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________________general description the MAX2424/max2426 highly integrated front-end ics provide the lowest cost solution for cordless and ism- band 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 trans- mit 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-mod- ulus prescaler can also be set to a direct mode, in which it acts as an lo buffer amplifier. four separate power- down inputs can be used for system power manage- ment, including a 0.5? shutdown mode. the MAX2424/max2426 come in a 28-pin ssop pack- age. ________________________applications cordless phones wireless telemetry wireless networks spread-spectrum communications two-way paging ____________________________features receive mixer with 35db image rejection adjustable-gain lna up to +2dbm combined receiver input ip3 4db combined receiver noise figure optimized for common receiver if frequencies: 10.7mhz (MAX2424) 70mhz (max2426) pa predriver provides up to 0dbm low current consumption: 23ma receive 20ma transmit 9.5ma oscillator 0.5a shutdown mode operates from single +2.7v to +4.8v supply MAX2424/max2426 900mhz image-reject receivers with transmit mixer ________________________________________________________________ maxim integrated products 1 28 27 26 25 24 23 22 21 20 19 18 17 16 15 1 2 3 4 5 6 7 8 9 10 11 12 13 14 gnd gnd gnd tank txon preout pregnd mod div1 vcoon rxon cap2 txin txin lnagain txout gnd gnd rxin gnd rxout cap1 ssop top view MAX2424 max2426 tank v cc v cc v cc v cc v cc ___________________pin configuration 19-1350 rev 3; 12/00 part MAX2424 eai -40? to +85? temp. range pin-package 28 ssop _______________ordering information functional diagram appears at end of data sheet. evaluation kit available max2426 eai -40? to +85? 28 ssop for pricing, delivery, and ordering information, please contact maxim/dallas direct! at 1-888-629-4642, or visit maxim? website at www.maxim-ic.com.
MAX2424/max2426 900mhz image-reject receiver with transmit mixer 2 _______________________________________________________________________________________ absolute maximum ratings dc electrical characteristics (v cc = +2.7v to +4.8v, no rf signals applied, lnagain = unconnected, v txin = v txin = 2.3v, v vcoon = 2.4v, v rxon = v txon = v mod = v div1 = 0.45v, pregnd = gnd, t a = -40? to +85?. typicals are at t a = +25?, v cc = 3.3v, unless otherwise noted.) (note 1) stresses beyond those listed under ?bsolute maximum ratings?may cause permanent damage to the device. these are stress rating s only, and functional operation of the device at these or any other conditions beyond those indicated in the operational sections of the specificatio ns is not implied. exposure to absolute maximum rating conditions for extended periods may affect device reliability. note 1: 25? guaranteed by production test, <25? guaranteed through correlation to worst-case temperature testing. note 2: calculated by measuring the combined oscillator and prescaler supply current and subtracting the oscillator supply current. note 3: calculated by measuring the combined oscillator and lo buffer supply current and subtracting the oscillator supply current. note 4: 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 5: calculated by measuring the combined transmit and oscillator supply current and subtracting the oscillator supply current. v cc to gnd ...........................................................-0.3v to +5.5v txin, txin differential voltage ..............................................+2v voltage on txout......................................-0.3v to (v cc + 1.0v) voltage on lnagain, txon, rxon, vcoon, div1, mod, txin, txin ............................-0.3v to (v cc + 0.3v) rxin input power..............................................................10dbm tank, tank input power ...................................................2dbm continuous power dissipation (t a = +70?) ssop (derate 9.50mw/? above +70?) ......................762mw operating temperature range ...........................-40? to +85? junction temperature ......................................................+150? storage temperature range .............................-65? to +165? lead temperature (soldering, 10s) .................................+300? parameter min typ max units receive supply current (note 4) 23 36 ma prescaler supply current (buffer mode) (note 3) 5.4 8.5 ma oscillator supply current supply-voltage range 2.7 4.8 v 9.5 14 ma prescaler supply current ( 64/65 mode) (note 2) 4.2 6 ma conditions v rxon = 2.4v, pregnd = unconnected v div1 = 2.4v pregnd = unconnected digital input voltage low 0.45 v shutdown supply current 0.5 rxon, txon, div1, vcoon, mod vcoon = rxon = txon = mod = div1 = gnd digital input current ? ?0 ? voltage on any one digital input = v cc or gnd digital input voltage high v 2.4 rxon, txon, div1, vcoon, mod 10 ? t a = +25? t a = -40? to +85? transmitter supply current (note 5) 20 32 ma v rxon = 0.45v, v txon = 2.4v, pregnd = unconnected
ac electrical characteristics (MAX2424/max2426 ev kit, v cc = +3.3v, f rxin = 915mhz, p rxin = -35dbm, v txin = v t xin = 2.3v (dc bias), v txin = 250mvp-p, f txin = 1mhz, v lnagain = 2v, v vcoon = 2.4v, rxon = txon = mod = div1 = pregnd = gnd, t a = +25?, unless otherwise noted.) MAX2424/max2426 900mhz image-reject receiver with transmit mixer _______________________________________________________________________________________ 3 (note 12) MAX2424 (notes 6, 7) t a = t min to t max (note 6) t a = +25? (notes 6, 7) (notes 6, 11) (note 10) receiver on or off lnagain = gnd (note 8) v lnagain = 1v (note 8) v lnagain = 1v (notes 6, 9) lnagain = v cc (notes 6, 9) v lnagain = v cc , t a = -40? to +85? (notes 6, 8) max2426 (notes 6, 7) v lnagain = v cc , t a = +25? (note 8) v lnagain = 1v (notes 6, 8) lnagain = v cc (notes 6, 8) conditions dbc 30 carrier suppression dbm 3.5 output third-order intercept (oip3) dbm -0.5 output 1db compression -10 -4.5 dbm -9.5 -7 -5 output power mhz 125 baseband 3db bandwidth mhz 800 1000 output frequency range ns 500 receiver turn-on time dbm -60 lo to rxin leakage -8 db -19 -17 input third-order intercept (iip3) 12 db 45 noise figure mhz 8.5 10.7 12.5 mhz 800 1000 input frequency range -16 12 18 24 19 25 55 70 85 if frequency range db 26 35 image frequency rejection db 20 22 24.5 conversion power gain 19 21 23.5 units min typ max parameter MAX2424 max2426 MAX2424 max2426 v lnagain = 1v lnagain = v cc -18 dbm -26 input 1db compression (note 13) ns 220 transmitter turn-on time dbm/hz -140 output noise density receiver (v rxon = 2.4v, f lo = 925.7mhz (MAX2424), f lo = 985mhz (max2426)) transmitter (v txon = 2.4v, f lo = 915mhz)
v div1 = 2.4v, z l = 50 ? , MAX2424/max2426 900mhz image-reject receiver with transmit mixer 4 _______________________________________________________________________________________ ac electrical characteristics (continued) (MAX2424/max2426 ev kit, v cc = +3.3v, f rxin = 915mhz, p rxin = -35dbm, v txin = v t xin = 2.3v (dc bias), v txin = 250mvp-p, f txin = 1mhz, v lnagain = 2v, v vcoon = 2.4v, rxon = txon = mod = div1 = pregnd = gnd, t a = +25?, unless otherwise noted.) parameter min typ max units oscillator phase noise oscillator frequency range 800 1100 mhz 82 dbc/hz (note 6) conditions note 6: guaranteed by design and characterization. note 7: image rejection typically falls to 30dbc at the frequency extremes. note 8: 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. note 9: two tones at p rxin = -45dbm each, f1 = 915.0mhz and f2 = 915.2mhz. note 10: time delay from v rxon = 0.45v to v rxon = 2.4v transition to the time the output envelope reaches 90% of its final value. note 11: output power typically falls to -10dbm at the frequency extremes. note 12: two tones at v txin = 125mvp-p, f1 = 1.0mhz, and f2 = 1.2mhz. note 13: time delay from v txon = 0.45v to v txon = 2.4v transition to the time the output envelope reaches 90% of its final value. note 14: using tank components l3 = 5.0nh (coilcraft a02t), c2 = c3 = c26 = 3.3pf, r6 = r7 = 10 ? . note 15: this approximates a typical application in which txout is followed by an external pa and a t/r switch with finite isolation. note 16: relative to the rising edge of preout. prescaler output level 500 mvp-p -11 -8 required modulus setup time (notes 6, 16) 10 ns z l = 100k ? | | 10pf 64/65 mode required modulus hold time (notes 6, 16) 0 ns 64/65 mode oscillator buffer output level (notes 6, 14) -12 dbm v div1 = 2.4v, z l = 50 ? 10khz offset (note 14) 72 standby to tx or standby to rx 35 oscillator pulling 8 khz 110 rx to tx with p rxin =- 45dbm (rx mode) to p rxin = 0dbm (tx mode) (note 15) MAX2424 max2426 MAX2424 max2426 MAX2424 max2426 70 t a = +25? t a = -40? to +85? oscillator and prescaler
MAX2424/max2426 900mhz image-reject receiver with transmit mixer _______________________________________________________________________________________ 5 28 32 30 38 36 34 42 40 24 26 -40 0 20 -20 40 60 80 receiver supply current vs. temperature MAX2424/6-01 temperature (?) i cc (ma) v cc = 2.7v v cc = 3.3v v cc = 4.8v rxon = v cc pregnd = unconnected includes oscillator current 37 35 33 31 29 27 25 23 21 39 -40 0 20 -20 40 60 80 transmitter supply current vs. temperature MAX2424/6-02 temperature ( c) i cc (ma) v cc = 2.7v v cc = 3.3v v cc = 4.8v txon = v cc pregnd = unconnected includes oscillator current 0 1.0 0.5 2.5 2.0 1.5 4.0 3.5 3.0 4.5 -40 0 20 -20 40 60 80 shutdown supply current vs. temperature MAX2424/6-03 temperature ( c) i cc ( a) v cc = 2.7v v cc = 3.3v v cc = 4.8v vcoon = gnd 25 20 15 10 5 0 -5 -10 -15 -20 0 0.5 1.0 1.5 2.0 receiver gain vs. lnagain MAX2424/6-04 lnagain voltage (v) receiver gain (db) adjustable gain max gain lna partially biased lna off avoid this region rxon = v cc 18 22 20 26 24 -40 0 20 -20 40 60 80 MAX2424 receiver gain vs. temperature MAX2424/6-07 temperature ( c) receiver gain (db) v cc = 2.7v v cc = 3.3v v cc = 4.8v lnagain = v cc rxon = v cc -20 -15 -10 -5 0 5 0 0.5 1.0 1.5 2.0 receiver input ip3 vs. lnagain MAX2424/6-05 lnagain voltage (v) iip3 (dbm) adjustable gain avoid this region max gain lna partially biased lna off rxon = v cc 0 5 15 10 25 20 30 40 35 0 0.5 1.0 1.5 2.0 receiver noise figure vs. lnagain MAX2424/6-06 lnagain voltage (v) noise figure (db) adjustable gain avoid this region max gain lna partially biased lna off rxon = v cc div1 = v cc 3.0 4.0 3.5 5.0 5.5 4.5 -40 0 20 -20 40 60 80 receiver noise figure vs. temperature and supply voltage MAX2424/6-08 temperature ( c) noise figure (db) v cc = 3.3v lnagain = v cc rxon = v cc div1 = v cc v cc = 4.8v v cc = 2.7v -20 -16 -18 -8 -10 -6 -12 -14 -40 0 20 -20 40 60 80 receiver input ip3 vs. temperature MAX2424/6-09 temperature ( c) iip3 (dbm) v lnagain = 1v rxon = v cc v lnagain = 2v __________________________________________typical operating characteristics (MAX2424/max2426 ev kit, v cc = +3.3v; f lo( rx) = 925.7mhz (MAX2424), 985mhz (max2426); f rxin = 915mhz, p rxin = -35dbm, f lo(tx) = 915mhz, v txin = v txin = 2.3v (dc bias), v txin = 250mvp-p, f txin = 1mhz, v lnagain = 2v, v vcoon = 2.4v, rxon = txon = mod = div1 = pregnd = gnd, t a = +25?, unless otherwise noted.)
MAX2424/max2426 900mhz image-reject receiver with transmit mixer 6 _______________________________________________________________________________________ ______________________________t ypical operating characteristics (continued) (MAX2424/max2426 ev kit, v cc = +3.3v; f lo( rx) = 925.7mhz (MAX2424), 985mhz (max2426); f rxin = 915mhz, p rxin = -35dbm, f lo(tx) = 915mhz, v txin = v txin = 2.3v (dc bias), v txin = 250mvp-p, f txin = 1mhz, v lnagain = 2v, v vcoon = 2.4v, rxon = txon = mod = div1 = pregnd = gnd, t a = +25?, unless otherwise noted.) -20 10 -10 0 30 20 50 40 60 0 400 800 1200 1600 2000 receiver image rejection vs. rf frequency MAX2424/6-11 rf frequency (mhz) image rejection (db) rxon = v cc -9 -8 -4 -5 -3 -6 -7 -40 0 20 -20 40 60 80 MAX2424 receiver output 1db compression point vs. temperature MAX2424/6-10 temperature ( c) 1db compression point (dbm) v cc = 2.7v v cc = 4.8v rxon = v cc v cc = 3.3v 0 10 20 30 5 15 25 35 40 1 10 100 1000 receiver image rejection vs. if frequency MAX2424/6-12 if frequency (mhz) image rejection (db) rxon = v cc MAX2424 max2426 0 25 30 15 20 10 5 35 40 45 50 0 -60 -40 -20 -80 -100 800 600 1000 1200 1400 rxin input impedance vs. frequency MAX2424/6-13 frequency (mhz) real impedance ( ? ) imaginary impedance ( ? ) real imaginary rxon = v cc -300 -200 -250 -50 -100 -150 0 50 100 150 800 600 1000 1200 1400 1600 1800 2000 txout output impedance vs. frequency MAX2424/6-16 frequency (mhz) real or imaginary impedance ( ? ) real imaginary txon = v cc 5 -25 -15 -20 -5 -10 0 transmitter output power vs. input voltage MAX2424/6-14 input voltage (mvp-p) output power (dbm) 10 100 1000 v cc = 2.7v v cc = 4.8v v cc = 3.3v txon = v cc -14 -2 -3 -4 -5 -6 -7 -8 -9 -10 -11 -12 -13 -40 0 20 -20 40 60 80 transmitter output power vs. temperature MAX2424/6-15 temperature ( c) output power (dbm) v cc = 2.7v v cc = 4.8v v cc = 3.3v txon = v cc -100 -80 -90 -60 -70 -40 -50 -30 -10 -20 0 910 912 913 914 911 915 916 917 919 918 920 transmitter output spectrum MAX2424/6-17 frequency (mhz) power (dbm) double-side band fundamental lo txon = v cc -7 -5 -6 -3 -4 0 -1 -2 1 -40 0 -20 20 40 60 80 transmitter 1db compression point vs. temperature MAX2424/6-18 temperature ( c) output 1db compression (dbm) v cc = 4.8v v cc = 3.3v v cc = 2.7v txon = v cc
MAX2424/max2426 900mhz image-reject receiver with transmit mixer _______________________________________________________________________________________ 7 -36 3 0 -3 -6 -9 -12 -15 -18 -21 -24 -27 -30 -33 1 10 100 1000 transmitter baseband frequency response MAX2424/6-20 frequency (mhz) txout (dbc) txon = v cc -100 10 0 -10 -20 -30 -40 -50 -60 -70 -80 -90 10 100 1000 power vs. txin voltage MAX2424/6-19 txin voltage (mvp-p) power (dbm) txon = v cc note: txin is total voltage for two tones (peak-to-peak) fundamental im3 level ______________________________t ypical operating characteristics (continued) (MAX2424/max2426 ev kit, v cc = +3.3v; f lo( rx) = 925.7mhz (MAX2424), 985mhz (max2426); f rxin = 915mhz, p rxin = -35dbm, f lo(tx) = 915mhz, v txin = v txin = 2.3v (dc bias), v txin = 250mvp-p, f txin = 1mhz, v lnagain = 2v, v vcoon = 2.4v, rxon = txon = mod = div1 = pregnd = gnd, t a = +25?, unless otherwise noted.) ______________________________________________________________pin description supply voltage input for the receive low-noise amplifier. bypass with a 47pf low-inductance capacitor to gnd (pin 7 recommended). 6 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. 5 ground connection 4 v cc rxin gnd single-ended, 330 ? if output. ac couple to this pin. 3 receive bias compensation pin. bypass with a 47pf low-inductance capacitor and 0.01? to gnd. do not make any other connections to this pin. 2 supply-voltage input for master bias cell. bypass with a 47pf low-inductance capacitor and 0.1? to gnd (pin 28 recommended). 1 function pin 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. 10 pa predriver output. see figure 1 for an example matching network, which provides better than 2:1 vswr from 902mhz to 928mhz. 9 lnagain txout ground connection for receive low-noise amplifier. connect directly to ground plane using multiple vias. 7 gnd rxout cap1 v cc name ground connection for signal-path blocks, except lna 8 gnd supply voltage input for the signal-path blocks, except lna. bypass with a 47pf low-inductance capac- itor and 0.01? to gnd (pin 8 recommended). 11 v cc
MAX2424/max2426 900mhz image-reject receiver with transmit mixer 8 _______________________________________________________________________________________ _________________________________________________pin description (continued) function pin name transmit bias compensation input. bypass with a 47pf low-inductance capacitor and 0.01? to gnd. do not make any other connections to this pin. 14 transmit mixer? inverting baseband/if input. txin, txin form a high-impedance, differential input port. see figure 1. 13 cap2 txin transmit mixer? noninverting baseband/if input. txin, txin form a high-impedance, differential input port. see figure 1. 12 txin 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. 21 ground connection for the prescaler. connect pregnd to ground for normal operation. leave uncon- nected to disable the prescaler and the output buffer. connect mod and div1 to ground and leave pre- out unconnected when disabling the prescaler. 20 preout pregnd modulus control for the d ivide-by-64/65 prescaler. drive mod high for divide-by-64 mode. drive mod low for divide-by-65 mode. 19 drive vcoon with a logic high to turn on the vco, phase shifters, vco buffers, and prescaler. to dis- able the prescaler, leave the pregnd pin unconnected. 17 drive rxon and vcoon with a logic high to enable the lna, receive mixer, and if output buffer. see power management section. 16 mod vcoon rxon 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. 15 txon 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 divide- by-64/65 operation. drive this pin low when in shutdown to minimize shutdown current. 18 div1 supply-voltage input for prescaler. bypass with a 47pf low-inductance capacitor and 0.01? to gnd (pin 20 recommended). 22 v cc differential oscillator tank port. see applications information for information on tank circuits or on using an external oscillator. 24 tank supply-voltage input for vco and phase shifters. bypass with a 47pf low-inductance capacitor to gnd (pin 26 recommended). 23 v cc differential oscillator tank port. see applications information for information on tank circuits or on using an external oscillator. 25 tank ground connection for vco and phase shifters 26 gnd ground (substrate) 27 gnd ground connection for master bias cell 28 gnd
MAX2424/max2426 900mhz image-reject receiver with transmit mixer _______________________________________________________________________________________ 9 v cc v cc 17 16 15 18 19 21 1000pf varactors: alpha smv1299-004 or equivalent when using differential source, remove resistors and replace capacitors with shorts. for single-ended source, drive only modulator input. choose r a and r b values as shown in transmitter section. receive if output (330 ? ) 27 23 26 3 20 22 1 28 receive rf input transmit rf output 5 9 7 6 0.01 f 47pf v cc v cc 0.1f v cc v cc v cc 2 47pf 47pf 0.1f 8.2nh 12nh 22nh 18nh 47pf 47pf 47pf 8 11 0.01f 47pf 0.01f vco tank components for 915mhz rf frequency 47pf 14 0.01f 47pf rxin txout gnd v cc v cc gnd gnd cap2 gnd cap1 v cc v cc txon rxon vcoon div1 mod preout txon rxon vcoon div1 mod to pll gnd rxout 100nh gnd pregnd 47pf 24 25 vco adjust c3 1k 47k 47pf 1k c2 c26 r6 r7 v cc v cc r a r b 10k 10k l3 tank modulator input 12 0.01 f* txin modulator input 13 0.01 f* txin lnagain lnagain 4 47pf 10 MAX2424 max2426 tank * MAX2424 max2426 device 10 20 r6, r7 ( ? ) 2.0 4.0 c26 (pf) 6.8 3.3 l3 (nh) 3.3 8.0 c2, c3 (pf) figure 1. typical operating circuit
MAX2424/max2426 900mhz image-reject receiver with transmit mixer 10 ______________________________________________________________________________________ _______________detailed description the following sections describe each of the functional blocks shown in the functional diagram. 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 lna- gain 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 applica- tions where linearity is a concern. as the lnagain volt- age reaches 1v, the lna is fully biased into class a mode, and the gain is monotonically adjustable for lna- gain 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 image- reject mixer consisting of an input buffer with two out- puts, each of which is fed to a double-balanced mixer. a quadrature lo drives the local-oscillator (lo) port of 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 sec- ond 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 bal- anced mixer and a pa driver amplifier. the mixer inputs are accessible via the txin and txin pins. an equiva- lent 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 multi- ply 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 r a and r b (figure 1). the total series impedance of r a and r b should be approxi- mately 100k ? . frequency modulation (fm) is realized by modulating the vco tuning voltage. apply the appropriate differen- tial and common-mode voltages to txin and txin to control transmitter output power (figure 3). lna partially biased. avoid this mode the lna operates in a class c manner lna capacitively bypassed, minimum gain, maximum ip3 mode lna at maximum gain (remains monotonic) lna gain is monotonically adjustable 1.5 < v lnagain v cc 1.0 < v lnagain 1.5 0.5 < v lnagain < 1.0 0 < v lnagain 0.5 lnagain voltage (v) table 1. lna modes MAX2424 max2426 txin 2m 1.5 a 1.5 a vmixer input txin figure 2. txin, txin equivalent circuit
MAX2424/max2426 900mhz image-reject receiver with transmit mixer ______________________________________________________________________________________ 11 for example, if v cc = 3.3v and p out = -8dbm, choose r t = 100k ? for sufficient current through the divider, so that bias currents for txin and txin have little effect over temperature. set v txin = 2.3v to satisfy common- mode voltage range requirements at v cc = 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 2 2 and use it for v diff . a -8dbm transmitter output requires 250mvp-p / 2 2 = 88.4mv. v txin = 2.3v + 0.0884v = 2.3884v r t = r1 + r2 + r3 solve for resistors r1, r2, and r3 with the following equations: 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. phase shifter the MAX2424/max2426 uses passive networks to pro- vide quadrature phase shifting for the receive if and lo signals. because these networks are frequency selec- tive, 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). local oscillator (lo) the on-chip lo is formed by an emitter-coupled differ- ential pair. an external lc resonant tank sets the oscil- lation 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. prescaler the on-chip prescaler operates in two different modes: as a dual-modulus divi de-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 con- trolled by the mod pin. drive mod high to operate the prescaler in di vide-by-64 mode. drive mod and div1 low to operate the prescaler in divide-by-65 mode. r3 v x r v r2 v v x r v r1 r r2 r3 t cc txin t cc t = = () = txin txin r t = r 1 + r 2 + r 3 v diff = v txin - v txin r1 i r2 r3 2m v cc 1.5 a txin 1.5 a txin MAX2424 max2426 figure 3. biasing txin and txin for fm
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. power management the MAX2424/max2426 supports four different power- management 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 upcon- verter mixer and pa predriver. vcoon must be high for the transmitter to operate. when txon is low, the trans- mitter is off. to disable all chip functions and reduce the supply cur- rent 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 capaci- tors c2, c3, and both halves of the varactor diode capacitance set the resonant frequency as follows: where c d1 is the capacitance of one varactor diode. choose tank components according to your application needs, such as phase-noise requirements, tuning range, and vco gain. high q inductors such as air- core micro springs yield low phase noise. use a low- tolerance 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 l t . 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. c = 1 1 c2 1 c3 2 c eff d1 ++ ? ? ? ? ? ? + c26 f = 1 2l3c r eff () () ? ? ? ? ? ? MAX2424/max2426 900mhz image-reject receiver with transmit mixer 12 ______________________________________________________________________________________ MAX2424 max2426 tank l3 6.8nh l4 100nh r5 1k r4 1k 1/2 d1 1/2 d1 d1 = alpha smv1299-004 see figure 1 for r6, r7, c2, c3, c26, and l3 component values. c1 47pf vco_ctrl r7 r6 c3 r8 47k c2 c26 v cc tank figure 4. oscillator tank schematic using the on-chip vco
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 equiva- lent 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 cho- sen 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. oscillator tank pc board layout the parasitic pc board capacitance, as well as pcb trace inductance and package inductance, affect oscil- lation 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. 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 v cc supply via 50 ? resistors. if a differential lo source such as the max2620 is available, ac-couple the inverting output into tank . MAX2424/max2426 900mhz image-reject receiver with transmit mixer ______________________________________________________________________________________ 13 MAX2424 tank 50 ? 50 ? 50 ? 47pf 50 ? ext lo external lo level is 0dbm from a 50 ? source. v cc 47pf v cc tank figure 6. using an external local oscillator MAX2424 max2426 tank l1 l t l t l2 l3 l4 l5 r1 r2 r3 c i 1/2 d1 1/2 d1 c1 c2 v cc v tune tank figure 5. series-coupled resonant tank for wide tuning range and low phase noise
MAX2424/max2426 900mhz image-reject receiver with transmit mixer _________________________________________________________functional diagram rxon txon cap2 txout cap1 rxin lnagain 90 90 0 0 MAX2424 max2426 phase shifter 1/64/65 bias rxout div1 mod preout pregnd tank tank vcoon txin 0 txin ________________________________________________________package information ssop.eps maxim cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a maxim product. no circu it patent licenses are implied. maxim reserves the right to change the circuitry and specifications without notice at any time. maxim integrated products, 120 san gabriel drive, sunnyvale, ca 94086 408-737-7600 ____________________ 14 ? 2000 maxim integrated products printed usa is a registered trademark of maxim integrated products.

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