Datenblatt-Suchmaschine für elektronische Bauteile |
|
LM1868 Datenblatt(PDF) 7 Page - National Semiconductor (TI) |
|
|
LM1868 Datenblatt(HTML) 7 Page - National Semiconductor (TI) |
7 / 10 page 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-NoiseLow Frequency Oscillation Twist speaker leads Orient R10 and C20 parallel to the axis of the loopstick Locate C11 away from the loopstick TLH7909 – 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) 1r3 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 mA 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 mA 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 mA 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 mA 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 |
Ähnliche Teilenummer - LM1868 |
|
Ähnliche Beschreibung - LM1868 |
|
|
Link URL |
Privatsphäre und Datenschutz |
ALLDATASHEETDE.COM |
War ALLDATASHEET hilfreich? [ DONATE ] |
Über Alldatasheet | Werbung | Kontakt | Privatsphäre und Datenschutz | Linktausch | Hersteller All Rights Reserved©Alldatasheet.com |
Russian : Alldatasheetru.com | Korean : Alldatasheet.co.kr | Spanish : Alldatasheet.es | French : Alldatasheet.fr | Italian : Alldatasheetit.com Portuguese : Alldatasheetpt.com | Polish : Alldatasheet.pl | Vietnamese : Alldatasheet.vn Indian : Alldatasheet.in | Mexican : Alldatasheet.com.mx | British : Alldatasheet.co.uk | New Zealand : Alldatasheet.co.nz |
Family Site : ic2ic.com |
icmetro.com |