Spectrum analyzer RBW filtering
Gaussian vs. Rectangular RBW filtering
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- as a practical example, let's take a look at the analogue television RF spectrum
- in real life analogue transmitters, linearizer circuit alignment is a must
- (modern digital transmitters are capable of automatic alignment)
- for the alignment, vision modulation (pulsing), FM sound modulation and NICAM QPSK modulation is removed
- in Finland we use the PAL B/G VSB with intercarrier FM sound at 5.5 MHz and digital NICAM at 5.85 MHz
- here is a plot using a spectrum analyzer with gaussian (10k) RBW filter response
- here is a plot using a spectrum analyzer with rectangular (10k) RBW filter response
- all basic SPA settings are identical (auto), but as you can see, you can sweep faster with gaussian filtering
- for clarity, here the two plots above are overlayed (4k) to emphasize the difference
- analogue FM and digital sound carrier 1. order IMD 350 kHz sidebands are visible within the DSB area
- this IMD is sometimes more evident in common mode transmitters due to vision amplifier non linearity
- in this case, IMD is generated more severely during the peak sync pulse period
- to analyse this problem, triggering and gating can be used, but that is a different story
Updated 09.02.2004