射频通讯RF基础二

- CC112x Used !
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Practical Range Estimation - Range Testing CC112x in Cape Town – 868 MHz, 14dBm, 1.2kbps
VSWR 1 S11dB 20log() 20log VSWR 1
Forward Power (PFWD) ZTx S11 ZAntenna Reflected Power (PREF)
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Adding an antenna – 4. Antenna Directivity / Gain
*2
6
Adding an antenna – 2. Antenna Bandwidth
Properly designed antenna’s should cover the range of frequencies over which the antenna can operate correctly with sufficient bandwidth.
4
Adding an antenna – Major Antenna Parameters effecting Range Distance
2. Bandwidth (BWTX) 3. Efficiency (TX) 4. Directivity (DTX)
2. Bandwidth (BWRX) 3. Efficiency (RX) 4. Directivity (DRX)
RF Basics II
Richard Wallace
1
RF Basics II
• RF Basics I covered the following topics:
– – – – – Output Power Sensitivity Level Operating Frequency Blocking / Selectivity Regulatory Requirements
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Adding an antenna – Basic Function of an Antenna
• Transmit mode: Transform RF signals into electromagnetic waves, propagating into free space
Receive mode: Transform electromagnetic waves into RF signals
•
Gain, G. is the maximum radiation beam of the highest beam. This parameter takes into account VSWR mismatch and energy losses.
– Important to remember that antennas do not amplify RF. Since antennas cannot create energy, the total power radiated is the same as an isotropic antenna. Any additional energy radiated in the directions it favors is offset by equally less energy radiated in all other directions. IEEE Gain Definition: GIEEE = Radiated Power / Delivered Power = O D
BW = 100 ( (FH – FL) / FC)
S11
5
VSWR
3
2
-9.5 dB
1.5
• There is a direct relationship between Q and bandwidth
1
Frequency fL fH fc
fH fL Q fc
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Adding an antenna – 3. Radiation Efficiency (r) & Overall Efficiency (O)
• RF Basics II will show the impact of the previously covered topics in a typical compact wireless product and cover additional topics:
– Adding an antenna – Practical Range Expectation with Real Life Test Example – Choosing the optimum ISM band
Adding an antenna – 1. Mismatch and VSWR
Forward Power (PFWD) ZTx ZAntenna Reflected Power (PREF)
Output Power delivered to antenna
S11
*1 *3
*2
*4
*1 *3 (S11) *4
915 MHz 868 MHz 433 MHz
315 MHz
169 MHz
23.8
44.4
9.4
17.5
Balance between operating frequency and available board space for antenna and GND.
• Lower frequency increases the range
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Practical Range Estimation - Range Testing CC112x in Cape Town – 868 MHz, 14dBm, 1.2kbps Transmit antenna (2.2dB: GTX) Direct transmission
Receive antenna (2.2dB: GRX)
–
•
–
Directivity, D. Antenna directivity is usually measured in dBi, or decibels above isotropic sphere antenna.
The directional antenna has a maximum directivity greater than 0dB.
•
TX
RX Range
Hale Waihona Puke OBS: The antenna is a key component for
the successful design of a wireless communication system.
3
Adding an antenna – Not a broadband 50 ohm impedance
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Adding an antenna – 5. Frequency “v” Size “v” Antenna Efficiency
λ/4 (cm) 3.1 8.2 8.6 17.3 λ/4 (inch) 1.2 3.2 3.4 6.8
Frequency 2.4 GHz
Antenna Feed Ground Plane
– Theoretically, reducing the frequency by a factor of two doubles the range (line of sight)
• Lower frequency increases the antenna size
– Lower frequency requires a larger antenna (larger wavelength). – For a limited board size, lowering the frequency decreases radiated efficiency.
Current Voltage Power
/4
*1
/4 *1
Antenna Feed
/4
Ground Plane
All monopole antennas are derivatives of a simple dipole where one ¼ wavelength radiator is in air and one ¼ wavelength radiator is imaged into the GND and serves as the second radiator.
hTX : 1000m TX
Reflection law Θi=Θr Θi
Reflected transmission
Θr
hRX: 1m RX
d?
Link Budget = Output Power + Transmit Antenna gain (GTX) + Receiver Antenna gain (GRX) – Sensitivity Link Budget = 14 + 2 + 2 – 120 = 138 dB
•
Resonance Frequency is the electrical resonance is related to the electrical length of the antenna. Polarization the direction of the electric field to the electromagntic wave.
1. Mismatch (ZTX)
PFWD PREF
PRAD
PRAD
1. Mismatch (ZRX)
PFWD RX PREF