What is your TNC doing? - Transmit Signal

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

Transmit Signal

The transmit signal is the data signal coming from the TNC modem going to the radio. This signal carries the intelligence to be transmitted. The type of signal modulation will depend on the modem.

Audio Modulation. The transmit signal leaves the TNC through a pin of the TNC radio connector. This pin may be called: AFSK out, microphone audio, transmit audio, or transmit data. The wire carrying this signal will connect to the radio at a pin called mic, microphone audio, mic in, AFSK, MOD, or data in. This pin is present on the microphone jack and sometimes on an auxiliary jack.

Hand-Held Radios. When a microphone is used, the Transmit Signal line carries the voice signal from the mic to the radio for transmission. Most hand-held radios also apply a dc voltage to this line to power the microphone. If the TNC is not made to handle the dc voltage, it may cause the transmitted signal to be distorted. It is also possible, on some radios, for this dc voltage to cause the radio to be constantly keyed. Many TNCs require one of the following solutions:

1) Place a 0.1 microfarad capacitor in the Transmit Signal wire to block the dc voltage.

2) Place an audio transformer across the Transmit Signal wire to allow ac (audio) frequencies to pass through the wire but not dc voltages.

3) Modify the TNC to include an isolation circuit. One disadvantage to this is that you may have to re-modify the TNC when you want to use a base radio.

DFSK Modulation. The TNC transmit data pin is connected to the data input pin of the radio. Inside the radio, the data input pin is connected directly to the varicap (bypassing the audio circuits of the radio). A few radios provide this connection. If it is not provided, you may be able to modify the radio. To use DFSK, the radio must use true FM (frequency modulation) and not PM (phase modulation). The bandwidth of the radio may also need to be modified, as discussed later.

FM Transmission Quality

The transmitting station is responsible for the quality of the signal. One way to evaluate the transmitted signal is to receive it and listen to it. We've probably all listened to several voice stations. If the radio volume control is left at one setting, we've noticed that some stations sound soft while other stations sound loud and even distorted. For our listening enjoyment, we can change the volume control. In addition, the ear is a marvelous part of our body and can understand many distorted voices. However, a TNC is not manufactured with the capability to change the volume control; and electronic circuits are not designed as well as our ears. Therefore, transmission quality is more critical for packet stations than for voice stations.

TNC Output Drive Level. Voice operators can affect the quality of the transmitted signal by how loud they speak and how close they hold the microphone to their mouth. Packet stations can produce a quality signal by properly adjusting the TNC output drive level. This is normally done by changing a potentiometer or jumper inside the TNC. An output level that is too low is comparable to a voice that is too soft. The receiving TNC may not be able to hear it well enough to decode the packet. High output level is comparable to a loud and possibly distorted voice. The receiving TNC is designed for a certain range of "loudness". If the received signal is outside that range, the TNC will be unable to properly decode the information.

Two-meter FM voice radios typically need to transmit a signal with 3.5 kHz deviation for the signal to be properly received at the remote station. If you are fortunate to have access to a deviation meter, adjust the output drive level of the TNC until the radio is sending a signal with 3.5 kHz deviation. Otherwise, listen to other signals and your signal. Then adjust the TNC drive level until your signal has the same loudness as other signals on the frequency. (Your TNC manual should give more specifics for your model of TNC. Output drive level may also be called AFSK output level, output-level control, or transmit audio level.)

Radio Deviation. Radios are designed to take a certain input drive level and produce a certain output deviation level. The radio's input drive level is the voice signal from the microphone or the transmit signal from the TNC. If the input drive level to the radio is correct, the quality of the signal (or deviation level) will be good.

Radios also have adjustments. You can adjust the amount of deviation that is produced by a specific input signal. Making an adjustment in a radio is normally more difficult than making an adjustment in a TNC. If a radio is also going to be used for voice, it is best to adjust the radio properly for voice and perform any necessary adjustments for packet in the TNC.

Note: The amount of power transmitted by the radio does not affect deviation. Radio power only affects the signal-to-noise ratio.

Impedance. Most TNCs are manufactured to expect an impedance of 600 ohms connected at the Transmit Signal pin. If your radio input connection is not between 200 and 1800 ohms, you will need to add an impedance matching transformer across the Transmit Signal line. (As a rule of thumb, for impedances to match, they must be within a 3 to 1 ratio.)

Transmitted Tones. Another factor for a quality signal is that the tones must be the correct tones. Some TNCs have potentiometers (pots) in the circuits that produce the transmitted tones. Pots, by their design, have a tendency to become ill-adjusted. If this happens, the TNC no longer sends the correct tones resulting in either many retries or a total inability to communicate with other TNCs. The steps to adjust the pots vary from TNC to TNC. Check with your TNC manual or manufacturer for specific instructions. (Kantronics TNCs do not have pots in these circuits; instead they use switched- capacitance filters that provide stable tones without the need for adjustments.)

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-tx.html

What is your TNC doing? - Transmit Signal

Transmit Signal

Radio to TNC Connection

FM Transmission Quality