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
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

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

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

Well, I may have to go back to the old question, because it's not
exactly a DSSS depending on cross correlation. What are the possible
bad effects when I keep connecting amplifiers? Shorter bandwidth?
Smaller IP3? There should be something I don't know.


Mike Andrews wrote:
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

Can I know how did you get that number of 50Hz? Thanks.

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

wrote in message
oups.com...
Can I know how did you get that number of 50Hz? Thanks.

Boltzmans constant times temperature

KT for any resistor at room temperature =-174 dBm/Hz. That's as good as you
get without cryogenic cooling.

Add 17 dBm to increase bandwidth from 1 Hz to 50 Hz = -157 dBM/Hz

Add 3 more to get to MDS and it's -154 dBm, your signal level.

W4ZCB

On 11 Oct 2006 12:13:20 -0700, wrote:

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

The problem was, you got some prior answers and then wanted to argue
about it.

The frequency is 70MHz.

Then, assuming some type of filtering around that frequency that's
more or less where your system will likely oscillate.

One reason superhetrodyne receivers were developed to replace TRF
receivers was to develop system gain at different frequencies so as to
minimize the chance of oscillation. Another, of course was to achieve
better selectivity that with older technology was better accomplished
at lower frequencies.

Even a single conversion receiver usually develops its gain at three
different frequencies: r-f, i-f and post detection.

On 11 Oct 2006 13:33:26 -0700, wrote:

I'm dealing with a DSSS current signal at 70MHz. Before I start
designing anything, I am calculating the signal amplitude.

You did not specify the chip rate or base band bit rate and spreading
function. For GPS signals the chip rate is 1.023 MHz, and the signal
spectrum is about 1 MHz wide, so the front end selectivity should be
as close to this as possible.

For a terrestrial 70 MHz DSSS system, there are usually going to be
several independent DSSS transmitters sharing this 1 MHz bandwidth (or
whatever your chip rate is), some being close to the receiver and some
very distant.

If you want to receive the distant signal and use the whole 1 MHz
bandwidth all the way through the 140 dB gain, a weak distant
transmitter producing only 0.1 uV at your receiver antenna terminal
would be amplified to 1 V. However a local DSSS transmitter sharing
the same 70 MHz band with a different PN sequence could produce 1 mV
at your antenna terminal, which should also be amplified by 140 dB to
10 kV, which clearly is impossible ! The strong signals would saturate
the later amplifying stages and the weak signal would not get through.

A reasonable gain/bandwidth distribution for a 70 MHz system would be
to use a low loss band pass filter with a few MHz bandwidth at 70 MHz,
to take out any VHF band I TV signals and FM broadcast signals at 100
MHz. Use a low noise amplifier stage, followed by a filter with deep
sides which is matched to the bandwidth of the service (1 MHz for
GPS). Then perhaps one stage, possibly with AGC and then the
despreader, followed by a filter matching the bit rate (not chip rate)
of the signal, followed by multiple AGC amplification stages to
amplify the despread signal before the actual BPSK or whatever
detector.

In terrestrial systems at 70 MHz the noise temperature is well above
300 K, thus the antenna noise figure is much worse than 3 dB, so
trying to keep the receiver system noise figure below 3 dB might not
be too productive. It might even be better to suffer an additional 1
dB noise figure hit and use steep filters tailored to the chip rate in
front of the first LNA.

Paul OH3LWR