Multitasking in SDR (DVB-T and FM)?

I am interested in knowing whether LimeSDR (or any SDR) can do multi-tasking? For example, Will I be able to run an FM receiver as well as DVB (or watch TV) at the same time in the same device? I understand that there is only one analog RFFE, but is there a way in which both operations can be achieved?

Provided the signals fit within the device bandwidth, yes. For example, SDRangel supports having multiple different demodulators configured and I’ve not tried this, but would assume that you just “digitally tune” from the radio front-end centre frequency.

Ok. Good to know. Will try it. I was under the impression that it is not possible to do both the things simultaneously.

Well, it’s not quite that simple. There are four limiting factors on how many distinct signals you can receive at one time:

  1. Frequency and bandwidth. If you want to look at a DVB signal at 400MHz, and a broadcast FM signal at 98.1MHz, you would need to be able to have both signals in the passband of the SDR, and be sampling fast enough to meet Nyquist’s limit on them, so you would need roughly 300MHz bandwidth and be sampling at 600MSample/sec. Some SDR chips (not the Lime, unfortunately) have dual receive paths that are totally independent, so you can treat them as two SDRs. The Lime’s dual receive paths are more intended for MIMO (multiple in, multiple out) where the signals are at the same frequency.
  2. Transfer path capacity. You would have to get the samples into the processing element (your computer). Pushing 600MSample/sec as per the example would really stress even USB3.
  3. Processor capacity. Once you pulled all that into your computer, you could have to then process the signals. Not only do you have the overhead of the FM demodulator and the DVB demodulator, but you would also have to have a component taking one of the signals (probably the FM signal) and moving it from being off-center to being centered, and that takes some processing.
  4. Dynamic range. If the DVB signal is down around -90dBm/Hz, and the FM signal is blasting away at -40dBm/Hz, you might have the situation where the DVB signal is below the noise floor of the SDR if you have the AGC set for the FM signal, or conversely if you crank the AGC to pull the DVB signal out of the noise, you overload the SDR with the FM signal.
    Now, I made an assumption that may not be correct - if the DVB signal is “close” to the FM signal, then you might be able to do this better, as the bandwidth needed to cover them both would be less. And if they are both of roughly the same power, the dynamic range issue goes away. But it’s not as simple as saying “yes you can” or “no you cannot”, we need to know a bit more of the specifics.

Item 1 on your list, you would be heading directly towards ITAR (International Traffic in Arms Regulations). For each individual restricted item sold you would need the person who wanted to buy the product to provide signed legal documents prior to the sale, government import approval, governmental export approval prior to sale for the individual and country. The country must be on an approved list for that category of item. And part of the legally binding contract would be that the product could not be re-sold or re-exported to a different person or country without new contracts. The legal documentation even cover how the product can eventually be scrapped.
“Regulations”-> “EAR (Export Administration Regulations)”->“Category 3 - Electronics Design Development and Production”

The friendlier sounding EAR (Export Administration Regulations) which is administered by BIS (Bureau of Industry and Security) currently sanction what can be exported globally without such draconian restrictions, and for ADC’s that is currently (last updated 2018-10-29, it changes often):

  • a.5.a.1. A resolution of 8 bit or more, but less than 10 bit, with a “sample rate” greater than 1.3 Giga Samples Per Second (GSPS);
  • a.5.a.2. A resolution of 10 bit or more, but less than 12 bit, with a “sample rate” rate greater than 600 Mega Samples Per Second (MSPS);
  • a.5.a.3. A resolution of 12 bit or more, but less than 14 bit, with a “sample rate” greater than 400 Mega Samples Per Second (MSPS);
  • a.5.a.4. A resolution of 14 bit or more, but less than 16 bit, with a “sample rate” greater than 250 Mega Samples Per Second (MSPS); or
  • a.5.a.5. A resolution of 16 bit or more with an “sample rate” greater than 65 Mega Samples Per Second (MSPS);

I agree with everything else you said, but I thought that I would highlight why you will not see cheap, high dynamic range, ultra wide band ADC’s, at least not until ITAR/EAR restrictions are relaxed more.

Regarding point 1.
LMS7002M has double ADCs, for quadrature sampling - thus the sampling rate can be Nyquist’s limit / 2.

Source: LMS7002M-DataSheet