them at 12 miles before mods.

But I did -- assuming white noise, you should gain somewhere in the
neighborhood of a 6x improvement in signal power vs. noise power, which
works out to around 8dB. It probably doesn't actually do that well, so call
it at least 6dB. It's interesting that you see a 2x range improvement,
because radar path loss goes as distance squared, so you'd expect to need
12dB improvement.

Of course when one is really proud of ones modifications one tends to look
harder...

"gudmundur" wrote in message
...
Because that is just it..... It is a cheap radar with only AN 8 MHZ
FACTORY
SUPPLIED I.F. BANDWIDTH which is typical of these units. The newer ones
have at least 2 bandwidths, and perhaps 3 on the really expensive units.
I would not re-invent the wheel, I just want an answer in db snr.

My answer is this, The screen full of echos looks real good after I do my
voodoo. Now, can someone just answer the question?

In article ,

says...

Why not just use the narrow IF that is already in the radar for the
wider pulse width?

73
Gary K4FMX

On Fri, 06 Feb 2004 21:16:04 -0000, (gudmundur)
wrote:

Okay, let me restate, since I seem to be mis-understood,

The I.F. is stagger tuned, with swamping resistors to kill the Q. It is
running at 60mhz with a 6db bandwidth of 8mhz. It is part of a 'short
pulse'
marine radar unit. The pulses were like maybe .06microseconds at about
1khz
rep rate.

The transmitter now squirts out 'long pulses' of about .8microseconds.
I
want to raise the value of the swamp resistors, and 'un-stagger' the
tuning
to get a 1.5mhz bandwidth at the 6db point.

So the original question was "If I reduce the bandwidth from 8mhz to
1.5mhz,
and everything else remains the same, what will happen to the signal to
noise ratio in terms of db snr?

I have done similar mods before, and the units work better than ever.
Just
getting the swamp resistors up in value helps receiver gain more than
the
numbers would apparently indicate, as I can see small craft at 24 miles,
when
I could hardly see them at 12 miles before mods.

But I did -- assuming white noise, you should gain somewhere in the
neighborhood of a 6x improvement in signal power vs. noise power, which
works out to around 8dB. It probably doesn't actually do that well, so call
it at least 6dB. It's interesting that you see a 2x range improvement,
because radar path loss goes as distance squared, so you'd expect to need
12dB improvement.

Of course when one is really proud of ones modifications one tends to look
harder...

"gudmundur" wrote in message
...
Because that is just it..... It is a cheap radar with only AN 8 MHZ
FACTORY
SUPPLIED I.F. BANDWIDTH which is typical of these units. The newer ones
have at least 2 bandwidths, and perhaps 3 on the really expensive units.
I would not re-invent the wheel, I just want an answer in db snr.

My answer is this, The screen full of echos looks real good after I do my
voodoo. Now, can someone just answer the question?

In article ,

says...

Why not just use the narrow IF that is already in the radar for the
wider pulse width?

73
Gary K4FMX

On Fri, 06 Feb 2004 21:16:04 -0000, (gudmundur)
wrote:

Okay, let me restate, since I seem to be mis-understood,

The I.F. is stagger tuned, with swamping resistors to kill the Q. It is
running at 60mhz with a 6db bandwidth of 8mhz. It is part of a 'short
pulse'
marine radar unit. The pulses were like maybe .06microseconds at about
1khz
rep rate.

The transmitter now squirts out 'long pulses' of about .8microseconds.
I
want to raise the value of the swamp resistors, and 'un-stagger' the
tuning
to get a 1.5mhz bandwidth at the 6db point.

So the original question was "If I reduce the bandwidth from 8mhz to
1.5mhz,
and everything else remains the same, what will happen to the signal to
noise ratio in terms of db snr?

I have done similar mods before, and the units work better than ever.
Just
getting the swamp resistors up in value helps receiver gain more than
the
numbers would apparently indicate, as I can see small craft at 24 miles,
when
I could hardly see them at 12 miles before mods.

Hi,

The I.F. is stagger tuned, with swamping resistors to kill the Q. It is
running at 60mhz with a 6db bandwidth of 8mhz. It is part of a 'short pulse'
marine radar unit. The pulses were like maybe .06microseconds at about 1khz
rep rate.

The transmitter now squirts out 'long pulses' of about .8microseconds. I
want to raise the value of the swamp resistors, and 'un-stagger' the tuning
to get a 1.5mhz bandwidth at the 6db point.

So the original question was "If I reduce the bandwidth from 8mhz to 1.5mhz,
and everything else remains the same, what will happen to the signal to
noise ratio in terms of db snr?

I don't pretend to be able to answer this question completely as the subject
is more complicated than it would seem. However, all things being equal, SNR (
Ps/Pn) is inversely proportional to bandwidth so you would expect it to
increase by 10 x log(8/1.5) ~ 7dB for the values you give.

It is important though that bandwidth and pulse length match for optimum
results and in "Threshold Signals", MIT Rad Labs Vol. 24, they show
experimental evidence that this is indeed the case (B x Tau = 1). However you
intend to reduce the bandwidth by a factor of ~5 and to increase the pulse
width by ~13 (a B x Tau increase from 0.5 to 2.5) and the graphs say that this
should decrease the visible threshold by ~3dB.

There are a few other things I would mention. An increase in pulse-width
will increase the power dissipation in the transmitter by the same factor,
should reduce target discrimination (1us is 300m and so particularly important
close-in) and that a wider bandwidth is helpful in simplifying tuning or
reducing constraints on the system AFC.



Cheers - Joe, G3LLV

Hi,

The I.F. is stagger tuned, with swamping resistors to kill the Q. It is
running at 60mhz with a 6db bandwidth of 8mhz. It is part of a 'short pulse'
marine radar unit. The pulses were like maybe .06microseconds at about 1khz
rep rate.

The transmitter now squirts out 'long pulses' of about .8microseconds. I
want to raise the value of the swamp resistors, and 'un-stagger' the tuning
to get a 1.5mhz bandwidth at the 6db point.

So the original question was "If I reduce the bandwidth from 8mhz to 1.5mhz,
and everything else remains the same, what will happen to the signal to
noise ratio in terms of db snr?

I don't pretend to be able to answer this question completely as the subject
is more complicated than it would seem. However, all things being equal, SNR (
Ps/Pn) is inversely proportional to bandwidth so you would expect it to
increase by 10 x log(8/1.5) ~ 7dB for the values you give.

It is important though that bandwidth and pulse length match for optimum
results and in "Threshold Signals", MIT Rad Labs Vol. 24, they show
experimental evidence that this is indeed the case (B x Tau = 1). However you
intend to reduce the bandwidth by a factor of ~5 and to increase the pulse
width by ~13 (a B x Tau increase from 0.5 to 2.5) and the graphs say that this
should decrease the visible threshold by ~3dB.

There are a few other things I would mention. An increase in pulse-width
will increase the power dissipation in the transmitter by the same factor,
should reduce target discrimination (1us is 300m and so particularly important
close-in) and that a wider bandwidth is helpful in simplifying tuning or
reducing constraints on the system AFC.



Cheers - Joe, G3LLV

Hi,

Sorry, I should have said "increase the visible threshold by ~3dB".

In other words, a signal would have to be ~3dB stronger to be perceived against
the background noise.


Cheers - Joe, G3LLV

Hi,

Sorry, I should have said "increase the visible threshold by ~3dB".

In other words, a signal would have to be ~3dB stronger to be perceived against
the background noise.


Cheers - Joe, G3LLV