8 MHz bandwidth - is there a reason for this that can be explained?

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br0adband
Posts: 2
Joined: Thu Sep 24, 2015 12:38 pm

8 MHz bandwidth - is there a reason for this that can be explained?

Postby br0adband » Thu Sep 24, 2015 1:06 pm

Hi there,

New user here, not new to monitoring/scanning and even SDR tools nowadays but new to this forum specifically. I am a potential SDRPlay buyer at this point and I had a question or two about the hardware and the 8 MHz bandwidth capability - I almost want to call it a limitation but I'm doing my best not to. Having said that, the question then becomes:

Is there a specific reason why 8 MHz is the "limitation" (there, I said it) in terms of the bandwidth, meaning was the choice to put that cap (see, I used a different word that time) on the bandwidth done so to keep the potential of images/ghosts/intermod to a minimum based on the rest of the design hardware, or is it actually just a limitation (dammit) of the hardware chosen to make up the RSP itself?

I ask because here in the US and specifically where I live there are several systems in the UHF range (750 to 950 MHz in my example) that happen to use trunking and they have a frequency spread of pretty much 10 MHz. One system in particular has frequency allotments from 851.4625 to 859.7375 so technically that's even wider than 10 MHz (almost 10.5 MHz) so, the width of the potential receive bandwidth window is a bit relevant for someone like myself. I'd be able to monitor it just fine with 8 MHz, I'm just wondering if that's a truly set in stone thing with the RSP or it might be "opened up" with a future update of some kind, or if the next version of the RSP (assuming there will be one and it's being designed at this time) will offer a larger bandwidth window to work with.

I know most people won't have issues with such a thing, and I personally may never ever actually sit here at my laptop monitoring a full 10 MHz of spectrum at any given time - signals would basically be nothing but needles on the spectrum and there'd be quite a few of them during busy daytime hours so that's probably not going to happen. I just wanted to ask the question so I'd have the answer and the info for future use.

I'm on the fence as the saying goes with respect to either SDRPlay or Airspy (yeah, I know it's dangerous to mention the competition's product here, sorry) - I find the SDRPlay to be better suited for my purposes in many respects, especially with the fact that it uses the USB-B connector which I consider to be vastly more secure and stable than the microUSB port on the Airspy. It's the little things like this that make me lean towards the RSP over Airspy, honestly. The lower cost is nice, obviously, but I have to really get serious about the things that matter when it comes down to spending this kind of money for a device and hopefully having it last for years to come without issues. microUSB ports wear out, obviously, and they're not very sturdy overall whereas a USB-B connector is very sturdy and will last much longer (that's an opinion based on using a lot of USB-B port devices over the years). The actual USB cables tend to be of much higher quality (and better shielded too) for USB-B port connections so again, it's another little thing that leans me towards the RSP.

Been reading every review that I find and I honestly haven't found anyone rail against the RSP so far - I consider that to be a very good thing, and not a little one in that respect.

Thanks for your time, and congrats to the creator/developer of SDRPlay and well wishes for the future. I hope to become an owner in the next few months (being retired on a somewhat limited income does tend to cut into my ability to buy all the cool new toys I want) and so far it looks like the RSP is "the winner" in the race so far.

Have fun, always...
Last edited by br0adband on Thu Jan 01, 1970 12:00 am, edited 0 times in total.
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The difference between genius and stupidity? Genius has limits.

Tech_Support
Posts: 202
Joined: Mon Jun 01, 2015 7:00 pm

Re: 8 MHz bandwidth - is there a reason for this that can be explained?

Postby Tech_Support » Thu Sep 24, 2015 1:40 pm

Dear Broadband,
The 8 MHz bandwidth (4 MHz in I and Q in ZIF mode = 8 MHz double sideband) is set by the analog filters on the RF tuner chip within the RSP. These are currently automatically calibrated, so as things stand, if you try to monitor 10 MHz of Spectrum, you will see attenuation at the extreme edges of what you are monitoring. That said, this should still work fine as the filters are well downstream of the RF front end and so the attenuation should not impact the NF by any significant degree. The only thing to be aware of is that if you have strong unwanted signals in the middle of the segment of spectrum that you are monitoring, you will find that your dynamic range is a little more limited than if everything lay comfortably within the pass-band of the filter.
Longer term, it may be possible to slightly widen the filters (or even narrow them). The tuner does have a manual trim mode for the filters, but at the moment the API that we have from Mirics does not allow for any manual trimming of the filters. Whilst we cannot at this stage make any promises, it is possible that having the ability to tweak the filter may be something that we can add as an advanced feature at some point in the future. Even then, we cannot guarantee any specific bandwidth due to on-chip component variations. The ultimate limit will be determined by the maximum ADC sample rate, beyond which aliasing will become a serious problem. The maximum sample rate for I and Q is around 10 MS/s and so this gives a practical limit of around 10 MHz.

We try to avoid making comparisons with other products, but in our understanding the Airspy is also a good product. Whichever you choose, we hope you get many years of enjoyment from using it.

Sincerely

SDRplay Tech_Support
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br0adband
Posts: 2
Joined: Thu Sep 24, 2015 12:38 pm

Re: 8 MHz bandwidth - is there a reason for this that can be explained?

Postby br0adband » Thu Sep 24, 2015 4:14 pm

Thanks for the response, it's basically what I expected based on my understanding of the hardware chosen to make up the RSP, no worries. It's still the favorite right now - I'm doing a pros/cons list for both the RSP and Airspy and the RSP is considerably ahead with more pros and less cons so far. :D

Have fun, always...
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The difference between genius and stupidity? Genius has limits.

CaptainNemo
Posts: 114
Joined: Mon Jan 05, 2015 1:22 pm
Location: Genoa - Italy

Re: 8 MHz bandwidth - is there a reason for this that can be explained?

Postby CaptainNemo » Fri Sep 25, 2015 9:16 am

Tech_Support wrote:... The tuner does have a manual trim mode for the filters, but at the moment the API that we have from Mirics does not allow for any manual trimming of the filters. Whilst we cannot at this stage make any promises, it is possible that having the ability to tweak the filter may be something that we can add as an advanced feature at some point in the future. ...


Generally speaking, in the Software Defined Radio arena, making available as much API as possible is -- by-definition -- the right way.
You (sdrplay tech team & mirics team I suppose) just did a good job in this way with the last release, removing the "gap" 380-420 or so.
Well done! Go on this way!
(Do not forget to expose API for the DSP processor on board the MSi2500 if possible)

I'm an old-fashion sql programmer/functional analyst so I guess I'll never be able to develop sw for sdrplay
but I'm sure a lot of people in the "open and young community" could exploit the features of sdrplay.

c. nemo
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