LM1865N

LM1865 Advanced FM IF System 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 signal 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) Y Y Y Meter output proportional to signal level Stop detector with open-collector output Adjustable signal level mute stop threshold controlled either by ultrasonic noise in the recovered audio or by the meter output Adjustable deviation mute stop threshold Separate time constants for signal level and deviation mute stop Dual threshold AGC eliminates need for local distance switch and offers improved immunity from third order intermodulation products due to tuner overload User control of both AGC thresholds Excellent signal to noise ratio AM rejection and system limiting sensitivity Y Y Features Y Y Y On-chip buffer to provide gain and terminate two ceramic filters Low distortion 0 1% typical with a single tuned quadrature coil for 100% modulation Broad off frequency distortion characteristic Low THD at minimum AFT offset Y Y Y Y Block Diagram Order Number LM1865M or LM1865N See NS Package Number M20B or N20A FIGURE 1 TL H 7509 – 1 C1995 National Semiconductor Corporation TL H 7509 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 17 Package Dissipation (Note 1) Storage Temperature Range Operating Temperature Range Max Voltage on Pin 16 (Stop Output) 16V 2 0W b 55 C to a 150 C b 20 C to a 85 C 16V 215 C 220 C See AN-450 ‘‘Surface Mounting Methods and Their Effect on Product Reliability’’ for other methods of soldering surface mount devices Soldering Information Dual-In-Line Package Soldering (10 seconds) Small Outline Package Vapor Phase (60 seconds) Infrared (15 seconds) 260 C Electrical Characteristics Test Circuit TA e 25 C V a e 12V S1 in position 2 S2 in position 1 and S3 in position 2 unless indicated otherwise Parameter STATIC CHARACTERISTICS Supply Current Pin 9 Regulator Voltage Operating Voltage Range Pin 18 Output Leakage Current Pin 16 Stop Low Output Voltage Pin 16 Stop High Output Leakage Current Pin 15 Audio Output Resistance Pin 1 Buffer Input Resistance Pin 3 Buffer Output Resistance Pin 20 Wide Band Input Resistance Pin 8 Meter Output Resistance DYNAMIC CHARACTERISTICS fMOD e 400 Hz fo e 10 7 MHz Deviation e g 75 kHz b 3 dB Limiting Sensitivity Conditions Min Typ Max Units 33 57 (See Note 2) Pin 20 Open VIF e 0 S3 in Position 1 S1 in Position 1 S2 in Position 3 S2 in Position 2 V14 e V9 73 01 03 01 47 Measured at DC Measured at DC Measured at DC 350 350 2 1 45 mA V 16 V mA V mA kX X X X kX IF Only (See Note 3) VIN Pin 1 e 10 mVrms at 10 7 MHz VIF e 10 mVrms V14 e V9 VIF e 10 mVrms V14 e V9 (See Note 4) V14 e V9 VIF e 1 mV 30% AM Mod VIF e 10 mV 30% AM Mod VIF e 10 mV VIF e 10 mV Tune until V14 e V9 VIF e 10 mV VIF e 10 mV S2 in Position 3 fMOD e 0 Offset e (Frequency for Pin 16 Low) b (Frequency where V14 e V9) V14 e V9 S1 in Position 3 (See Note 5) VIF e 10 mV fMOD e 100 kHz S2 in Position 3 Amount of Deviation where V16 x Low 19 275 70 50 50 60 22 320 84 60 60 01 01 0 15 g 50 120 25 470 mVrms dB mVrms dB dB dB Buffer Voltage Gain Recovered Audio Signal-to-Noise AM Rejection Minimum Total Harmonic Distortion THD at Frequency where V14 e V9 (Zero AFT Offset) THD g 10 kHz from Frequency where V14 e V9 AFT Offset Frequency for Low Stop Output at Pin 16 Ultrasonic Mute Stop Level Threshold 0 35 0 45 % % % kHz 60 kHz 2 Electrical Characteristics Test Circuit TA e 25 C V a e 12V S1 in position 2 S2 in position 1 and S3 in position 2 unless indicated otherwise (Continued) Parameter Conditions Min Typ Max Units DYNAMIC CHARACTERISTICS fMOD e 400 Hz fo e 10 7 MHz Deviation e g 75 kHz (Continued) Pin 13 Mute Stop Threshold Voltage V14 e V9 S1 in Position 4 S2 in Position 3 V13 where V16 x Low S2 in Position 4 S1 in Position 1 VIF e 10 mV S1 in Position 1 V14 e V9 VIF e 10 mV Increase IF Input until I AGC e 0 1 mA Pin 20 e 30 mVrms VIF e 100 mVrms Increase Signal to Pin 20 until IAGC e 0 1 mA VIN Pin 20 e 100 mV VIF e 100 mVrms VIN Pin 20 e 100 mV VIF e 100 mVrms (See Note 6) VIF e 10 mV VIF e 300 mV VIF e 3 mV 100 5 220 mV Amount of Muting (LM1965 Only) Amount of Muting with Pin 13 and Pin 16 Grounded Narrow Band AGC Threshold Wide Band AGC Threshold Pin 18 Low Output Voltage (LM1865 and LM1965 only) Pin 18 High Output Voltage (LM2065 only) Pin 8 Meter Output Voltage 66 0 210 12 02 11 7 01 11 26 300 22 05 dB dB mVrms mVrms V V V V V Note 1 Above TA e 25 C derate based on TJ(max) e 150 C and iJA e 60 C W Note 2 All data sheet specifications are for V a e 12V may change slightly with supply Note 3 When the IF is preceded by 22 dB gain in the buffer excellent system sensitivity is achieved Note 4 Measured with a notch at 60 Hz and 20 Hz to 100 kHz bandwidth Note 5 FM modulate RF source with a 100 kHz audio signal and find what modulation level expressed as kHz deviation results in V16 x 12V Test Circuit TL H 7509 – 2 FIGURE 2 3 Typical Performance Characteristics (from Test Circuit) FM Limiting Characteristics and AM Rejection Pin 8 Meter Output Voltage vs IF Input Level FM Limiting Characteristics a THD % THD vs OFF Tuning (Single Tuned Quadrature Coil) Deviation Mute Stop Threshold as a Function of AFT Load Resistor Supply Current vs Supply Voltage Pin 14 AFT Current vs Tuning TL H 7509 – 3 Coils and ceramic filters are available from Toko America 1250 Feehanville Drive Mount Prospect IL 60056 (312) 297-0070 Murata 2200 Lake Park Drive Smyrna GA 30080 (404) 436-1300 4 Application Circuit TL H 7509 – 4 FIGURE 3 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 Typical Value 0 01 mF 0 01 mF 0 01 mF 10 mF 0 01 mF 50 mF 2 2 mF 5 mF 0 1 mF 0 01 mF 25 mF 0 01 mF Tuner Dependent Tuner Dependent Meter Dependent 5k1 25k 5k 10k Pot 12k Comments AC coupling for wide band AGC input Buffer and AGC supply decoupling IF decoupling capacitors Meter decoupling capacitor AC coupling for IF output Regulator decoupling capacitor affects S N floor Level mute stop time constant AFT decoupling affects stop time Disables noise mute stop AC coupling for noise mute stop threshold adjust Supply decoupling AGC output decoupling capacitor Wide band AGC threshold adjust 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 Level mute stop threshold adjustment Noise mute stop threshold adjustment decrease resistor for lower S N at threshold for optimum performance over temp and gain variation set this resistor value so that the signal level mute stop threshold occurs in the radio at 45dB S N ( g 3 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 minimize effect variation of Q has on both minimum THD and AFT offset 10 7 MHz quadrature coil QUL l 70 R10 R11 R12 R13 L1 T1 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 CF1 CF2 Murata SFE10 7ML or equivalent 10 7 MHz ceramic resonators provide selectivity good group delay characteristics important for low THD of system 5 Typical Application LAYOUT CONSIDERATIONS Although the pinout of the LM1865 has been chosen to minimize layout problems some care is required to insure stability The ground terminal on CF1 should return to both the input signal ground and the buffer ground pin 19 The ground terminal on CF2 should return to the ground side of C4 The quadrature coil T1 and inductor L1 should be separated from the input circuitry as far as possible PC Layout (Component Side) TL H 7509 – 6 PERFORMANCE CHARACTERISTICS OF TYPICAL APPLICATION WITH TUNER The following data was taken using the typical application circuit in conjunction with an FM tuner with 43 dB of gain a Meter Output and Signal-to-Noise vs Tuner Input 5 5 dB noise figure and 30 dB of AGC range The tuner was driven from a 50X source 75 ms of de-emphasis was used on the audio output pin 15 The 0 dB reference is for g 75 kHz deviation at 400 Hz modulation Total Harmonic Distortion vs Tuner Input AM Rejection vs Tuner Input TL H 7509 – 7 b 3 dB limiting e 0 9 mV 30 dB quieting e 1 4 mV Level stop mute threshold e 1 4 mV Deviation mute window ( b 3 dB) e g 45 kHz 6 Application Notes ADJUSTABLE MUTE STOP THRESHOLD The threshold adjustments for the mute and stop functions are controlled by the same pins Thus the term mute stop will be used to designate either function The adjustable mute stop threshold in the LM1865 allows for user programming of the signal level at which muting or stop indication takes place The adjustment can be made in two mutually exclusive ways The first way is to take a voltage divider from the meter output (pin 8) to the off channel mute input (pin 13) When the voltage at pin 13 falls below 0 22V an internal comparator is tripped causing muted or causing the stop output to go low Adjustment of the voltage divider ratio changes the signal level at which this happens The second method of mute stop detection as a function of signal level is to use the presence of ultrasonic noise in the recovered audio to trip the internal comparator As the signal level at the antenna of the radio drops the amount of noise in the recovered audio both audible and ultrasonic increases The recovered audio is internally coupled through a high pass filter to pin 13 which is internally biased above the comparator trip point Large negative-going noise spikes will drive pin 13 below the comparator trip point and cause mute stop action A simplified circuit is shown in Figure 4 Since the input to the comparator is noise the output of the comparator is noise Consequently a mute stop filter on pin 12 is required to convert output noise spikes to an average DC value This filter is not necessary if pin 13 is driven from the meter Adjustment of the mute stop threshold in the noise mode is accomplished by adjusting the pole of the high pass filter coupled to the comparator input This is done with a series capacitor resistor combination R9 C11 from pin 13 to ground As the pole is moved higher in frequency (i e R9 gets smaller) more ultrasonic noise is required in the recovered audio in order to initiate mute stop action This corresponds to a weaker signal at the antenna of the radio In choosing the correct value for R9 it is important to make sure that recovered audio below 75 kHz is not sufficient to cause mute stop action This is because stereo and SCA information are contained in the audio signal up to 75 kHz Also note that the ultrasonic mute stop circuit will not operate properly unless a tuner is connected to the IF This is because at low signal levels the noise at the tuner output dominates any noise sources in the IC Consequently driving the IC directly with a 50X generator is much less noisy than driving the IC with a tuner and therefore not realistic The RC filter on pin 12 not only filters out noise from the comparator output but controls the ‘‘feel’’ when manually tuning For example a very long time constant will cause the mute to remain active if you rapidly tune through valid strong stations and will only release the mute if you slowly tune to a valid station Conversely a short time constant will allow the mute to kick in and out as one tunes rapidly through valid stations The advantage in using the noise mute stop approach versus the meter driven approach is that the point at which mute stop action occurs is directly related to the signal-tonoise ratio in the recovered audio Furthermore the mute stop threshold is not subject to production and temperature variations in the meter output voltage at low signal levels and thus might be able to be set without a production adjustment of the radio The noise mute stop threshold is very insensitive to temperature and gain variations Proper operation of this circuit requires that the signal level mute stop threshold be set at a signal level that achieves 45 dB S N ( g 3 dB) in mono in a radio In an electronically tuned radio the signal level stop threshold can be set to a much larger level by gain reducing the tuner (ie pulling the AGC line) in scan mode and then releasing the AGC once the radio stops on a station In an environment where temperature variations are minimal and manual adjustment of the signal level mute stop threshold is desired then the meter driven approach is the best alternative TL H 7509 – 8 FIGURE 4 Simplified Level Mute Stop Circuit 7 Application Notes (Continued) STOP TIME An electronically tuned radio (ETR) pauses at fixed intervals across the FM band and awaits the stop indication from the LM1865 If within a predetermined period of time no stop indication is forthcoming the controller circuit concludes that there is no valid station at that frequency and will tune to the next interval There are several time constants that can affect the amount of time it takes the LM1865 to output a valid stop indication on pin 16 In this section each time constant will be discussed Deviation Stop Time Constant An offset voltage is generated by the AFT if the LM1865 is tuned to either side of a station Since deviation stop detection in the LM1865 is detected by the voltage at pin 14 it is important that this voltage move fast enough to make the deviation stop decision within the time allowed by the controller The speed at which the voltage at pin 14 moves is governed by the RC time constant R5 C9 This time constant must be chosen long enough to remove recovered audio from pin 14 and short enough to allow for reasonable stop detection time Signal Level Stop Using Ultrasonic Noise Detection As previously mentioned the R6 C8 time constant on pin 12 is necessary to filter the noise spikes on the output of the internal comparator in the LM1865 This time constant also determines the level stop time When the voltage at pin 12 is above a threshold voltage of about 0 6V the stop output is low The maximum voltage at pin 12 is about 0 8V The level stop time is dominated by the amount of time it takes the voltage at pin 12 to fall from 0 8V to 0 6V The voltage at pin 12 follows an exponential decay with RC time constant given by R6 C8 For example if R6 e 25k and C8 e 2 2 mF the stop time is given by t e b(24k) (2 2 mF) fin Signal Level Stop Using the Meter Output Pin 8 As mentioned previously R6 C8 is not necessary when the meter output is used to drive pin 13 Consequently this time constant is not a factor in determining the stop time However the speed at which the meter voltage can move may become important in this regard This speed is a function of the resistive load on pin 8 and filter capacitance C5 AGC Time Constant In tuning from a strong station to a weaker station above the level stop threshold the AGC voltage will move in order to try to maintain a constant tuner output The AGC voltage must move sufficiently fast so that the tuner is gain increased to the point that the level stop indicates a valid station This time constant is controlled by R11 and C13 DISTORTION COMPENSATION CIRCUIT The quadrature detector of the LM1865 has been designed with a special circuit that compensates for distortion generated by the non-linear phase characteristic of the quadrature coil This circuit not only has the effect of reducing distortion but also desensitizes the distortion as a function of tuning characteristic As a result low distortion is achieved with a single tuned quad coil without the need for a double tuned coil which is costly and difficult to adjust on a production basis The lower distortion has been achieved without any degradation of the noise floor of the audio output Futhermore the compensation circuit first-order cancels the effect of quadrature coil Q on distortion When measuring the total harmonic distortion (THD) of the LM1865 it is imperative that a low distortion RF generator be used In the past it has been possible to cancel out distortion in the generator by adjustment of the quadrature coil This is because centering the quadrature coil at other than the point of inflection on the S-curve introduces 2nd harmonic distortion which can cancel 2nd harmonic distortion in the generator Thus low THD numbers may have been obtained wrongly Large AFT offsets asymmetrical off tuning characteristic and less than minimum THD will be observed if alignment of the quadrature coil is done with a high distortion RF generator Care must also be taken in choosing ceramic filters for the LM1865 It is important to use filters with good group delay characteristics and wide enough bandwidth to pass enough FM sidebands to achieve low distortion 0 8J 06 which yields t e 15 ms It should be noted that the 0 6V threshold at pin 12 has a high temperature dependence and can move as much as 100 mV in either direction 8 Application Notes (Continued) The LM1865 has been carefully designed to insure low AFT offset current at the point of minimum THD AFT offset current will cause a non-symmetric deviation mute stop window about the point of minimum THD No external AFT offset adjustment should be necessary with the LM1865 The amount of resistance in series with the 18 mH quadrature coil drive inductor L1 has a significant effect on the minimum THD This series resistance is contributed not only by R13 but also by the Q of L1 The Q of L1 should be as high as possible (ie Q l50) in order to avoid production problems with the Q variation of L1 Once R13 has been optimized for minimum THD adjustment on a radio by radio basis should be un-necessary DUAL THRESHOLD AGC (AUTOMATIC LOCAL DISTANCE SWITCH) There is a well recognized need in the field for gain reducing (AGCing) the front end (tuner) of an FM receiver This gain reduction is important in preventing overload of the front end which might occur for large signal inputs Overloading the front end with two out-of-band signals one channel spacing apart and one channel spacing from center frequency or two channel spacings apart and two channel spacings from center frequency will produce a third order intermodulation product (IM3) which falls inband This IM3 product can completely block out a weaker desired station The AGC in the LM1865 has been specially designed to deal with the problem of IM3 With the LM1865 system a low AGC threshold is achieved whenever there are strong out-of-band signals that might generate an interfering IM3 product and a high AGC threshold is achieved if there are no strong out-of-band signals The high AGC threshold allows the receiver to obtain its best signal-to-noise performance when there is no possibility of an IM3 product The low AGC threshold allows for weaker desired stations to be received without gain-reducing the tuner It should be noted that when the AGC threshold is set low there will be a signal-to-noise compromise but is assumed that it is more desirable to listen to a slightly noisy station than to listen to an undesired IM3 product The simplified circuit diagram (Figure 5 ) of the AGC system shows how the dual AGC thresholds are achieved Vm e 1V corresponds to a fixed in-band signal level (defined as VNB) at the tuner output VNB will be referred to as the ‘‘narrow band threshold’’ VWB also corresponds to a fixed tuner output which can either be an in-band or out-ofband signal This fixed tuner output will be called the ‘‘wide band threshold’’ Always VWB l VNB R11 and C13 define the AGC time constant A reverse AGC system is shown This means that VAGC decreases to gain-reduce the tuner The LM1865 AGC output is an open-collector current source capable of sinking at least 1 mA TL H 7509 – 9 FIGURE 5 Dual Threshold AGC I1 e GM1 Vm only if Vm l 1V otherwise I1 e 0 Gm1 VWB e constants IAGC e Gm2 Vo where Gm2 e I1 26 mV and Vo l VWB otherwise IAGC e 0 9 Application Notes (Continued) First examine what happens with a single in-band signal as we vary the strength of this signal Figures 6 and 7 illustrate what happens at the tuner and AGC outputs In Figure 7 there is no AGC output until the tuner output equals the wide band threshold At this point both SW2 and SW1 are closed and the AGC holds the tuner output in Figure 6 relatively constant Another simple case to examine is that of the single out-ofband signal Here there is no AGC output even if the signal exceeds VWB There is no output because the ceramic filters prevent the out-of-band signal from getting to the input of the IF With no signal at the IF input there is no meter output and SW1 is open which means No AGC FIGURE 6 FIGURE 7 TL H 7509–10 Figures 8 and 9 illustrate what happens at the tuner and AGC outputs when the strength of an in-band signal is varied in the presence of a strong out-of-band signal (i e greater than VWB) which is held constant at the tuner input For this example the in-band signal at the tuner output will be referred to as VD (desired signal) and the out-of-band signal as VUD (undesired signal) In Figure 9 we see that there is no AGC output until the tuner output exceeds the narrow band threshold VNB At this point Vm l 1V and SW1 closes Further increase of the desired signal at the tuner input results in an AGC current that tries to hold the desired signal at the tuner output constant This gain reduction of the tuner forces the undesired signal at the tuner output to fall At the point that VUD reaches the wide band threshold no further gain reduction can occur as Vo would fall below VWB (refer to Figure 5 ) At this point control of the AGC shifts from the meter output (narrow band loop) to the out-of-band signal (wide band loop) Here VUD is held constant along with the AGC FIGURE 8 Prime indicates referenced to tuner input TL H 7509 – 11 FIGURE 9 10 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 constant by the AGC When compared to the simple case of a single in-band signal we see that because of the presence of a strong out-ofband 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-reduced 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 simplified 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 accomplished 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 decreases 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 TL H 7509 – 12 FIGURE 10 Narrow Band Threshold Circuit TL H 7509 – 13 FIGURE 11 Affect of Meter Load on Narrow Band Threshold 11 Application Notes (Continued) WIDE BAND AGC THRESHOLD ADJUSTMENT There are a number of criteria that determine where the wide band threshold should be set If the threshold is set too high protection against IM3 will be lost If the threshold is set too low the front end under certain input conditions may be needlessly gain-reduced sacrificing signal-to-noise performance Ideally the wide band threshold should be set to a level that will insure AGC operation whenever there are out-of-band signals strong enough to generate an IM3 product of sufficient magnitude to exceed the narrow band threshold Ideally this level should be high enough to allow for a single in-band desired station to AGC the tuner only after the maximum signal-to-noise has been achieved In order to insure that the wide band loop is activated whenever the IM3 exceeds the narrow band threshold VNB determine the minimum signal levels for two out-of-band signals necessary to produce an IM3 equal to VNB Then arrange for the wide band loop to be activated whenever the tuner output exceeds the rms sum of these signals There are many combinations of two out-of-band signals that will produce an IM3 of a given level However there is only one combination whose rms sum is a minimum at the tuner output IM3 at the tuner output is given according to the equation IM3 e aVUD12 VUD2 (assuming no gain reduction) (1) where a e constant dependent on the tuner VUD1 e out-of-band signal 400 kHz from center frequency applied to tuner input VUD2 e out-of-band signal 800 kHz from center frequency and 400 kHz away from VUD1 applied to tuner input In general due to tuned circuits within the tuner the tuner gain is not constant with frequency Thus if the tuner is kept fixed at one frequency while the input frequency is changed the output level will not remain constant Figure 12 illustrates this It can be shown that for a given IM3 the combination of VUD1 and VUD2 that produces the smallest rms sum at the tuner output is given by the equations VUD1 e 1 12  A1 a J A2 IM3 (2) VUD2 e 0 794  A2 A12 IM3 2a J 2 (3) Therefore in order to guarantee that the AGC will be keyed for an IM3 e VNB we need only satisfy the condition VWB s 2 0VNB a (A1) (1 12)  A1 a J ( A2 VNB a A2 (0 794)  A2 A12 VNB 2a J( 2 (4) The right hand term of equation (4) defines an upper limit for VWB called VWBUL VWBUL is the rms sum of all the signals at the tuner output for two out-of-band signals VUD1 and VUD2 as expressed in equations (2) and (3) applied to the tuner input TL H 7509 – 14 Define A e tuner gain at center frequency A1 e tuner gain at f o a 400 kHz A2 e tuner gain at f o a 800 kHz FIGURE 12 12 Application Notes (Continued) In order to make the calculation in equation (4) the constants a A1 A2 must first be determined This is done by the following procedure 1 Connect together two RF generators and apply them to the tuner input Since the generators will terminate each other remove the 50X termination at the tuner input 2 Connect a spectrum analyzer to the tuner output Most spectrum analyzers have 50X input impedances To make sure that this impedance does not load the tuner output use a FET probe connected to the spectrum analyzer The tuner output should be terminated with a ceramic filter 3 Disconnect the AGC line to the tuner Make sure that the tuner is not gain-reduced 4 Adjust the two RF generators for about 1 mV input and to frequencies 400 kHz and 800 kHz away from center frequency (Figure 13 ) 5 Note the three output levels in volts 6 Knowing the tuner input levels for VUD1 and VUD2 and the resulting IM3 just measured ‘‘a’’ is calculated from the formula ae IM3 VUD12 VUD2 (5) If the wide band threshold was set to VWBUL then when a single in-band station reached the level VWBUL at the tuner output AGC action would start to take place For this reason it is hoped that VWBUL is above the level that will allow for maximum signal-to-noise If however this is not the case consideration might be given to improving the intermodulation performance of the tuner The lower limit for VWB is the minimum tuner output that achieves the best possible signal-to-noise ratio in the recovered audio In general it is desirable to set VWB closer to the upper limit rather than the lower limit This is done to prevent AGC action within the narrow band loop except when there is a possibility of an IM3 greater than VNB The wide band threshold at the pin 20 input to the LM1865 is fixed at 12 mVrms Generally speaking if pin 20 were driven directly from the tuner output VWB would be too low Therefore in general pin 20 is not connected directly to the tuner output Instead the tuner output is attenuated and then applied to pin 20 Increasing attenuation increases the wide band threshold VWB Pin 20 has an input impedance at 10 7 MHz that can be modeled as a 500X resistor in series with a 19 pF capacitor giving a total impedance of 940X K b58 Thus an easy way to attenuate the input to pin 20 is with the arrangement shown in Figure 14 Notice that pin 20 must be AC coupled to the tuner output and that C1 is a bypass capacitor R1 adjusts the amount of attenuation to pin 20 The wide band threshold will roughly increase by a factor of (R1 a 940X) 940X AGC CIRCUIT USED AS A CONVENTIONAL AGC If for some reason the dual AGC thresholds are not desired it is easy to use the LM1865 as a more conventional LM3189 type of AGC This is accomplished by AC coupling the pin 20 input after the ceramic filters rather than before the filters Thus as with the LM3189 only in-band signals will be able to activate the AGC where all levels are in volts rms A typical value for ‘‘a’’ might be 2 c 106 7 A1 and A2 are calculated according to the following formulas A1 e V1 VIN f o a 400 kHz (6) A2 e V2 VIN f o a 800 kHz (7) TL H 7509 – 16 TL H 7509 – 15 FIGURE 14 Wide Band Threshold Adjustment FIGURE 13 Spectrum Analyzer Display of Tuner Output 13 Simplified Diagram 14 TL H 7509 – 17 Advanced FM IF System Physical Dimensions inches (millimeters) Small Outline IC Package (M) Order Number LM1865M NS Package Number M20B 15 LM1865 Advanced FM IF System Physical Dimensions inches (millimeters) (Continued) Molded Dual-in-Line Package (N) Order Number LM1865N 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 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