G8MNY > TECH 01.04.25 07:30z 68 Lines 3337 Bytes #101 (0) @ WW
BID : 29318_GB7CIP
Subj: Passive CW headphone filter
Path: ED1ZAC<ED1ZAC<GB7CIP
Sent: 250401/0717Z @:GB7CIP.#32.GBR.EURO #:29318 [Caterham Surrey GBR]
From: G8MNY@GB7CIP.#32.GBR.EURO
To : TECH@WW
By G8MNY (Updated Mar 04)
(8 Bit ASCII graphics use code page 437 or 850, Terminal Font)
This filter is a plug in adapter that has no batteries but gives very good
performance.
Level
ÚÄÄÄÄÄÄÄÄ10RÄÄÄÄÄÄÄÂÄÄ¿ 0dB¿ ..
Rig o through o ³H/Phone ³ . .
Jack ³ Socket -10dB´ . .
<ÄÄÄÄo DOUBLE POLE C/O oÄÄ)ÄÄÄÄ< ³ . .
<¿ \ \ ³ Ú< -20dB´ . .
³ o cw filter o ³ ³ ³ . .
³ ³ u47 ³ ³ ³ -30dB´ . .
³ === 88mH||) 120R ³ ³ . .
³ ÃÄ3K3ÄÂÄÄÄÄ¿ ||) ³ ³ -40dB´ . Measured .
³ ³ ³ ||)88mH ||) ³ ³ ³ . Performance .
³ ||) u47³ ||)ÄÄ1K5ÄÄ´ ³ ³ -50dB´ . .
³ ||) === ||) === ³ ³ ³ . .
³ ||)88mH ³ ³ u47³ ³ ³ -60dBÅÄÄÄÂÄÄÄÂÄÄÄÂÄÄÄÂÄÄÄÂÄÄÄÂÄÄÄÂÄÄÄÂÄÄÄÂÄ
ÀÄÄÄÁÄÄÄÄÄÁÄÄÄÄÁÄÄÄÄÄÄÄÁÄÄÁÄÄÄÙ 0 200 400 600 800 1k 1k2 1k4 1k6 1k8
Frequency Hz
The through position attenuator values of 10R & 120R 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!
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Eric ON4CBL says try a long resonant tube of a LS...
RESONANT QUARTER WAVE OF A TUBE BAFFLE GOOD FOR MAKING RESONANT SPEAKER ON CW!
Eg Tube diameter: 0,1 metres, Length: 1 metres
This is a resonant quarter wave at:118,835 Hz
A tube placed over an audio speaker can be used to enhance a specific frequency
such as a CW signal. Use a tube resonated to your preferred cw beat note &
place over the speaker. Other frequencies not at your chosen beat note will be
attenuated. 73 - Eric
Why Don't U send an interesting bul
73 De John, G8MNY @ GB7CIP
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