[CTSARA] SARA Digital Bootcamp Sunday 9/19

[Jon Perelstein]

We'll be holding the third session of the digital bootcamp this Sunday night at 
9pm.  This session will focus on getting your first transmission.  For this 
first transmit session, we'll try PSK on FM in the VHF band.  Unless otherwise 
specified, we will meet on the New Canaan repeater, but do our transmitting at 
145.110 MHz.  IMPORTANT:  Yaesu rigs think that 145.110 is a repeater frequency 
(it's not) and will automatically apply -600 KHz offset (shift) to transmissions 
at that frequency.  You must turn off the offset (shift)!!!

Please feel free to contact me if you have an questions from our previous 
sessions.

Jon
KB1QBZ


*****



So How Do I Hook Up My Interface for Transmit?
-------------------
Hooking up the interface depends on your rig, on your interface, and on your 
computer.  For purposes of this material, I'm going to assume that you have a 
USB interface, and not an older serial port interface.  

First, let's remember what's happening here.  The characters that you type into 
your keyboard are going to be converted by FLDIGI into a set of computer codes 
that cause the computer's soundcard to generate specific, discrete tones.  Those 
tones are going to flow from the soundcard through the interface into the 
microphone input of the rig, where they will be used to modulate your carrier 
(sideband or FM).  Similarly, tones coming out of the rig will flow through the 
interface to the soundcard where they will be converted into computer codes that 
FLDIGI can then interpret as characters.

The Donner interface and the RIGblaster interfaces come with driver software 
that you have to load on your computer to make the interface think that it is 
talking to a serial port on your computer.  The instructions that come with both 
the Donner and the RIGblasters are pretty clear about how to install the driver 
software.  The SignaLink does not imitate a COM port and there's no driver 
software to be set up.  

On the other side, there's the issue of how you connect to your rig.  There are 
two basic methods, depending on the rig

1.  Connect into the microphone jack of the rig.  You will be removing your 
rig's microphone and connecting the output of the interface into the microphone 
input jack.

2.  Connect into the DATA jack.  If the rig has a DATA jack (may be called AUX 
or PACKET), you can connect into that instead.  Most newer rigs have a data jack 
for digital/packet transmissions.  By connecting into the data jack, you do not 
have to remove your microphone.  Note:  This is not the same thing as a CAT 
jack, which is used for controlling the rig through your computer (e.g., 
frequency, power setting).

The data jack is, essentially, an alternate microphone input (and alternate 
audio output).  It receives audio from your soundcard interface and feeds it 
into the rig (the microphone part) and also takes audio from the rig and feeds 
it back to the soundcard interface (the audio output part).

Rigs with data jacks have specific digital modes.  For example, on the FT-897, 
there is a specific mode for digital (meaning upper sideband SSB transmission) 
and a specific mode for packet (meaning FM transmission).  On the Icom 756, you 
set the mode to upper sideband or FM, and then press an additional button to 
select digital.  Rigs without data jacks do not specific digital modes.  For 
example, the K2 does not have a specific digital mode; you just put the rig into 
upper sideband and you're all set.    

If you have a data jack and are using it, there is an important difference 
between the regular sideband or FM mode and the digital or packet mode.  In the 
regular mode, both the digital jack AND the microphone are hot; the mic will 
pick up and transmit noises in the shack, which can be very embarrassing if 
you're saying nasty things about the person on the other end.  In digital or 
packet mode, the mic is not hot and will not pick up noise in the shack.  Note 
that the PTT button on most mics does not cut the mic from the rig -- all it 
does is trigger the PTT relay in the rig.

Once you have everything hooked up, you have to tell FLDIGI where to get its 
input and output signals.  This is done on the Configuration >> Soundcard menu. 
 Unlike most digital software, you do not select a COM port.  Instead, you 
select the soundcard you are using.  When you go to the Configuration >> 
Soundcard screen, you will see drop-downs for selecting your receive soundcard 
and your transmit soundcard.  If you're using a RIGblaster or Donner, you're 
going to wind up selecting the internal soundcard in the computer (you do have 
to know what that soundcard is called in your computer).  With a SignaLink, the 
soundcard is going to be called something like USB Codec.  The FLDIGI help has 
material showing examples of how to select the settings.

For now, that's about all the setup you need to do.  The default configuration 
of FLDIGI will work well with PSK.

Now comes the fun of setting the power and the gain (volume).

Power Settings - FM
-------------------
FM, as you know, is a continuous duty cycle mode, which means that the 
transmitter is always transmitting at the power that is set.  That's the reason 
why rigs that are 100 watts on SSB are only 50 watts or 40 watts on FM -- the 
manufacturer reduces the power to avoid overheating the rig on transmit. 
 However, digital transmissions over FM usually last longer than voice 
transmissions.  Whereas the typical voice transmission is no more than 1 or 2 
minutes at a time, the typical digital transmission might be 3-5 minutes, and 
sending a long data list can take 20 or 30 minutes.  Because of that, it is 
common practice to further reduce the power on your rig to only 50% of the 
manufacturer's rated FM power.  Most people reduce to 10 watts or so because 
usually that's all that's needed for digital, even if you're trying to transmit 
FM from Stamford to Bridgeport (assuming you have a decent antenna).

Setting power levels for SSB is somewhat more complicated and much more critical 
so that you don't overheat your rig.  That will be explained in the next class.

Gain Settings - FM
------------------
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 preventing 
ALC from kicking in.  Unfortunately, FM always uses ALC because FM wants 
constant amplitude on its signal (otherwise it would be AM and not FM).  The 
next best thing is to watch the modulation.  If you have a modulation meter 
built into your rig, start a transmission through FLDIGI but don't actually 
transmit any text.  Adjust the gain control (see below) until you get as much 
deflection as you're going to get on the rig's modulation meter, then back off 
the gain to about half that.  That should be a good setting.  Have someone who 
is somewhat close to you (a mile or so but no closer) monitor your signal to 
make sure that it is a good setting.  In this case, that means the listener 
should watch your signal on the waterfall to make sure that you are not creating 
harmonics or sidebands.  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 a modulation meter 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.

If you're using a SignaLink or 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.

To access the audio output level on your computer:  If you're running Windows, 
you probably have a little speaker icon on the taskbar (lower right of your 
screen).  Clicking on that gives you a control to adjust volume.  The question 
is which volume are you adjusting.  There should be a "Mixer" button or 
"Advanced" button or "Properties" button on it, and by clicking that you get to 
the point where you can select which soundcard you're controlling.  Select the 
soundcard that is feeding into the interface and adjust volume on that. 
 Alternatively, Control Panel >> Sounds will get you to the screen where you can 
select your device and set audio output.

Sorry, but I don't know how to set audio output on Macs or Linux machines.

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?
------------
We'll get into a discussion of that in another lesson, but for now position your 
bandwidth cursor (the yellow lines and white line) so that it's centered at 1000 
(the center white line will be at 1000 on the waterfall).  That means that it 
will be 1000 Hz (1 KHz) up from the center of the frequency you have tuned on 
the rig.  For example, if your rig is tuned to 145.110 MHz (145110 KHz) and you 
have your bandwidth cursor centered at 1000 Hz in the waterfall, your PSK tones 
will be at 145.111 MHz (145111 KHz), which is perfectly acceptable given that we 
use +/- 15 KHz steps in tuning FM around here (some areas of the country use +/- 
20 KHz steps). 

*****

See you Sunday night.