射频通讯RF基础二

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TX
RX Range
OBS: The antenna is a key component for
the successful design of a wireless communication system.
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Adding an antenna – Not a broadband 50 ohm impedance
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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)
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.
• 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
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
- CC112x Used !
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Practical Range Estimation - Range Testing CC112x in Cape Town – 868 MHz, 14dBm, 1.2kbps
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Practical Range Estimation with Real Life Test Example
Transmit Gain (GTX)
Receiver Gain (GRX)
Tx Output Power
Range
Rx Sensitivity
Frequency
Antenna Heights
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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.
Typical Lab Equipment Impedance One Specific Antenna Impedance
S11
Smith chart shows how the impedance varies with frequency. Useful tool to find the values of the antenna matching component values. VSWR circles can be used to see how well the antenna is matched.
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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
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
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Adding an antenna – 5. Wavelength () All Monopole Antennas are Derivatives of Dipole
AC current through an inductor lags the voltage by 90 degrees *1: /2 Dipole produces the most power at the ends of the antenna with little power at the feedline.
*2
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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.
RF Basics II
Richard Wallace
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RF Basics II
• RF Basics I covered the following topics:
– – – – – Output Power Sensitivity Level Operating Frequency Blocking / Selectivity Regulatory Requirements
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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
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
BW = 100 ( (FH – FL) / FC)
S11
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VSWR
3
2
-9.5 dB
1.5
• There is a direct relationship between Q and bandwidth
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Frequency fL fH fc
fH fL Q fc
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Adding an antenna – 3. Radiation Efficiency (r) & Overall Efficiency (O)
1. Mismatch (ZTX)
PFWD PREF
PRAD
PRAD
1. Mismatch (ZRX)
PFWD RX PREF
5. Wavelength ()
TX
6. Height above GND (hTX)
6. Height above GND (hRX)
7. Range Distance
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TI defines the antenna’s bandwidth in Hertz when VSWR less than 2:1, or return loss of greater than -9.5dB
Infinite
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• Bandwidth (BW) can be defined in percentage of the operating frequency:
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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
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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.
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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)
– 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.
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Practical Range Estimation with Real Life Test Example – 868 MHz, 14dBm, 1.2kbps
- 1020 packets received with 0% PER at 25km. -Standard omnidirectional antenna - No PAs or LNAs used
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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