RF and EVM measurement results

In order to provide as much as possible information we have performed some though RF and signal quality measurements on our design. We are using the direct conversion technique for generation of the QPSK spectrum. This technique requires careful PCB design in order to minimize unwanted feedback of the modulated output back to the input of the IQ modulator. This is much harder to achieve at 13cm than for 23cm due to practical RF aspects like undesired feedback and coupling, shielding etc. The measurement results below show the typical spectral performance and EVM values for both the 23cm and 13cm boards. As you can see, with a typical value of 750m% EVM for the 23cm edition and 1.6% on 13cm and the very low adjacent channel power emission we achieve much better values than most other amateur designs.

Figure 1. Measured performance for the 23cm edition of the IQ modulator.

Figure 2. Dutch D-ATV project under test.

We decided to make seperate designs for 13cm and 23cm in order to achieve maximum performance for both frequency bands. This means that we payed more attention in achieving low phase noise for our custom VCO designs. With our VCO design for 13cm we achieve a typical phasenoise level of -87dBc/Hz @10 kHz distance of the carrier. For 23cm this value becomes even better with typical -92 dBc/Hz @ 10 kHz offset. The results of the measurements on the 13cm IQ modulator board are given below. The vector signal analysis result shows that the EVM is approximately 1.6%. This value is caused by a slight IQ imbalance (small mismatch between I and Q amplitude levels). This slight mismatch will not be an issue in practice (you need a €100.000 measurement device to detect these imperfections !).


Figure 3. Vector signal analysis and spectral performance for the 13cm IQ modulator board.