Measuring BER on a Continuously Transmitted Sequence (Flex Frame)

Some transmitters send a bit sequence continuously rather than once per frame - a free-running PN 1) part number, or 2) packet number generator is the common case. The sequence is then at an arbitrary point when the acquisition begins, and its phase is not aligned to the frame. Flex Frame measures BER Bit Error Ratio - A ratio of the number of errors to data bits received on a digital circuit. on a signal like this, and it does not need a synchronization allocation to do it, but it does need Autodetect Bit Sequence Offset to be selected.

With that check box cleared, the payload sequence is assumed to start at the allocation's first symbol. A continuously transmitted sequence does not, so the reference never matches the signal, synchronization fails, and every value in the error summary goes invalid - not only BER, but 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., frequency error and the rest.

To measure BER on a continuously transmitted sequence

Set Configuration Mode first - the payload parameters used in the later steps are not available until it is set to Advanced.

Set Configuration Mode to Advanced. Basic mode has no payload parameters, so BER cannot be configured there.
Set Symbol Rate to the symbol rate of the signal. The measurement Span is adjusted automatically to suit the symbol rate.
On the Frame Definition tab, select the allocation and set its Sequence to the sequence being transmitted - PN9, PN11, PN15, PN20, PN23, or Custom with Custom Sequence Type set to Bit Sequence. Set Modulation to match the signal.
Select Include in BER Calculations.
Select Autodetect Bit Sequence Offset. This is the setting that lets the demodulator find where the transmitted sequence actually starts.
If the Result Length spans more than one frame, set Payload Bits Across Frames to Continuous Over Frames so the reference sequence does not restart at each frame boundary.

BER, Bit Errors and Bit Count then appear in the Syms/Errs table and in the Frame Summary.

Synchronization

No synchronization allocation is needed for this kind of signal, and none should be configured. An allocation whose bit sequence offset is autodetected cannot serve as a synchronization source, so Synchronization cannot be selected for it. When the frame contains no known sequence, the demodulator synchronizes from the generic characteristics of the signal's modulation format instead. See About Synchronization.

If BER comes back high and stays high no matter how clean the signal is, and the frame contains only one allocation, the cause is carrier phase rotation. An autodetected sequence cannot also serve as the phase reference, so the demodulator assumes one of the rotations permitted by the modulation format; if that assumption is wrong, the recovered bits are wrong. Defining a second allocation that is used for synchronization resolves the rotation.