On Dec 10, 2008, at 12/10 8:57 AM, Alfred Frugoli wrote:
> When I had the first circuit built I tried reversing the keying on the
> "modem" (in this case software) end. What resulted was a very hard
> to copy
> signal (as tested sending from 1 rig to another in the shack - with
> appropriate attenuation of the signal of course).
Interesting. Perhaps the transistor was only marginally pulling down
the keying line?
If some characters, such as O (oh) and 9 gets through more reliably
than other characters, such as Space or Z then it might indeed be
insufficient current drive causing your woes. The wrong baud rate
will also cause copy of some characters to be easier than others -
knowing which Baudot characters get through better might give a
better clue of what is wrong.
Did you try to reduce the base resistor in an attempt to drive more
base current? This solution won't always work because the serial
port may be current limited anyway, but if the base resistor of the
circuit was designed for RS-232 and the serial port you are using is
spec'ed instead for RS-423, it may just be a case of insufficient
base drive.
The next alternative is to wire a second NPN transistor to the first
NPN transistor as a Darlington configuration. This should generally
work.
To find the current needed to key your rig, you can connect the
keying input of the rig through a resistor to ground and change the
value of the resistor until the FSK output "just" keys. At that
point, if you measure the voltage across the resistor, you should be
able to figure out the minimum current that you need to drive your
rig's FSK keying line.
73
Chen, W7AY
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