Packet Primer - TNC Audio Levels

[Budd Churchward]

While browsing around looking for more information about Packet Radio, I came across this web page where John Ackerman, N8UR, gives us three methods for setting the audio levels on our packet stations. He begins, however, with a great description of what is going on with the TNC and the Radio and why we have to set these levels in the first place. I think you will enjoy this:

http://www.febo.com/packet/layer-one/transmit.html

Budd

[Bill Vodall]

> While browsing around looking for more information about Packet Radio, I
> came across this web page where John Ackerman, N8UR, gives us three methods
> for setting the audio levels on our packet stations. He begins, however,
> with a great description of what is going on with the TNC and the Radio and

Those are some great pages to have available.

Don't forget a somewhat updated technique using the RTL dongle:

https://xastir.org/index.php/HowTo:Set_Deviation_via_RTL

Now if only there was some way as easy to do it for 9600 baud...

73
Bill

[Keith Williamson]

It's the best web page I've found on this topic also. Taking a cue from it, I put a 22K ohm resistor in series between the UDRC AF-IN (really the L SPKR out) and the 1200 DATA IN on my Alinco DR135. I tried 10K and 56K also before settling on 22K. With the resistor in place, the LO Drive control in alsamixer now allows me to adjust the deviation. Before, even with the Lo drive turned all the way down, I was over deviating. 

To measure and adjust the deviation, I am using an RTL-SDR dongle on my Linux laptop and running the Gnu Radio Companion to determine the "Bessel 0" point with a fixed 1200Hz tone. With the LO drive set to where I get the Bessel 0 (carrier null), I know that the modulation index is 2.41 which means the 1200Hz deviation is 2.88KHz (1200 * 2.41). Measuring the 1200Hz peak-to-peak voltage at that drive level and then switching to the 2200Hz tone, I measured the 2200Hz peak-to-peak voltage. Taking the ratio of the 2200Hz voltage to the 1200Hz voltage and multiplying by the 1200Hz deviation of 2.88KHz yielded a 2200Hz peak deviation of 4.05KHz. Going back and referencing the above article by N8UR, I backed off on the Lo drive by one notch which brings my deviation in line with his recommendations. 

It's not quite as accurate as a calibrated service monitor, but it's the best low-cost method I've come across. 

Cheers,

Keith

KF7DRV

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[Robert Sears]

I am attempting to have an RTL setup ready to use on Tuesday to demonstrate how to set the transmit levels on the UDRC so you get the proper deviation using the carrier null method.

Robert/KF7VOP

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[Keith Williamson]

Cool. What software are you using to detect the carrier null? 

I started playing with the gnuradio companion after watching HB9UF's you tube video: https://www.youtube.com/watch?v=iAQgNHbXv44

Also, KP4MD's video (Windoze approach): https://www.youtube.com/watch?v=lWcFqsh7ZvQ

Now I'm totally hooked on learning gnu radio companion! Amazing open source software. 

-Keith

On Mon, Nov 14, 2016 at 3:57 PM, Robert Sears <rts...@comcast.net> wrote:

I am attempting to have an RTL setup ready to use on Tuesday to demonstrate how to set the transmit levels on the UDRC so you get the proper deviation using the carrier null method.

Robert/KF7VOP

On 11/14/2016 2:18 PM, Budd Churchward wrote:

While browsing around looking for more information about Packet Radio, I came across this web page where John Ackerman, N8UR, gives us three methods for setting the audio levels on our packet stations. He begins, however, with a great description of what is going on with the TNC and the Radio and why we have to set these levels in the first place. I think you will enjoy this:

http://www.febo.com/packet/layer-one/transmit.html

Budd

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[Robert Sears]

I probably will just use SDR# (Windows) as my laptop is nowhere near powerful enough to run GNU radio. I tried it a long time ago with moderate success. Since we don't need very much bandwidth it might run fine for this application, but I don't have time to relearn it by Tuesday.

I am also working on a little portable setup using a Raspberry PI, 3.5 inch TFT and Adafruit's "Freq Show" Pi RTL-SDR program.

I have had Freq Show running before and it's easy to use so will be able to loan that out.

Robert/KF7VOP

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