[RTTY] PK900

[Ford Peterson]

Kok Chen wrote:

> I had a PK900 a long time ago but gave it to the U of Texas Ham club.
> 
> Isn't there a soft parameter you can set which squelches print?  I 
> forgot what AEA called it now.  It is to prevent the screenfulls of 
> line noise on the display when a signal is not there, but pretty 
> useless for DX.  I am sure you can disable that particular feature.
> 
> 73
> Chen, W7AY

There is a parameter like the one you describe.  It relates to packet transmissions.  

I think I figured out the problem, but haven't come up with a solution yet.  The DSP board is inserted right at the audio-in connector.  A capacitor gets clipped and the DSP board is placed in series with the audio signal path.  Looking at the input and output of the DSP board on the scope, I can see clearly why the print goes bad at 4uV signal levels.

The DSP unit does very poorly on weak signals.  The dynamic range is just not there.  At 3uV, the audio level is clearly visible on the O'Scope, but the output of the DSP is more like noise.  In fact, the output of the DSP unit with no signal applied is filled with about 50mV of hash-like noise.

The audio levels at the back of the radio are more than adequate.  The 4.7K output impedance of the IC746 is not a problem for the very high input impedance on the PK900.  And with a reasonable signal level on the radio, the output at that port on the receiver is about 225mV RMS.  The PK900 manual suggests 200mV.  When the signal level drops, so does the amplitude of that audio.  When it gets to about 100mV, I lose print entirely.

I'm going to try and amplify the radio output to a much higher level.  A level where I can barely "see" atmospheric noise coming through the DSP.  That should give me more than adequate signal levels for weak signal work.  If all I wanted to copy was strong signals, I'd just use an old Hamcom interface.

Thanks for the feedback.  If anybody has had experience with this, please let me know.

Ford-N0FP
ford at cmgate.com