Electronics > RF, Microwave, Ham Radio

Question about wideband SDRs

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mehdi:
Hi
I was thinking why some SDRs (like bladeRF and USRP B200) have lower limits (300MHZ and 70MHZ respectively)
This is the limitation of underlying RF chipset (for example LimeMicro's) and now there are wideband SDRs that support DC-3.8 or DC-6 GHZ
(like the upcoming LimeSDR)
Someone told me supporting lower frequencies introduces some design problems. So my question is whether newer devices  have problems (lower quality/sensitivity/selectivity/etc) in HF/VHF or those problems are fixed and today's technology has addressed them?
Are there any tradeoffs when designing/building wideband SDRs/receivers?
Thanks in advance

Fank1:
Most likely limited by the local oscillator in the chip.
SDR is nothing more than direct conversion with 2 mixers and a quadrature LO feeding them.
Google YU1LM he has some easy to build designs that work well for the low bands.

KE5FX:
Wideband (meaning HF-microwave) SDR receivers tend to work by running a local oscillator at several GHz, dividing it down as necessary to perform direct downconversion of the target frequency to baseband.   This technique works well for the most part, but it runs out of steam at low frequencies because the receiver is almost as sensitive at odd harmonics of the LO as it is at the fundamental.  If you were to extend a typical AD9361 or Lime Micro-based SDR's coverage down to the LF/lower-HF region by adding more divider stages, you would need to add increasingly elaborate front-end filtering to avoid interference from strong signals in the lower VHF region such as FM and TV broadcast, pagers, and who knows what else.

The best way to get lower-frequency coverage out of these chipsets is with an upconverter that translates the LF-HF spectrum up to a VHF or UHF IF that the SDR can handle.  That still requires filtering but at least it doesn't need to be bank-switched, the way it would be if you tried to perform direct conversion all the way down to LF.

As RF ADCs get better, it starts to make more sense to switch the antenna directly to the ADC input for low-band coverage.  But this isn't feasible yet for the popular ultra-low cost SDR dongles.

mehdi:

--- Quote from: KE5FX on July 17, 2016, 06:59:04 pm ---...
As RF ADCs get better, it starts to make more sense to switch the antenna directly to the ADC input for low-band coverage.  But this isn't feasible yet for the popular ultra-low cost SDR dongles.

--- End quote ---

So that's why we don't have wide-band direct-conversion SDRs? (To my knowledge, current direct-conversion based systems only support HF and some parts of VHF)
If I understood you correctly, with the mass-production of advanced and high-speed ADCs, we can have HF-Microwave direct-conversion receivers which somehow fix the problem we have now (filtering, harmonics, etc.). Right?

KE5FX:

--- Quote from: mehdi on July 18, 2016, 09:32:54 am ---
--- Quote from: KE5FX on July 17, 2016, 06:59:04 pm ---...
As RF ADCs get better, it starts to make more sense to switch the antenna directly to the ADC input for low-band coverage.  But this isn't feasible yet for the popular ultra-low cost SDR dongles.

--- End quote ---

So that's why we don't have wide-band direct-conversion SDRs? (To my knowledge, current direct-conversion based systems only support HF and some parts of VHF)
If I understood you correctly, with the mass-production of advanced and high-speed ADCs, we can have HF-Microwave direct-conversion receivers which somehow fix the problem we have now (filtering, harmonics, etc.). Right?

--- End quote ---

Yes, that's where things are headed, although slowly.  The best 12/14-bit ADCs are capable of direct conversion up to 1.5 GHz or so in their first Nyquist zone, with no filtering beyond an LPF in the front end.  Unfortunately they cost multiple thousands of dollars per part, they have demanding power requirements, and they can only talk to JESD204B cores that run on higher-speed FPGAs.  So low-cost receivers are going to have to rely on mixing for a while until the mass market catches up.

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