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AN RFMD® APPLICATION NOTE
Low/Medium Power Channel Selective
Repeater Application
Using the RF2051 Wideband Synthesizer/PLL with Integrated Mixers
Overview
A cellular repeater is a system of duplex reception, amplification, and transmission used to enhance uplink (UL) and downlink
(DL) signals in areas of low radio coverage. This enhancement expands the coverage of cellular network base transceiver stations (BTS) at low cost. The signals at the input and output ports of the two transceivers (TRX), each dedicated for the UL or DL
direction of signal reception, amplification, and re-transmission, are multiplexed at a directional donor and local area antennas
by means of diplexers in FDD systems such as GSM and WCDMA FDD.
For the DL path, assuming a fixed link with the BTS, a signal of relatively slow-varying power is amplified to a fixed level suitable
for adequate radio coverage and QoS in the target local area. For the UL path, a signal of varying power due to slow-fading from
user equipment within the local target area is received and amplified at a fixed power level before re-transmission to the BTS.
Signal integrity on both the DL and UL paths should be maintained over the widest possible range of input levels in terms of
modulation accuracy (EVM) and spectral content. In addition, the levels of in-repeater-band wideband noise and spurious
emissions at the repeater output should comply with 3GPP requirements [1,2].
RF MICRO DEVICES®, RFMD®, Optimum Technology Matching®, Enabling Wireless Connectivity™, PowerStar®, POLARIS™ TOTAL RADIO™ and UltimateBlue™ are trademarks of RFMD, LLC. BLUETOOTH is a trademark owned by Bluetooth SIG, Inc., U.S.A. and licensed for use by RFMD. All other trade names, trademarks and registered trademarks are the property of their respective owners. ©2010, RF Micro Devices, Inc.
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Low/Medium Power Channel Selective Repeater Application
DOWNLINK P ATH
VREF
IF VGA
IF SAW
BAL
UN
DET
DL PA
RF SAW
RF SAW
÷1
DL LNA
÷1
Charge
Pump
Dup lexer
MUX
Frac-N
Se qGen
Fr ac- N
Seq Gen
Phase/
Freq Det
Charge
Pump
MUX
Phase/
Freq Det
÷N
÷R
÷1
UL LNA
DONOR
ANTENNA
÷R
÷N
Dup lexer
LOCAL ARE A
ANTENNA
CXO
RF2051
BAL
UN
VVA
RF2051
÷1
UL PA
RF SAW
RF SAW
IF SAW
BAL
UN
BAL
UN
VVA
DET
VREF
UPLI NK PATH
Figure 1. Generic repeater architecture with RF2051
Figure 1 shows a generic architecture of a channel-selective repeater. The corresponding operations of the UL and DL TRX subsystems are equivalent. An RF signal with power levels between SMIN and SMAX at the TRX input is amplified by an LNA stage,
down-converted to IF, filtered and amplified by a VGA, up-converted back to RF and, finally, amplified by the power amplification
stages at a nearly constant output power level P0. The output power is dynamically maintained at the prescribed level for slowly
varying input power via a closed loop power control scheme, which detects the average output power via a detector and logamp, and compares it with a reference level voltage obtained from power calibration.
Down-conversion to IF provides certain advantages:
• Effective filtering of interfering signals outside the repeater pass band associated with the dedicated BTS and carrier frequency
• Wide dynamic range and robust power control design via the IF VGA
• Better isolation between the UL-TRX and DL-TRX paths
The selection of the IF, along with the specification of the front- and back-end RF filtering is critical in attenuating higher order
mixer products to levels below the in- and out-of-repeater-band emission limits.
RF2051, with its integrated PLL and two mixers, augmented by suitable LNA, IF VGA, and driver/PA subsystems, is ideally
suited to perform the down- and up-conversion functions of the architecture shown in Figure 1 in a compact and efficient manner. It provides a high spurious free dynamic range, with low noise and low spurious content TRX paths.
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DO WNLINK PAT H (21 10-2 170M Hz)
UPLI NK PATH (1 920- 1980M Hz)
Low/Medium Power Channel Selective Repeater Application
Figure 2. Experimental prototype of the repeater implementation with RF2051
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Low/Medium Power Channel Selective Repeater Application
Figure 3. Schematic of the prototype
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Low/Medium Power Channel Selective Repeater Application
General Repeater Characteristics
Of particular interest, especially in the UL path, is the dynamic range of the TRX subsystem. This range is the difference
between the maximum and minimum input signal that the TRX system can accommodate for a fixed output power level P0. In
general, the maximum allowable input signal SMAX is constrained by the minimum available gain of the receiver, the compression point of the TRX system, and regulatory limits of ACPR, spectrum emissions, and in-repeater-band intermodulation products for a multi-carrier input scenario. On the other hand, the minimum possible input signal SMIN is a function of the maximum
available gain GO, along with the desirable output power level P0, and the in-repeater-band wideband noise. In RF2051, GO is
approximately 70dB, while there is sufficient attenuation control range in the IF VGA to support a high level input before a saturation of any of the stages occur. Ultimately, the DR of the repeater will be constrained by its wideband noise and linearity performance.
The cascaded NF, along with total gain, will determine the thermal noise floor at the output of the TRX. For large values of gain
required for low input signal levels, the cascaded NF mainly depends on the front end and 1st mixer NF while the output noise
power increases monotonically with system gain. The level of the thermal noise at the output of the TRX as a function of the
effective system noise BW (3.84MHz for WCDMA) will be given by
N MAX  dBm  = – 173.9(dBm  Hz) + log 10 10 BW + NF  dB  + P O – S MIN
(1)
Given the maximum allowable adjacent channel noise to signal power ratio
ACPR = N MAX – P O
(2)
at the output, the minimum input signal level SMIN can be obtained from equations (1) and (2).
On the other hand, the P1dB and IP3 points of the two mixers define the cascaded linearity parameters of the TRX paths. For
WCDMA, the ACPR is approximately related with the OIP3 of the system via
ACPR  – 20.7 + 1.6  PAR + 2   S MAX – IIP3 
(3)
where the UL PAR is less than 5 dB. The maximum acceptable ACPR for the given channel output power PO will determine the
cascaded required IP3 and P1dB points, as well as the maximum input signal level SMAX. A summary level diagram of a 10dBm
WCDMA repeater with the RF2051 is shown in Table 1.
Table 1: Level Diagram for 10dBm Repeater
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Low/Medium Power Channel Selective Repeater Application
Repeater Demonstrator
The characteristics and performance of a low power (10dBm), channel-selective WCDMA repeater are demonstrated employing a prototype system built around two RF2051 ICs, along with LNAs (RF2374), drivers (RF2374), PAs (SXA-389B), and IF VGA
stages (SBB-2089 and third party VGA). Figure 2 and Figure 3 show the repeater demonstrator board and the schematic diagram of its UL path, while the schematic diagram of the DL is similar. The LNA stages are specified with a nominal gain of
16.5dB, an IIP3 of 9dBm, and an NF of 1.2dB, while the driver and PA stages have a nominal linear gain of 30dB and an output P1dB of 25dBm. An IF SAW filter with a 5MHz nominal bandwidth is centered at the selected IF of 172.8MHz.
20
80
15
70
10
60
5
50
O u tp u t
Power
(d B m )
0
40
-5
30
-10
Pout
Pout
Pout
Pout
G a in
G a in
G a in
G a in
-15
-20
-25
- 100
(d B m-) A T T = 0 d B
(d B m-) A T T = 1 0 d B
(d B m-) A T T = 2 0 d B
(d B m-) A T T = 3 0 d B
(d B-) A T T = 0 d B
(d B-) A T T = 1 0 d B
(d B-) A T T = 2 0 d B
(d B-) A T T = 3 0 d B
20
10
0
- 90
- 80
-70
- 60
- 50
- 40
-30
-20
In p u t S ig n a l P o w e r (d B m )
Figure 4. Output Power and Gain Performance
-20
ACPRACPRACPRACPRACPRACPR-
-25
-30
ATT=0dB (64
- DPCH)
ATT=10dB (64
- DPCH)
ATT=20dB (64
- DPCH)
ATT=0dB (1- DPCH)
ATT=10dB (1
- DPCH)
ATT=20dB (1
- DPCH)
-35
WCDMA
ACPR-40
(dB)
-45
-50
-55
-60
-75
-70
-65
-60
-55
-50
-45
-40
-35
Input Signal Power (dBm)
Figure 5. ACPR versus Input Signal Power: WCDMA 64-DPCH-1C(TM1) and 1-DPCH
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Low/Medium Power Channel Selective Repeater Application
Figure 4 and Figure 5 summarize the performance of the repeater, in terms of its power and gain, as well as the dynamic range
constraints due to wideband noise and intermodulation. Figure 5 illustrates the effect of the wideband noise for low input signals and correspondingly high gain settings, while the maximum input signal is limited by the intermodulation performance
and the maximum acceptable adjacent channel power ratio. It is worth pointing out that the ACPR, on the left of the of its minimum point in Figure 5, increases linearly with decreasing signal level or increasing gain, whereas on the right of this point it
increases approximately 2dB per every 1dB increase in signal input level. In this case, an input signal range from -60dBm up
to -30dBm can be accommodated while maintaining an ACPR level below 35dB and meeting the spectrum emissions
requirements1 with adequate margin. This 30dB dynamic range and output power can be extended by higher gain and an IP3
power amplifier.
References
1
3GPP TS 25.106, UTRA Repeater Radio Transmission and Reception (Release 8), V8.0.2, 2008-03.
2
3GPP TR 45.050, Background for Radio Frequency (RF) Requirements, Annex E: Repeater Scenarios, V8.10.0, 2008.
3
RFMD, RF2051 High Performance Wideband RF Synthesizer/VCO with Integrated RF Mixers, DS080513.
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