PROPERLY SETTING THE TRANSMIT DEVIATION FOR PACKET The importance of properly setting your transmit audio level, which directly affects your transmit deviation, cannot be understated. The graph at the end of this article shows the relationship between the signal level required for 99% copy and the transmit deviation. On the right side, this relationship is shown in relative terms: performance in decibels, and power required at the transmit end. It is easily seen that a transmit audio level even slightly higher than ideal affects the path quality tremendously. Note that lower deviations have a less severe effect. Two factors cause the degradation in performance at deviations above 3.5 kHz: First, some of the transmitted signal's energy falls outside of the receiver's passband, resulting in a lower overall signal and waveform distortion. Second, the squelch circuit response time becomes slower and is more likely to "false", or operate improperly. With an excessive deviation, the squelch circuit will close for a few milliseconds in the middle of receiving data, causing a total loss of that packet. While both of these factors occur in voice communications, small amounts of distortion and signal loss are hardly noticeable. Nearly all TNCs have the transmit audio set too high when new. While the deviation limiter circuit in your radio will take care of most of the problem, this circuit usually permits about 5 kHz of deviation. Some radios do not have a limiter circuit, or it doesn't work very well, or is mis- adjusted. Signals with 7 kHz deviation have been monitored. It is recommended that the transmit deviation be set to 3.0 kHz using a deviation meter or service monitor. If you don't have this test equipment, first try sending out a packet message in your local area, or asking your local network sysop - chances are someone will be happy to help. If not, the following procedure will get you close. The transmit deviation adjustment is made by varying the audio output level of the TNC. Most TNCs have an adjustment potentiometer accessible without opening the case, but you may have to go inside. To find out for sure, read the manual. It is not recommended that you adjust the deviation limiter on the radio, since distortion will result. The deviation limiters on older rigs also don't work very well, either. 1. Obtain another receiver to listen to your transmitted signal. 2. Locate the audio output adjustment potentiometer on your TNC. (Consult the manual). 3. Put your TNC into "CALibrate" mode. (Check your manual on how to do this. With most TNCs you can type CAL at the "cmd:" prompt and then press the letter K to key the radio (transmit). Pressing D makes it "dither" (oscillate) between transmit tones rapidly, which is a better way of setting deviation than with a single tone. If necessary, use only the lower tone to set deviation. Pressing Q shuts off the transmitter and returns you to the "cmd:" prompt) 4. While listening on the receiver, adjust the TNC's audio output upwards until the audio on the receiver does not get any louder. Find this point carefully by repeating the adjustment a few times. This is where the radio's limiter begins limiting the deviation, usually 5 to 7 kHz. 5. Now adjust the TNC's audio output level downwards until the audio on the receiver is about one-half as loud as it was. (Note: most TNC manuals say to leave the audio level at maximum - but this is often 5 kHz deviation, much too high). The audio on the receiver should sound soft but not faint. At this point the transmit deviation should be between 2 and 4 kHz, which is much better than 5 to 7 kHz. Some combinations of TNC and radio are very difficult to adjust, because the adjustment is very sensitive, giving you either too much audio or not enough audio. The fix for this is to add a resistor in series with the transmit audio line to the radio. Something in the range of 10k or 20k Ohms should work fine. Received Signal Strength for 99% Copy vs Transmit Deviation ________________________________________ 0.8-| |- -12dB 160 | . . | | . . | 0.7-| . . | | . . | | . Rcvr w/tight IF -->. | 0.6-| . . .|- -9 dB 80 | . . . | | . . . | 0.5-| . . . | | . . . | | . . . | 0.4-| . . . |- -6 dB 40 | . . . | | . . . | 0.3-| . . . |- -3 dB 20 | . . . <--- | | . . .' Typical | 0.2-| ' .........'. ' Rcvr. |- 0 dB 10 | | | | 0.1 |_______________________________________ | ^ ^ 0 1 2 3 4 5 6 7 | | Transmit Deviation (kHz) | | | | Relative Performance in Decibels ---- | Power Needed to Work a 10 Watt Path ---- (Relative to 3 kHz Deviation) The text of this article was originally written by Bill Slack, NX2P.