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G8MNY  > TECH     07.10.18 09:30l 49 Lines 2449 Bytes #2 (0) @ WW
BID : 40811_GB7CIP
Subj: Variable RF Attenuator
Path: ED1ZAC<ED1ZAC<CX2SA<GB7CIP
Sent: 181007/0720Z @:GB7CIP.#32.GBR.EURO #:40811 [Caterham Surrey GBR]
From: G8MNY@GB7CIP.#32.GBR.EURO
To  : TECH@WW

By G8MNY                                     (Updated Mar 10)
(8 Bit ASCII graphics use code page 437 or 850, Terminal Font)

While working on an old pulse generator that goes up to 50MHz, I was interested
to know how the variable attenuator works. It used a ganged log-antilog pot to
make quite usable approximation to this table.

                                   Attenuation   Log Var     Anti Var
              2k5                     40dB       2500.0        0.0
  50 o)Log<(o 50        35dB       1406.0        0.4
 Output     Var       Input       30dB        790.0        1.05
        51        51              25dB        443.0        2.5
                        20dB        248.0        5.0
              3k                    15dB        136.0       10.65
         Antilog<Ŀ                 10dB         70.7       22.5
             Var                    7dB         44.8       40.0
                     6dB         37.7      151.0
                                       5dB         30.4       64.0
                                       3dB         17.6      120.5
                                       2dB         11.6      215.0 
                                     0.1dB          0.5     3000.0
HOW IT WORKS
At maximum loss the 2k5 in series with the signal & the 3k is shorted out, the
input & output Z is mainly due to the two 51.

o)2k5(o     o)44.4(o o)44.4(o     o)0.5(o
                                                   0.1dB   
  51    51         131   131 =  51    51         51     51 
    40dB              7dB          7dB              
                                80    80             3k      
                  
                                           approx

At minimum loss the 2k5 is a short & the antilog to earth is nearly open
circuit @ 3k. The cleaver bit is mechanically arranging that log & antilog
tracks produce the correct resistance to maintain the 50 match at all other
settings. Note 7dB is half way around the pot with about equal tracks!

To work well to 50MHz all the leads are quite short & the Rs are not wire wound
types!


Why Don't U send an interesting bul?

73 de John G8MNY @ GB7CIP


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