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[aprssig] ICOM 706 MKii G Packet experience.

Stephen H. Smith wa8lmf2 at aol.com
Sun Nov 25 04:10:50 UTC 2007


Andrew Rich wrote:
> Gudday
>  
> I thought I would share this experience - others may be able to help.
>  
> I bought a KPC3 for 1200 baud to use outboard with the ICOM 706 Mk iiG
>  
> Seemed to work well enough - using the "data" port on the ICOM 706. I 
> seemed to be copying most packets on ISS and PCSAT.
>  
> I then migrated to AGWPE sound card and I noticed that it was not 
> copying packets 100%.
>  
> I had a look on the Sound Card tuning aid and I saw something odd.
>  
> It looked like 1200/2200 but was sort of riding on a low frequency 
> sine wave, in that the packet tones where not symetrical about zero volts.
>  
> OK, time to drag out the CRO. Sure enough saw the same thing on the CRO.
>  
> Time to try something different - the "phones" connection on the front 
> panel - perfect nicely balanced packet.
>  
> Fiddled with the 9600 / 1200 baud lines and settings in the ICOM 706. 
> Not much joy.
>  
> Before I dig deeper (service manual), anybody seen similar ?
>  
> I


1)   What kind of computer and sound system were you using with AGW? 

2)  What kind of interface/isolation were you using between the radio 
and the computer.


Some general observations and possibilities in no particular order........


o     Motherboard-based sound systems are notorious for picking up noise 
and hash from the computer's timing chains and clocks and DC-DC power 
converters since they typically share the power supply and ground/common 
busses with all the digital stuff on the motherboard.   The result is a 
lot of low-level trash superimposed on what ever you feed into them.  
You don't hear this junk on the cheap speakers lacking bass response 
typically used with computers, but critical DSP applications such as ham 
sound card programs will be affected.

o     Most motherboard-based sound systems are brain-dead "gutless 
wonders" that substitute massive software drivers for dedicated 
hardware, and dump the sound processing tasks on the host CPU instead of 
using dedicated hardware.  In turn, the sound processing competes for 
CPU "air time" and interrupts with all the other software running on the 
CPU at the same time. The sampling rate and timing of the audio A-to-D 
conversion actually varies depending on the number of other tasks 
running at the same time.   The timing errors and varying sample rates 
of many motherboard-based systems is enough to severely degrade the 
decode performance of AGW.

Traditional add-on sound cards and external USB-connected sound systems  
use dedicated hardware with accurate crystal-controlled timebases and 
normally are much more accurate and consistent, and tend to be much 
quieter.

o     If you don't totally isolate the computer sound system from the 
radio with audio transformers, you have a common metallic chassis 
ground/earth between the two devices. In turn, this allows 50/60 Hz AC 
leakage currents from the power supplies of either the computer, the 
radio or both, and hash/noise from the computer's switching power 
supply,  to circulate on the common shield wires of the audio connections.

In turn, these circulating currents (a.k.a. "ground loops") can cause 
several 10s or 100s of millivolts of noise and hum to be superimposed on 
the TX and RX audio.    Note that completely isolating the PC and radio 
requires both transformer-coupling the TX and RX audio lines (typically 
with 600/600 ohm telephone-type audio transformers salvaged from old 
modems or answering machines)  -AND-  isolating the PTT line for keying 
the transmitter with either opto-isolators or relays. 

o     The TNC has a very definite lower limit to it's audio frequency 
response determined by the audio coupling capacitors on it's input.  
(The TNC designer knows that nothing lower than several hundred Hz needs 
to pass and sizes the caps accordingly.)  The computer sound system has 
an audio frequency response that is essentially flat clear to 5 or 10 Hz 
(or even to DC). Any low frequency hum or hash picked up by cable ground 
loops will pass much more readily into the computer sound input, with 
less loss, than into the hardware TNC. 

o     Another possible source for the low-frequency hum or hash is the 
refresh clocks for LCD displays in the radio which are often scanned at 
only a few tens or hundreds of Hz.   The speaker or headphones output 
typically has far less low-frequency response than the dedicated 
data/packet jack which is often DC-coupled to preserve the DC levels of 
9600-baud packet.   Small value coupling capacitors in series with the 
audio isolation transformers can reduce this kind of low-frequency " 
garbage.

o     Finally, if you are using the MIC input of the computer sound 
system rather than a LINE level one, you may need to attenuate the 
receive audio fed into it by something like 50:1 to 100:1 to prevent the 
mic amp from overloading. A simple resistive voltage divider consisting 
of something like a 100K resistor and a 1K resistor will usually do it.  




--

Stephen H. Smith    wa8lmf (at) aol.com
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