Spectracom GSG-5 series, User Manual With Scpi Manual

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Where k is the Bolzmann’s constant, T is the ambient temperature (in Kelvin), and BN is the
bandwidth (in Hertz).
For example, an ideal GPS L1 C/A code filter would have a passband of 2MHz, and noise
power passed by the filter at a temperature of 290 K would be equal to -141 dBW.
The
ambient noise power spectral density
is given by the equation:
N
T
= kT = 4.00 x 10
-21
W/Hz = -204 dBW/Hz = -174 dBm/Hz
By definition, carrier-to-noise density is the carrier power divided by the noise power spectral
density. The GPS ICD specifies that the received signal level at the surface level is -130 dBm or
better. Carrier-to-noise density is then:
C/N
0
= -130 dBm/(-174 dBm/Hz) = 44 dB Hz
C/N
0
(not SNR) is the figure that the receivers typically display as an indication of quality for
the received, digitally modulated signal. If the receiver has bandwidth of 6 MHz, SNR would
be:
SNR = 44 dB Hz/(6 x 10
6
Hz) = 44 – (10 x log
10
(6 x 10
6
)) dB = -23.8 dB.
If a stronger input signal for the receiver is required, while maintaining the same C/N
0
, addi
tional noise needs to be introduced into the transmitted signal. One may think of this as having
an active antenna at the receiver input. The signal level is higher, but so is the noise level.
Interaction of Transmit Power and External Attenuation
When you change the values in the Transmit Power dialog, you may notice that other settings
may change as a consequence of the changes made. For example, if you have Transmit Power
set to -70 dBm, and External Attenuation set to 5.0 dB, the unit actually transmits signals at -
65 dBm to compensate for the external losses.
Note, however, that manually adjusting the attenuation to 10 dB in such a situation will cause
the Transmit Power to drop to -75 dBm as a consequence. This is a result of the hardware con
figuration, as the unit cannot deliver more than a total of -65 dBm. The Transmit Power setting
gives the power level at the end of your antenna cable.
Adjusting Transmit Power: Best practices
In general, when changing the Transmit Power setting, it is recommended to follow this order:
1.
Set the External Attenuation
2.
Set the Transmit Power
3.
Set the Noise Bandwidth
4.
Set the Carrier-to-Noise Density
5.
Set the Noise Offset (this can be done at any time without affecting the other settings)
Adjusting Power/Noise via SCPI command
If you use the SCPI protocol to change the power/noise settings, use the order above to do
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CHAPTER
3
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User Manual GSG-5/6 Series Rev. 26
3.6 "Options" Menu