PDF LM1865 Data sheet ( Hoja de datos )

Número de pieza	LM1865
Descripción	Advanced FM IF System
Fabricantes	National Semiconductor
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No Preview Available ! www.DataSheet4U.com February 1995 LM1865 Advanced FM IF System General Description Reduced external component cost improved performance and additonal functions are key features to the LM1865 FM IF system The LM1865 is designed for use in electronically tuned radio applications It contains both deviation and sig- nal level stop circuitry in addition to an open-collector stop output The LM1865 generates a reverse AGC voltage (ie decreasing AGC voltage with increasing signal) Features Y On-chip buffer to provide gain and terminate two ce- ramic filters Y Low distortion 0 1% typical with a single tuned quadra- ture coil for 100% modulation Y Broad off frequency distortion characteristic Y Low THD at minimum AFT offset Y Meter output proportional to signal level Y Stop detector with open-collector output Y Adjustable signal level mute stop threshold controlled either by ultrasonic noise in the recovered audio or by the meter output Y Adjustable deviation mute stop threshold Y Separate time constants for signal level and deviation mute stop Y Dual threshold AGC eliminates need for local distance switch and offers improved immunity from third order in- termodulation products due to tuner overload Y User control of both AGC thresholds Y Excellent signal to noise ratio AM rejection and system limiting sensitivity Block Diagram DataSheet4U.com DataShee C1995 National Semiconductor Corporation TL H 7509 FIGURE 1 Order Number LM1865M or LM1865N See NS Package Number M20B or N20A TL H 7509 – 1 RRD-B30M115 Printed in U S A DataSheet4U.com 1 page www.DataSheet4U.com Application Circuit et4U.com FIGURE 3 TL H 7509 – 4 IC External Components (See Application Circuit) Component C1 C2 C3 C4 C5 C6 C7 C8 C9 C10 C11 C12 C13 R1 R2 R3 R4 R5 R6 R7 R8 R9 R10 R11 R12 R13 L1 T1 CF1 CF2 Typical Value Comments 0 01 mF AC coupling for wide band AGC input 0 01 mF Buffer and AGC supply decoupling 0 01 mF IF decoupling capacitors 10 mF Meter decoupling capacitor 0 01 mF AC coupling for IF output 50 mF Regulator decoupling capacitor affects S N floor 2 2 mF 5 mF DaLAeFtvaTeSldmehcueoteueptsl4itnoUgp .atcifmfoeecmtcsosntsotpanttime 0 1 mF Disables noise mute stop 0 01 mF AC coupling for noise mute stop threshold adjust 25 mF Supply decoupling 0 01 mF AGC output decoupling capacitor Tuner Dependent Wide band AGC threshold adjust Tuner Dependent Meter Dependent 5k1 25k 5k Gain set and bias for IF R2 a R3 e 330X to terminate ceramic filter Sets full-scale on meter Deviation mute stop window adjustment Mute stop filter affects stop time Level mute stop threshold adjustment 10k Pot Level mute stop threshold adjustment 12k Noise mute stop threshold adjustment decrease resistor for lower S N at threshold for optimum performance over temp and gain varia- tion set this resistor value so that the signal level mute stop threshold 10k 50k 3k9 62X 18 mH Qul50 10 7 MHz TDK Electronics TPO410-180K or equivalent Qul70 10 7 MHz L to resonate w 82 pF 10 7 MHz TOKO KAC-K2318HM or equivalent TL H 7509–5 occurs in the radio at 45dB S N (g3 dB) in mono Load for open-collector stop output AGC output load resistor for open-collector output Sets Q of quadrature coil affecting THD S N and recovered audio Optimises minimum THD Sets signal swing across quadrature coil High Q is important to mini- mize effect variation of Q has on both minimum THD and AFT offset 10 7 MHz quadrature coil QUL l 70 Murata SFE10 7ML or equivalent 10 7 MHz ceramic resonators provide selectivity good group delay characteristics important for low THD of system 5 DataSheet4U.com DataShee 5 Page www.DataSheet4U.com Application Notes (Continued) voltage while VD is allowed to increase VD will increase until it reaches the level of the wide band threshold at the tuner output When this occurs VUD is no longer needed to keep Vo l VWB as VD takes over the job Thus VUD will drop as the amount of AGC increases while VD is held con- stant by the AGC When compared to the simple case of a single in-band sig- nal we see that because of the presence of a strong out-of- band signal AGC action has occurred earlier For the simple case AGC started when VD t VWB For the two signal case above AGC started when VD t VNB Thus the LM1865 achieves an early AGC when there are strong adjacent channels that might cause IM3 and a later AGC when these signals aren’t present For the range of signal levels that the tuner was gain-re- duced and VD k VWB there was loss in signal-to-noise in the recovered audio as compared to the case where there was no gain reduction in this interval Note however that the tuner is not desensitized by the AGC to weak desired stations below the narrow band threshold NARROW BAND AGC THRESHOLD ADJUSTMENT Both the narrow band and wide band AGC thresholds are user adjustable This allows the user to optimize the AGC response to a given tuner Referring to Figure 5 when the meter output exceeds 1V a comparator closes SW1 A sim- plified circuit diagram of this comparator is shown in Figure 10 The 1K resistor in series with pin 8 allows for an upward adjustment of the narrow band threshold This is accom- plished by externally loading pin 8 with a resistor Figure 11 illustrates how this adjustment takes place From Figure 11 it is apparent that loading the meter output not only moves the narrow band threshold but also de- creases the meter output for a given input In general one chooses the narrow band threshold based on what signal-to-noise compromise is considered acceptable et4U.com FIGURE 10 Narrow Band Threshold Circuit DataSheet4U.com TL H 7509 – 12 DataShee TL H 7509 – 13 FIGURE 11 Affect of Meter Load on Narrow Band Threshold DataSheet4U.com 11 11 Page

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