tl/h/7909 lm1868 am/fm radio system february 1995 lm1868 am/fm radio system general description the combination of the lm1868 and an fm tuner will pro- vide all the necessary functions for a 0.5 watt am/fm radio. included in the lm 1868 are the audio power amplifier, fm if and detector, and the am converter, if, and detector. the device is suitable for both line operated and 9v battery applications. features y dc selection of am/fm mode y regulated supply y audio amplifier bandwidth decreased in am mode, reducing amplifier noise in the am band y am converter agc for excellent overload characteristics y low current internal am detector for low tweet radiation block diagram tl/h/7909 1 order number LM1868N see ns package number n20a note: see table for coil data c 1995 national semiconductor corporation rrd-b30m115/printed in u. s. a.
absolute maximum ratings if military/aerospace specified devices are required, please contact the national semiconductor sales office/distributors for availability and specifications. supply voltage (pin 19) 15v package dissipation 2.0w above t a e 25 c, derate based on t j(max) e 150 c and i ja e 60 c/w storage temperature range b 55 cto a 150 c operating temperature range 0 cto a 70 c lead temperature (soldering, 10 sec.) 260 c electrical characteristics test circuit, t a e 25 c, v s e 9v, r l e 8 x (unless otherwise noted) parameter conditions min typ max units static characteristics e am e 0, e fm e 0 supply current am mode, s1 in position 1 22 30 ma regulator output voltage (pin 16) 3.5 3.9 4.8 v operating voltage range 4.5 15 dynamic characteristicseam mode f am e 1 mhz, f mod e 1 khz, 30% modulation, s1 in position 1, p o e 50 mw unless noted maximum sensitivity measure e am for p o e 50 mw, 816 m v maximum volume signal-to-noise e am e 10 mv 40 50 db detector output e am e 1mv 40 60 85 mv measure at top of volume control overload distortion e am e 50 mv, 80% modulation 2 10 % total harmonic distortion (thd) e am e 10 mv 1.1 2 % dynamic characteristicsefm mode f fm e 10.7 mhz, f mod e 400 hz, d f e g 75 khz, p o e 50 mw, s1 in position 1 b 3 db limiting sensitivity 15 45 m v signal-to-noise ratio e fm e 10 mv 50 64 db detector output e fm e 10 mv, d f e g 22.5 khz 40 60 85 mv measure at top of volume control am rejection e fm e 10 mv, 30% am modulation 40 50 db total harmonic distortion (thd) e fm e 10 mv 1.1 2 % dynamic characteristicseaudio amplifier only f e 1 khz, e am e 0, e fm e 0, s1 in position 2 power output thd e 10%, r l 8 x v s e 6v 250 325 mw v s e 9v 500 700 mw bandwidth am mode, p o e 50 mw 11 khz fm mode, p o e 50 mw 22 khz total harmonic distortion (thd) p o e 50 mw, fm mode 0.2 % voltage gain 41 db typical performance characteristics (test circuit) all curves are measured at audio output vs voltage quiescent supply current fm limiting characteristics fm if am rejection tl/h/7909 2 2
typical performance characteristics (continued) all curves are measured at audio output (test circuit) am characteristics recovered audio vs supply amplifier only gain vs frequency audio output, r l e 8 x power dissipation vs power out, r l e 16 x power dissipation vs power audio amplifier only distortion vs frequency tl/h/7909 3 test circuit note: see table for coil data tl/h/7909 4 3
typical application fm performance (88 mhz108 mhz) am performance (525 khz1650 khz) # 30 db quieting sensitivity: 3.5 m v # maximum sensitivity: 100 m v/m # b 3 db limiting sensitivity: 7 m v # 20 db quieting sensitivity: 250 m v/m # tweet * worst case: 5% 100 mv/m: 1.5% * tweet is an audio tone produced by the 2nd and 3rd harmonic of the if beating against the received signal. it is measured as an equivalent modu- lation level: i.e., a 30% tweet has the same amplitude at the detector as a desired signal with 30% modulation. tl/h/7909 5 4
pc board layout tl/h/7909 6 component side typical performance characteristics typical application all curves are measured at audio output tl/h/7909 7 tl/h/7909 8 5
ic external components (application circuit) component typical comments value c1 100 pf removes tuner lo from if input c2 0.1 m f antenna coupling capacitor c4, c5 0.01 m f fm if decoupling capacitors c6, c9 0.005 m f am smoothing/fm de-emphasis network, de-emphasis pole is r5 1k ( given by. f1 j 1 2 q (c6 a c9) # r4 r6 r4 a r6 j c10 10 m f regulator decoupling capacitor c11 0.1 m f regulator decoupling capacitor c12 10 m f ac coupling to volume control c13 0.1 m f power supply decoupling c14 50 m f power supply decoupling c15 0.1 m f audio amplifier input coupling r7 3k roll off signals from detector in the am band to prevent radiation c16 0.001 m f ( c17 100 m f power amplifier feedback decoupling, sets low frequency supply rejection r8 16k am detector bias resistor component typical comments value r9 240k set agc time constant c19 1 m f ( c7 10 m f if coupling c8 0.1 m f if coupling c20 0.1 m f high frequency load for audio amplifier, required to stabilize r10 5 x ( audio amplifier c21 250 m f output coupling capacitor r1 6k2 sets q of quadrature coil, determining fm thd and recovered audio r2 12k if amplifier bias r r3 5k6 sets gain of am if and q of am if output tank r4 10k detector load resistor r6 50k volume control c18 0.02 m f power supply decoupling r11, r12 150 x terminates the ceramic filter, biases fm if input stage d1 1n4148 optional. quickens the agc response during turn on coil and tuning capacitor specifications c1 am ant 140 pf max 5.0 pf min fm 20 pf max 4.5 pf min am osc 82 pf max 5.0 pf min toko cy2-22124pt trimmers 5 pf l1 640 m h, q u e 200 am antenna r p e 3k5 @ f e 796 khz 1 mv/meter induces (at secondary) approximately 100 m v open circuit at the secondary l0, l2 360 m h, q u l 80 @ f e 796 khz toko rwo-6a5105 or equivalent toko america 1250 feehanville drive mount prospect, il 60056 (312) 297-0070 tl/h/7909 9 l4 swg y 20, n e 3 (/2 t, inner diameter e 5mm l5 swg y 20, n e 3 (/2 t, inner diameter e 5mm l6 l e 0.44 m h, n e 4 (/2 t, qu e 70 l7 swg y 20, n e 2 (/2 t, inner diameter e 5mm cf2 10.7 mhz ceramic filter murata murata sfe 10.7 ma or 2200 lake park drive equivalent smyrna, ga 30080 (404) 436-1300 t1 q u l 70 @ 10.7 mhz, l to resonate w/82 pf @ 10.7 mhz toko kac-k2318 or equivalent tl/h/7909 10 t2 q u l 14 @ 455 khz, l to resonate w/180 pf @ 455 khz toko 159gc-a3785 or equivalent tl/h/7909 11 cf1 toko cfu-090d or equivalent bw l 4.8 khz @ 455 khz tl/h/7909 12 t3 apollo electronics ns-107c or equivalent tl/h/7909 13 6
layout considerations am section most problems in an am radio design are associated with radiation of undesired signals to the loopstick. depending on the source, this radiation can cause a variety of problems including tweet, poor signal-to-noise, and low frequency os- cillation (motor boating). although the level of radiation from the lm1868 is low, the overall radio performance can be degraded by improper pcb layout. listed below are layout considerations association with common problems. 1. tweet: locate the loopstick as far as possible from de- tector components c6, c9, r4, and r5. orient c6, c9, r4, and r5 parallel to the axis of the loopstick. return r8, c6, c9, and c19 to a separate ground run (see typical applica- tion pcb). 2. poor signal-to-noise/low frequency oscillation: twist speaker leads. orient r10 and c20 parallel to the axis of the loopstick. locate c11 away from the loopstick. tl/h/7909 14 in general, radiation results from current flowing in a loop. in case 1 this current loop results from decoupling detector harmonics at pin 17; while in case 2, the current loop results from decoupling noise at the output of the audio amplifier and the output of the regulator. the level of radiation picked up by the loopstick is approximately proportional to: 1) 1/r 3 ; where r is the distance from the center of the loopstick to the center of the current loop; 2) sin i , where i is the angle between the plane of the current loop and the axis of the loopstick; 3) i, the current flowing in the loop; and 4) a, the cross-sectional area of the current loop. pickup is kept low by short leads (low a), proper orientation ( i j 0sosin i j 0), maximizing distance from sources to loopstick, and keeping current levels low. fm section the pinout of the lm1868 has been chosen to minimize layout problems, however some care in layout is required to insure stability. the input source ground should return to c4 ground. capacitors c13 and c18 form the return path for signal currents flowing in the quadrature coil. they should connect directly to the proper pins with short pc traces (see typical application pcb). the quadrature coil and input cir- cuitry should be separated from each other as far as possi- ble. audio amplifier the standard layout considerations for audio amplifiers ap- ply to the lm1868, that is: positive and negative inputs should be returned to the same ground point, and leads to the high frequency load should be kept short. in the case of the lm1868 this means returning the volume control ground (r6) to the same ground point as c17, and keeping the leads to c20 and r10 short. circuit description (see equivalent schematic) am section the am section consists of a mixer stage, a separate local oscillator, an if gain block, an envelope detector, agc cir- cuits for controlling the if and mixer gains, and a switching circuit which disables the am section in the fm mode. signals from the antenna are ac-coupled into pin 7, the mixer input. this stage consists of a common-emitter ampli- fier driving a differential amp which is switched by the local oscillator. with no mixer agc, the current in the mixer is 330 m a; as the agc is applied, the mixer current drops, decreasing the gain, and also the input impedance drops, reducing the signal at the input. the differential amp con- nected to pin 8 forms the local oscillator. bias resistors are arranged to present a negative impedance at pin 8. the frequency of oscillation is determined by the tank circuit, the peak-to-peak amplitude is approximately 300 m a times the impedance at pin 8 in parallel with 8k2. after passing through the ceramic filter, the if signals are applied to the if input. signals at pin 11 are amplified by two agc controlled common-emitter stages and then applied to the pnp output stage connected to pin 13. biasing is ar- ranged so that the current in the first two stages is set by the difference between a 250 m a current source and the darlington device connected to pin 12. when the agc threshold is exceeded, the darlington device turns on, steering current away from the if into ground, reducing the if gain. current in the if is monitored by the mixer agc circuit. when the current in the if has dropped to 30 m a, corresponding to 30 db gain reduction in the if, the mixer agc line begins to draw current. this causes the mixer current and input impedance to drop, as previously described. the if output is level shifted and then peak detected at detector cap c1. by loading c1 with only the base current of the following device, detector currents are kept low. drive from the agc is taken at pin 14, while the am detector output is summed with the fm detector output at pin 17. fm section the fm section is composed of a 6-stage limiting if driving a quadrature detector. the if stages are identical with the exceptions of the input stage, which is run at higher current to reduce noise, and the last stage, which is switched off in the am mode. the quadrature detector collectors drive a level shift arrangement which allows the detector output load to be connected to the regulated supply. audio amplifier the audio amplifier has an internally set voltage gain of 120. the bandwidth of the audio amplifier is reduced in the am mode so as to reduce the output noise falling in the am band. the bandwidth reduction is accomplished by reducing the current in the input stage. regulator a series pass regulator provides biasing for the am and fm sections. use of a pnp pass device allows the supply to drop to within a few hundred millivolts of the regulator out- put and still be in regulation. 7
equivalent schematic tl/h/7909 15 8
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lm1868 am/fm radio system physical dimensions inches (millimeters) molded dual-in-line package (n) order number LM1868N ns package number n20a life support policy national's products are not authorized for use as critical components in life support devices or systems without the express written approval of the president of national semiconductor corporation. as used herein: 1. life support devices or systems are devices or 2. a critical component is any component of a life systems which, (a) are intended for surgical implant support device or system whose failure to perform can into the body, or (b) support or sustain life, and whose be reasonably expected to cause the failure of the life failure to perform, when properly used in accordance support device or system, or to affect its safety or with instructions for use provided in the labeling, can effectiveness. be reasonably expected to result in a significant injury to the user. national semiconductor national semiconductor national semiconductor national semiconductor corporation europe hong kong ltd. japan ltd. 1111 west bardin road fax: ( a 49) 0-180-530 85 86 13th floor, straight block, tel: 81-043-299-2309 arlington, tx 76017 email: cnjwge @ tevm2.nsc.com ocean centre, 5 canton rd. fax: 81-043-299-2408 tel: 1(800) 272-9959 deutsch tel: ( a 49) 0-180-530 85 85 tsimshatsui, kowloon fax: 1(800) 737-7018 english tel: ( a 49) 0-180-532 78 32 hong kong fran 3 ais tel: ( a 49) 0-180-532 93 58 tel: (852) 2737-1600 italiano tel: ( a 49) 0-180-534 16 80 fax: (852) 2736-9960 national does not assume any responsibility for use of any circuitry described, no circuit patent licenses are implied and national reserves the right at any time without notice to change said circuitry and specifications.
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