Datenblatt-Suchmaschine für elektronische Bauteile |
|
LM1863M Datenblatt(PDF) 10 Page - National Semiconductor (TI) |
|
|
LM1863M Datenblatt(HTML) 10 Page - National Semiconductor (TI) |
10 / 14 page Applications Information (Continued) The RF AGC threshold has been carefully chosen to pre- vent overloading the mixer which would cause distortion and tweet problems However the threshold level is suffi- ciently large to minimize the possibility of strong adjacent stations de-sensitizing the radio by activating the RF AGC and thereby gain reducing the RF front end The RF AGC output Pin 3 is an open collector NPN transis- tor This collector must be tied to a positive voltage through a load resistor R8 Furthermore decoupling is required (C11 and C12) in order to insure that the RF AGC does not induce significant distortion in the recovered audio Howev- er the tradeoff between good THD performance and fast stop time is not too severe for the RF AGC because large changes in the RF AGC level are unlikely when moving be- tween adjacent channels This is because the selectivity in the RF stage is not great enough to cause abrupt signal level changes at the mixer input as the radio is tuned Thus since the RF AGC does not have to follow abrupt signal level changes the time constant on the AGC output can be relatively long which allows for good THD performance C12 is required in order to insure good RF decoupling of signals at the RF AGC output and sets the non-dominant pole The RF AGC 10 mA threshold is fixed at 6 mVrms at the mixer input However due to the gain of the RF stage and losses through the RF transformers this level may be differ- ent when referenced to the antenna input For the applica- tion circuit shown the RF threshold occurs at 2 mVrms at the dummy antenna input Thus the RF AGC threshold can effectively be adjusted by altering the gain of the RF stage The value of R8 also has some affect on the RF AGC threshold of the application circuit Smaller values will tend to increase the threshold while larger values will tend to reduce the threshold GAIN DISTRIBUTION The purpose of this section is to clarify some of the trade- offs involved in redistributing gain from one portion of the radio to another An AM radio basically has three gain blocks consisting of the RF stage the mixer and the IF stage The total gain of these three blocks must be suffi- ciently large as to insure reception of weak stations Given then a fixed amount of required gain how does distributing this gain among the three blocks affect the radio perform- ance Large amounts of gain in the RF stage will have the effect of decreasing the RF AGC threshold A decreased RF AGC threshold means that it is more likely that strong adjacent stations can activate the RF AGC and desensitize the radio Also a lot of RF gain implies large signals across the RF varactor diodes which is undesirable for good tracking and can result in overloading these varactors which can cause cross modulation On the other hand high RF gain insures good noise performance and improved THD High mixer gain implies large signal swings at the mixer out- put especially on AGC transients These large signal swings could cause the mixer ouput transistors to saturate and also could overload the IF stage On the other hand redistributing the gain from the IF to the mixer would im- prove the noise performance of the radio The gain of the mixer can be controlled moving the tap on the mixer output transformer T4 Since the output signal level of the IF is held constant by the AGC increasing gain in the IF has the effect of reducing the signal level at the IF input Noise sources at the IF input therefore become a larger percentage of the IF input signal thereby degrading the SN floor of the radio For this rea- son the LM1863 employs 20 dB of IF AGC The IF gain of the LM1863 is adjustable by changing the tap across the IF ouput coil or by changing the ratio of R24 to R4 The gain distribution for the application circuit is as follows Gain Distribution TLH5185 – 23 VG e 0 dB (10 mV) V1 eb16 dB V2 ea10 dB (Pin 20) V3 ea33 dB (Pin 11) VO ea84 dB (Pin 14) The IF gain could also be varied by changing the value of R6 across the IF output coil However it is a good idea to maintain a high Q IF tank in order to achieve good adjacent channel rejection In order to prevent distortion due to over- loading the IF amplifier it is important that the impedance Pin 14 sees looking into the IF output tank T5 does not go below 3K ohms The above gain distribution is prior to any AGC action in the radio This distribution represents a good compromise be- tween the various tradeoffs outlined previously LEVEL CONTROLLED LOCAL OSCILLATOR Tracking of the RF varactors with the local oscillator varac- tor is a serious consideration in order to insure adequate performance of the ETR radio Due to non-linear capaci- tance versus voltage characteristic of the varactor large signals across these varactors will tend to modulate their capacitance and cause tracking problems This problem is compounded further if the level of the signals across the varactors change In an AM radio the local oscillator fre- quency changes a ratio of two to one The Q of the oscilla- tor tank remains fairly constant over this range Thus since Q e RP 0L e Constant this implies that RP(RP e un- loaded parallel resistance of the tank) must change two to one The internal level-control loop prevents the two to one change in AC voltage across the tank which the change in the RP would otherwise cause Phase jitter of the local oscillator is very important in regard to AM stereo where L-R information is contained in the phase of the carrier Local oscillator jitter has the effect of modulating the L-R channel with phase noise thus degrad- ing the stereo signal to noise performance Great care has been taken in the design of the LM1863 local oscillator to insure that phase jitter is a minimum In fact the dominant source of phase jitter is the high impedance resistor drive to the varactor The thermal noise of the resistor modulates the varactor voltage thus causing phase jitter VARACTOR TUNED RF STAGE Electronically tuned car radios require the use of a tuned RF stage prior to the mixer Many of the performance charac- 10 |
Ähnliche Teilenummer - LM1863M |
|
Ähnliche Beschreibung - LM1863M |
|
|
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 |