英文原文.txt

毕业论文全套~ 本课题主要设计一种气体泄漏检测系统。

GOPTICAL LINK = –39 dB. 

Alternatively, this value could have been read directly 
off of the gain curves in the Appendix. 

To offset this loss, amplifiers must be added prior to the 
transmitter and/or after the receiver. The placement of 
the amps will affect both the noise performance and 
distortion, as described below. 

Noise 

Before calculating the noise of the optical link, the overall 
link requirements should be converted into a noise 
figure. Due to the SNR requirement of 12 dB, the minimum 
detectable signal will need to be –72 dBm (i.e., 
–60 dBm – 12 dB). Therefore, the total noise in any 
given channel bandwidth must be less than –72 dBm. 
A rearranged equation 22 on page 12 can then be used 
to determine the maximum acceptable EIN: 

equation 31, 

EINTOTAL LINK = NOISECHANNEL – BWdB, Hz 

EINTOTAL LINK < –72 dBm – 10 log (35 x 106) 

EINTOTAL LINK < –147 dBm/Hz. 

Converting this to a noise figure with equation 12 on 
page 9 gives: 

NFTOTAL LINK = EIN + 174 dBm/Hz 

NFTOTAL LINK < –147 dBm/Hz + 174 dBm/Hz 

NFTOTAL LINK < 27 dB. 

Now, the actual performance of the optical link can 
computed. The performance curves resented in the 
appendix (or equations 15, 17, and 18 on page 10) predict 
that the above specified transmitter and receiver 
pair, with an optical loss of 3 dB, will have a NF of better 
than approximately: 

NFOPTICAL LINK = 54 dB. 

The preamplifier and postamplifier also will affect the 
total link NF. If, for example, the selected components 
are a preamp with a gain of 30 dB and an NF of 5 dB, 
and a postamp with a gain of 9 dB and an NF of 6 dB, 
then Friis’ formula (equation 19 on page 11) can be 
used to cascade the noise figures. First, the optical link 
and the postamp should be cascaded as follows: 

equation 32, 

NFOPTICAL LINK AND POSTAMP = 

NFPOSTAMP RATIO – 1)

(,

NFOPTICALLINK RATIO , + ------------------------------------------------------------------


GOPTICAL LINK, RATIO 
NFOPTICAL LINK AND POSTAMP = 

6 dB 

.54 dB . .
. 
10 .
. 

. 10 .(10 –1 )

10 + ------------------------------------


.–39 dB . 

. 10 .
10


NFOPTICAL LINK AND POSTAMP = 275,000 = 54.4 dB. 

Next, the result can be cascaded with the NF of the 
preamp: 

NFTOTAL LINK = 

(NFOPTICAL LINK AND POSTAMP – 1

NFPREAMP RATIO, + ----------------------------------------------------------------------------------------


GPREAMP 

.5 dB .. . 
.10 ..275 000–1.

NFTOTAL LINK = 10 + .-----------,---------------------.

30dB 

..--------------.. 

. 10 
.10 .. 

NFOPTICAL LINK = 278 = 24.4 dB. 

Thus, the link satisfies the 27 dB NF requirement. 

Agere Systems Inc. 


Application NoteApril 2001 RF and Microwave Fiber-Optic Design Guide 

Link Design Calculation (continued) 

Dynamic Range 

Next, it must be ensured that the maximum signals do 
not saturate the transmitter. With the 30 dB preamp 
chosen, the maximum input per channel to the optical 
transmitter would be: 

SCHANNEL, Tx (MAX) = –35 dBm + 30 dB 

SCHANNEL, Tx (MAX) = –5 dBm 

Since there are five channels, the total RF power into 
the transmitter is: 

STOTAL, Tx (MAX) = –5 dBm + 10 log(5) 

STOTAL, Tx (MAX) = 2 dBm 

The 1 dB compression of the laser is greater than 
13 dBm, therefore, the optical link has at least another 
11 dB of dynamic range before the laser transmitter 
begins to clip significantly. 

(Alternatively, a rough check of the dynamic range of 
only the optical portion of the link without any amplifiers 
can be calculated quickly with equation 24 on page 
13.) 

DR1dB = P1dB – EINdBm/Hz – BWdB, Hz 

DR1dB = +13 dBm –(–120 dBm/Hz) –10log (35 x 106) 

DR1dB = 57 dB 

The dynamic range requirements of the entire application 
are: 

equation 33, 

DRAPPLICATION = [SCHANNEL, RF (MAX) – 
SCHANNEL, RF (MIN)] + SNR 

DRAPPLICATION = [–35 dBm – (–60 dBm)] + 12 dB 

DRAPPLICATION = 37 dB 

This quick calculation indicates that, by adding a preamplifier 
with an appropriate gain and reasonable 
noise, the optical link can satisfy the dynamic range 
requirements of the application for a single channel 
input. 

Distortion 

For a multitone input, the third-order intermodulation 
terms must not interfere with other signals. As a quick 
check of such distortion, the spur-free dynamic range 
can be calculated: 

equation 34, 

SFDR = 2/3 (IIP3dBm – EINdBm/Hz – 10 log BW) 

SFDR = 2/3 (+ 25 dBm – (–120 dBm/Hz) – 
10log(35 x 106 Hz)) 

SFDR = 47 dB 

which again indicates that with the correct choice of a 
preamp, the optical link can satisfy the 37 dB dynamic 
range calculated above. 

To verify that the amplifier chosen is correct, the actual 
power of the intermodulation terms can be calculated 
using equation 25 on page 14: 

C/I2 TONE = 2 (IIP3 – SCHANNEL, Tx (MAX)) 

C/I2 TONE > 2 (25 dBm – (–5 dBm)) 

C/I2 TONE > 60 dB 

This corresponds to an intermodulation signal level of: 

I2 TONE <= SCHANNEL, Tx(MAX) – C/I2 TONE 

I2 TONE < < –5 dBm – 60 dB 

I2 TONE < –65 dBm 

The distortion can then be compared with the lowest 
power signal at the transmitter input. Although the sensitivity 
of the total link needs to be better than –72 dBm, 
at the transmitter the minimum input signal will be 
30 dB higher due to the preamp: 

SCHANNEL, Tx (MIN) = –72 dBm + 30 dB 

SCHANNEL, Tx (MIN) = –42 dBm 

Since I2 TONE is well below this lowest input signal of 
–42 dBm, two tones of –5 dBm in each can be input 
into the transmitter without creating third-order distortion 
terms above the noise floor. 

Agere Systems Inc. 


Application Note 
RF and Microwave Fiber-Optic Design Guide April 2001 

Link Design Calculation (continued) 

Distortion (continued) 

Alternatively, if the input signal of all five channels were 
raised equally to the maximum, and if they were 
equally spaced in frequency, then their intermodulation 
products could accumulate, as described in the section 
on Large Number of Carriers, page 15. 

Using the counting factor of equation 28 on page 15, 
the C/I would decrease by 12.5 dB: 

C/I5 CHANNELS = C/I2 TONE – [6 dB + 10 log (x)] 

C/I5 CHANNELS = 60 dB – 12.5 dB 

C/I5 CHANNELs = 47.5 dB 

Similarly, the intermodulation term would increase by 

12.5 dB to: 
I5 CHANNELS = –5 dBm – 47.5 dB 
I5 CHANNELS = –52.5 dBm 
The result is still better than the minimum detectable 
signal of –45 dBm. (In fact, for this case, the –45 dBm 
is more demanding than necessary. Since it was 
assumed that all the channels were raised to the same 
power, the lowest power signals that would need to be 
detected would be within the SNR of the carrier power 
level. Since the SNR is only 12 dB, the C/I5 CHANNELS 
of 47.5 dB far exceed the linearity requirements of this 
application.) 

In summary, the above described link should work well 
in remoting an X-band antenna. 

Agere Systems Inc. 


Application NoteApril 2001 RF and Microwave Fiber-Optic Design Guide 

Selection of Optical Fiber Components 

Although Agere Systems does not manufacture optical 
fiber cable, the performance of the transmitters and 
receivers in linear fiber-optic links depends on the characteristics 
of the optical fiber being used. The chief 
parameters for the cable are wavelength, loss, dispersion, 
ruggedization, and connectorization. 

Wavelength and Loss 

The fundamental part of an optical fiber consists of an 
inner core and an outer cladding of glass, as shown in 
Figure 14. When light is launched in this inner core at a 
low enough angle, it will remain trapped by the effect of 
total internal reflection and propagate down the length 

of the fiber. Total internal reflection occurs because the 
inner core is composed of a glass with a slightly higher 
index of refraction than that of the outer cladding. 

While propagating down the fiber, some of the light is 
lost due to optical absorption from impurities in the 
glass, scattering from non-uniformities in the material, 
and bend loss (when the fiber bend radius is smaller 
than roughly 1 inch). The unavoidable scattering and 
absorption losses depend on the wavelength of the 
light, as shown in Figure 15. Due to the two minima of 
this curve, the most common fiber wavelengths used 
today are 1310 nm and 1550 nm, although some 
840 nm fiber is still used for some less demanding 
applications. Typically, the loss in single-mode fiber at 
1310 nm fiber is less than 0.4 dB/km; and at 1550 nm, 

0.25 dB/km. 
Figure 14. Precise Indices of Refractions Enables Total Internal Reflection in Optical Fiber 
CORE 
CLADDING 
1-1225F 
WAVELENGTH, nm 
800 1000 1200 1400 1800 
10.0 
OPTICAL LOSS, dB/km 

0.1 

0.1 

1-1226F 

Figure 15. Scattering and Absorption Losses vs. Wavelength 

Agere Systems Inc. 


Application Note 
RF and Microwave Fiber-Optic Design Guide April 2001 

Selection of Optical Fiber Components 

(continued) 

Dispersion 

In addition to loss, a given fiber will have a characteristic 
dispersion, which also depends on the wavelength 
of the optical signal. If a square pulse of light is 
launched into a fiber, it will emerge at the end somewhat 
rounded and broadened due to a combination of 
chromatic dispersion (wavelength spreading) and 
modal dispersion. This modal dispersion occurs in multimode 
fibers, such as the one shown in Figure 14 on 
page 19. These fibers have cores with diameters 
50 μm or larger, which allow light to follow a number of 
different paths, each with differing lengths and transit 
times. An optical signal that is sent down such a fiber 
will therefore break up into some combination of these 
modes and become smeared out by the time it reaches 
the end of the fiber. 

To avoid this dispersion problem of multimode fiber, 
single-mode fiber is always used for Lucent’s fiber-
optic links. Single-mode fiber has a core with a typical 
diameter of 9 μm, which allows only a single spatial 
mode to propagate straight down the center. This single 
transverse mode also will experience some chro


20 

10 

0 

–10 

–20 

DISPERSION(ps/nm * km) 

matic dispersion, albeit much smaller than modal 
dispersion due to the fact that light of different wavelengths 
travels at different speeds in a fiber. Figure 16 
shows the wavelength dependency of this dispersion 
for standard telecommunication fiber and for special 
dispersion shifted fiber. Roughly 90% of fiber presently 
installed is the standard type centered at 1310 nm. 

The bandwidth limit due to dispersion is given approximately 
by: 

equation 35, 

BW = λ 1/(2s·η(L)) 

where s is the fiber dispersion coefficient in ps/(kmnm), 
λ is the wavelength spread of the optical signal, 
and L is the length of the fiber. This wavelength spread 
includes both the intrinsic linewidth of the unmodulated 
laser and any additional wavelength chirp which may 
result from modulating the laser. (This chirp will be discussed 
more fully in the section on Distributed-Feedback 
(DFB) vs. Fabry-Perot (FP) Lasers, page 25.) As 
an example, a laser with a spectral width of 1 nm transmitting 
down a 10 km fiber with a dispersion coefficient 
of 5 ps/km-nm would produce a blurred output for signals 
faster than 10 GHz. In most cases, this chromatic 
dispersion is not a limitation for single-mode DFB 
transmitters used with fiber of the correct wavelength. 

WAVELENGTH 

1-1227F 

1100 1200 1400 1600 1