RSP1A Frequency span and fading

[Tony_M]

Hello!

I'm about to order SDRplay RSP1A, but I need to have few answers regarding this product.

1. First - why SDRplay devices makes those slopes at both ends of spectrum? (marked with red arrows):



Why this bothering me? Because even cheap RTL-SDR dongle don't show this "issue". Yes, I see this as an issue, since it resembles vignetting in photography or narrow viewing angles in screen-related electronics (TVs, laptops etc). It robs ends of usable bandwidth.

2. Second - can I go with lower bandwidth than 2 Mhz? Especially in case, when I want to record something and save some HDD space by recording small chunk of spectrum (lets say 250 Khz).

[Tech_Support]

Hello Tony_M,
The roll off you are describing is a deliberate feature designed to minimise the effects of aliasing. There are both analog and digital filters incorporated into the hardware which are necessary to reduce the impact of aliasing from signals just outside of the visible spectrum. The analog bandwidth is automatically selected to be the nearest available bandwidth that is lower than the sample rate. In this case it is 1.536 MHz.
If you want a greater span without roll-off, select a higher sample rate.
To answer your second question, to obtain a smaller amount of viewable spectrum, simply apply higher levels of decimation. a 2 MHz sample rate with a decimation factor of 8 will give you 250 kHz of visible spectrum.

Sincerely

Tech_Support

[Tony_M]

Thank's for clear answer!

Best regards,
Tony_M

[Tech_Support]

Hello again Tony_M,
I should add that if you select low IF mode, rather than Zero IF mode, you will see less roll-off with a 2 MHz span. The reason for this is that the ADC sample rate is set to 6 MHz as you are sampling an IF and so whilst the analog filter is still set to 1.536 MHz, this is a much gentler roll-off filter than the digital filter which now rolls off at 6 MHz.

Sincerely

Tech_Support