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G8MNY > TECH 20.04.09 15:35l 43 Lines 2133 Bytes #999 (0) @ WW
BID : 24025_GB7CIP
Read: GUEST
Subj: Variable RF Attenuator
Path: IZ3LSV<IK2XDE<DB0RES<DK0WUE<7M3TJZ<UA6ADV<GB7CIP
Sent: 090419/2219Z @:GB7CIP.#32.GBR.EU #:24025 [Caterham] $:24025_GB7CIP
From: G8MNY@GB7CIP.#32.GBR.EU
To : TECH@WW
By G8MNY (New Mar 09)
(8 Bit Graphics use code page 437 or 850)
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.
2k5 Attenuation Log Var Anti Var
50ê o)ÄÄÂÄÄÄLog<ÄÄÄÂÄÄ(o 50ê 40dB 2500.0ê 0.0ê
Output³ ³ Var ³ ³ Input 35dB 1406.0ê 0.4ê
³ 51ê 51ê ³ 30dB 790.0ê 1.05ê
³ ÀÄÄÄÄÂÄÄÄÄÄÙ ³ 25dB 443.0ê 2.5ê
³ 3k ³ 20dB 248.0ê 5.0ê
³ Antilog<Ä¿ ³ 15dB 136.0ê 10.65ê
³ Var ³ ³ 10dB 70.7ê 22.5ê
³ ³ ³ 7dB 44.8ê 40.0ê
ÀÄÄÄÄÄÄÄÄÄÄÄÁÄÄÄÄÙ 5dB 30.4ê 64.0ê
3dB 17.6ê 120.5ê
0.1dB 0.5ê 3000.0ê
HOW IT WORKS
At maximum loss the 2k5 in series with the signal & the 3k 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
ÀÄÄÁÄÄÄÄÄÄÁÄÄÙ ÀÄÄÁÄÄÄÄÄÄÁÄÄÙ ³ 80ê 80ê ³ ³ 3k ³
ÀÄÄÁÄÄÄÄÄÄÁÄÄÙ ÀÄÄÄÄÄÄÁÄÄÄÄÄÄÙ
At minimum loss the 2k2 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!
Why Don't U send an interesting bul?
73 de John G8MNY @ GB7CIP
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