What is your TNC doing? - Receive Signal

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

Receive Signal

The receive signal comes from the antenna to the radio and then to the TNC. This signal is used for two things inside the TNC: 1) to receive signals that can be decoded into packets, and 2) to determine carrier detect.

Radio to TNC Connection

Audio Modulation. The wire that carries the receive signal from the radio to the TNC attaches to the TNC at a pin often called: receive audio, audio signal, or receive data. There are several places the Receive Signal wire may attach to a radio: the speaker or headphone jack, the mic jack, or an auxiliary jack.

When connecting to the speaker or headphone jack, the Receive Signal wire is connected to the tip of the plug. For this type of connection, many TNC manufacturers provide a pre-wired cable.

Some radios have a pin on the mic jack for the receive signal. This pin may be called: Rx audio output, receive data, SP (speaker), AF, ANO, or data out. The Receive Signal wire may be connected to this pin. Other radios provide a similar connection on an auxiliary jack.

DFSK Modulated signals need to come directly from the discriminator of the radio and not through the audio circuits of a voice radio. Some radios may provide this signal at a data-connector pin. If this pin is not provided, the radio must be modified to connect the Receive Signal wire directly to the discriminator output. A true FM-modulated radio must be used; not a PM (phase modulated) radio. Modems that receive DFSK modulation are normally for speeds that require a wider bandwidth than is standard in a voice radio (discussed below).

FM Audio Receive Volume

Radio receive volume is set with the following steps (when using energy carrier detect, which is default on most TNCs):

1) Some TNCs have a threshold knob that needs to be set first. With the radio off, turn the knob all the way clockwise. Then turn the knob counterclockwise until the DCD LED goes out. Turn the radio on before continuing.

2) Open the radio squelch so you hear noise when no signal is present.

3) Adjust radio volume as low as possible.

4) Increase radio volume until the TNC RCV LED comes on. (This LED is labeled DCD on some units.) Then set the volume level slightly higher.

5) Increase radio squelch until the TNC RCV (DCD) LED goes off.

If the radio's squelch is set at a point where noise is heard, the TNC will interpret the noise as a signal and will not transmit. If the radio's volume is set too high, it can overdrive the TNC input circuit and cause distortion of the receive signal.

As always, to transfer maximum power, impedances must match. In this case, the radio speaker output impedance must match the TNC receive input impedance. However, the radio volume control can be increased to overcome most impedance mismatches. Only in extreme cases will distortion be caused.

FM Audio Equalization

Audio-modulated packet is transmitted using two tones. The receiving modem works best if the two received tones have the same amplitude. When the tones are received at different amplitudes, modem performance suffers; frames are decoded incorrectly; and retries are required. Some TNCs provide adjustments (with jumpers, potentiometers, or commands) to equalize the amplitude of the two tones.

Theoretically the tones should be of equal amplitude when received, however in the real world theories are not accurate. The complete explanation of the need for equalization is deep in FM theory and beyond the scope of this book. In general, the transmitting TNC sends the tones to the transmitting radio with equal amplitudes. The radio pre-emphasizes some tones, changing their amplitude, to improve the signal-to-noise ratio. The receiving radio de-emphasizes these tones to return them to equal before sending them to the TNC. Pre-emphasis and de-emphasis are normal sections of the audio circuits in FM voice radios. However, there are slight variations between what one radio will pre-emphasize and what the next will de-emphasize. Therefore, there are times when it is necessary for the TNC to also perform some equalization.

In extreme cases, you may need to set equalization differently depending on which station you want to communicate with. In other words, the pre-emphasis of one station may be so much different from the pre-emphasis of another station that you may need to use a different equalization setting for each station. Otherwise, communications may be unsuccessful. In this case, you will not be able to communicate with both stations at the same time. The type of carrier detect used may also affect how sensitive the TNC receive circuit is to different amplitudes (equalization).

When using voice, near perfect equalization is not a problem because the ear is a marvelous part of our body. If you listen to the same person on radios with different de-emphasis, you may notice that he does not sound exactly the same. However, your ear can still understand the voice. The TNC is listening for only two tones. As the differences in amplitudes increase, the TNC has more difficulty hearing them both equally well.

Carrier Detect

The receive signal is also used by the TNC to detect carrier. In other words, to detect if the frequency is in use. Carrier detect is used in the determination of when to transmit. If a carrier is detected, the TNC will not transmit. However, this does not necessarily mean packet signals are being received. Depending on the type of carrier detection used, packet signals are not always present when carrier is detected.

It is possible to use an external means of carrier detect. In which case, the received signal also goes through an external circuit (may be built into radio). The external circuit is then connected to a separate TNC connector pin for external carrier detect (XCD, SQ, or squelch input). The receive signal is still connected as described above, to provide reception of packet data. In most Kantronics units, external carrier detect is selected by the command CD EXTERNAL.

There are basically two kinds of carrier detect: energy and data.

Energy Carrier Detect. Many TNCs use energy carrier detect as a default. Energy is any signal the receive circuits detect. This includes packet signals, voice signals, and any type of noise. If you are listening to the frequency, anything you can hear is considered energy. On most Kantronics units, the command CD INTERNAL selects energy carrier detect.

Data Carrier Detect. Data carrier detect circuits are built in several different ways. However, they all have the same purpose: to determine if the receive signal contains data that is in the format defined by the receive modem. When a signal containing "real" data is being received, carrier detect is asserted and the TNC will not transmit. On the other hand, if a signal is being received that does not contain real data, the TNC will transmit. This allows us to open the squelch to where noise is heard and copy weak packet signals. Another benefit is the ability to transmit even when plagued by some RF interference. Data carrier detect is selected in most Kantronics TNCs with the command CD SOFTWARE.

The Kantronics CD SOFTWARE command looks for transitions in the signal that are typical of 1200 baud. The timing and regularity of the changes between Mark and Space are used to detect a 1200 baud signal. On the other hand, the TAPR DCD state machine looks for "real" data by looking for the specific audio tones used for 1200 baud packet: 1200 Hz and 2200 Hz. Similarly, the DE1200 modem for the Kantronics Data Engine looks for any sinewave; in this case, any modems that use audio tones can co-exist on the same channel.

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