Dear 13dka,
The noise bumps you see at low gains are not actually a fault. They simply relate to the use of sigma-delta ADCs. These converters use a highly oversampled sigma-delta modulator to 'shape' quantisation noise from lower to higher frequencies. Decimation filtering is then used to increase the resolution at the expense of reduced bandwidth. These converters rely upon having either signals or noise to be large enough to 'toggle' the converter or the sigma-delta modulator will not shape the noise in the desired way. At very low gains if the AC signal (including noise) is not sufficient to overcome the DC offset at the input of the ADCs, then the noise shaping will not work in the correct way. There is no reason for you to have the gain set this low as you do not have strong signals present within the receiver bandwidth.
Regarding the DC centre spike, this is entirely due to the use of Zero IF mode in the tuner. The R820T2 based RTL stick does not use zero IF, it only supports the use of a low IF and as a consequence you will not see a DC offset spike. If you had an E4000 based RTL stick, you would see the same thing as the Elonics E4000 also used zero IF. You can select Low IF mode and this will not give a DC offset, but the maximum bandwidth that can be used in Low IF mode is 1.536 MHz.
You don't say what SDR software you are using, but if you are using HDSDR or SDR#(v 1361 or earlier) in the in the advanced setting within the ExtIO window, you can enable DC offset compensation and this will substantially suppress the DC spike and virtually all cases. The latest versions of SDR console also use the latest features of the API of which, DC offset compensation is just one.
Finally, the observation that you have made in that switching between 249 MHz to 251 MHz causes a spurious TETRA signal to appear is an artefact of the way architecture works. From 60 MHz to 250 MHz and from 420 MHz to 2 GHz, the RSP uses a direct conversion (zero IF) architecture. Form DC to 60 MHz and from 250 MHz to 420 MHz, the RSP uses a block converter to shift the input spectrum to an IF before it is then converted to either a zero IF or a low IF. The use of the block converter can give rise to spurious mixing products, where harmonics of the first LO create higher intermediate frequencies which can in turn mix with harmonics of the synthesizer to produce 'false' in-band signals. In HDSDR or SDR# (version 1361 or earlier), open up the advanced tab and under 'LO frequency plan' deselect 'Auto' and manually pick one of the available IFs. This will generally help move spurious mixing products outside of the receiver bandwidth. If you specifically want to receive a signal at 251 MHz, the easiest thing may simply be to set the LO to 249 MHz and make sure the IF bandwidth is 5 MHz or more and 251 MHz will fall within the bandwidth of the receiver.
The RSP is a highly configurable and flexible receiver with many more modes of operation than a RTL based dongle. it does take a bit of time to get to understand how to get the best out of the device for a particular use case.
As well as this particular community forum, if you are a Facebook user, you might want to consider joining the independent Facebook group for the RSP at:
https://www.facebook.com/groups/sdrplay/?fref=ts
Here you will find a community of over 1800 RSP users who are very knowledgeable and helpful for how to get the best out of the RSP for a particular application. Alternatively, as this forum is a community forum and not specifically a support forum, if you have any particular problems with your RSP, please feel free to contact
support@sdrplay.com
Please remember to quote your RSP serial number, OS and application S/W when contacting support, so we are best equipped to help you.
Sincerely
SDRplay Tech_Support
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