Message 27245 in TECH.WW
From: G8MNY
Date: Tue, 04 Aug 26 06:49:00 Z
Newsgroups: TECH.WW
Subject: PSK31 interface isolation
Message-ID: <1092_GB7CIP>
Path: N2MH4|GB7BED|EI2GYB|GB7CIP
R:260804/0652Z 34843@N2MH4.#SENC.NC.USA.NOAM LinBPQ6.0.24
R:260804/0652Z 15355@GB7BED.#21.GBR.EU LinBPQ6.0.25
R:260804/0651Z 57242@EI2GYB.DGL.IRL.EURO LinBPQ6.0.25
R:260804/0649Z @:GB7CIP.#32.GBR.EURO #:1092 [Caterham Surrey GBR] $:1092_GB7CIP
From: G8MNY@GB7CIP.#32.GBR.EURO
To : TECH@WW
By G8MNY (Updated Nov 09)
(8 Bit ASCII graphics use code page 437 or 850, Terminal Font)
________________________________________________________
Having just got going on this mode with an IBM Laptop & ICOM IC735. I was
concerned about damaging either with a direct connection!
This applies to all modes like FT8 RTTY SSTV AMTOR etc, where a PC/RIG is
connected to anything with medium to high DC current supply with a different
earth or has RF currents!
So I have made an isolated barrier TO PROTECT THE LAPTOP's SOUND CARD FROM
R.F.I. & getting ZAPPED & also the Rig from the Laptop's RF QRM. As well as
doing that it also STOPS any HUM LOOPS & DAMAGING DC PATHS the can also burn
out RIG's MIC/LS & the PC interfaces!
For this rig it gave a nice 1 plug solution...
LAPTOP ICOM IC735
PNP ______________ Pin 3
small 1:1 signal
transformers
Of course for other Rigs with different pinouts etc. refer to handbooks.
TRANSFORMERS
They need to be low capacitance primary to secondary & could be from scrap
modems/answering machines etc. about 1 cm cube in size. A 1:1 ratio is not that
important as long as the Z presented does not attenuate the signal, & it has a
flat enough responce at the Z used over the comms AF. If the transformer has
tapings, these could be used to set the best signal level. On LS line drive,
levels must be use not to saturate the transformer.
TX PTT
The RS232 port is initially protected by a series 1k in the plug to reduce the
chance of zapping the laptop's RS232 port. (P.S. U will only do this once!)
The use of a PNP to follow the opto coupler increases the PTT current ability
(so it could easily drive a 50mA rig PTT) as well as keeping down the RS232
load current. If you can't find a PNP then use a NPN like this..
___ 1 OPTO 5
There are many types of opto isolators most should work OK once the pin outs
are found.
SERIES Rs
A good idea but not essential is to copy the series R in the plug at the PC end
if you can, as this does stop some RF & excessive LS currents if the leads get
shorted etc.
MIC SOCKET
If you using a mic socket (rather than line) for the input either on the laptop
or the rig, you will need an attenuator ideally in the mic plug so that RF+hum
pickup are attenuated too.
level 1K Plug components will need
Inside mic plug
BOXING UP
The whole lot is mounted on stripboard, & housed in a small plastic box making
sure the 2 sets of leads are kept away from each other.
IN USE
With a local aerial, this simple isolation significantly reduces the QRM
compared to a direct connection.
With a small nearby SW PLL SSB Rx with a small whip aerial it produces a good
demonstration of the mode & waterfall display to analyse signals.
Steve, G1KQH @ GB7COV writes...
As well as using a similar circuit, I also use a Plug in USB Sound Card
which you can get NEW on ebay for a couple of quid each. Which again helps
to save the PCs internal guts from blowing!
See my Tech Bul on "Rig DC Power & RF Hazards".
Why don't U send an interesting bul?
73 de John G8MNY @ GB7CIP
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