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19-1350 Rev 2; 2/99
ANUAL N KIT M LUATIO ATA SHEET EVA WS D FOLLO
900MHz Image-Reject Receivers with Transmit Mixer
____________________________Features
o Receive Mixer with 35dB Image Rejection o Adjustable-Gain LNA o Up to +2dBm Combined Receiver Input IP3 o 4dB Combined Receiver Noise Figure o Optimized for Common Receiver IF Frequencies: 10.7MHz (MAX2424) 70MHz (MAX2426) o PA Predriver Provides up to 0dBm o Low Current Consumption: 23mA Receive 20mA Transmit 9.5mA Oscillator o 0.5A Shutdown Mode o Operates from Single +2.7V to +4.8V Supply
________________General Description
The MAX2424/MAX2426 highly integrated front-end ICs provide the lowest cost solution for cordless and ISMband radios operating in the 900MHz band. Both devices incorporate a receive image-reject mixer (to reduce filter cost) as well as a versatile transmit mixer. The devices operate from a +2.7V to +4.8V single power supply, allowing direct connection to a 3-cell battery stack. The receive path incorporates an adjustable-gain LNA and an image-reject downconverter with 35dB image suppression. These features yield excellent combined downconverter noise figure (4dB) and high linearity with an input third-order intercept point (IIP3) of up to +2dBm. The transmitter consists of a double-balanced mixer and a power amplifier (PA) predriver that produces up to 0dBm (in some applications serving as the final power stage). It can be used in a variety of configurations, including BPSK modulation, direct VCO modulation, and transmitter upconversion. For devices featuring transmit as well as receive image rejection, refer to the MAX2420/MAX2421/MAX2422/MAX2460/MAX2463 data sheet. The MAX2424/MAX2426 have an on-chip local oscillator (LO), requiring only an external varactor-tuned LC tank for operation. The integrated divide-by-64/65 dual-modulus prescaler can also be set to a direct mode, in which it acts as an LO buffer amplifier. Four separate powerdown inputs can be used for system power management, including a 0.5A shutdown mode. The MAX2424/MAX2426 come in a 28-pin SSOP package.
MAX2424/MAX2426
_______________Ordering Information
PART MAX2424EAI MAX2426EAI TEMP. RANGE -40C to +85C -40C to +85C PIN-PACKAGE 28 SSOP 28 SSOP
___________________Pin Configuration
TOP VIEW
VCC 1 CAP1 2 RXOUT 3 GND 4 RXIN 5 28 GND 27 GND 26 GND 25 TANK
________________________Applications
Cordless Phones Wireless Telemetry Wireless Networks Spread-Spectrum Communications Two-Way Paging
VCC 6 GND 7 GND 8 TXOUT 9 LNAGAIN 10 VCC 11 TXIN 12 TXIN 13
MAX2424 MAX2426
24 TANK 23 VCC 22 VCC 21 PREOUT 20 PREGND 19 MOD 18 DIV1 17 VCOON 16 RXON 15 TXON
Functional Diagram appears at end of data sheet.
CAP2 14
SSOP ________________________________________________________________ Maxim Integrated Products 1
For free samples & the latest literature: http://www.maxim-ic.com, or phone 1-800-998-8800. For small orders, phone 1-800-835-8769.
900MHz Image-Reject Receiver with Transmit Mixer MAX2424/MAX2426
ABSOLUTE MAXIMUM RATINGS
VCC to GND ...........................................................-0.3V to +5.5V TXIN, TXIN Differential Voltage ..............................................+2V Voltage on TXOUT......................................-0.3V to (VCC + 1.0V) Voltage on LNAGAIN, TXON, RXON, VCOON, DIV1, MOD, TXIN, TXIN............................-0.3V to (VCC + 0.3V) RXIN Input Power..............................................................10dBm TANK, TANK Input Power ...................................................2dBm Continuous Power Dissipation (TA = +70C) SSOP (derate 9.50mW/C above +70C) ......................762mW Operating Temperature Range ...........................-40C to +85C Junction Temperature ......................................................+150C Storage Temperature Range .............................-65C to +165C Lead Temperature (soldering, 10sec) .............................+300C
Stresses beyond those listed under "Absolute Maximum Ratings" may cause permanent damage to the device. These are stress ratings only, and functional operation of the device at these or any other conditions beyond those indicated in the operational sections of the specifications is not implied. Exposure to absolute maximum rating conditions for extended periods may affect device reliability.
DC ELECTRICAL CHARACTERISTICS
(VCC = +2.7V to +4.8V, no RF signals applied, LNAGAIN = Unconnected, VTXIN = V TXIN = 2.3V, VVCOON = 2.4V, VRXON = VTXON = VMOD = VDIV1 = 0.45V, PREGND = GND, TA = -40C to +85C. Typicals are at TA = +25C, VCC = 3.3V, unless otherwise noted.) PARAMETER Supply-Voltage Range Oscillator Supply Current Prescaler Supply Current (/ 64/65 mode) (Note 1) Prescaler Supply Current (buffer mode) (Note 2) Receive Supply Current (Note 3) Transmitter Supply Current (Note 4) Shutdown Supply Current Digital Input Voltage High Digital Input Voltage Low Digital Input Current VDIV1 = 2.4V VRXON = 2.4V, PREGND = unconnected VRXON = 0.45V, VTXON = 2.4V, PREGND = unconnected VCOON = RXON = TXON TA = +25C = MOD = DIV1 = GND TA = -40C to +85C RXON, TXON, DIV1, VCOON, MOD RXON, TXON, DIV1, VCOON, MOD Voltage on any one digital input = VCC or GND 1 2.4 0.45 10 PREGND = unconnected CONDITIONS MIN 2.7 9.5 4.2 5.4 23 20 0.5 10 TYP MAX 4.8 14 6 8.5 36 32 UNITS V mA mA mA mA mA A V V A
Note 1: Calculated by measuring the combined oscillator and prescaler supply current and subtracting the oscillator supply current. Note 2: Calculated by measuring the combined oscillator and LO buffer supply current and subtracting the oscillator supply current. Note 3: Calculated by measuring the combined receive and oscillator supply current and subtracting the oscillator supply current. With LNAGAIN = GND, the supply current drops by 4.5mA. Note 4: Calculated by measuring the combined transmit and oscillator supply current and subtracting the oscillator supply current.
2
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900MHz Image-Reject Receiver with Transmit Mixer
AC ELECTRICAL CHARACTERISTICS
(MAX2424/MAX2426 EV kit, VCC = +3.3V, fRXIN = 915MHz, PRXIN = -35dBm, VTXIN = V TXIN = 2.3V (DC bias), VTXIN = 250mVp-p, fTXIN = 1MHz, VLNAGAIN = 2V, VVCOON = 2.4V, RXON = TXON = MOD = DIV1 = PREGND = GND, TA = +25C, unless otherwise noted.) PARAMETER CONDITIONS MIN TYP MAX UNITS
MAX2424/MAX2426
RECEIVER (VRXON = 2.4V, fLO = 925.7MHz (MAX2424), fLO = 985MHz (MAX2426)) Input Frequency Range IF Frequency Range Image Frequency Rejection VLNAGAIN = VCC, TA = +25C (Note 7) Conversion Power Gain VLNAGAIN = VCC, TA = -40C to +85C (Notes 5, 7) VLNAGAIN = 1V (Note 7) LNAGAIN = GND (Note 7) Noise Figure Input Third-Order Intercept (IIP3) Input 1dB Compression LO to RXIN Leakage Receiver Turn-On Time LNAGAIN = VCC (Notes 5, 7) VLNAGAIN = 1V (Notes 5, 7) LNAGAIN = VCC (Notes 5, 8) VLNAGAIN = 1V (Notes 5, 8) LNAGAIN = VCC VLNAGAIN = 1V Receiver on or off (Note 9) -19 MAX2424 MAX2426 MAX2424 MAX2426 (Notes 5, 6) MAX2424 (Notes 5, 6) MAX2426 (Notes 5, 6) 800 8.5 55 26 20 19 19 18 12 -16 4 12 -17 -8 -26 -18 -60 500 5 dB dB dBm dBm ns 10.7 70 35 22 21 24.5 23.5 25 24 dB 1000 12.5 85 MHz MHz dB
TRANSMITTER (VTXON = 2.4V, fLO = 915MHz) Output Frequency Range Baseband 3dB Bandwidth Output Power Output 1dB Compression Output Third-Order Intercept (OIP3) Carrier Suppression Output Noise Density Transmitter Turn-On Time (Note 12) (Note 11) TA = +25C TA = TMIN to TMAX (Note 5) -9.5 -10 -0.5 3.5 30 -140 220 (Notes 5, 10) 800 125 -7 -5 -4.5 1000 MHz MHz dBm dBm dBm dBc dBm/Hz ns
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3
900MHz Image-Reject Receiver with Transmit Mixer MAX2424/MAX2426
AC ELECTRICAL CHARACTERISTICS (continued)
(MAX2424/MAX2426 EV kit, VCC = +3.3V, fRXIN = 915MHz, PRXIN = -35dBm, VTXIN = V TXIN = 2.3V (DC bias), VTXIN = 250mVp-p, fTXIN = 1MHz, VLNAGAIN = 2V, VVCOON = 2.4V, RXON = TXON = MOD = DIV1 = PREGND = GND, TA = +25C, unless otherwise noted.) PARAMETER OSCILLATOR AND PRESCALER Oscillator Frequency Range Oscillator Phase Noise (Note 5) 10kHz offset (Note 13) Standby to TX or Standby to RX Oscillator Pulling MAX2424 MAX2426 MAX2424 MAX2426 800 82 72 8 35 70 110 500 TA = +25C TA = -40C to +85C -11 -12 10 0 -8 mVp-p dBm ns ns kHz 1100 MHz dBc/Hz CONDITIONS MIN TYP MAX UNITS
RX to TX with PRXIN =-45dBm (RX mode) MAX2424 to PRXIN = 0dBm (TX mode) (Note 14) MAX2426 ZL = 100k | | 10pF VDIV1 = 2.4V, ZL = 50, ZL = 50 / 64/65 mode / 64/65 mode
Prescaler Output Level Oscillator Buffer Output Level (Notes 5, 13) Required Modulus Setup Time (Notes 5, 15) Required Modulus Hold Time (Notes 5, 15) Note 5: Note 6: Note 7: Note 8: Note 9: Note 10: Note 11: Note 12: Note 13: Note 14: Note 15:
Guaranteed by design and characterization. Image rejection typically falls to 30dBc at the frequency extremes. Refer to the Typical Operating Characteristics for a plot showing Receiver Gain vs. LNAGAIN Voltage, Input IP3 vs. LNAGAIN Voltage, and Noise Figure vs. LNAGAIN Voltage. Two tones at PRXIN = -45dBm each, f1 = 915.0MHz and f2 = 915.2MHz. Time delay from VRXON = 0.45V to VRXON = 2.4V transition to the time the output envelope reaches 90% of its final value. Output power typically falls to -10dBm at the frequency extremes. Two tones at VTXIN = 125mVp-p, f1 = 1.0MHz, and f2 = 1.2MHz. Time delay from VTXON = 0.45V to VTXON = 2.4V transition to the time the output envelope reaches 90% of its final value. Using tank components L3 = 5.0nH (Coilcraft A02T), C2 = C3 = C26 = 3.3pF, R6 = R7 = 10. This approximates a typical application in which TXOUT is followed by an external PA and a T/R switch with finite isolation. Relative to the rising edge of PREOUT.
4
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900MHz Image-Reject Receiver with Transmit Mixer
__________________________________________Typical Operating Characteristics
(MAX2424/MAX2426 EV kit, VCC = +3.3V; fLO(RX) = 925.7MHz (MAX2424), 985MHz (MAX2426); fRXIN = 915MHz, PRXIN = -35dBm, fLO(TX) = 915MHz, VTXIN = V TXIN = 2.3V (DC bias), VTXIN = 250mVp-p, fTXIN = 1MHz, VLNAGAIN = 2V, VVCOON = 2.4V, RXON = TXON = MOD = DIV1 = PREGND = GND, TA = +25C, unless otherwise noted.)
RECEIVER SUPPLY CURRENT vs. TEMPERATURE
MAX2424/6-01
MAX2424/MAX2426
TRANSMITTER SUPPLY CURRENT vs. TEMPERATURE
MAX2424/6-02
SHUTDOWN SUPPLY CURRENT vs. TEMPERATURE
4.0 3.5 VCOON = GND
MAX2424/6-03
42 40 38 36 ICC (mA) 34 32 30 28 26 24 -40 -20 0 20 40 60 80 TEMPERATURE (C) RXON = VCC PREGND = UNCONNECTED INCLUDES OSCILLATOR CURRENT VCC = 4.8V VCC = 3.3V VCC = 2.7V
39 37 35 33 ICC (mA) 31 29 27 25 23 21 -40 -20 0 TXON = VCC PREGND = UNCONNECTED INCLUDES OSCILLATOR CURRENT 20 40 60 80 TEMPERATURE (C) VCC = 4.8V
4.5
3.0 ICC (A) 2.5 2.0 1.5 1.0 0.5 0 -40 -20 0 20 40 60 80 TEMPERATURE (C) VCC = 3.3V VCC = 2.7V VCC = 4.8V
VCC = 3.3V VCC = 2.7V
RECEIVER GAIN vs. LNAGAIN
MAX2424/6-04
RECEIVER INPUT IP3 vs. LNAGAIN
MAX2424/6-05
RECEIVER NOISE FIGURE vs. LNAGAIN
35 30 NOISE FIGURE (dB) 25 20 15 10 AVOID THIS REGION 0 0.5 1.0 RXON = VCC DIV1 = VCC LNA ADJUSTABLE MAX PARTIALLY GAIN GAIN BIASED
MAX2424/6-06
25 20 15 RECEIVER GAIN (dB) 10 5 0 -5 -10 -15 -20 0 LNA OFF
LNA PARTIALLY BIASED
5 LNA OFF LNA ADJUSTABLE MAX PARTIALLY GAIN GAIN BIASED AVOID THIS REGION
40 LNA OFF
0
ADJUSTABLE GAIN
MAX GAIN
IIP3 (dBm)
-5
-10
AVOID THIS REGION RXON = VCC
-15 5 RXON = VCC -20 0 0.5 1.0 1.5 2.0 0
0.5
1.0
1.5
2.0
1.5
2.0
LNAGAIN VOLTAGE (V)
LNAGAIN VOLTAGE (V)
LNAGAIN VOLTAGE (V)
MAX2424 RECEIVER GAIN vs. TEMPERATURE
MAX2424/6-07
RECEIVER NOISE FIGURE vs. TEMPERATURE AND SUPPLY VOLTAGE
MAX2424/6-08
RECEIVER INPUT IP3 vs. TEMPERATURE
VLNAGAIN = 1V
MAX2424/6-09
5.5 LNAGAIN = VCC RXON = VCC DIV1 = VCC
26 RECEIVER GAIN (dB)
LNAGAIN = VCC RXON = VCC
-6 -8 -10 IIP3 (dBm)
5.0 NOISE FIGURE (dB)
VCC = 4.8V
24 22
VCC = 4.8V
4.5 VCC = 3.3V 4.0 VCC = 2.7V 3.5
-12 -14 -16 -18 RXON = VCC
20
VCC = 3.3V
VCC = 2.7V
VLNAGAIN = 2V
18 3.0 -40 -20 0 20 40 60 80 -40 -20 0 20 40 60 80 TEMPERATURE (C) TEMPERATURE (C)
-20 -40 -20 0 20 40 60 80 TEMPERATURE (C)
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5
900MHz Image-Reject Receiver with Transmit Mixer MAX2424/MAX2426
______________________________Typical Operating Characteristics (continued)
(MAX2424/MAX2426 EV kit, VCC = +3.3V; fLO(RX) = 925.7MHz (MAX2424), 985MHz (MAX2426); fRXIN = 915MHz, PRXIN = -35dBm, fLO(TX) = 915MHz, VTXIN = V TXIN = 2.3V (DC bias), VTXIN = 250mVp-p, fTXIN = 1MHz, VLNAGAIN = 2V, VVCOON = 2.4V, RXON = TXON = MOD = DIV1 = PREGND = GND, TA = +25C, unless otherwise noted.)
MAX2424 RECEIVER OUTPUT 1dB COMPRESSION POINT vs. TEMPERATURE
MAX2424/6-10
RECEIVER IMAGE REJECTION vs. RF FREQUENCY
MAX2424/6-11
RECEIVER IMAGE REJECTION vs. IF FREQUENCY
35 IMAGE REJECTION (dB) 30 25 20 15 10 5 0 RXON = VCC 1 10 100 1000 MAX2424 MAX2426
MAX2424/6-12
-3 1dB COMPRESSION POINT (dBm) -4 -5 -6 VCC = 3.3V -7 -8 RXON = VCC -9 -40 -20 0 20 40 60 80 TEMPERATURE (C) VCC = 2.7V VCC = 4.8V
60 RXON = VCC 50 IMAGE REJECTION (dB) 40 30 20 10 0 -10 -20 0 400 800 1200 1600
40
2000
RF FREQUENCY (MHz)
IF FREQUENCY (MHz)
RXIN INPUT IMPEDANCE vs. FREQUENCY
50 45 40 REAL IMPEDANCE () 35 30 25 20 15 10 5 0 600 800 1000 1200 1400 FREQUENCY (MHz) 0 -25 10 -20 REAL -40 IMAGINARY
MAX2424/6-13
TRANSMITTER OUTPUT POWER vs. INPUT VOLTAGE
MAX2424/6-14
TRANSMITTER OUTPUT POWER vs. TEMPERATURE
-2 -3 -4 -5 -6 -7 -8 -9 -10 -11 -12 -13 -14
MAX2424/6-15
RXON = VCC
-100
5 0 OUTPUT POWER (dBm) -5
TXON = VCC
VCC = 4.8V VCC = 3.3V
IMAGINARY IMPEDANCE ()
-80
OUTPUT POWER (dBm)
-60
VCC = 4.8V
VCC = 2.7V -10 -15 -20
VCC = 2.7V
VCC = 3.3V
TXON = VCC -40 -20 0 20 40 60 80
100 INPUT VOLTAGE (mVp-p)
1000
TEMPERATURE (C)
TXOUT OUTPUT IMPEDANCE vs. FREQUENCY
MAX2424/6-16
TRANSMITTER OUTPUT SPECTRUM
MAX2424/6-17
TRANSMITTER 1dB COMPRESSION POINT vs. TEMPERATURE
0 -1 -2 -3 -4 -5 -6 -7 TXON = VCC -40 -20 0 20 40 60 80 VCC = 2.7V VCC = 3.3V VCC = 4.8V
MAX2424/6-18
150 REAL OR IMAGINARY IMPEDANCE () 100 50 0 -50 -100 -150 -200 -250 -300 TXON = VCC IMAGINARY REAL
0 -10 -20 -30 POWER (dBm) -40 -50 -60 -70 -80 -90 -100 LO TXON = VCC DOUBLE-SIDE BAND FUNDAMENTAL
1 OUTPUT 1dB COMPRESSION (dBm)
600 800 1000 1200 1400 1600 1800 2000 FREQUENCY (MHz)
910 911 912 913 914 915 916 917 918 919 920 FREQUENCY (MHz)
TEMPERATURE (C)
6
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900MHz Image-Reject Receiver with Transmit Mixer
______________________________Typical Operating Characteristics (continued)
(MAX2424/MAX2426 EV kit, VCC = +3.3V; fLO(RX) = 925.7MHz (MAX2424), 985MHz (MAX2426); fRXIN = 915MHz, PRXIN = -35dBm, fLO(TX) = 915MHz, VTXIN = V TXIN = 2.3V (DC bias), VTXIN = 250mVp-p, fTXIN = 1MHz, VLNAGAIN = 2V, VVCOON = 2.4V, RXON = TXON = MOD = DIV1 = PREGND = GND, TA = +25C, unless otherwise noted.)
POWER vs. TXIN VOLTAGE
0 -10 -20 POWER (dBm) -30 -40 -50 -60 -70 -80 -90 -100 10 100 TXIN VOLTAGE (mVp-p) 1000 IM3 LEVEL FUNDAMENTAL TXON = VCC NOTE: TXIN IS TOTAL VOLTAGE FOR TWO TONES (PEAK-TO-PEAK)
MAX2424/6-19
MAX2424/MAX2426
TRANSMITTER BASEBAND FREQUENCY RESPONSE
3 0 -3 -6 -9 -12 -15 -18 -21 -24 -27 -30 -33 -36 1
MAX2424/6-20
10
TXOUT (dBc)
TXON = VCC 10 100 1000
FREQUENCY (MHz)
______________________________________________________________Pin Description
PIN 1 2 3 4 5 6 7 8 9 NAME VCC CAP1 RXOUT GND RXIN VCC GND GND TXOUT FUNCTION Supply-Voltage Input for Master Bias Cell. Bypass with a 47pF low-inductance capacitor and 0.1F to GND (pin 28 recommended). Receive Bias Compensation Pin. Bypass with a 47pF low-inductance capacitor and 0.01F to GND. Do not make any other connections to this pin. Single-ended, 330 IF Output. AC couple to this pin. Ground Connection Receiver RF Input, single ended. The input match shown in Figure 1 maintains an input VSWR of better than 2:1 from 902MHz to 928MHz. Supply Voltage Input for the Receive Low-Noise Amplifier. Bypass with a 47pF low-inductance capacitor to GND (pin 7 recommended). Ground Connection for Receive Low-Noise Amplifier. Connect directly to ground plane using multiple vias. Ground Connection for Signal-Path Blocks, except LNA PA Predriver Output. See Figure 1 for an example matching network, which provides better than 2:1 VSWR from 902MHz to 928MHz. Low-Noise Amplifier Gain-Control Input. Drive this pin high for maximum gain. When LNAGAIN is pulled low, the LNA is capacitively bypassed and the supply current is reduced by 4.5mA. This pin can also be driven with an analog voltage to adjust the LNA gain in intermediate states. Refer to the Receiver Gain vs. LNAGAIN Voltage graph in the Typical Operating Characteristics, as well as Table 1. Supply Voltage Input for the Signal-Path Blocks, except LNA. Bypass with a 47pF low-inductance capacitor and 0.01F to GND (pin 8 recommended).
10
LNAGAIN
11
VCC
_______________________________________________________________________________________
7
900MHz Image-Reject Receiver with Transmit Mixer MAX2424/MAX2426
_________________________________________________Pin Description (continued)
PIN 12 13 14 15 16 17 NAME TXIN TXIN CAP2 TXON RXON VCOON FUNCTION Transmit Mixer's Noninverting Baseband/IF Input. TXIN, TXIN form a high-impedance, differential input port. See Figure 1. Transmit Mixer's Inverting Baseband/IF Input. TXIN, TXIN form a high-impedance, differential input port. See Figure 1. Transmit Bias Compensation Input. Bypass with a 47pF low-inductance capacitor and 0.01F to GND. Do not make any other connections to this pin. Drive TXON and VCOON with a logic high to enable the transmit IF variable-gain amplifier, upconverter mixer, and PA predriver. See Power Management section. Drive RXON and VCOON with a logic high to enable the LNA, receive mixer, and IF output buffer. See Power Management section. Drive VCOON with a logic high to turn on the VCO, phase shifters, VCO buffers, and prescaler. To disable the prescaler, leave the PREGND pin unconnected. Drive DIV1 with a logic high to disable the divide-by-64/65 prescaler and connect the PREOUT pin directly to an oscillator buffer amplifier, which outputs -8dBm into a 50 load. Drive DIV1 low for divideby-64/65 operation. Drive this pin low when in shutdown to minimize shutdown current. Modulus Control for the Divide-by-64/65 Prescaler. Drive MOD high for divide-by-64 mode. Drive MOD low for divide-by-65 mode. Ground connection for the Prescaler. Connect PREGND to ground for normal operation. Leave unconnected to disable the prescaler and the output buffer. Connect MOD and DIV1 to ground and leave PREOUT unconnected when disabling the prescaler. Prescaler/Oscillator Buffer Output. In divide-by-64/65 mode (DIV1 = low), the output level is 500mVp-p into a high-impedance load. In divide-by-1 mode (DIV1 = high), this output delivers -8dBm into a 50 load. AC couple to this pin. Supply-Voltage Input for Prescaler. Bypass with a 47pF low-inductance capacitor and 0.01F to GND (pin 20 recommended). Supply-Voltage Input for VCO and Phase Shifters. Bypass with a 47pF low-inductance capacitor to GND (pin 26 recommended). Differential Oscillator Tank Port. See Applications Information for information on tank circuits or on using an external oscillator. Differential Oscillator Tank Port. See Applications Information for information on tank circuits or on using an external oscillator. Ground Connection for VCO and Phase Shifters Ground (substrate) Ground Connection for Master Bias Cell
18
DIV1
19
MOD
20
PREGND
21
PREOUT
22 23 24 25 26 27 28
VCC VCC TANK TANK GND GND GND
8
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900MHz Image-Reject Receiver with Transmit Mixer MAX2424/MAX2426
VCC 1 0.1F 47pF 28 2 0.1F RECEIVE RF INPUT 47pF GND 8.2nH 47pF 12nH VCC 0.01F* 47pF 22nH 47pF 9 TRANSMIT RF OUTPUT 18nH TXOUT TXIN 12 10k RA RB VCC MODULATOR INPUT GND RXOUT GND CAP1 PREGND VCC 20 VCC 23 47pF 26 27 0.01F 3 RECEIVE IF OUTPUT (330) DEVICE MAX2424 MAX2426 VCO TANK COMPONENTS FOR 915MHz RF FREQUENCY L3 (nH) 6.8 3.3 C2, C3 (pF) 3.3 8.0 C26 (pF) 2.0 4.0 R6, R7 () 10 20 VCC VCC 22 47pF 0.01F VCC
5
RXIN
MAX2424 MAX2426
TXIN
13
10k MODULATOR INPUT 0.01F* VCC
6 47pF VCC 11 0.01F 47pF 8 7
VCC 100nH GND TANK 25 R7
VARACTORS: ALPHA SMV1299-004 OR EQUIVALENT 1k C2 47k
VCC
L3 GND TANK 24 R6
C26
VCO ADJUST 47pF
1000pF 14 0.01F 47pF MOD DIV1 VCOON LNAGAIN 47pF 4 GND TXON 15 TXON 10 LNAGAIN RXON CAP2 PREOUT 21 19 18 17 16 TO PLL MOD DIV1 VCOON RXON
C3
1k
* WHEN USING DIFFERENTIAL SOURCE, REMOVE RESISTORS AND REPLACE CAPACITORS WITH SHORTS. FOR SINGLE-ENDED SOURCE, DRIVE ONLY MODULATOR INPUT. CHOOSE RA AND RB VALUES AS SHOWN IN TRANSMITTER SECTION.
Figure 1. Typical Operating Circuit
_______________________________________________________________________________________ 9
900MHz Image-Reject Receiver with Transmit Mixer MAX2424/MAX2426
_______________Detailed Description
The following sections describe each of the functional blocks shown in the Functional Diagram.
MAX2424 MAX2426
TXIN 1.5A 2M TXIN 1.5A VMIXER INPUT
Receiver
The MAX2424/MAX2426's receive path consists of a 900MHz low-noise amplifier, an image-reject mixer, and an IF buffer amplifier. The LNA's gain and biasing are adjustable via the LNAGAIN pin. Proper operation of this pin provides optimum performance over a wide range of signal levels. The LNA has four modes determined by the DC voltage applied on the LNAGAIN pin. See Table 1, as well as the relevant Typical Operating Characteristics plots. At low LNAGAIN voltages, the LNA is shut off and the input signal capacitively couples directly into mixer to provide maximum linearity for large-signal operation (receiver close to transmitter). As the LNAGAIN voltage increases, the LNA turns on. Between 0.5V and 1V at LNAGAIN, the LNA is partially biased and behaves like a Class C amplifier. Avoid this operating mode for applications where linearity is a concern. As the LNAGAIN voltage reaches 1V, the LNA is fully biased into Class A mode, and the gain is monotonically adjustable for LNAGAIN voltages above 1V. See the receiver gain, IP3, and Noise Figure vs. LNAGAIN plots in the Typical Operating Characteristics for more information. The downconverter is implemented using an imagereject mixer consisting of an input buffer with two outputs, each of which is fed to a double-balanced mixer. A quadrature LO drives the local-oscillator (LO) port of
Figure 2. TXIN, TXIN Equivalent Circuit
each mixer. An on-chip oscillator and an external tank circuit generates the LO. Its signal is buffered and split into two phase shifters, which provide 90 of phase shift across their outputs. This pair of LO signals is fed to the mixers. The mixers' outputs then pass through a second pair of phase shifters, which provide a 90 phase shift across their outputs. The resulting mixer outputs are then summed together. The final phase relationship is such that the desired signal is reinforced and the image signal is canceled. The downconverter mixer output appears on the RXOUT pin, a single-ended 330 output.
Transmitter
The MAX2424/MAX2426 transmitter consists of a balanced mixer and a PA driver amplifier. The mixer inputs are accessible via the TXIN and TXIN pins. An equivalent circuit for the TXIN and TXIN pins is shown in Figure 2. Because TXIN and TXIN are linearly coupled to the mixer stage, they can accept spectrally shaped input signals. Typically, the mixer can be used to multiply the LO with a baseband signal, generating BPSK or ASK modulation. Transmit upconversion can also be implemented by applying a modulated IF signal to these inputs. For applications requiring image rejection on the transmitter, refer to the MAX2420/MAX2421/ MAX2422/MAX2460/MAX2463 data sheet. Set the common-mode voltage at TXIN, TXIN to 2.3V by selecting appropriate values for RA and RB (Figure 1). The total series impedance of RA and RB should be approximately 100k. Frequency modulation (FM) is realized by modulating the VCO tuning voltage. Apply the appropriate differential and common-mode voltages to TXIN and TXIN to control transmitter output power (Figure 3).
Table 1. LNA Modes
LNAGAIN VOLTAGE (V) 0 < VLNAGAIN 0.5 MODE LNA capacitively bypassed, minimum gain, maximum IP3 LNA partially biased. Avoid this mode-- the LNA operates in a Class C manner LNA gain is monotonically adjustable LNA at maximum gain (remains monotonic)
0.5 < VLNAGAIN < 1.0
1.0 < VLNAGAIN 1.5 1.5 < VLNAGAIN VCC
10
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900MHz Image-Reject Receiver with Transmit Mixer
Phase Shifter
VCC R1 i TXIN
MAX2424/MAX2426
MAX2424 MAX2426
1.5A
R2 TXIN
2M
The MAX2424/MAX2426 uses passive networks to provide quadrature phase shifting for the receive IF and LO signals. Because these networks are frequency selective, both the RF and IF frequency operating ranges are limited. Image rejection degrades as the IF and RF moves away from the designed optimum frequencies. The MAX2424/MAX2426's phase shifters are arranged such that the LO frequency is higher than the RF carrier frequency (high-side injection).
R3 RT = R1 + R2 + R3 VDIFF = VTXIN - V TXIN
1.5A
Local Oscillator (LO)
The on-chip LO is formed by an emitter-coupled differential pair. An external LC resonant tank sets the oscillation frequency. A varactor diode is typically used to create a voltage-controlled oscillator (VCO). See the Applications Information section for an example VCO tank circuit. The LO may be overdriven in applications where an external signal is available. The external LO signal should be about 0dBm from 50, and should be AC coupled into either the TANK or TANK pin. Both TANK and TANK require pull-up resistors to V CC. See the Applications Information section for details. The local oscillator resists pulling caused by changes in load impedance that occur as the part is switched from standby mode, with just the oscillator running to either transmit or receive mode. The amount of LO pulling is affected if a signal is present at the RXIN port in transmit mode. The most common cause of pulling is imperfect isolation in an external transmit/ receive (T/R) switch. The AC Electrical Characteristics table contains specifications for this case as well.
Figure 3. Biasing TXIN and TXIN for FM
For example, if VCC = 3.3V and POUT = -8dBm, choose RT = 100k for sufficient current through the divider, so that bias currents for TXIN and TXIN have little effect over temperature. Set VTXIN = 2.3V to satisfy commonmode voltage range requirements at VCC = 3.3V. Use the Transmit Output Power vs. Input Voltage graph in the Typical Operating Characteristics to determine the input voltage (in mVp-p) required to produce the desired output. Divide this value by 22 and use it for VDIFF. A -8dBm transmitter output requires 250mVp-p / 22 = 88.4mV. VTXIN = 2.3V + 0.0884V = 2.3884V RT = R1 + R2 + R3 Solve for resistors R1, R2, and R3 with the following equations: R3 = V TXIN x RT
Prescaler
The on-chip prescaler operates in two different modes: as a dual-modulus divide-by-64/65, or as an oscillator buffer amplifier. The DIV1 pin controls this function. When DIV1 is low, the prescaler is in dual-modulus divide-by-64/65 mode; when it is high, the prescaler is disabled and the oscillator buffer amplifier is enabled. The buffer typically outputs -8dBm into a 50 load. To minimize shutdown supply current, pull the DIV1 pin low when in shutdown mode. In divide-by-64/65 drive mode, the division ratio is controlled by the MOD pin. Drive MOD high to operate the prescaler in divide-by-64 mode. Drive MOD and DIV1 low to operate the prescaler in divide-by-65 mode.
R2 = VTXIN - V TXIN R1 = RT - R2 - R3
(
VCC
) x VR
T
CC
Since the transmit and receive sections typically require different LO frequencies, it is not recommended to have both transmit and receive active at the same time.
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11
900MHz Image-Reject Receiver with Transmit Mixer MAX2424/MAX2426
To disable the prescaler entirely, leave PREGND and PREOUT unconnected. Also connect the MOD and DIV1 pins to GND. Disabling the prescaler does not affect operation of the VCO stage. Choose tank components according to your application needs, such as phase-noise requirements, tuning range, and VCO gain. High Q inductors such as aircore micro springs yield low phase noise. Use a lowtolerance inductor (L3) for predictable oscillation frequency. Resistors R6 and R7 can be chosen from 0 to 20 to reduce the Q of parasitic resonance due to series package inductance LT. Keep R6 and R7 as small as possible to minimize phase noise, yet large enough to ensure oscillator start-up in fundamental mode. Oscillator start-up with be most critical with high tuning bandwidth (low tank Q) and high temperature. Capacitors C2 and C3 couple in the varactor. Light coupling of the varactor is a way to reduce the effects of high varactor tolerance and increase loaded Q. For a wider tuning range, use larger values for C2 and C3 or a varactor with a large capacitance ratio. Capacitor C26 is used to trim the tank oscillator frequency. Larger values for C26 will help negate the effect of stray PCB capacitance and parasitic inductor capacitance (L3). Choose a low-tolerance capacitor for C26.
VCC
Power Management
The MAX2424/MAX2426 supports four different powermanagement features to conserve battery life. The VCO section has its own control pin (VCOON), which also serves as a master bias pin. When VCOON is high, the LO, quadrature LO phase shifters, and prescaler or LO buffer are all enabled. Stabilize VCO by powering it up prior to transmitting or receiving. For transmit-to-receive switching, the receiver and transmitter sections have their own enable control inputs, RXON and TXON. With VCOON high, bringing RXON high enables the receive path, which consists of the LNA, image-reject mixers, and IF output buffer. When this pin is low, the receive path is inactive. The TXON input enables the upconverter mixer and PA predriver. VCOON must be high for the transmitter to operate. When TXON is low, the transmitter is off. To disable all chip functions and reduce the supply current to typically 0.5A, pull VCOON, DIV1, MOD, RXON, and TXON low.
___________Applications Information
Oscillator Tank
The on-chip oscillator requires a parallel-resonant tank circuit connected across TANK and TANK. Figure 4 shows an example of an oscillator tank circuit. Inductor L4 provides DC bias to the tank ports. Inductor L3, capacitor C26, and the series combination of capacitors C2, C3, and both halves of the varactor diode capacitance set the resonant frequency as follows: fr = 1 2 L3 CEFF
MAX2424 MAX2426
TANK R7
L4 100nH C2 R5 1k 1/2 D1 L3 6.8nH R6 C26 C3 1/2 D1 R4 1k D1 = ALPHA SMV1299-004 C1 47pF
R8 47k
VCO_CTRL
TANK
( )(
)
CEFF =
1 + C26 1 1 2 C2 + C3 + C D1
SEE FIGURE 1 FOR R6, R7, C2, C3, C26, AND L3 COMPONENT VALUES.
Figure 4. Oscillator Tank Schematic Using the On-Chip VCO
where CD1 is the capacitance of one varactor diode.
12
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900MHz Image-Reject Receiver with Transmit Mixer MAX2424/MAX2426
MAX2424 MAX2426
LT
TANK
L1
L3
R1
1/2 D1 R2 Ci C2 1/2 D1 LT TANK L2 L5 R3 L4 VTUNE C1 VCC
Figure 5. Series-Coupled Resonant Tank for Wide Tuning Range and Low Phase Noise
For applications that require a wide tuning range and low phase noise, a series-coupled resonant tank may be required as shown in Figure 5. This tank will use the package inductance in series with inductors L1, L2, and capacitance of varactor D1 to set the net equivalent inductance which resonates in parallel with the internal oscillator capacitance. Inductors L1 and L2 may be implemented as microstrip inductors, saving component cost. Bias is provided to the tank port through chokes L3 and L5. R1 and R3 should be chosen large enough to de-Q the parasitic resonance due to L3 and L5 but small enough to minimize the voltage drop across them due to bias current. Values for R1 and R3 should be kept between 0 and 50. Proper high frequency bypassing (C1) should be used for the bias voltage to eliminate power supply noise from entering the tank.
VCC
MAX2424
TANK
50 47pF VCC 50
50
TANK 50 47pF
EXT LO EXTERNAL LO LEVEL IS 0dBm FROM A 50 SOURCE.
Oscillator Tank PC Board Layout
The parasitic PC board capacitance, as well as PCB trace inductance and package inductance, affect oscillation frequency, so be careful in laying out the PC board for the oscillator tank. Keep the tank layout as symmetrical, tightly packed, and close to the device as possible to minimize LO feedthrough. When using a PC board with a ground plane, a cut-out in the ground plane (and any other planes) below the oscillator tank reduces parasitic capacitance.
Figure 6. Using an External Local Oscillator
Using an External Oscillator
If an external 50 LO signal source is available, it can be used as an input to the TANK or TANK pin in place of the on-chip oscillator (Figure 6). The oscillator signal is AC coupled into the TANK pin and should have a level of about 0dBm from a 50 source. For proper biasing of the oscillator input stage, pull up the TANK and TANK pins to the VCC supply via 50 resistors. If a differential LO source such as the MAX2620 is available, AC-couple the inverting output into TANK.
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13
900MHz Image-Reject Receiver with Transmit Mixer MAX2424/MAX2426
_________________________________________________________Functional Diagram
LNAGAIN RXIN
90
0
RXOUT
CAP1 RXON TXON CAP2 BIAS
DIV1 MOD 0 90
/ 1/64/65
PREOUT PREGND TANK TANK VCOON
PHASE SHIFTER
0 TXOUT
MAX2424 MAX2426
TXIN TXIN
________________________________________________________Package Information
SSOP.EPS
14
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