These two configurations of attenuator pad are undoubtedly familiar, but finding their resistance values for some particular dB of attenuation can be a little inconvenient.

This code in GWBASIC does that task. Note that the impedance level, the ohms values of the R_{source} and the R_{load } is stated in line 100 so you can change that if you care to.

10 CLS:SCREEN 9:COLOR 15,1:A$="pad @ ### ohms: "

20 PRINT "save "+CHR$(34)+"padcalc.bas"+CHR$(34):PRINT

30 PRINT "save "+CHR$(34)+"a:\padcalc.bas"+CHR$(34):PRINT:PRINT:GOTO 70

40 XX=1:CC=(1-SGN(CC))*3:RETURN

50 IF XX=5 THEN GOSUB 40

60 LOCATE 3+XX*3,1:RETURN

70 C$=" ##.## dB Tee "+A$+"Series = #####.## ohms Shunt = #####.## ohms"

80 D$=" ##.## dB Pi "+A$+"Series = #####.## ohms Shunt = #####.## ohms"

90 E$="Attenuation in dB ( 62 dB max. )"

100 RS=50:REM ************ Source & load impedance value. **********

110 LOCATE 18,1:PRINT " "

120 COLOR 15,1:LOCATE 18,1:PRINT E$;:INPUT DB

130 IF DB>62 THEN GOTO 110

140 K=10^(-DB/20):R1=RS:IF DB<=0 THEN GOTO 110

150 R2=RS*(1-K)/(1+K):REM Series R for tee attenuator.

160 R3=(R1-R2)*(R1+R2)/2/R2:REM Shunt R for tee attenuator.

170 R4=R1*(1+K)/(1-K):REM Shunt R for pi attenuator.

180 R5=R1*R4/(R4-R1)-R1*R4/(R4+R1):REM Series R for pi attenuator.

190 XX=XX+1:GOSUB 50:COLOR 14-CC,1

200 PRINT USING D$;DB,R1,R5,R4:PRINT USING C$;DB,R1,R2,R3:PRINT:GOTO 110

Although written in GWBASIC, this code will also work with Q-Basic.

To get a whole set of results into a conveniently readable form, the algorithm of this code was ported into Excel and the results were printed out in two charts below, one for 50 ohms and the other for 75 ohms impedance level.

If you would like to have a copy of the Excel file, just e-mail me at ambertec@ieee.org and I will send it to you.

The next issue is this:

Suppose you have a multiplicity of pads that you will use in cascade with each other and you want each pad to dissipate the same amount of power as each of the others does. How do you allocate the dB attenuations?

The GWBASIC code below does that calculation:

10 CLS:SCREEN 9:COLOR 15,1:DIM A(10),P(10),A$(10),B$(10),PL(10)

20 PRINT "save "+CHR$(34)+"equipads.bas"+CHR$(34):PRINT

30 PRINT "save "+CHR$(34)+"a:equipads.bas"+CHR$(34):PRINT:PRINT

40 A$(1)="##.## dB = ##.## dB + ##.## dB":B$(1)=" #.### + #.### "

50 FOR J=2 TO 5:A$(J)=A$(J-1)+" + ##.## dB":B$(J)=B$(J-1)+" + #.### ":NEXT J

60 C$="Examples of equal power pads and their fractions "

70 C$=C$+"of input power absorbed:":PRINT C$:PRINT:ON ERROR GOTO 240

80 READ ADB,N:GOSUB 90:GOTO 80:DATA 3,2,10,4,30,6

90 ASUM=0:FOR J=1 TO 10:A(J)=0:NEXT J:FOR J=1 TO N

100 NUM=(N+1-J)*10^(ADB/10)+J-1:DEN=(N-J)*10^(ADB/10)+J

110 A(J)=10*LOG(NUM/DEN)/2.30259:ASUM=ASUM+A(J):NEXT J:P(0)=1

120 FOR J=1 TO N:P(J)=P(J-1)/(10^(A(J)/10)):PL(J)=P(J-1)-P(J):NEXT J

130 IF N=2 THEN PRINT USING A$(1);ASUM,A(1),A(2)

140 IF N=2 THEN PRINT USING B$(1);PL(1),PL(2)

150 IF N=3 THEN PRINT USING A$(2);ASUM,A(1),A(2),A(3)

160 IF N=3 THEN PRINT USING B$(2);PL(1),PL(2),PL(3)

170 IF N=4 THEN PRINT USING A$(3);ASUM,A(1),A(2),A(3),A(4)

180 IF N=4 THEN PRINT USING B$(3);PL(1),PL(2),PL(3),PL(4)

190 IF N=5 THEN PRINT USING A$(4);ASUM,A(1),A(2),A(3),A(4),A(5)

200 IF N=5 THEN PRINT USING B$(4);PL(1),PL(2),PL(3),PL(4),PL(5)

210 IF N=6 THEN PRINT USING A$(5);ASUM,A(1),A(2),A(3),A(4),A(5),A(6)

220 IF N=6 THEN PRINT USING B$(5);PL(1),PL(2),PL(3),PL(4),PL(5),PL(6)

230 RETURN

240 PRINT:INPUT "How many dB and with how many pads ";ADB,N:IF N>6 THEN N=6

250 GOSUB 90:GOTO 240

As before, this code will also run in Q-Basic.

The following is an example of the results of running this code:

Three quick examples are presented at the start of things. These are examples of 3 dB using two pads, 10 dB using four pads and 30 dB using six pads.

In the example shown, I chose to ask for 12 dB using four pads. The decibel values of the four pads from Pad 1 thru Pad 4 are as shown in that order from left to right and each of those pads is shown to dissipate 23.4% of the total power input.

Numerically checking this, we say that the attenuation is 10 * Log_{10} ( 1 - 4 * 0.234 ) = - 11.938 dB where the target had been -12 dB. The difference can be ascribed to numerical accuracy.

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Posted by: optical attenuators | June 22, 2012 at 06:09 AM