Tuning N9600A Deviation

More info on the N9600A2 1200/9600 TNC here: www.tarpn.net/d

This video demonstrates how to set the transmit deviation for an N9600A TNC using the built-in 999Hz tuning tone and a cheap USB SDR.

In the test setup, an N9600A2 TNC is connected to a TAIT TM8105 radio set to transmit on 145.510 MHz. Software is SDR Console v3.0.14 on a Windows 10 PC. Receive bandwidth on the SDR software is set to 250kHz, AGC on, WFM.

The TX DEV potentiometer on the TNC is at minimum at first, then slowly increased while holding the TEST TX button. Proper deviation is reached when the center spike in the spectrum display is dipped to minimum. This takes advantage of the first "Bessel Null", which happens when FM deviation is 2.405 times the frequency of the transmitted tone (999Hz in this case). 2.4kHz provides minimum-shift deviation for 9600 baud transmitted data.

After this video was taken, the setup was tested against an IFR 1200 service monitor to verify transmit deviation. The monitor showed 2.42kHz, which is perfect for 9600 or 1200 baud data.


It's also useful to measure the peak-to-peak voltage of the transmit audio signal fed to the radio after proper deviation is set, using an oscilloscope. In this case it was 690mV. This will serve as a "good enough" deviation setting in the future for this particular radio, and likely for similar radios of the same model.