Re: [TenTec] RE: PSK Help

[Jerry Volpe <kg6tt@tomorrowsweb.com>]

From my experience running PSK there are a few issues to consider:

Audio to mic or audio in on rig (you can use either) must be clean, 
undistorted, without hum, or rf feedback artifacts.
- Try to keep your sound card's audio out slider away from the extremes 
(min or max) in order to have the least distortion and noise from the 
computer itself. If you can't get it somewhere in the middle 70% or so 
of its range and still have controllable output with the Omni's PWR and 
MIC controls than you should add or remove some attenuation within the 
rig to computer interface. Also it is best to find the balance where the 
Omni's MIC control can also be set somewhere away from the min or max 
settings.
- Be sure you have audio isolation between the sound card audio in and 
out and the Omni audio in and out (PTT too). Some may say they do ok 
without signal isolation but they are in the minority. If you don't have 
one they can be bought inexpensively or built easily and cheaply. Your 
interface should present approximately 1000 ohms (hopefully more) 
impedance to the audio in on the Omni. Some isolation transformers are 
1:1 which gives you 600 ohms or more. However some isolation 
transformers are step-up/step-down and can present 8 ohms or less if 
inserted incorrectly.
- Use good quality shielded wire between sound card and rig.
- Good practice to include ferrite beads on interface cables. Sound card 
audio out to rig would be the most sensitive as you are transmitting.

Two ways to check your signal quality:
1: Best approach is to use a monitor scope. I know few hams actually 
have them these days but it is still the best approach.
2. Checking the IMD value of your transmitted audio on another receiver 
running Digipan. It only needs to be a reasonably stable receiver with 
audio out to a second computers audio in connection.

What to watch out for:
1. Make sure you are not running in and ALC indicated range. Keep the 
mic gain down just below the point where the ALC LED begins to light.
2. Do NOT use the Omni's PROC.
3. Be sure to complete the sound card calibration procedure within 
Digipan. This is usually a pretty simple process and only has to be done 
once. Your actual sound card may be operating on an internal clock that 
is somewhat above or below what Digipan expects as default. This can 
lead to your Transmit and Receive tones being somwhat removed from one 
another or forcing Digipan to 'hunt' more for the incoming signal each 
time it goes into transmit.

Watch how Digipan is locking onto stations. Does it take a few seconds 
or more to begin to properly decode the incoming signal? If so this can 
indicate that Digipan is hunting in between the time that the other 
station ends a transmission and you begin yours. If it is hunting on 
background noise your transmit tones may be significantly removed from 
the frequency the other station is expecting to hear them. etc.... There 
are a few things that you can do to minimize this....
- Insure that your audio in as shown on your waterfall or spectrum 
display is adjusted to the point where background noise is barely 
indicating. If the background noise level is too high it will cause 
Digipan to hunt randomly when the other station ends a transmisison (It 
also makes it much more difficult for Digipan to properly decode).
- Use Digipan's Frequency Lock function once you have sync'd on an 
incoming signal. The Frequency Lock on most PSK programs works on your 
transmit tones. So even if Digipan begins to hunt you will transmit 
exactly where the other station expects.

I can't over emphasized the issue of getting clean audio from the sound 
card to the rig and transmitting that audio without ALC or PROC. Some 
will argue the following point, but nearly 30 years of Ten-Tec usage has 
clearly indicated to me that Ten-Tec's are very sensitive to their 
proximity to strong RF fields. Even 30 watts can be strong if the 
antenna is close or RF is coming back into the shack via the outer 
shield of the transmission line... etc.  I should own stock in a ferrite 
bead factory for all the cores I have drapped on my Ten-Tec's and the 
equipment that is interfaced to them. You don't need a lot of RF 
generated audio distortion to make a PSK output signal unreadable... and 
probably hardly noticeable in SSB or CW.

Hope some of this is useful.

Jerry, KG6TT
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