Shaped OQPSK (Digital Demod)

Menu Path: MeasSetup > Digital Demod Properties > Format tab > Format: box

Shaped OQPSK is similar to Offset QPSK Quadrature phase shift keying and can be thought of dually as:

an OQPSK Offset Quadrature Phase Shift Keying: A type of QPSK modulation that offsets the bit streams on the I and Q channels by a half bit. This reduces amplitude fluctuations and helps improve spectral efficiency. scheme using a special differential bit mapping and frequency-pulse shaping filter
CPM Continuous Phase Modulation using a special set of symbols with restrictions on bit ordering (and a special frequency-pulse filter). The I and Q streams of bits are separately restricted state transitions of +1/-1/0 and then the I and Q streams are combined.

The Shaped OQPSK demodulator behaves like the 89600 VSA OQPSK demodulator (IQ traces), instead of like the CPM demodulator (FSK Frequency Shift Keying: A form of modulation using multiple carrier frequencies to carry the digital information. The most common is the two frequency FSK system using the two frequencies to carry the binary ones and zeros. traces).

To automatically configure the VSA for measuring signals that conform to the IRIG 106-04 standard, on the Format tab, click Preset to Standard > Other and select SOQPSK-TG (IRIG 106-04). Note that the SOQPSK-TG preset sets the symbol rate to 5 MHz Megahertz: A unit of frequency equal to one million hertz or cycles per second. although the standard does not define a specific symbol rate.

For more information about SOQPSK-TG, see the following document: http://www.irig106.org/docs/106-05/chapter2.pdf

To automatically configure the VSA for measuring signals that conform to the SOQPSK-A or SOQPSK-B standards, on the Format tab, click Preset to Standard > Other and select SOQPSK-A or SOQPSK-B.

Things to note

When using Shaped OQPSK, note the following:

An even number must be specified for Points / Symbol.
The metrics in the Error Summary table use the same computations as other demodulation formats with the following exceptions:
- Amplitude droop is not computed for Shaped OQPSK.
- The VSA uses two points-per-symbol (instead of one point-per-symbol) to compute the following:
- Offset EVM is computed in addition to the regular EVM Error vector magnitude (EVM): A quality metric in digital communication systems. See the EVM metric in the Error Summary Table topic in each demodulator for more information on how EVM is calculated for that modulation format..
The symbol rate is the frequency of the I (or Q) clock, not the 2x frequency sometimes associated with the I to Q clock frequency.
Demodulated traces show a symbol at every I and Q detection (decision) point. Therefore, all demodulated traces (except the symbol table) show twice as many points as the Result Length, at twice the symbol rate.