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[aprssig] Deviation meters

Ray Wells vk2tv at exemail.com.au
Thu Apr 5 07:16:28 UTC 2007


'Scott Miller' wrote:

>>Audio drive to the transmitter play an equally, and maybe more, 
>>important role.
>>    
>>
>
>And this is really what I'm hoping to come up with a tool to set - i.e., set
>your tracker/TNC to calibrate mode (alternating tones) and set the audio
>drive level to get appropriate deviation.  Assuming the transmitter's not so
>out of whack that at 3.2 kHz deviation it's clipping the high tone already.
>
>  
>
I wish you success. Such a tool will be greeted with much enthusiasm.

>>More important is having the transmitted amplitude of the two 
>>tones in 
>>    
>>
>
>You're preaching to the choir.  =] 
>
But it's amazing how many are not in the choir!

> Of course, as long as we've got D700's
>and such out there, there's no getting around the fact that some stations
>will transmit with no pre-emphasis.  I made a point of adding a pre-emphasis
>circuit to my T2-135 board, since the DR-135T doesn't provide pre-emphasis
>on the TNC input (not the one from the internal header, anyway).  I may add
>a jumper on future versions to bypass it if needed, though.
>
>  
>
It certainly complicates matters when mixed standards are employed on 
the same network.

>>In the absense of a service monitor or oscilloscope, and if a simple, 
>>peak reading device is used, adjust the audio drive until the desired 
>>level is achieved, then back it off just a tad.
>>    
>>
>
>This is what I've encouraged users to do.  Still, it'd be nice to have a
>small board that'd give you at least a basic indication that you've got it
>set right, even if it wasn't very sensitive and required the radio to be
>tuned to a known frequency.  I've got an Agilent service monitor, myself,
>and it's wonderful for checking my relative tone levels and deviation, but
>even used and several years old it was more than $3,000.  I'd like to be
>able to point people to an affordable option that's somewhere between the
>extremes of 'service monitor' and 'set it by ear'.
>
>  
>
Like yourself, I have a service monitor, a Motorola R2400, but we're 
amongst a lucky minority.

I think that an oscilloscope is the most useful tool one can have for 
making audio adjustments, and with software scopes available, such a 
tool is within the reach of most. If one can monitor a "known" 
transmitter (or transmitters), it's fairly elementary to adjust one's 
own. Such a method does not return a precise deviation result but it's a 
whole lot better than guesswork.

Ray vk2tv

>Scott
>N1VG
>
>
>
>
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>  
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