PSK Modulated Carriers Jagged Waveform

THarrington,

I’m assuming by top of the carrier you are looking at the signal shape in the frequency domain? If yes, then what you are seeing is likely just the fact that the VSG25 has a small pattern memory buffer and cannot transmit long sequences of unique symbols. The most symbols we can fit in a output buffer is 512. Ensure your “Data” setting in the VSG25 contains a large enough sequence of bits to ensure randomness. You can do this by inserting several PN9 sequences, one for each bit per symbol. This will ensure you have the maximum number of random symbols that will “fill out” the top of the carrier.

In general, this shouldn’t affect the measurement. If you need a more random looking signal, the VSG60 has the ability to transmit extremely long PRBS sequences (PN23).

If you would like to share a screenshot of the behavior, I can verify that my response is correct.

Andrew

Yes this is a known limitation of the VSG25 design. There is no reconstruction filter after the DAC, so you will see DAC images and multiples of the sample rate. For modulated signals like QPSK, the amount of oversampling will determine how far out/close they are.

You can see a picture of this behavior in the production manual, section 6.5.4.
https://signalhound.com/sigdownloads/VSG25/VSG25A%20User%20Manual.pdf

Andrew