What is your TNC doing? - Ground|Characteristics|Trouble

Chapter 5 continued - TNC to Radio Wiring and Adjustments - Chapter Table of Contents

Ground

The Ground wire between the TNC and radio completes the PTT circuit and serves as a reference for the transmit and receive signals. The wire from the TNC Ground pin should be connected to a pin on the radio called Ground (GND), Earth ground (E), PTT ground, or Shield. The pre-wired speaker cable that comes with many TNCs usually has ground wired and is sufficient for most radios. Some radios have a pin named Mic Ground. It is normally unnecessary to connect this ground. Connecting the Mic Ground pin in some radios has been known to cause hum on the transmitted signal.

Radio Characteristics

Switching Time

The time it takes a radio to switch from transmit to receive and from receive to transmit is important when considering packet efficiency. During switching time nothing can be received or transmitted, but collisions can be caused. The TNC starts its transmission routines as soon as it asserts the radio PTT. However, it takes time for the radio to reach full power output. During this time, other stations cannot detect that the radio is transmitting, so they may also begin their transmission routines and collisions occur. The time it takes to switch from transmit to receive is also deadtime because nothing can be received. When the remote TNC needs to respond, it must wait enough time for the other station to return to receive. This slows down communications and decreases efficiency.

The TXDELAY parameter allows you to adjust for differences in switching time. Stations with fast radios will be able to set this parameter short and increase efficiency. Some TNCs default TXDELAY to 30, which is 300 milliseconds. That doesn't sound like a very long time. If we consider 1200 baud packet, it takes 300 milliseconds to send 45 characters. (1200 bits per second / 8 bits per character = 150 characters per second. 150 * 0.3 seconds = 45 characters per 300 ms). However, 9600 baud is 8 times faster. The TNC could have transmitted 360 characters during 300 milliseconds. As the baud rate increases, switching time becomes more important.

Bandwidth

Bandwidth is the group of frequencies needed to transmit and receive a given modulated signal. Although you set the frequency read-out of a radio to one frequency, the radio actually transmits and receives using several frequencies surrounding the one you set. The frequencies that can be used are determined by the filters in the radio. If the filters are too narrow, the entire signal will not be transmitted (or received), thus making it difficult to correctly interpret the message. If the filters are too wide, there will be other signals present that add noise to the intended message and make it difficult to understand. When transmitting digital data, the signal is changing between two states. The bandwidth of the radio must be wide enough to allow both states to be transmitted and received.

Audio Modulation is restricted by the range of tones that can pass through the audio amplifier of the radio. The highest audio tone that can be sent through most voice radios is 3000 Hz and the lowest tone is about 500 Hz. Standard 1200 baud packet uses a 1200 Hz tone and a 2200 Hz tone.

DFSK Modulation is limited by the bandwidth of the radio's IF filters. The TNC modem is connected to the radio at the discriminator and varicap, bypassing the audio circuits. The standard G3RUH 9600 baud modem shifts the frequency 3000 Hz above the carrier and 3000 Hz below the carrier. James Miller, G3RUH, recommends using an IF bandwidth of 15 kHz to achieve best performance. The modem can be modified for different speeds. The deviation and bandwidth for specific baud rates are shown in the following chart.

Baud Rate 4800 9600 19200 38400 64000

(using FM)
Deviation +/- (in kHz) 1.5 3 6 12 20

IF Bandwidth (in kHz) 8 15 30 60 100

Baud Rate	4800	9600	19200	38400	64000
(using FM)
Deviation +/- (in kHz)	1.5	3	6	12	20
IF Bandwidth (in kHz)	8	15	30	60	100

You can roughly estimate the bandwidth needed for a specific speed by multiplying the baud rate by two (answer is in Hertz).

Troubleshooting

TNC RCV (DCD) LED always lit

1) Check the radio squelch setting. If the squelch is set too loose, the TNC will hear a constant noise and detect carrier. If possible, you may want to change to a type of carrier detect that will detect real data instead of just noise. 2) Check to see if the radio pin is providing squelched or unsquelched audio. Some TNCs cannot decode unsquelched signals. 3) On some TNCs, check the threshold knob setting. If it is not turned far enough counterclockwise, the DCD LED will stay lit continuously.

Radio constantly keyed

When connecting a cable between the TNC and radio, if the radio goes into transmit and stays in transmit, double check the cable for correct wiring. If the correct pins are connected, you may need to add a 0.1 番 capacitor or an isolation transformer in the Transmit Signal line.

Hum on Transmitted Signal

It may be reported to you that you have hum on your signal, or you may listen to your signal on another radio and hear hum on your signal but not on other signals. Hum is often caused when mic ground is connected between the TNC and radio. Since mic ground is often not common with PTT ground, a ground loop is created by connecting both grounds. A ground loop, caused by a difference in voltage or current between PTT ground and mic ground, often causes hum on the transmitted signal. The ground used to complete the PTT circuit is the only ground necessary between the TNC and radio.

Ground Loops

When there seems to be no logical reason for a problem, check your entire system for ground loops. A ground loop occurs when there is a difference in voltage or current on the ground of one unit compared to the ground of another unit. This can often be very difficult to track down and fix.

Battery Saver

Battery Saver modes in hand-held radios must be turned OFF. Otherwise, carrier will not be properly detected.

Order the book What is your TNC doing? Dealer List

Ham Radio Articles Home Page

URL: http://prairie.lakes.com/~medcalf/ztx/tnc-bk/ch5-gnd.html