[CTSARA] Digital Bootcamp Tonight

[Jon Perelstein]

Sorry for getting this out late.  I was having too much fun working digital DX 
on 30 meters and forgot to finish this document.  I did add 8 new countries for 
my DXCC -- all in central Asia or Western Pacific.  We'll meet at 9pm after the 
SARA net.

*****


For this session, we'll try PSK on SSB in the HF band.  Unless otherwise 
specified, we will meet on the New Canaan repeater, but do our transmitting at 
about 14.080 MHz (we'll see where the band is clear).  Remember that all digital 
modes operate on USB when using SSB.

So How Do I Hook Up My Interface for Transmit?
----------------------------------------------

Should be same as for VHF FM, but most of you will be using a different rig than 
you did last week, which means you have to read the material from last week and 
apply it to your HF rig.


Power Settings - SSB
--------------------

It's very very very important to remember that digital transmissions over SSB 
are continuous duty cycle.  

Normal SSB phone is not a continuous duty cycle mode.  When transmitting, the 
transmitter puts out different power levels in response to your voice.  Rig 
designers count on that and expect, on average that you will be at full power 
less than about 30% of the time when you are transmitting.  That's how they can 
rate a rig at 100 watts on SSB.  

HOWEVER, for SSB digital, from the moment you start the transmit until the 
moment you end it, your rig will be transmitting at the full power to which it 
is set.  Therefore, with SSB, we want to back off on the power.  The general 
rule of thumb is that you want to limit your power to no more than 30% of max 
for that rig.  Just as importantly, there are non-official but accepted 
standards for different modes.  Usually, you don't want to be using more than 
about 30 watts for common, narrow modes like PSK, MFSK, DominoEX, etc.  Any more 
than that risks creating serious QRM for other users, even if your gain is 
properly set and you're not splattering.

There are two methods of controlling the power, depending on your rig:

1.  The vast majority of rigs measure power at the input, meaning that when you 
set the power on your rig, you're setting the power you want in.  The rig 
doesn't care what power is coming out since that is a function of the audio 
going in.  For these rigs, the best approach is to set the power level at or 
near the max power of the rig, and then control the actual output with the input 
audio gain/volume setting on the computer. 

2.  A few rigs, such as the Elecraft K2 and K3, measure the power coming out and 
attempt to always drive the rig at the power set.  For these rigs, the best 
approach is to set the power level at the transmit power you want.  So, for 
example, if you have a 100 watt K2 and want your PSK transmissions at 25 watts, 
you would set the output power on the K2 to 25 watts.


Gain Settings - SSB
-------------------

Digital modes require at least a certain volume level (gain) coming from the 
computer and going into the rig.  However, just like with a voice transmission, 
you have to watch the gain settings to avoid overdriving and distorting your 
signal.

On SSB, good settings can be obtained by watching the ALC level and adjusting 
gain so that ALC is 1-2 bars/dots or less.  Thus, you would adjust gain to get 
the power you want and then check ALC.  If your ALC is at 1-2 bars/dots or less, 
you're fine.  If not, you want to back off on the gain (and thus the power) 
until you have the ALC down in the desired range.  You can test all this before 
you send any actual text. 

W1HKJ (the creator of FLDIGI) has some good screen shots of what signals should 
look like -- and should not look like -- at 
http://www.w1hkj.com/FldigiHelp-3.20/Modes/index.htm.  Look carefully at BPSK - 
Overdriven.  

With PSK, someone receiving your signal should see an imd of -20 or lower. 
 Remember, in minus numbers, lower is a bigger number.  -25 is lower than -20. 
 imd readings appear on the bottom line of the FLDIGI screen and are designated 
"imd".

Once you have it set nicely without sending text, try sending some text to see 
if that changes the situation.  Normally it won't, but it's worth testing.

If you don't have ALC metering on your rig, you're going to have to go with the 
"have someone else look at my signal approach".  Go into transmit mode without 
text and adjust the gain while someone else watches your signal.  Again, once 
you have it right, try it with text also.

There are separate meters you can buy or make (e.g., an imd meter).


Adjusting Gain
--------------

So how do I adjust gain?  There are two different gain adjustments -- the audio 
output level on your computer, and the input gain on your rig.  If you're 
feeding in through the mic, it's the mic gain.  If you have a separate data 
jack, the data jack usually has its own separate gain control and you may need 
to access a menu to set that gain.  Read The Manual.  Remember, this is not the 
AF or RF gain, those are for receive.

On the SignaLink, and some of the RIGblasters, there is there is a third gain 
adjustment on the interface itself.

So which one do you use?  All of them.  The best approach is to play with all of 
them so that they're each at some midway-ish point.  As with most things, having 
one of them at an extreme (all the way high or all the way low) gets you into a 
non-linear response area which will cause some distortion (it may not be enough 
to notice -- but then again it may be enough to notice).  You just have to try 
adjusting things until you get a decent result.  Don't panic if you can't get 
them all midway-ish, but try to keep any of them from all the way high or all 
the way low.

When you access the audio level controls on your computer, make sure you're 
adjusting for the soundcard that you are actually using.  There are provisions 
in the soundcard volume setting/mixer to select which soundcard you are 
adjusting.

By the way, just to give you even more choices, some operating systems will let 
you adjust the master gain separately from the gain for an individual program 
such as FLDIGI.


Transmitting
------------

The FLDIGI "Beginner's Guide" and "Online Documentation" do excellent jobs of 
describing how to use the buttons that send FLDIGI (and your rig) into transmit 
and into receive, and I'm not going to repeat that here.  Remember the 
following:

1.  The T/R button simply toggles from whatever you're in (transmit or receive) 
to whatever you're not in (receive or transmit).
2.  The Tx and Rx buttons are specific for starting transmit or starting receive 
(respectively).  
3.  Hitting escape a few times will immediately force you into receive even if 
you haven't finished transmitting what's in the transmit buffer.

Review the FLDIGI "Beginner's Guide" and "Online Documentation" for explanations 
of how to use the transmit buffer.



Where Do I Position On The Waterfall?
-------------------------------------

Assuming that you've got FLDIGI set for upper sideband (upper left just above 
the receive window), FLDIGI will show you a waterfall that starts at the actual 
dialed-in frequency.  In other words, the 0 position on the waterfall is your 
dialed-in frequency.  As you move up the waterfall, you are moving above that 
frequency (assuming you have selected upper sideband).  FLDIGI is normally 
showing you how many Hz above your dialed-in frequency you're going to transmit. 
 For example, if you have your rig set at 14.070.15, and the cursor on FLDIGI is 
centered around 1100 on the waterfall scale, this means that you will be 
transmitting at 14,070,150 Hz + 1000 Hz, or 14,071.150 Hz (14.071.15).

Most rigs have a sweet spot of around 1000 Hz up where you get the most power 
out for the same power in.  If possible, that's where you want your signal.  As 
you move further out on the waterfall, you will actually need to crank up the 
gain a bit in order to maintain the same power.

*****

See you tonight.



Jon
KB1QBZ