I asked what I don't know. I am expecting a helpful answer.

The frequency is 70MHz.


Wes Stewart wrote:
On 11 Oct 2006 11:38:03 -0700, wrote:

What do you mean by overload? How come there is too much garbage as
long as my NF is low?

I actually need 143dB gain. What kind of problem I will have if I just
use 6 stages of amplifiers, like 1stage(20dB, NF2.4dB) 2nd stage(30dB,
NF3db), 3rd stage(43dB, NF 6.2dB), 4th stage(43dB NF6.2dB). It seems my
NFtotal is pretty low. What's my problem?

Your problem is you don't know what you're doing.

What frequency is this oscillator going to run at?

I asked what I don't know. I am expecting a helpful answer.

The frequency is 70MHz.


As Dale gave you an answer that is correct, you might reason with him, I
recognize that the last answer wasn't very helpful.

Basically, with 20 dB of gain, your noise figure will be well established
with the NF of that first stage. It will actually deteriorate a tenth of a
dB or so for the further stages down the line, but not very much.

If you need additional gain, it's best obtained after some selectivity. 140
plus dB of gain on a single frequency is going to be VERY difficult to tame
and keep from oscillation in it's own right.

W4ZCB

It seems that I know the Friis equation.

I am asking because it seems difficult, and maybe impossible. I've
never heard of this high gain before, but I dont' know why I cann't
stack amplifiers and I am looking for somebody who can tell me what
exactly the problem is: you see the NF is nice, and I can get the SNR
if the band is narrow enough.

It also seems that I "don't know what I'm doing". So please explain a
little if you want me to understand. Thanks.

Harold E. Johnson wrote:
I asked what I don't know. I am expecting a helpful answer.

The frequency is 70MHz.


As Dale gave you an answer that is correct, you might reason with him, I
recognize that the last answer wasn't very helpful.

Basically, with 20 dB of gain, your noise figure will be well established
with the NF of that first stage. It will actually deteriorate a tenth of a
dB or so for the further stages down the line, but not very much.

If you need additional gain, it's best obtained after some selectivity. 140
plus dB of gain on a single frequency is going to be VERY difficult to tame
and keep from oscillation in it's own right.

W4ZCB

wrote in message
ps.com...
It seems that I know the Friis equation.

I am asking because it seems difficult, and maybe impossible. I've
never heard of this high gain before, but I dont' know why I cann't
stack amplifiers and I am looking for somebody who can tell me what
exactly the problem is: you see the NF is nice, and I can get the SNR
if the band is narrow enough.

It also seems that I "don't know what I'm doing". So please explain a
little if you want me to understand. Thanks.

Well, how about a little additional information? What is it that you're
trying to accomplish? A gain of 140 dB would amplify a signal of strength 8
x 10^-10 Watts (About the equivalent of a received signal of 10 dB over
"S"-9) to a kiloWatt. I don't think you're trying to do that, so what
convinces you that you NEED 140 dB of gain?

Regards
W4ZCB

I'm dealing with a DSSS current signal at 70MHz. Before I start
designing anything, I am calculating the signal amplitude. It's going
to be 5nArms. If I have a 50ohm resistor, it's -153dBm. I'd be happy to
know any solid proof that it's not doable because it's my job now.

In theory DSSS signal can work under noise level, but can it be so
much?


Harold E. Johnson wrote:
wrote in message
ps.com...
It seems that I know the Friis equation.

I am asking because it seems difficult, and maybe impossible. I've
never heard of this high gain before, but I dont' know why I cann't
stack amplifiers and I am looking for somebody who can tell me what
exactly the problem is: you see the NF is nice, and I can get the SNR
if the band is narrow enough.

It also seems that I "don't know what I'm doing". So please explain a
little if you want me to understand. Thanks.

Well, how about a little additional information? What is it that you're
trying to accomplish? A gain of 140 dB would amplify a signal of strength 8
x 10^-10 Watts (About the equivalent of a received signal of 10 dB over
"S"-9) to a kiloWatt. I don't think you're trying to do that, so what
convinces you that you NEED 140 dB of gain?

Regards
W4ZCB

I googled and saw GPS signal is -150dBm. So it's detectable, but it's
much higher frequency. What if it's 70Mhz?

wrote:
I'm dealing with a DSSS current signal at 70MHz. Before I start
designing anything, I am calculating the signal amplitude. It's going
to be 5nArms. If I have a 50ohm resistor, it's -153dBm. I'd be happy to
know any solid proof that it's not doable because it's my job now.

In theory DSSS signal can work under noise level, but can it be so
much?


Harold E. Johnson wrote:
wrote in message
ps.com...
It seems that I know the Friis equation.

I am asking because it seems difficult, and maybe impossible. I've
never heard of this high gain before, but I dont' know why I cann't
stack amplifiers and I am looking for somebody who can tell me what
exactly the problem is: you see the NF is nice, and I can get the SNR
if the band is narrow enough.

It also seems that I "don't know what I'm doing". So please explain a
little if you want me to understand. Thanks.

Well, how about a little additional information? What is it that you're
trying to accomplish? A gain of 140 dB would amplify a signal of strength 8
x 10^-10 Watts (About the equivalent of a received signal of 10 dB over
"S"-9) to a kiloWatt. I don't think you're trying to do that, so what
convinces you that you NEED 140 dB of gain?

Regards
W4ZCB

wrote in message
ps.com...
I googled and saw GPS signal is -150dBm. So it's detectable, but it's
much higher frequency. What if it's 70Mhz?

Well, the GPS signal is detected all right, but they sure don't put all
their gain at the signal frequency.

W4ZCB

On 11 Oct 2006 13:41:07 -0700, wrote in om:

wrote:
I'm dealing with a DSSS current signal at 70MHz. Before I start
designing anything, I am calculating the signal amplitude. It's going
to be 5nArms. If I have a 50ohm resistor, it's -153dBm. I'd be happy to
know any solid proof that it's not doable because it's my job now.

In theory DSSS signal can work under noise level, but can it be so
much?


Harold E. Johnson wrote:
wrote in message
ps.com...
It seems that I know the Friis equation.

I am asking because it seems difficult, and maybe impossible. I've
never heard of this high gain before, but I dont' know why I cann't
stack amplifiers and I am looking for somebody who can tell me what
exactly the problem is: you see the NF is nice, and I can get the SNR
if the band is narrow enough.

It also seems that I "don't know what I'm doing". So please explain a
little if you want me to understand. Thanks.

Well, how about a little additional information? What is it that you're
trying to accomplish? A gain of 140 dB would amplify a signal of strength 8
x 10^-10 Watts (About the equivalent of a received signal of 10 dB over
"S"-9) to a kiloWatt. I don't think you're trying to do that, so what
convinces you that you NEED 140 dB of gain?

I googled and saw GPS signal is -150dBm. So it's detectable, but it's
much higher frequency. What if it's 70Mhz?

It probably isn't going to make a lot of difference whether it's 25 cm. or
40 cm; the principles are the same for DSSS decoding.

GPS signals make heavy use of PN sequences so that they can be detected
using autocorrelation techniques. They're down below the terrestrial noise
threshold, and require autocorrelation techniques for synchronization and
detection. But you probably know that already. This probably will be true
of your solution as well, and especially so at those power levels.

If you're going to be receiving and detecting signals at those power
levels, you'll need to use something other than *just* amplification to
pull the modulation out.

One presumes you'll have a known short spreading sequence that you can
use to try to sync up on the signal, or (if it's a longer sequence)
that you'll have a rough idea of the code epoch, so that you can try
to sneak up on it from behind, as it were.

The point here is that a _lot_ of the process gain comes from the
autocorrelation and despreading processes. You may not need all that
analog gain. Getting it up to ... something like -30 dBm or -15 dBm
may be enough -- or even overkill.

--
Mike Andrews, W5EGO

Tired old sysadmin

wrote in message
ups.com...
I'm dealing with a DSSS current signal at 70MHz. Before I start
designing anything, I am calculating the signal amplitude. It's going
to be 5nArms. If I have a 50ohm resistor, it's -153dBm. I'd be happy to
know any solid proof that it's not doable because it's my job now.

In theory DSSS signal can work under noise level, but can it be so
much?

Well, I have no idea of what a DSSS signal is, so I can't be of much help.
Whatever it is, it better be in a very narrow bandwidth. At that signal
level, it will take a 50 Hz bandwidth to achieve minimum discernable signal
(MDS) with a front end NF of 2.4 dB.

If you're talking about using autocorrelation to bring it up out of the
noise, all bets are off depending on how much time you can sacrifice.

If you're planning on bringing this signal to -10 dBm, better prepare for a
lot of shielding and decoupling, not to mention filtering and frequency
stability at 70 MHz center frequency.

W4ZCB

Harold E. Johnson wrote:
wrote in message
ups.com...
I'm dealing with a DSSS current signal at 70MHz. Before I start
designing anything, I am calculating the signal amplitude. It's going
to be 5nArms. If I have a 50ohm resistor, it's -153dBm. I'd be happy to
know any solid proof that it's not doable because it's my job now.

In theory DSSS signal can work under noise level, but can it be so
much?

Well, I have no idea of what a DSSS signal is, so I can't be of much help.
Whatever it is, it better be in a very narrow bandwidth. At that signal
level, it will take a 50 Hz bandwidth to achieve minimum discernable signal
(MDS) with a front end NF of 2.4 dB.

If you're talking about using autocorrelation to bring it up out of the
noise, all bets are off depending on how much time you can sacrifice.

If you're planning on bringing this signal to -10 dBm, better prepare for a
lot of shielding and decoupling, not to mention filtering and frequency
stability at 70 MHz center frequency.

W4ZCB

Some comments:

DSSS is direct sequence spread spectrum. It is wideband on the front
end.

It is probably not a valid comparison to compare DSSS detection levels
with GPS. GPS encodes much redundancy into the signal to enhance S/N,
and unless the DSSS encoder does something similar we can't use the
same detection level numbers. To say DSSS works under the noise level
assumes some gain due to encoding, and probably also assumes an
information bandwidth much less than the spread spectrum bandwidth.

Here are the main reasons you can't cascade a lot of gain at the
operating frequency. Others have mentioned this--so this is just a
summary with clarification:

1. Stability. A lot of gain at a single frequency is difficult to do
without causing stability problems. Even careful shielding between
stages doesn't`always work. At 70 Mhz, about 40 dB is the best I can
do reliably and even then I would use two separate metal enclosures,
each with 20 dB of gain. Above, say, 60 dB all kinds of things go
wrong--enclosures don't shield well enough, power supplies don't
decouple enough, interconnects talk to each other, etc. With
single-chip MMICs and LNAs it's really easy to build something and see
for yourself.

2. Intermodulation and blocking. Any unwanted signal at the antenna
would be amplified--assume you are trying to detect a -150 dBm signal
and someone in the house has a wireless headphone, which transmits at
about 72 MHz. With 140 dB of gain your amplifiers would saturate
completely and pass none of the wanted signal.

Radio design uses a careful balance between gain, mixing, AGC and
filtering to minimize stability and overload problems. For narrowband
signals you want to apply selectivity as soon in the signal train as
you can. For spread spectrum you do the same: a filter as narrow as
you can get it and still pass the spectrum, then your despreader at the
lowest level you can make it, then a narrow band filter that just
passes the information, and finally more gain and the detector.

3. Components
Some components such as crystal filters and mixers work better at
lower signal levels before a lot of gain is applied. This is really
just an extension of reason 2.


Hope this helps.

Glenn Dixon AC7ZN