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[aprssig] paccom schematic

Ray Wells vk2tv at exemail.com.au
Sat Sep 1 21:37:40 UTC 2007


Nick Pugh wrote:

> Yes the tones are clean and not distorted. The high tone is about 6 db 
> lower in amplitude the low frequency tone.
>
> nick
> ----- Original Message ----- From: "Ray Wells" <vk2tv at exemail.com.au>
> To: "TAPR APRS Mailing List" <aprssig at lists.tapr.org>
> Sent: Friday, August 31, 2007 4:15 PM
> Subject: Re: [aprssig] paccom schematic
>
>
>> Nick Pugh wrote:
>>
>>> I have a Pacomm Picopacket It will tx ox but is decoding 1 out of 4 
>>> rx packets The input level is set to about 300 mv pp
>>> does the group have any suggestions??
>>> nick
>>>
>>> ------------------------------------------------------------------------ 
>>>
>>>
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>> Have you looked at the audio to the TNC with a CRO to see what the 
>> tones look like? That's the first thing I would do.
>>
>> Ray vk2tv
>>
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Hi Nick,

You've just discovered your problem. The tones must be the same 
amplitude going to the TNC, or very close to it.

Some modem chips are particularly non-cooperative when the high tone is 
lower in amplitude than the low tone.

I wonder where you're getting the audio from in the radio?

The usual sequence is that tones of equal amplitude are fed into the 
transmitter. Pre-emphasis cause the high tone to be approximately 6dB 
higher than the low tone when transmitted. At the other end, de-emphasis 
in the rx restores the tones to equal amplitude to go to the TNC.

Causes?

1. No pre-emphasis at the tx
2. Your rx is applying additional "slope" at the point you're grabbing 
the audio.

My first attempt at packet was using a Multi 750 2m rig and for 
convenience I grabbed speaker audio and had exactly your problem. The 
high tone at the speaker was about 8dB down on the low tone.

You obviously have a CRO so start looking around the rx for a point that 
provides flat audio.

Ray vk2tv




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