The Importance of Dynamic Range in Fiber Optic Links
Traditionally satellite transponders were 36MHz wide and the Spurious Free Dynamic Range (SFDR) of an RF over fiber link was less important. Ultra-wide dynamic range fiber optic links were a “nice-to-have” because the same product could be used for both the up and down links. However, with High Throughput Satellites (HTS), that have transponder bandwidths of 500 or 800MHz, the signals have increased hugely; consequently so have the demands on dynamic range. This means that ultra-wide dynamic range fiber optic links are now a “must-have” for anyone transporting HTS signals. Within this document we compare the SFDR and real dynamic range obtained with HTS satellite signals.
Calculating Spurious Free Dynamic Range
SFDR is a measure of the dynamic range of a fiber optic link and is relative to 1Hz of bandwidth – which is not particularly relevant as most signals are much greater than 1Hz.
To calculate SFDR you must use the thermal noise floor (kTB), the noise figure (nf) and the third order intercept point (IP3).
The thermal noise floor is -174dBm/Hz.
The noise figure is the Signal to Noise Ratio (SNR) degradation caused by the fiber optic link and is a product of the design of the link – it is usually stated in the supplier’s data along with the P1dB which will allow you to calculate SFDR.
The 1dB compression point (P1dB) is the power level at which the gain drops by 1 dB from its small signal value.
The third order Intercept Point or IP3 is a theoretical point at which the third-order distortion signal amplitude equals the input signal. In a fiber optic link this is typically 12dB above the P1dB. The calculation for SFDR is:
SFDR = 2/3((P1dB+12) – (Thermal Noise Floor + Noise Figure of the link))
ViaLite SFDR = 2/3((-1+12) – (-174+20)) = 110 dB/Hz 2/3
Therefore the SFDR is affected by the P1dB and the nf of a fiber optic link. It is a useful measure to compare the performance of fiber optic links when looking at dynamic range.
The Dynamic Range (DR) is the difference between the minimum and maximum signal that you can put through a link for a given traffic bandwidth. The minimum detectable signal (MDS) is usually assumed to be just above the system noise floor and the maximum signal level is the largest peak power without distortion. Unlike gain or noise figure, dynamic range cannot be improved using amplification, but the window can be moved to suit higher or lower signals.
Calculating Minimum Detectable Signal (MDS)
The calculation for MDS is:
MDS = Thermal Noise Floor+10log(traffic bandwidth in Hertz) + Noise Figure of the link
Typically High Throughput Satellites (HTS) have a traffic bandwidth (BW) of 500 or 800MHz. ViaLite MDS for a 500MHz bandwidth is:
ViaLite MDS for 500MHz BW = -174+(10log(500×106)+20 = -67dBm
P1dB, Minimal Detectable Signal and Dynamic Range
As described above, the dynamic range is the difference between the MDS and the P1dB. So, for a ViaLite fiber optic link with a traffic bandwidth of 500MHz the dynamic range is:
DR = P1dB-MDS
ViaLite DR = -1dBm-(-67dBm) = 66dB
The dynamic range as described above does not take into consideration any Signal to Noise Ratio (SNR) or any back off (signal power reduction) to meet Intermodulation Ratio (IMR) requirements for higher order modulation schemes. These are normally customer or application specific but need to be taken into consideration – especially if the fiber optic link has a narrow dynamic range.
To calculate the true usable dynamic range, the SNR in dB needs to be added to the MDS and, to ensure no distortion occurs at the maximum input signal, signal levels may need to be reduced to meet IMR specifications – this further reduces the usable range.
If you take a typical IMR of 40dBc, the maximum signal will be reduced by 8dB and, at a typical SNR of 20dB, the minimum input signal is increased by 20dB as shown below:
Usable DR = (P1dB-back off) – (MDS+SNR)
ViaLite DR = (-1dBm-8dB) – (-67dBm+20db) = 38dB (Usable Range – Max -9dBm to Min-47dBm)
The following table shows a comparison of Dynamic Range/Usable Dynamic Range based upon the SFDR of different brands of fiber optic link products.
|ViaLite||Brand D||Brand E||Brand F|
|DR at 500MHz*||65.8dB||61.8dB||50.8dB||32.8dB|
|DR to meet 20dB SNR & 40dBc IMR**||37.8dB|
(-46.8 to -9dBM)
(-40.8 to -18dBm)
(-24 to -28.8dBm)
|% of ViaLite DR||100%||89%||60%||12.7%|
** At 800MHz the dynamic range is 2dB narrower and 5dB narrower for full-band (1500MHz)
What does this mean for RF over Fiber links in HTS ground stations?
HTS satellites use more bandwidth and this means that there is a need for higher dynamic range RF over fiber links. Fiber links that have an SFDR of 100dB/Hz 2/3 or less are difficult or impossible to use in HTS applications.
For further information on specific dynamic range requirements or calculations, please email: firstname.lastname@example.org.