PSK Modes

General Description

Narrow band modes such as PSK31 are low symbol rate, single carrier differential Binary PSK (called 2-PSK or BPSK) or Quadrature PSK (4-PSK or QPSK). With digital phase modulation the phase changes abruptly, and without additional measures wide sidebands would be created. To prevent this, all these modes also include 100% raised-cosine amplitude modulation (ASK) at the symbol rate, which 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 straightforward 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 (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.

To counter this lack of robustness under adverse conditions, the PSKR ("R" for "robust"), series of modes has been developed. Using a similar design as the MFSK modes with a convolutional encoder and an interleaver these modes provide a much more robust link at the expense of the data speed which is divided by half when compared to the standard BPSK mode. Soft bits decoding was also added to maximize the probabilities of decoding the right sequence.

With these modes, a very linear transmitter is required. Over-driven operation results in excessive bandwidth, poorer reception and difficult tuning. However, the sensitivity is such that very little power is usually required.


The BPSK modes are unconnected, manually controlled message asynchronous simplex chat modes, used without Forward Error Correction.  The default calling mode is BPSK31.  PSK63F is a mode ideally suited for keyboard to keyboard chat.  The PSK-R modes are designed for use with data transfer applications such as pskmail, flarq or other automated-repeat-request applications.

Coding and Character Set

A binary varicode with ASCII-256 user interface is used. Lower case characters are sent faster. Modulation is bit-wise symbol synchronous, differential.

The QPSK modes use binary convolution to generate two dibits per varicode bit at the same symbol rate.  Rate R=1/2, Constraint length K=5. No interleaver is used. Two-bit quadrature modulation is based on a differential code table.

The PSKR modes use binary convolution to generate two bits per varicode bit at the same symbol rate.  These modes us Rate R=1/2, Constraint length K=7 with diagonal interleavers, variable in size to keep the timing between redundant information constant.  The number of interleavers for each baud rate is 40, 80 and 160 for PSK125R, PSK250R and PSK500R respectively.  PSKR modes use the MFSK varicode which provides a small increase in average speed of about 13%.

The PSK63FEC (also PSK63F in short) is identical to the PSKR modes but for the absence of an interleaver.

Operating Parameters
Mode Symbol Rate Typing Speed1 Duty Cycle2 Bandwidth3 ITU Designation4
BPSK315 31.25 baud 50 wpm ~ 80% 62.5 Hz 63H0G1B
BPSK63 62.5 baud 100 wpm ~ 80% 125 Hz 125HG1B
BPSK125 125 baud 200 wpm ~ 80% 250 Hz 250HG1B
BPSK250 250 baud 400 wpm ~ 80% 500 Hz 500HG1B
500 baud
800 wpm
~ 80%
1000 Hz
QPSK31 31.25 baud 50 wpm ~ 80% 62.5 Hz 63H0G1B
QPSK63 62.5 baud 100 wpm ~ 80% 125 Hz 125HG1B
QPSK125 125 baud 200 wpm ~ 80% 250 Hz 250HG1B
QPSK250 250 baud 400 wpm ~ 80% 500 Hz 500HG1B
500 baud
800 wpm
~ 80%
1000 Hz
62.5 baud
55 wpm
~ 80%
125 Hz
125 baud
110 wpm
~ 80%
250 Hz
250 baud
220 wpm
~ 80%
500 Hz
500 baud
440 wpm
~ 80%
1000 Hz


1. WPM is based on an average 5 characters per word, plus word space. Values based on sending 100 "paris " words.
2. Transmitter average power output relative to a constant carrier of the same PEP value.
3. This is the "Necessary Bandwidth" as defined by the ITU.
4. A summary of the ITU Designation system can be found at

5. Default and normal calling mode.