# Selectivity and sensitivity in SDR Hardwares

Finally we worked on RTL-SDR and HackRF One and USRP in practical test and different moudlation like GFSK,DVB-T,ATSC,OFDM,DMR.With all of them we can see spectrum, it is not final purpose, we emphasize ,purpose is data in digital radio communication.So which SDR hardware can handle  data extracting with different moudation and protocols like GSM or even LTE and so on.

So we want to how we can select best sdr hardware according our purpose and our budget.So please for buy new SDR check these important paramters.We can define sensitiviy with helping this link.

The sensitivity of an electronic device, such as a communications system receiver, or detection device, such as a PIN diode, is the minimum magnitude of input signal required to produce a specified output signal having a specified signal-to-noise ratio, or other specified criteria.

Sensitivity is sometimes improperly used as a synonym for responsivity.[citation needed][1]

The sensitivity of a microphone is usually expressed as the sound field strength in decibels (dB) relative to 1 V/Pa (Pa = N/m2) or as the transfer factor in millivolts per pascal (mV/Pa) into an open circuit or into a 1 kilohm load.[citation needed]

The sensitivity of a loudspeaker is usually expressed as dB / 2.83 VRMS at 1 metre.[citation needed] This is not the same as the electrical efficiency; see Efficiency vs sensitivity.

The sensitivity of a hydrophone is usually expressed as dB re 1 V/µPa.[citation needed]

Sensitivity in a receiver is normally defined as the minimum input signal S i {\displaystyle S_{i}} S_{i} required to produce a specified signal-to-noise S/N ratio at the output port of the receiver and is defined as the mean noise power at the input port of the receiver times the minimum required signal-to-noise ratio at the output of the receiver:

S i = k ( T a + T r x ) B ⋅ S o N o {\displaystyle S_{i}=k(T_{a}+T_{rx})B{\cdot }{\frac {S_{o}}{N_{o}}}} S_{i}=k(T_{a}+T_{{rx}})B{\cdot }{\frac {S_{o}}{N_{o}}}

where

S i {\displaystyle S_{i}} S_{i} = sensitivity [W]
k = Boltzmann’s constant
T a {\displaystyle T_{a}} T_{a} = equivalent noise temperature in [K] of the source (e.g. antenna) at the input of the receiver
T r x {\displaystyle T_{rx}} T_{{rx}} = equivalent noise temperature in [K] of the receiver referred to the input of the receiver
B = bandwidth [Hz]
S o N o {\displaystyle {\frac {S_{o}}{N_{o}}}} {\frac {S_{o}}{N_{o}}} = Required SNR at output [-]

Selectivity is a measure of the performance of a radio receiver to respond only to the radio signal it is tuned to (such as a radio station) and reject other signals nearby in frequency, such as another broadcast on an adjacent channel.

Selectivity is usually measured as a ratio in decibels (dBs), comparing the signal strength received against that of a similar signal on another frequency. If the signal is at the adjacent channel of the selected signal, this measurement is also known as adjacent-channel rejection ratio (ACRR).

Selectivity also provides some immunity to blanketing interference.

So we here introduce some of important paramters in sensitivity and selectivity.

• Frequency precision( Oscillator Precision )

SDR hardware has a paramter that is called Oscillator Precision ,only you should buy a SDR hardware with 1ppm or more less, In case over 1pmm, i advise do not wasting your money.

Buying a SDR with 8bit ADC,is wasting money.i advise  that once  buy a high quality SDR Hardware  instead buy some SDR,that every time you have to switch high quality SDR hardware, So You should buy SDR hardware with 12bit or better more than 12 bit like blade RF (for low cost case) and USRP for (high cost case).

• USB3

I strongly advice buy a SDR with USB3, USB2 in higher and complicated protcol is not good and ideal result.

• Programmable Logic Gates

For FPGA filter idealization, i advise use a SDR that is in FPGA with more than 40k,For example HackRF one use CPLD ,So it is very weak in receiving.

• Output pwoer

Output power is not important,because in practical we should design power amplifier, but it is good in range 10dB.

HackRF One Ettus B200 Ettus B210 BladeRF x40 RTL-SDR LimeSDR
Frequency Range 1MHz-6GHz 70MHz-6GHz 70MHz-6GHz 300MHz-3.8GHz 22MHz-2.2GHz 100kHz-3.8GHz
RF Bandwidth 20MHz 61.44MHz 61.44MHz 40MHz 3.2MHz 61.44MHz
Sample Depth 8 bits 12 bits 12 bits 12 bits 8 bits 12 bits
Sample Rate 20MSPS 61.44MSPS 61.44MSPS 40MSPS 3.2MSPS 61.44MSPS (Limited by USB 3.0 data rate)
Transmitter Channels 1 1 2 1 0 2
Receivers 1 1 2 1 1 2
Duplex Half Full Full Full N/A Full
Interface USB 2.0 USB 3.0 USB 3.0 USB 3.0 USB 2.0 USB 3.0
Programmable Logic Gates 64 macrocell CPLD 75k 100k 40k (115k avail) N/A 40k
Price $299$686 $1,119$420 ($650) ~$10 $299 ($289 pre-order)