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I’m running a VSG25A with Windows 10 and the provided Signalhound VSG software. I created a simple BPSK transmitter with a 100 kHz symbol rate and selected “None” for filter. For data, I selected “PN9” (which I’m assuming is a 9-stage linear shift register?). When I run it, I get the spectrum shown in the attached images. It should be just a simple sin(x)/x curve. Any ideas?
Gary,
There are a few things going on here. It sounds like you are aware there is no baseband reconstruction filter on the VSG25A. But we also apply an inverse sinc correction to the in-band data to flatten it.
PN9 is a PRBS9 sequence (linear feedback shift register) that basically repeats every 511 symbols. We upsample each symbol by a factor of 4 to produce 2044 I/Q samples, which get transmitted in a loop.
Even with no raised cosine filter, the upsample process will result in gaps in the sin(x)/x curve.
Hopefully this helps.
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