[RTTY] afsk to fsk

[Kevin Normoyle]

I posted a while back with subject line "Optoisolators"
commenting on the much superior optoisolators with mosfet outputs that
are available now, based on what I saw Clint, KK7UQ doing
 (I was wondering why people still used the 4n33)
see
http://lists.contesting.com/archives//html/RTTY/2005-01/msg00151.html

I use the PS7141-1A  for both FSK and for amp keying.
(I used one wired for 300mA Iload to key a negative-voltage
Yaesu FL2100B...works great. Also used on AL-80B ).

You can wire them up to get twice the output current by using two legs 
(see data sheet)
So Iload=300mA to over 2A is easily done, with minimal equivalent Ron in 
the output.

Just add a series resistor and a blocking diode to the input and that's it.

I've not used this next part, but if you're worried about output current:
this guy can handle it:

PS710E-1A 
Ron = 0.15 ohms
Il = 2A   (that's right...2A)
ton = 2ms
toff = 0.2ms
Vl = 80v


(datasheet at http://www.cel.com/pdf/datasheets/ps710e1a.pdf )

Mouser has it for $5.27 in stock. I little pricey I guess.  Not too bad compared to reed relay?

Apparently you can even make it handle Iload=4A if you hook up pins 6 and 5 in parallel

(see the data sheet Connection C)

The main thing is that the mosfet output helps solve a lot of problems 
with not
enough current in the output.

I referenced Clint Hurd on my original post, and he kindly emailed me 
with some
useful info about why he switched (I followed his lead)

I hope Clint doesn't mind if I  include his comment to me for posterity 
here. I found it useful,
and maybe others will also.

-Kevin
KE6RAD

---------------------------

Hi Kevin - Jack, W7LD forwarded your comments on the rtty reflector to me
re: driving FSK and PTT with optoisolators.  Thought I would add some ideas.

I switched over to the "solid state relay" ps7141-1a part for driving PTT
and FSK a year ago, because of the noted problems with the 706 MkII G .  The
4n33 drives the voltage to just under 0.8 volts when on, which is marginal
in all cases, but flat doesn't work in the 706 MkII G (data port only - ACC
port OK).  Oddly, that rig has different circuits for receiving PTT in the
ACC connector and the Data Port.  As you point out, the R on is lower, hence
the voltage drops to about 0.2volts on the rigs that I have measured.  A
better choice in all cases.  Too bad it isn't pin compatible with the 4n33 -
it required an artwork change on my interface board.

When driving FSK, I have seen a problem with RF feedback into the FSK line
which causes the signal to stay in the "low" state even when the control
signal would have put it back into the "high" state.  I use CAT5 cable
(twisted pair) to drive all the lines from my interface, and this is the
only time I have seen a problem like this.  A solution is to put a 0.1uFd
cap across the output pins of the PS7141-1A.  Same thing goes for CW keying
on some rigs.  Most testing was done on a 706 MkII.  When you open the
output of an opto isolator or the solid state relay, it goes from low
impedance to ground, to high impedance to +V through the pull up on the rig.
This is when the rig buffer circuit seems to have a problem with RF coming
in on this line.

It seems counter intuitive that a cap would solve this, but it seems to work
reliably.  (Same problem on the 4n33).  If the line were replaced with a
shielded twisted pair, grounded at the rig, it would probably work fine also
without the cap.

With regard to the FT101 PTT - opto isolators just don't work - they burn
out.  I have found that a relay is the only good solution here.  The lastest
artwork of the kk7uq model II interface has circuit on there to put on a
relay for those cases of older rigs that need it.

Thanks for your interest,

73, Clint Hurd KK7UQ