0
\$\begingroup\$

we know that GPS receiver can reach sensitivty of -168dBm, and also it is possible to make receivers with similar sensetitvity can operate at another bands/frequencies .

but I just want to know if possible to build amplifier with input of -168 dBm ? I want increase signal power at some -90s dBm range to make the system able to receive high bitrate at very weak signals "-160s dBm" .

I know the noise will also amplified , but the SNR still unchanged or slightly.

thanks,

\$\endgroup\$
3
\$\begingroup\$

-168 dBm is very tiny and equivalent to ~1.7 nV RMS in a 50 ohm system over a measly and pathetic bandwidth of only 3.8 Hz at about 30degC.

If that bandwidth is no good then you'll have to start cooling the radio receiver front end. For instance a 50 ohm resistor generates -168 dBm when cooled to -100 degC but with a slightly improved bandwidth of 6.6 Hz.

Realistically, the "best" formula around that I know of is this: -

Minimum signal power = -154dBm + 10.log\$_{10}\$(data rate)

At a data rate of 1 Hz, mimimum power a receiver needs at about 27degC is -154dBm. At 10 bps this is -144dBm and at 100 bps it's -134dBm.

You can probably see the trend; every tenfold increase in BW requires a signal power that is 10dB more.

At 1M bps, the receiver requires -94 dBm etc...

So, it looks like cooling is the only option but, unfortunately, even when cooled at -272 degC, a 50 ohm resistor will produce -168 dBm over only a 1kHz bandwidth.

| improve this answer | |
\$\endgroup\$
  • \$\begingroup\$ Also see the related Shannon-Hartley theorm. \$\endgroup\$ – Phil Frost Oct 22 '14 at 21:23
  • \$\begingroup\$ that is mean amplifying -168 dBm to -94 dBm does not improve data rate , right ? \$\endgroup\$ – man abduo Oct 22 '14 at 21:45
  • \$\begingroup\$ Again, it's about amplifying noise and signal equals no net benefit. \$\endgroup\$ – Andy aka Oct 22 '14 at 22:11
3
\$\begingroup\$

Sure, you can amplify anything you want. In fact, any amplifier is just fine with a -168 dBm input, provided 168 dBm isn't over its maximum input power specification (which I guess is true of just about every amplifier). If you want a -168 dBm input to become -90 dBm, then you need a 78 dB amplifier. That's nothing special.

However, that won't do anything to improve your situation. Every GPS receiver already has an amplifier. The problem is, as you mention, that the SNR is unchanged at best. In practice, an amplifier has active electronics that adds additional noise. This additional noise is usually specified as a noise figure. It isn't the low power of the GPS signal that's problematic, it's that the signal-to-noise ratio is insufficient to reliably extract the signal from the noise. Amplification doesn't change that.

| improve this answer | |
\$\endgroup\$
  • 2
    \$\begingroup\$ However, the GPS signal has very specific characteristics that permit it to be detected at very low levels, even below the background noise. It can be correlated with a reference signal. This process adds significant processing gain which directly increases the effective signal-to-noise ratio. This is not true for a random low level signal whose waveshape and other characteristics are not known. \$\endgroup\$ – Barry Oct 22 '14 at 20:23
  • \$\begingroup\$ but I didnt find any commircal amplifiers has input range of -168 dBm ?! I read about some GPS receivers can distinguish signal at noise levels up to 30 dB , like u-blox . \$\endgroup\$ – man abduo Oct 22 '14 at 20:24
  • \$\begingroup\$ u-blox.com/images/downloads/Product_Docs/… "This special jamming-mitigation method requires considerable processing power, which u-blox 5 / 6 supplies with an ARM® processor. Under control of this processor, this proprietary combination of hardware and software can reduce jamming signals by 30 dB compared to conventional products" \$\endgroup\$ – man abduo Oct 22 '14 at 20:26
  • \$\begingroup\$ "Anti-jamming" usually refers to the reduction of a small number of specific, higher strength signals in the same frequency range as GPS so that they do not affect position accuracy. How that works would be a different question. \$\endgroup\$ – David Oct 22 '14 at 20:37
  • \$\begingroup\$ @Barry That's true, but that doesn't mean GPS has some magical property that allows it to be detected at the same SNR if the overall power is higher. For any modulation, no matter how robust, there is a minimum SNR at which it can be detected. Amplification of signal and noise doesn't increase SNR. \$\endgroup\$ – Phil Frost Oct 22 '14 at 21:21

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.