Tuning HF Digital Signals

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Proper tuning of the radio is critical for copying HF digital modes (Amtor, Gtor, Pactor, Rtty). The TNC has a much narrower definition of correct tuning than our ears do. If a voice signal is slightly mis-tuned, we can often still understand the message. If a digital signal is mis-tuned, the TNC will not be able to copy the message. Answers to the following questions can help us learn to tune our radios for best reception.

What does the TNC expect?
What is the bandwidth of the signal?
Where is the signal in the passband?
How is the noise eliminated?

What does the TNC expect?

The TNC expects to receive the same tones it transmits. A TNC usually has a bargraph of LEDs that light in a certain manner when the signal is properly tuned. While this accomplishes the task, additional information may help us eliminate the noise that is also received with a normal SSB signal. (By convention, digital modes use LSB on all bands.)

Digital information is sent as a signal that changes between two tones. The "standard" tones are 2125 Hz and 2295 Hz; however the European standard is 1275 and 1445. If we do the math, the difference between 2125 and 2295 is 170 Hz. The difference between 1275 and 1445 is also 170 Hz. Therefore, a signal sent with tones of 1275 and 1445 can be received as tones at 2125 and 2295 and carry the same digital information. The difference between the tones (or the shift) is the important factor, not the specific transmitted tones.

You do not need to be concerned with which tones another station transmits to you. When a radio receives a signal in its passband, the radio produces tones that you hear from the radio's speaker. As you move the tuning dial, the frequencies of the tones change because their location in the passband changes. More technically, early in the radio's receive circuits the radio mixes the receive signal with another signal based on the dial setting (VFO). This mixing results in a frequency the circuits of the radio are designed to handle. As you move the tuning dial, the VFO input to the mixer changes, therefore changing the output (tone). The TNC's bargraph is an aid for you to tune the radio to the tones expected by the TNC.

Besides the two tones needed for decoding the digital message, the radio is also receiving other noise that is within the passband of the radio. A noisy signal will be harder for the TNC to copy. Knowing the bandwidth of a signal helps you choose the right filters to receive a signal with the least noise.

What is the bandwidth of the signal?

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.

A rule of thumb for determining the bandwidth of an SSB digital signal is:

1.2 * shift + baud rate = bandwidth

The number 1.2 is derived from theorems and laws that are beyond the scope of this article. The shift of a two-tone digital signal is the difference between the two tones. For example, Rtty is often sent with the tones 2125 Hz and 2295 Hz. The mathematical difference (2295 minus 2125) is 170 Hz. Baud rate is the number of possible transitions (changes from high tone to low tone or vice versa) per second. Amateurs normally send Rtty at 45 baud. So, the bandwidth of Rtty is:

1.2 * 170 + 45 = 249 Hz

The chart below shows the bandwidth of some popular digital modes. The standard shift for most amateur digital modes has been 170 Hz; however, the standard shift for 300 baud packet is 200 Hz, therefore some TNCs (notably older AEA units) also use 200 Hz for other digital modes. When using SSB, this small difference in shift is not critical for proper reception.


                          Bandwidth       Bandwidth

Mode         Baud Rate    170 Hz Shift    200 Hz Shift



Rtty          45          249             285

Rtty          75          279             315

Amtor        100          304             340

Pactor       100          304             340

Pactor       200          404             440

Gtor         100          304             340

Gtor         200          404             440

Gtor         300          504             540

Packet       300          504             540

A shift of 1000 is used for 1200 baud HF packet, so its bandwidth is approximately 2400 Hz.

Where is the signal in the passband?

The passband of a radio determines the frequencies that are passed to the audio circuits. A typical voice passband allows you to listen to tones from about 500 to 3000 Hz. That's nearly 2500 Hz, which is 5 to 10 times wider than the typical HF digital signal (as shown in the chart above).

The tones your TNC wants to hear determine where the signal will be in the passband. The standard 2125 and 2295 tones will be at the upper end of the voice passband, while the European standard of 1275 and 1445 will be below the middle of the passband. For packet, the KAM uses 1600 and 1800 Hz tones which will be in the center of the passband. (You can change the KAM's expected tones with the SHIFT, MARK, and SPACE commands.)

Although the TNC is listening for only two tones, the noise around those tones decreases the signal-to-noise ratio and affects how well the TNC can copy the message.

How is the noise eliminated?

Filters are used to eliminate the noise in the receive signal. The filters available to you depend on the radio (or external audio filter) you are using. Since the TNC is expecting two tones, you need to be sure that any filter you use will pass those tones to the TNC (or if possible you might set the TNC to accept the tones produced by the filter). For example, if your TNC is expecting 2125 and 2295 Hz and the filter only passes tones from 1100 to 1600, the TNC will never be able to decode the message. Filters are often described by their center frequency and bandwidth. Below are some possible types of filters.

Passband Tuning or IF Shift. These two types of filters have similar results and one or the other is often present on a SSB radio. Effectively the filter is narrowing the frequencies you can listen to. Some radios move the passband above or below its normal position. For example normally the passband passes frequencies that allow you to listen to 500 through 3000 Hz. These filters may be used to change the passband, so it might allow you to listen to frequencies from 1500 through 4000 Hz; however, the audio circuits only pass frequencies up to 3000 Hz. So this example would effectively narrow the frequencies to 1500 through 3000 Hz. (These frequencies are used for example only - the radio knob (or control) for passband tuning and IF shift normally doesn't show the frequencies. The control's normal position is usually indicated and you can change the control up or down.) Other radios may actually change the width of the filter with similar listening results. Passband tuning and IF shift filters normally narrow the passband from one side only, eliminating either some of the high frequencies or some of the low frequencies.
Narrow filters. Some radios allow selection of narrower passband filters. Normal voice SSB filters are approximately 2500 Hz wide. Narrow filters are often 2000 Hz or 500 Hz wide. These filters are often only available when the TNC is wired to the radio's FSK jack and the radio's Rtty mode is selected. The article Which Jack for HF Radio-to-TNC Cables? discusses wiring the TNC to an HF radio.
Audio filters. These filters are usually external filters, although the Yaesu FT-990's digital filters are built-in audio filters. Modern filters often use DSP technology. While the passband tuning, IF shift, and narrow filters work on the signal before it enters the audio stages of the radio, the audio filters work on the signal after it has passed through the radio's audio circuits. Units like the Timewave 59+ are wired between the radio and TNC receive signal and allow you to set a center frequency and bandwidth so that only those frequencies are passed to the TNC.

Efficient use of spectrum. Filters allow us to use the spectrum more efficiently. When narrow filters are used, many more conversations can exist within the same range of frequencies. Remember a voice passband is about 2500 Hz wide and the digital signal only needs about 500 Hz or less. If you try to work a 500 Hz mode with a 2500 Hz filter, the noise (or other stations) around your desired signal may make reception difficult to impossible and the TNC may have a hard time decoding the message correctly.

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