I've been playing with some lab equipment recently in order to learn more about digital modulation techniques. I've connected R&S signal generator SMIQ 03B with the R&S spectrum analyzer FSIQ 3 and observed various modulated signals. I've managed to set up I-Q signal view, eye diagramm and constellation diagramm for BPSK, QPSK and 8PSK. But QAM16 gives me some trouble. Here are some examples:
For all my signals there are some common settings on both signal generator and analyzer:
- Symbol rate: 100 ksymbols/s
- Carrier frequency: 1 GHz
- Output filter: squared cosine/0.9
- IF bandwidth: 10 MHz
And here are some nice looking examples for various constellations: BPSK, QPSK and 8PSK. The problem is with QAM16 - the constellation is not expected 4x4 cell but rather some circular shape with no distinct clusters.
Question: which parameter has to be set on signal analyzer to obtain the classical 4x4 constellation? I don't mean only some device specific parameter - is there any common-theory known parameter that defines the constellation shape?