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G8MNY > TECH 14.01.17 11:29l 49 Lines 2449 Bytes #999 (0) @ WW
BID : 31555_GB7CIP
Read: GUEST
Subj: Variable RF Attenuator
Path: IW8PGT<IR2UBX<SR1BSZ<GB7CIP
Sent: 170114/1024Z @:GB7CIP.#32.GBR.EURO #:31555 [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 2500.0ê 0.0ê
50ê o)ÄÄÂÄÄÄLog<ÄÄÄÂÄÄ(o 50ê 35dB 1406.0ê 0.4ê
Output³ ³ Var ³ ³ Input 30dB 790.0ê 1.05ê
³ 51ê 51ê ³ 25dB 443.0ê 2.5ê
³ ÀÄÄÄÄÂÄÄÄÄÄÙ ³ 20dB 248.0ê 5.0ê
³ 3k ³ 15dB 136.0ê 10.65ê
³ Antilog<Ä¿ ³ 10dB 70.7ê 22.5ê
³ Var ³ ³ 7dB 44.8ê 40.0ê
ÀÄÄÄÄÄÄÄÄÄÄÄÁÄÄÄÄÙ 6dB 37.7ê 151.0ê
5dB 30.4ê 64.0ê
3dB 17.6ê 120.5ê
2dB 11.6ê 215.0ê
0.1dB 0.5ê 3000.0ê
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 51ê.
o)ÄÄÂÄÄ2k5ÄÂÄÄ(o o)ÄÄÂÄ44.4ÄÂÄÄ(o o)ÄÄÂÄ44.4ÄÂÄÄ(o o)ÄÄÂÄÄ0.5êÄÂÄÄ(o
³ ³ ³ ³ ³ ³ ³ ³ ³ ³ ³ ³ ³ ³ 0.1dB ³ ³
³ 51ê 51ê ³ ³ 131ê 131ê³ = ³ 51ê 51ê ³ ³ 51ê 51ê ³
³ ³ 40dB ³ ³ ³ ³ 7dB ³ ³ ³ ³ 7dB ³ ³ ³ ÀÄÄÄÂÄÄÄÙ ³
³ ³ ³ ³ ³ ³ ³ ³ ³ 80ê 80ê ³ ³ 3k ³
ÀÄÄÁÄÄÄÄÄÄÁÄÄÙ ÀÄÄÁÄÄÄÄÄÄÁÄÄÙ ÀÄÄÁÄÄÄÄÄÄÁÄÄÙ ÀÄÄÄÄÄÄÁÄÄÄÄÄÄÙ
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 50ê 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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