FLDIGI Users Manual  3.21
PSK - BPSK, BPSKR, QPSK, and Multi-Channel Modems

Table of Contents

BPSK and QPSK modems

PSK are narrow band low symbol rate modes using either single or multiple carrier differential Binary Phase Shift Kying, BPSK, or Quadrature Phase Shift Keying, QPSK.

PSK63FEC and the PSKxxxR modes are forward error correcting modes. PSK63FEC is compatible with the MultiPsk mode of the same name. The PSKxxxR, or robust, modes use both forward error correction and interleaving to achieve about 4 dB s/n improvement over standard PSK. These modes are use primarily by the PskMail user community. They are the invention of John Douyere, VK2ETA, a member of the fldigi development team.

In addition to the binary phase shift keying the signal is 100% raised-cosine amplitude modulated at the symbol rate. This reduces the power to zero at the phase change. Because of this amplitude modulation, the signal bandwidth is relatively narrow. Synchronization at the receiver is straight forward because it can be recovered from the amplitude information. Differential PSK is used to provide continuous phase changes when idle (to maintain sync), and by allowing the receiver to measure phase difference from symbol to symbol, to reduce the effects of ionospheric Doppler phase changes which modulate the signal. The slower modes are more affected by Doppler, and the QPSK modes are particularly affected.

With no interleaver and limited coding length, the QPSK mode Forward Error Correction coding gain is limited, and under burst noise conditions on HF the performance is usually worse than the BPSK option at the same baud rate. In general the narrow-band BPSK modes work well on a quiet single-hop path, but give poor performance in most other conditions.

PSK63 signal transmitting text data - oscilloscope view

PSK63 signal transmitting text data - waterfall view

QPSK63 signal transmitting text data - oscilloscope - waterfall view

QPSK63 signal transmitting text data - waterfall view

The two oscilloscope views above clearly show the combined phase and amplitude modulation of these modes.

With these modes, a very linear transmitter is required. Over-driven operation results in excessive bandwidth, poorer reception and difficult tuning. Overdrive usually occurs by having the audio signal much too large. These are very sensitive modes and usually very little power is required. QRP operation of 80, 40, 30 and 20 meters can provide nearly 100% copy over multi-hop paths. In many instances PSK can provide better decoding than CW.

Setting up for a good clean on air signal that will receive the accolades of your QSO partners is easy. Follow the instructions on using the Tune button and you will have a clean on signal.

Good reception of PSK signals requires that the demodulator be phase locked to the incoming signal. Fldigi has both a fast acquire / slow tracking AFC system. Place the red bandwidth bar (see above) so that it overlies the desired signal and then press the left mouse button. The signal should quickly lock on a decoding should commence immediately. It is almost impossible to visually tell whether a BPSK or QPSK signal is being received. Under very high s/n you might be able to hear the difference, but that is even difficult for most operators. If you are not able to decode a signal that looks like a BPSK and the bandwidth of the signal matches the baud rate then it might be a QPSK signal. Just change mode a try reacquiring the signal.

Multi-Channel PSK modems

PSK63R20C signal transmitting text data - waterfall view

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