[RTTY] One AFSK data point.

[Kok Chen <chen@mac.com>]

Blame W4UK -- he made me do this!

Just kidding.  While exchanging e-mail with Jerry, it occurred to me 
that I should just temporarily configure the equipment in the shack so 
I can make some AFSK measurements.  After all, I have two rigs, two 
computers and the facility to both record and watch the spectrum.

The set up is the following:

Transmit: FT-990 AFSK through Packet connector,
Receive:  FT-1000MP (opened up to 2.4 kc so I can watch a good chunk of 
the spectrum),
AFSK generator: cocoaModem running RTTY mode on a G4 Cube using a 20 
bit D/A converter,
Instrumentation: spectrum analyzer in cocoaModem on a G4 Powerbook 
using a 16-bit A/D converter.

Both A/D and D/A are checked to make sure the signals are not clipped.  
For the test, I'd modulated with LTRS diddles.

Power output is set to 10 watts since I could not induce an extreme 
case of overloading with higher RF power (unless I add an extra audio 
amplifier stage).

Power from the -990 is dumped into a dummy load.  Power is picked up by 
the -1000MP with a foot long pickup wire wound around the dummy load.

The -990's ALC meter (nice analog meter) shows green to about 60% 
beyond which you are not supposed to go, but I did anyway to see how 
bad it could be.

When I ran the transmitter with nothing showing on the ALC meter (as in 
the needle didn't move, not even in the green zone), I get sidebands 
that are -70dB down about 200 Hz from the Mark frequency.  This means 
that the entire RTTY signal is contained in a 570 Hz bandwidth with 
-70dB cutoff (570=200+170+200, the 170 being the shift between Mark and 
Space).

Note: I perform waveshaping (a very sharp 450 Hz FIR filter on the 
transmitted AFSK signal) in my AFSK generator; a different AFSK modem 
may not produce such a tight signal.  However, you will see that 
overloading will destroy anything that you do to generate a clean RTTY 
signal...

Next, I overloaded the audio as much as I could (to full scale on the 
meter -- well passed the green zone), the -70 dB point is now about 1 
kc from the Mark frequency.  The RTTY signal is now over 2000 Hz wide 
(1000 +170 + 1000) at the -70 dB point.

With some ALC showing on the meter (about a third of the way in the 
green zone of the meter), the -70dB bandwidth is still over a kc wide.

So... I would recommend that anyone running AFSK on an FT-990, to run 
it with no ALC showing on the meter.  That, by the way, is how I also 
normally run with my FT-1000MP since my earlier measurements have shown 
that my PSK31 IMD starts climbing if any ALC indicator is at all 
showing.

You can push just about 100 watts through an FT-1000MP with no ALC 
showing, but the 3rd order IMD of my MP's finals start to deteriorate 
for PSK31, so I usually run around 25 watts on PSK31, until I find a 
weak DX, then I bump it up to 100 watts.  Even then I am clocking about 
-28dB IMD, which is still better than most of the stuff you see around 
14071.

The 3rd order IMD is not a problem with RTTY since only one tone is on 
at any one time (unless someone can point me to something I missed 
regarding IMD3 and FSK), so I usually push my RTTY signal to the full 
100 watts (don't tell Yaesu's warrantee department, though :-)

In case you think I am unusually harsh by using a 70 dB cutoff, 70 dB 
down from an S9+40 signal is an S3 to S4 signal.  Certainly it is a 
signal that would produce 100% print if not for an overmodulated signal 
nearby (and it does not have to be that near, it could be 1 kc away).   
And while an S9+40 VU4 station would make you suspect it is a slim, 
there are plenty of S9+40 signals during a contest, and even more of 
the S3 and S4 stations that they are completely covering up, not to 
mention the even S7 signals that are hard to copy because of the 
overmodulated signal.

Your mileage may vary, yada yada yada.  But I myself will try to run 
with no ALC showing on my transmitters.

Anyone setting up a 'clinic" should remember to intentionally 
overmodulate to show its effects.

73
Chen, W7AY

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