G8MNY > TECH 03.06.25 06:15z 49 Lines 2439 Bytes #93 (0) @ WW
BID : 33999_GB7CIP
Read: GUEST
Subj: Variable RF Attenuator
Path: ED1ZAC<ED1ZAC<GB7CIP
Sent: 250603/0608Z @:GB7CIP.#32.GBR.EURO #:33999 [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 2500R 0R0
50 ohm o)ÄÄÂÄÄÄLog<ÄÄÄÂÄÄ(o 50 ohm 35dB 1406R 0R4
Output³ ³ Var ³ ³ Input 30dB 790R 1R05
³ 51R 51R ³ 25dB 443R 2R5
³ ÀÄÄÄÄÂÄÄÄÄÄÙ ³ 20dB 248R 5R0
³ 3K ³ 15dB 136R 10R65
³ Antilog<Ä¿ ³ 10dB 70R7 22R5
³ Var ³ ³ 7dB 44R8 40R0
ÀÄÄÄÄÄÄÄÄÄÄÄÁÄÄÄÄÙ 6dB 37R7 151R0
5dB 30R4 64R0
3dB 17R6 120R5
2dB 11R6 215R
0.1dB 0R5 3000R
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 51Rs.
o)ÄÄÂÄÄ2K5ÄÂÄÄ(o o)ÄÄÂÄ44R4ÄÂÄÄ(o o)ÄÄÂÄ44R4ÄÂÄÄ(o o)ÄÄÂÄÄ0R5ÄÄÂÄÄ(o
³ ³ ³ ³ ³ ³ ³ ³ ³ ³ ³ ³ ³ ³ 0.1dB ³ ³
³ 51R 51R ³ ³ 131R 131R³ = ³ 51R 51R ³ ³ 51R 51R 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 80R 80R 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 50 ohm 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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