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G8MNY > TECH 11.03.04 16:34l 53 Lines 2668 Bytes #999 (0) @ WW
BID : 47793_GB7CIP
Read: DF5NK DB0FHN GUEST OE7FMI
Subj: Passive CW headphone filter
Path: DB0FHN<DB0RGB<OK0PPL<DB0RES<ON0AR<WB0TAX<ZL2TZE<GB7YFS<GB7CIP
Sent: 040311/1147Z @:GB7CIP.#32.GBR.EU #:47793 [Caterham] $:47793_GB7CIP
From: G8MNY@GB7CIP.#32.GBR.EU
To : TECH@WW
By G8MNY (Updated Mar 04)
This filter is a plug in adapter that has no batteries but gives very good
performance.
ÚÄÄÄÄÄÄÄÄ10êÄÄÄÄÄÄÄÄÄÂÄÄÄ¿ 0dB¿ ..
Rig o through o ³ Headphone ³ . .
Jack ³ Socket -10dB´ . .
<ÄÄÄÄo DOUBLE POLE C/O oÄÄÄ)ÄÄÄÄ< ³ . .
<¿ \ \ ³ Ú< -20dB´ . .
³ o cw filter o ³ ³ ³ . .
³ ³ u47 ³ ³ ³ -30dB´ . .
³ === 88mH ) 120ê ³ ³ . .
³ ÃÄÄÄ3K3ÄÄÂÄÄÄ¿ ) ³ ³ -40dB´ . Measured .
³ ³ ³ ) 88mH ) ³ ³ ³ . Performance .
³ ) u47 === )ÄÄ1K5ÄÄ´ ³ ³ -50dB´ . .
³ ) 88mH ³ ) === ³ ³ ³. .
³ ) ³ ³ u47 ³ ³ ³ -60dBÅÄÄÄÂÄÄÄÂÄÄÄÂÄÄÄÂÄÄÄÂÄÄÄÂÄÄÄÂÄ
ÀÄÄÄÁÄÄÄÄÄÄÄÄÁÄÄÄÁÄÄÄÄÄÄÄÁÄÄÄÁÄÄÄÙ 200 400 600 800 1K 1K2 1K4 1K6
Hz
The through position attenuator values of 10ê & 120ê may need adjusting to suit
your Rig & Headphones to have the same loss as the filter.
The 3 tuned circuits are all made from 0.47uF polyester caps & old telecomm
bifilar wound line loading coils connected in series to make 88mH. The input &
output tuned circuits are series ones matching the low impedance of the LS jack
& Headphones. The middle tuned circuit is a parallel one but the coil centre
tap has been used as the output load.
The coupling Rs of 3K3 & 1K5 are compromise between bandwidth & loss.
I boxed it up in a small plastic box that was a tight squeeze for the
components so they did not need any fixing. The I/P was a short wander lead
jack & the O/P was a socket in the box. The holes for the cable, switch &
socket, were moulded in with a soldering iron.
In use it will make beacons & CW much easier & less tiring to read. At around
800Hz your ears can resolve about 25Hz in frequency, this filter is much wider
& does not have bad ring effects that some narrow filters do.
As this is a post IF filter large signals in the IF bandwidth outside the CW
filter bandwidth will affect the Rig's AGC & thereby weaken/modulate signals
heard through the CW filter.
Using it takes the hard work out of resolving noisy signals. Switching the
filter out on a tuned in weak signal is very impressive!
/QSL
73 De John, G8MNY @ GB7CIP
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