{"id":804,"date":"2026-06-07T09:51:32","date_gmt":"2026-06-07T07:51:32","guid":{"rendered":"https:\/\/www.i8zse.it\/en\/?page_id=804"},"modified":"2026-06-07T09:51:54","modified_gmt":"2026-06-07T07:51:54","slug":"804-2","status":"publish","type":"page","link":"https:\/\/www.i8zse.it\/en\/modulation\/804-2\/","title":{"rendered":"PSK"},"content":{"rendered":"

PSK<\/strong> (Phase Shift Keying) modulation is one of the most refined and efficient techniques used in digital radio communications, and it has found in amateur radio an ideal field of application. The basic principle is simple but powerful: information is not transmitted by varying amplitude or frequency, but by modifying the phase of the carrier. This allows optimal use of available bandwidth while maintaining good noise resistance.<\/p>\n

Variants<\/h3>\n

In the simplest case, BPSK<\/strong> (Binary PSK), two phase states are used (typically 0\u00b0 and 180\u00b0). Each symbol is represented by a phase state, and this simplicity makes BPSK very robust, especially in the presence of weak or noisy signals. However, the receiver must be able to maintain very precise phase synchronization: small errors can lead to bit inversions.<\/p>\n

For this reason, in amateur radio practice a variant called DPSK<\/strong> (Differential PSK) is often preferred, where what matters is not the absolute phase value but the change between consecutive symbols. In practice, the receiver observes \u201chow\u201d the phase changes rather than \u201cwhere\u201d it is. This approach reduces system complexity and increases robustness under real-world conditions, at the cost of a slight theoretical performance loss.<\/p>\n

A further step in efficiency is represented by QPSK<\/strong> (Quadrature PSK), which uses four phase states (e.g., 0\u00b0, 90\u00b0, 180\u00b0, 270\u00b0). In this way, each symbol carries two bits instead of one. Differential variants such as DQPSK also exist, used when higher speed and simpler demodulation are desired simultaneously. However, as the number of phase states increases, the angular distance between symbols decreases, making the system more sensitive to noise and phase errors.<\/p>\n

Going further, we find modulations such as 8PSK or 16PSK, where 3 or 4 bits per symbol are transmitted respectively. These techniques are very spectrally efficient, but in HF amateur radio they are less common because they require cleaner and more stable signals\u2014conditions not always guaranteed due to fading, multipath, and atmospheric noise.<\/p>\n

One of the most important aspects of PSK is bandwidth control. Abrupt phase transitions can generate spurious components and widen the signal spectrum. To avoid this, pulse shaping<\/em> techniques are used, such as the raised cosine<\/em> or root raised cosine<\/em> filter, which smooth transitions and make the signal more compact and less intrusive to adjacent channels.<\/p>\n

PSK31<\/h3>\n

An iconic example in amateur radio is PSK31<\/strong>. Developed by British radio amateur Peter Martinez<\/strong> (G3PLX) and introduced to the amateur community in December 1998, it was designed to create a simple and efficient mode for keyboard-to-keyboard HF communication. It uses a symbol rate of exactly 31.25 baud\u2014a value chosen because it corresponds to an average typing speed of about 50 words per minute and can be easily derived from the 8 kHz sampling rate used in common sound cards (8000\/256 = 31.25 Hz).<\/p>\n

The most widely used version is based on DBPSK (Differential BPSK), which avoids the absolute phase synchronization problems of pure BPSK. As for occupied bandwidth, the \u201c31 Hz\u201d value corresponds to the symbol rate and represents the theoretical minimum occupancy; in practice, considering signal filtering, the bandwidth is about 50\u201360 Hz. This extraordinary spectral compactness allows many PSK31 QSOs to coexist in the same portion of spectrum occupied by a single SSB transmission.<\/p>\n

PSK31 uses a character encoding called Varicode<\/strong>: Martinez structured the alphabet so that, as in Morse code, more frequent characters have shorter codes, while rarer ones use longer codes. This improves overall transmission efficiency without increasing bandwidth.<\/p>\n

The pulse shaping used in PSK31 is of the raised cosine<\/em> type: this smooths phase transitions, reduces out-of-band spectral components, and minimizes splatter<\/em>, contributing to the signal\u2019s characteristic cleanliness and compactness.<\/p>\n

Alongside the basic version, there is also QPSK31, which introduces a form of convolutional coding with forward error correction (FEC). An interesting aspect is that the transition from BPSK31 to QPSK31 keeps both bandwidth and data rate essentially unchanged: the additional capacity of QPSK is not used to increase speed, but to introduce redundancy and improve reliability. A rate-1\/2 convolutional code is used, with decoding based on the Viterbi algorithm.<\/p>\n

This solution makes communication more robust, especially in the presence of burst errors typical of real HF channels. However, in conditions of very high uniform noise, simple BPSK can sometimes perform better in terms of decoding threshold.<\/p>\n

In the world of digital modes, PSK represented an important step forward compared to simpler techniques (such as FSK and its RTTY implementation), especially in terms of spectral efficiency. However, it requires a more linear transmit and receive chain, because it is sensitive to phase and amplitude distortions (for example clipping effects or RF amplifier nonlinearity).<\/p>\n

Today, even though many modern modes (such as FT8) use different and more complex schemes, PSK remains a fundamental technology for understanding digital communications\u2014not only in amateur radio. It is still widely used for real-time, human-readable QSOs where operators want direct interaction without relying entirely on automated decoding.<\/p>\n

In Summary<\/h3>\n

Ultimately, PSK is a perfect example of balance between theory and practice: elegant from a mathematical standpoint, yet extremely concrete in its applications. From the simplest variants to implementations with error correction, it remains one of the most significant and educationally valuable modulation schemes in the amateur radio landscape.<\/p>\n","protected":false},"excerpt":{"rendered":"

PSK (Phase Shift Keying) modulation is one of the most refined and efficient techniques used in digital radio communications, and it has found in amateur radio an ideal field of application. The basic principle is simple but powerful: information is not transmitted by varying amplitude or frequency, but by modifying the phase of the carrier. 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