Message 27790 in TECH.WW
From: G8MNY
Date: Wed, 12 Aug 26 08:40:00 Z
Newsgroups: TECH.WW
Subject: Variable RF Attenuator
Message-ID: <1790_GB7CIP>
Path: N2MH4|GB7BED|EI2GYB|GB7CIP
R:260812/0852Z 35298@N2MH4.#SENC.NC.USA.NOAM LinBPQ6.0.24
R:260812/0852Z 15749@GB7BED.#21.GBR.EU LinBPQ6.0.25
R:260812/0851Z 57558@EI2GYB.DGL.IRL.EURO LinBPQ6.0.25
R:260812/0840Z @:GB7CIP.#32.GBR.EURO #:1790 [Caterham Surrey GBR] $:1790_GB7CIP
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
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 and the 3K is shorted out,
the input and output Z is mainly due to the two 51Rs.
approx
At minimum loss the 2K5 is a short and the antilog to earth is nearly open
circuit @ 3K. The cleaver bit is mechanically arranging that log and 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 and the Rs are not wire
wound types!
Why Don't U send an interesting bul?
73 de John G8MNY @ GB7CIP
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