Faq Radio Fldigi


How do I tune the radio? USB? LSB?

Normally we use USB for digital modes. For some radios it is difficult or impossible to use a narrow filter on USB, and in such cases it may be better to use LSB. This makes no difference in principle, only the relationship between rig (VFO) frequency and center frequency of the signal changes - 'the math is different'.

What is a center frequency?

The psk modes use a carrier which is phase modulated. This signal is generated in the audio domain. Through a clever modulation scheme the modulated signal is quite narrow. In fact, a PSK125 has 2 sidebands of 62.5 Hz on either side of the carrier. The carrier is the center frequency, and as the sidebands are perfectly symmetrical you find the center frequency right in the center of the signal.
PSKmail uses Fldigi to generate the audio signal. To shift it into the HF domain we give it to an SSB transmitter .
In USB mode the center frequency = VFOfrequency + Audiofrequency.
In LSB mode the center frequency = VFOfrequency - Audiofrequency.
Example 1: If you generate the audio signal at a frequency of 1000 Hz and the TRX VFO is at 10147 kHz, the center frequency is 10148 in USB mode and 10146 in LSB mode.
Example 2: You want to use USB mode to work PI4TUE on 10148 and you use Fldigi to generarte a psk signal at 1500 Hz.
You have to tune the rig to 10146.5 kHz.

What determines the audio frequency? Is 1000 Hz a good choice?

That depends on which rig you use, and the center freqency of the filter(s) you want to use.
You want to use as narrow a filter as you can get, and that is not always possible with a 1000 Hz center frequency.

Why use a narrow filter when the waterfall covers 2500 Hz?

You will get the best performance with a 'matched filter', i.e. a filter which has a bandpass matched to the bandwidth of the signal. Only then the S/N ratio will be optimal. Using a wide filter for psk is nice when all signals in the passband have the same strength (alll are strong). Agc action and receiver overload will kill weak signals. Here is an example from 10 MHz, the signals on the high side of the spectrum are the APRS 300 Bd Packet channel and the new 'robust packet' signals…

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See how the pskmail channel at 1000 Hz is reduced in performance?
Using a wide filter for PSK is 'bad practice!'

Always 1000 Hz?

1000 Hz allows easy calculations :). But there is another reason which determines the audio frequency. That is the center frequency of the filter you want to use. The above picture shows a picture from a Ten Tec ORION, which is completely flexible in this respect as the filter is determined by the if dsp.
Two other examples:

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Mode = digital, which is automatically USB, 300 Hz filter, DSP at minimum bandwidth.
What you see is the noise content in the center of the filter passband. In digimode the FT897D is centered on 1000 Hz.

ICOM 756

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Mode = USB, filter is 2 kHz (there is no narrow filter yet), Passband tuning does not go under 1500 Hz, set to minimum. What you see is the passband noise, which will be a lot narrower as soon as we install a 250 Hz filter.
The audio center frequency has to be 1500 Hz here.

Please add your trx here….

Fldigi parameters

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insert here Screenshot-fldigi_-_config.png

import from old_wiki 15/12/2017

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