LM2065N ,16 V, 2 W, advanced FM IF systemElectrical Characteristics
Soldering Information
Dual-ln-Line Package
Soldering (10 seconds) ..
LM207J ,Operational AmplifiersLM107/LM207/LM307OperationalAmplifiersDecember1994LM107/LM207/LM307OperationalAmplifiersGeneralDesc ..
LM208 ,Operational AmplifierLM108/LM208/LM308OperationalAmplifiersDecember1994LM108/LM208/LM308OperationalAmplifiersGeneralDesc ..
LM208 ,Operational AmplifierLM108/LM208/LM308OperationalAmplifiersDecember1994LM108/LM208/LM308OperationalAmplifiersGeneralDesc ..
LM208AH ,Operational AmplifiersLM108A/LM208A/LM308AOperationalAmplifiersMay1989LM108A/LM208A/LM308AOperationalAmplifiersGeneralDes ..
LM20BIM7 ,2.4V, 10µA, SC70, micro SMD Temperature SensorFeaturesture of +30˚C. The temperature error increases linearly andn Rated for full −55˚C to +130˚C ..
LM4040B82IDCKR , PRECISION MICROPOWER SHUNT VOLTAGE REFERENCE
LM4040B82IDCKRG4 , PRECISION MICROPOWER SHUNT VOLTAGE REFERENCE
LM4040BEM3-2.5+T ,2.5 V, improved precision micropower shunt voltage reference with multiple reverse breakdown voltageLM404019-1787; Rev 4; 4/04Improved Precision Micropower Shunt VoltageReference with Multiple Revers ..
LM4040BEM3-3.0+T ,Improved Precision Micropower Shunt Voltage Reference with Multiple Reverse Breakdown VoltagesELECTRICAL CHARACTERISTICS—2.048V(I = 100µA, T = T to T , unless otherwise noted. Typical values ar ..
LM4040BEM3-4.1+T ,Improved Precision Micropower Shunt Voltage Reference with Multiple Reverse Breakdown Voltages LM4040Improved Precision Micropower Shunt VoltageReference with Multiple Reverse Breakdown Voltag ..
LM4040BEM3-5.0+T ,Improved Precision Micropower Shunt Voltage Reference with Multiple Reverse Breakdown Voltages LM4040Improved Precision Micropower Shunt VoltageReference with Multiple Reverse Breakdown Voltag ..
LM2065N
16 V, 2 W, advanced FM IF system
LM1865/LM1965/LM2065
Fi)? Natiqnal _
Semiconductor
LM1865/LM1965/LM2065 Advanced FM IF System
. . a Low distortion 0.1% typical with a single tuned quadra-
General Deseription ture coil for 100% modulation.
Broad off frequency distortion characteristic
Low THD at minimum AFT offset
Reduced external component cost, improved performance,
and additonal functions are key features to the LM1865/
LM1965/LM2065 FM F system. The LM1865 and LM2065
are designed for use in electronically tuned radio applitra- Meter output proportional to signal level
tions. These versions contain both deviation and signal level Mute function with mute disable and soft deviation
stop circuitry in addition to an open-collector stop output. mute for LM1965
The LM1865 and LM2065 differ only in the direction of the .
AGC output voltage they generate on pin 18. The LM1865 iti'ut""'t" with open-collector output for LM1865/
generates a reverse AGC voltage (is: decreasing AGC volt- . .
age with increasing signal) and the LM2065 generates a n Adjustable Signal.level_ mute/stop threshold, centrolled
forward AGC voltage (is: increasing AGC voltage with in- elther by ultrasonic nonse In the recovered audio or by
creasing signal.) The LM1965 has a reverse AGC character- the meter output
istic. The LM1965 is designed for use in manually tuned II Adjustable deviation mute/stop threshold
radios and provides a deviation and signal level mute func- n Separate time constants for signal level and deviation
tion in addition to a pin that disables the mute function when mute/stop
grounded. All three versions are offered in both 20 pin D.l.P.
and S.O. packages.
I: Dual threshold AGC eliminates need for locaI/distance
switch and offers improved immunity from third order in-
termodulation products due to tuner overload
Ftatyrtt . , . tt User control of both AGC thresholds
n On-chlp buffer to provide gan and terminate two CB- . . . . .
ramic filters II Excellent signal to noise ratio, AM rejection and system
limiting sensitivity
Block Diagram
CERAMIC
F|LTER
OUTPUT - l a
l'"" aumn sur
I necuuru OUT
cmmc xumn
FILTER t : INPUT 7 POWER
IF tlllhlmliTlltlE ennuun 1
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sums AFT out AND
SUPPLY
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MUTE/STDP MUTE/STOP MUTE DISABLE (UMeM) I
L - - Qt.".h0W2l11.r. - STOPOUTPUT (mum Peru
mm Order Number LM1865N,
LM1965N or LM2065N
INRESHM See NS Package Number N20A
ADJUST METEI
TL/H/7509-1
FIGURE 1
Absolute Maximum Ratings
If Mllltary/Aerospace specified devices are required,
please contact the National Semiconductor Sales
OmtmfDlatributors for availability and apetMeationa.
Supply Voltage, Pin 17
Package Dissipation (Note I)
Storage Temperature Range
Operating Temperature Range
Max Voltage on Pin 16 (Stop Output)
for LM1865, LM2065
Electrical Characteristics
-55''C to + 150°C
- 20'C to + 85°C
Soldering Information
Dual-ln-Line Package
Soldering (10 seconds) 260°C
Small Outline Package
Vapor Phase (60 seconds) 215'C
Infrared (15 seconds) 220°C
See AN-450 “Suriace Mounting Methods and Their Effect
on Product Reliability" for other methods of soldering sur-
face mount devices.
Test Circuit, TA ='-' 25°C V+ =12V; SI in position 2; S2 in position I; and S3 in position 2 unless indicated otherwise
Parameter l Conditions I Min I Typ I Max l Units
STATIC CHARACTERISTICS
Supply Current 33 45 mA
Pin 9, Regulator Voltage 5.7 V
Operating Voltage Range (See Note 2) 7.3 16 V
Pin 18, Output Leakage Current Pin 20 Open, " = 0, S3 in Position 1 0.1 IIA
Pin 16, Stop Low Outpu1Voltage (LM1865 Only) S1 in Position 1, S2in Position 3 0.3 V
Pin 16, Stop High Output Leakage Current S2in Position 2, V14 = V9 0.1 ILA
(LM1865 Only)
Pin 15, Audio Output Resistance 4.7 kn
Pin 1, Buffer Input Resistance Measured at DC 350 n
Pin 3, Buffer Output Resistance Measured at DC 350 n
Pin 20, Wide Band Input Resistance Measured at DC 2 n
Pin 8, Meter Output Resistance 1 kn.
DYNAMIC CHARACTERISTICSiMOD = 400 Hz, to = 10.7 MHz, Deviation = t 75 kHz
-3 dB Limiting Sensitivity IF Only (See Note 3) 60 120 erms
Buffer Voltage Gain VIN Pin 1 = 10 mVrms at 10.7 MHz 19 22 25 dB
Recovered Audio " == 10 mVrms. V14 = V9 275 320 470 mVrms
Signal-to-Noise " = 10 mVrms, V14 ' V9 (See Note 4) 70 84 dB
AM Rejection V14 = V9
" = 1 mV, 30% AM Mod 50 60 dB
" = 10 mV, 30% AM Mod 50 60 dB
Minimum Total Harmonic Distortion " = 10 mV 0.1 0.35 %
THD at Frequency where V14 = V9 " = 10 mV, Tune until V14 = V9 0.1 0.45 %
(Zero AFT Offset)
THD k 10 kHz from Frequency where V14 == V9 " = 10 mV 0.15 %
AFT Offset Frequency for Deviation Mute " = 10 mV, Audio == -3 dB, S2 in Position 4 i 62 kHz
(LM1965 Only) Offset = (Frequency for -3 dB Audio) -
(Frequency where V14 = V9)
AFT Offset Frequency for Low Stop Output at " = 10 mV, S2 in Position 3, fMOD = 0 i 50 kHz
Pin 16 (LM1865 and LM2065 Only) Offset = (Frequency for Pin 16 Low) -
(Frequency where V14 = V9)
Ultrasonic Mute/Stop Level Threshold V14 = V9, S1 in Position 3 (See Note 5) 60 kHz
" = 10 mV
'MOD = 100 kHz
S2 in Position 4 (LM1965)
S2 in Position 3 (LM1865/LM2065)
Amount of Deviation where Audio Mutes (LM1965)
Amount of Deviation where V16 - Low
(LM1865, LM2065)
EQOZW'l/SQBI-W'l/SQBI-W'l
LM1865/LM1965ILM2065
Electrical Characteristics
Test Circuit, TA == 25''C, V+ =
12V; S1 in position 2; S2 in position I; and S3 in position 2 unless indicated otherwise
(Continued)
Parameter I Condltlons l Mln I Typ I Max l Units
DYNAMIC CHARACTERISTICS) = 400 Hz, to = 10.7 MHz, Deviation = k75 kHz (Continued)
Pin 13 Mute/Stop Threshold Voltage V14 = V9, S1 in Position 4 220 mV
S2in Position 4 (LM1965)
S2 in Position 3 (LM1865, LM2065)
V13 where Audio Mutes (LM1965)
V13 where V16 - Low (LM1865, LM2065)
Amount of Muting (LM1965 Only) S2 in Position 4,31 in Position I, " = 10 mV 66 dB
Amount of Muting with Pin 13 and S1 in Position 1 0 dB
Pin 16 Grounded VI4, = V9, " = 10 mV
Narrow Band AGC Threshold Increase IF Input until I AGC = 0.1 mA 100 210 300 erms
Pin 20 = 30 mVrms (See Note 6)
Wide Band AGC Threshold " = 100 mVrms 5 12 22 mVrms
Increase Signal to Pin 20 until IAGC = 0.1 mA
(See Note 6)
Pin 18, Low Output Voltage VIN Pin 20 = 100 mV, " = 100 mVrms 0.2 0.5 V
(LM1865 and LM1965 only)
Pin 18, High Output Voltage (LM2065 only) VIN Pin 20 = 100 mV, " = 100 mVrms, (See Note 6) 11.7 V
Pin 8, Meter Output Voltage " = 10 pV 0.1 V
" = 300 [J.V 1.1 V
" = 3 mV 2.6 V
Note 1: Above TA = 25°C aerate based on Tum“) =
Note 2: All data sheet specifications are for V+ =
Nola & When the F is preceded by 22 dB gain in the
Note 4: Measured with a notch at 60 Hz and 20 Hz to
150°C and 8UA = GO'C/W.
12V may change slightly with supply.
buffer, excellent system sensitivity is achieved.
100 kHz bandwidth.
Note tk FM modulate RF source with a 100 kHz audio signal and find what modulation level, expressed as kHz deviation, results In audio mute for the LM1965 or
V16 - 12V for the LM1865/LM2065.
Note 6: S3 in Position 3 for LM2065.
TL/H/ 7509-2
Test Circuit
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FIGURE 2
Typical Performance Characteristics (from Test Circuit)
FM Ummng Characteristics
and AM Rejectlon
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AFT Load Resistor
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TL/H/7509-3
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Application Circuit
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FIGURE 3
IC External Components (See Application Circuit)
Component Typlcal Value Comments
C1 0.01 p.F AC coupling for wide band AGC input
C2 0.01 p.F Buffer and A60 supply decoupling
C3, C4 0.01 " IF decoupling capacitors
C5 10 pF Meter decoupling capacitor
C6 0.01 p.F AC coupling for IF output
C7 50 p.F Regulator decoupling capacitor. affects S/N floor
C8 2.2 p.F Level mute/stop time constant
09 5 pF AFT decoupling. affects stop time
C10 0.1 pF Disables noise mute/stop
C11 0.01 pF AC coupling for noise mute/stop threshold adjust
C12 25 pF Supply decoupling
C13 0.01 p.F AGC output decoupling capacitor
R1 Tuner Dependent Wide band AGC threshold adjust
R2, R3 Tuner Dependent Gain set and bias for IF; R2 + R3 = 3300 to terminate ceramic filter
R4 Meter Dependent Sets fulI-scale on meter
R5 5k1 Deviation mute/stop window adjustment
R6 25k Mute/stop filter, affects stop time
R7 5k Level mute/stop threshold adjustment
R8 10k Pot Level mute/stop threshold adjustment
R9 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
occurs in the radio at 45dB S/N (i3 dB) in mono.
R10 10k Load for open-collector stop output
R11 50k AGC output load resistor for open-collector output
R12 3k9 Sets 0 of quadrature coil affecting THD, sm and recovered audio
R13 620 Optimises minimum THD
L1 18 pH Qu>50 @ 10.7 MHz Sets signal swing across quadrature coil, High Q is important to mini.
TDK Electronics mize effect variation of Q has on both minimum THD and AFT offset.
TPO410-180K or equivalent
T1 Qu>70 tit 10.7 MHz, Lto 10.7 MHz quadrature coil: QUL > 70
resonate w/82 pF @ 10.7 MHz
'%,i 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
Typical Application
LAYOUT CONSIDERATIONS
Although the pinout of the LM1865/LM1965/LM2065 has
been chosen to minimize layout problems, some care is re-
quired 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 Slde)
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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
Slgnal-to-Nolse
" Tuner Input
Total Harmonlc Dlstortlon "
Tuner Input
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TL/H/7509-6
5.5 dB noise figure, and 30 dB of A60 range. The tuner was
driven from a 50ft source, 75 p5 of de-emphasis was used
on the audio output, pin 15. The 0 dB reference is for i 75
kHz deviation at 400 Hz modulation.
AM Relectlon " Tuner
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TUNER INPUT (W)
--3 dB Ilmltlng " 0.9 pv
30 dB quietlng = 1.4 pLV
Level stop/mute threshold - 1.4 WV
Devlatlon mute window (-3 dB) = A45 kHz
TUNER INPUT (uV)
TUNER INPUT bt)
TL/H/7509-7
59OZW'l/996lW'1/998L W1
LM1865/LM1965/LM2065
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/
LM1965/LM2065 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 Compara-
tor is tripped causing muted or causing the stop output to go
low. Adjustment of the voltage divider ratio changes the sig-
nal 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 sig-
nal 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 CII, from pin 13 to
ground. As the polo is moved higher in frequency (i.e., R9
gets smaller) more ultrasonic noise is required in the recov-
ered audio in order to initiate mute/stop action. This corre-
r---"'",.,-.,..,-"
RECOVERED
'm7sac"hriuif, -
sponds 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 oper-
ate 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, driv-
ing the IC directly with a 500 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 it 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 ver-
sus the meter driven approach is that the point at which
mute/stop action occurs is directly related to the signal-to-
noise 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 ad-
]ustment of the radio. The noise mute/stop threshold is very
insensitive to temperature and gain variations. Proper oper-
ation of this circuit requires that the signal level mute/stop
threshold be set at a signal level that achieves 45 dB S/N
(K3 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.
"'"''"'''"'""''""'""''-"t
MUTE/ STOP
CIRCUIT
HIGH FOR MUTE tm
STOP OUTPUT LOW
TL/H/7509-8
FIGURE 4. Simplified Level Mute/Stop Clrcult
Application Notes (Continued)
DEVIATION MUTE/STOP
As with the LM3189, the resistor connected between VREG
(pin 9) and the AFT (pin 14) sets the deviation mute/stop
window (see Typical Performance Characteristics). The
LM1965 was designed with a soft deviation mute. This
means that the audio is gradually muted as you off tune
from center frequency. Gradually muting avoids the problem
of an audio pop which would otherwise occur due to the
unavoidable DC voltage shift at the audio output that ac-
companies the muting action. Capacitor C9 on the AFT pin
sets the time constant for the deviation mute/stop indepen-
dent of the level mute/stop time constant. C9 should be
large enough to remove the audio from the AFT. The AFT
pulls high at low signal levels if the IF is driven directly from
a 500 generator and not a tuner. This is a result of a loss of
signal across the quad coil and a resulting phase shift in the
quadrature detector. This phase shift offsets the AFT. With
a tuner and sufficient IF gain, at low signal levels there will
be enough noise across the quad coil to prevent much of
this AFT shift. Thus, care should be taken when adjusting
the F gain (which is done by adjusting the ratio of R3 to R2)
to minimize the AFT shift. Grounding pin 16 on the LM1965
will disable the mute function.
STOP TIME
An electronically tuned radio (ETR) pauses at fixed intervals
across the FM band and awaits the stop indication from the
LM1865/LM2065. If within a predetermined period of time,
no stop indication is forthcoming, the controller circuit con-
cludes that there is no valid station at that frequency and
will tune to the next interval. There are several time con-
stants that can affect the amount of time it takes the
LM1865/LM2065 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/
LM2065 is tuned to either side of a station. Since deviation
stop detection in the LM1865/LM2065 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/LM1965/LM2065. 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 =
25k and C8 = 2.2 wi' the stop time is given by
t-- -f24k 2.2 F I -._..
( )( P- ) n (0.8)
which yields t = 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.
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. Howev-
er, 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. G5.
A60 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 A60 voltage
must move sufficiently fast so that the tuner is gain in-
creased 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/LM1965/LM2065
has been designed with a special circuit that compensates
for distortion generated by the non-linear phase characteris-
tic 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-or-
der cancels the effect of quadrature coil Q on distortion.
When measuring the total harmonic distortion (T HD) of the
LM1865/LM1965/LM2065, it is imperative that a low distor-
tion 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 quadra-
ture 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 num-
bers may have been obtained wrongly. Large AFT offsets
asymmetrical off tuning characteristic, and less than mini-
mum 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/LM1965/LM2065. It is important to use filters with
good group delay characteristics and wide enough band-
width to pass enough FM sidebands to achieve low distortion.
SQOZW'l/SSGLW‘l/SSSl-W'l
LM1865/LM1965/LM2065
Application Notes (Continued)
The LM1865/LM1965/LM2065 has been carefully designed
to insure low AFT offset current at the point of minimum
THD. AFT offset current will cause a non-symmetric devia-
tion mute/stop window about the point of minimum THD. No
external AFT offset adjustment should be necessary with
the LM1865/LM1965/LM2065. The amount of resistance in
series with the 18 pH quadrature coil drive inductor. L1, has
a significant effect on the minimum THD. This series resist-
ance is contributed not only by R13 but also by the Q of L1.
The a of L1 should be as high as possible (is: Q> 50) in
order to avoid production problems with the Q variation of
L1. Once R13 has been optimized for minimum THD, adjust-
ment on a radio by radio basis should be un-necessary.
DUAL THRESHOLD AGO
(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 fre.
quency. or, two channel spacings apart and two channel
spacings from center frequency, will produce a third order
intermodulation product (lMa) which falls inband. This IM3
product can completely block out a weaker desired station.
The AGC in the LM1865/LM1965/LM2065 has been spe-
cially designed to deal with the problem of Mg.
With the LM1865/LM1965/LM2065 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 re-
ceiver to obtain Its best signaI-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 signaI-to-
noise compromise, but is assumed that It is more desirable
to listen to a slightly noisy station than to listen to an unde-
sired IMa product. The simplified circuit diagram (Figure 5)
of the AGC system shows how the dual AGC thresholds are
achieved.
Vm = 1V corresponds to a fixed In-band signal level (de-
fined as VNB) at the tuner output. VNB will be referred to as
the "narrow band threshold". Vwe also corresponds to a
fixed tuner output which can either be an ln-band or out-ot.
band signal. This fixed tuner output will be called the "wide
band threshold". Always Vwa > 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/LM1965 AGC output is an open-coilector cur-
rent source capable of sinking at least 1 mA. The LM2065
AGC output is also an open collector current source capa-
ble of sourcing at least 1 mA. The A60 voltage can move
over the full range of the V+ supply.
ANTENNA
SELECTIVITY
Paar V T
amass a FIL Ell aurrsn nun IF - “O
J; ouwur
" a m”
V“ >ha mass 0mm .-4 "
com com
l mu aurrur mun outrur
TO CLOSE swz TO CLOSE sw1
swz 3 am
TL/Hl7509-9
FIGURE 5. Dual Threshold AGC
I1 = GM1Vm onlyltvm >1V
otherwise I1 - 0
thm, Vwa a constants
lAec _ Gm; Vo where Gm; -- I1/26 mV and
Vo > Vwa otherwise IAQC " 0
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 A60 outputs.
TUNER OUTPUT
SLOPE IS INVERSELY PROPORTIONAL
T0 LOOP GAIN OF WIDE BAND Mt CIRCUII’
m ------
m - - I
I Avi Yatit
I HGURES
REVERSE Mt OUTPUT I
tl WI“ VAN!
FIGURE 7 TL/H/7509-10
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 Fi -
ure 6 relatively constant.
Another simple case to examine is that of the single out-of-
band signal. Here there is no AGO output even if the signal
exceeds Wm. There is no output because the ceramic fil-
ters 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.
Figures tt and 9 illustrate what happens at the tuner and
AGC outputs when the strength of an in-band signal is var-
ied in the presence of a strong out-of-band signal (i.e.,
greater than ng) 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 > 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 con-
stant. This gain reduction of the tuner forces the undesired
signal at the tuner output to fall. At the point that VUD reach-
es the wide band threshold, no further gain reduction can
occur as Vo would fall below ng (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
wuss oumr
- - - ---,
N, " REACHES v“
w. - x -- - " IN-BAND SIGNAL
t ', (Vb)
DESIRED SIGNAL 1 N
amass VIII l N
VIII - I t I 'x
I I Vun mm 1 N UUT-OF mu SIGNAL
l l T0 LEVEL I (Van)
', I 0F WI! : h
,' VHS 1 I M (TUNER INPUT)
I t I FIGURE 8
mans: Am: umrur , l l
(TUNER INPUT)
Prime indicates teterenced to tuner Input
TL/H/7509- 1 1
FIGURE 9
9903W1/ 996lW1/998IW1
LM1865/LM1965/LM2065
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 > Vwa as VD takes over the lob. Thus VUD will
drop as the amount of AGC increases, while Vo 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 2 Wm. For the two signal case
above, AGC started when VD 2 VNB. Thus, the LM1865/
LM1965/LM2065 achieves an early AGC when there are
strong adjacent channels that might cause Na, 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 < 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 A60 to weak desired
stations below the narrow band threshold.
ummn lk
NARROW BAND AGO 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
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 " 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.
METER OUTPUT
Tit HIGH - SW1 CLOSED
UNI -SW1 OPEN
TL/H/7509-12
FIGURE 10. Narrow Band Threshold Clrcult
PIN fl METER OUTPUT (V)
------ METER LOAD = 33k
--_r- METER LOAD =1k
Vu TUNER
TL/H/7509- 1 3
FIGURE 11. Mice! of Meter Load on Narrow Band Threshold
Application Notes (Continued)
WIDE BAND AGO 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 lM3 prod-
uct 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 signaI-to-noise has been achieved.
In order to insure that the wide band loop is activated when-
ever the IM3 exceeds the narrow band threshold, VNB, de-
termine the minimum signal levels for two out-ot-hand sig-
nals necessary to produce an IMa equal to VNB. Then, ar-
range 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 out-
put. ms at the tuner output is given according to the
equation:
lM3 = aVUD12 VUD2 (assuming no gain reduction) (1)
where a = constant dependent on the tuner;
Vum = out-of-band signal 400 kHz from center frequen-
cy, applied to tuner input;
TUNER GAIN
Vuoz = out-of-band signal 800 kHz from center frequen-
cy and 400 kHz away from Vum, applied to tun-
er 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 illus-
trates this.
It can be shown that for a given Ns the combination of
Vum and Vupg that produces the smallest rms sum at the
tuner output is given by the equations:
_ Sl "r?)''
vUD1 .1.12(A1 a (2)
A12 lMa)‘/3
= 0.794 - -
Vuoz (A22 a (3)
Therefore, in order to guarantee that the AGC will be keyed
for an IM3 = VNB we need only satisfy the condition:
vw3s\/VNZB + [(A1)(1.12) (A2%)'/’]2 + [A2 (0.794) (A1 VNB) %]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, Vum and
VUD2 [as expressed in equations (2) and (3)], applied to the
tuner input.
- - -1 - - -
l 4 TUNER INPUT FREQUENCY
lo 10 + fit +
400 kHz MO Hi:
TL/H/7509-14
Define A = tuner gain at center frequency
A1 = tunergain at fo + 400 kHz
A2 = tunergain at fo + 800 kHz
FIGURE "
9903W1/996lW1/998LW1
LM1865/LM1965/LM2065
Application Notes (Continued)
In order to make the calculation in equation (4), the con-
stants 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 500. termination at the tuner input.
2. Connect a spectrum analyzer to the tuner output. Most
spectrum analyzers have son input impedances. To
make sure that this impedance does not load the tuner
output use a FET probe connected to the spectrum ana-
lyzer. The tuner output should be terminated with a ce-
ramic 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 fre-
quency (Figure 13).
5. Note the three output levels in volts.
6. Knowing the tuner input levels for Vum and VUD2 and
the resulting IM3 just measured. "a" is calculated from
the formula:
a = - (5)
V0012 V002
where all levels are in volts rms. A typical value for "a"
might be 2 M 106.
7. A1 and A2 are calculated according to the following for-
A1 = (6)
fo + 400 kHz
A2 = (7)
fo + 800 kHz
Illa v2
[0 fu+mkH1 fo+m "
[0:10] MHz
TL/H/7509-15
FIGURE 13. Spectrum Analyzer Display of Tuner Output
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 rea-
son it is hoped that VWBUL is above the level that will allow
for maximum signal-to-noise. It, however, this is not the
case, consideration might be given to improving the inter-
modulation performance of the tuner.
The lower limit for Wm is the minimum tuner output that
achieves the best possible signal-to-noise ratio in the recov-
ered audio. In general, it is desirable to set ng 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 lM3 greater than VNB.
The wide band threshold at the pin 20 input to the LM1865/
LM1965/ LM2065 is fixed at 12 mVrms. Generally speaking,
if pin 20 were driven directly from the tuner output. ng
would be too low. Therefore, in general, pin 20 is not con-
nected directly to the tuner output. Instead the tuner output
is attenuated and then applied to pin 20. Increasing attenua-
tion increases the wide band threshold, VWB.
Pin 20 has an input impedance at 10.7 MHz that can be
modeled as a 5000 resistor in series with a 19 pF capacitor,
giving a total impedance of 9400 A-58°. 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 Cl 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 + 94on)/94on.
AGC CIRCUIT USED AS A CONVENTIONAL AGC
It for some reason the dual AGC thresholds are not desired,
it is easy to use the LM1865/LM1965/LM2065 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.
3300 OUTPUT
IMPEDRNCE
CERAMIC FILTER
TONER M ==
TO P1" N
TL/H/7509-16
FIGURE 14. Wide Band Threshold Adlustment
IE"! on!
Advanced FM IF System
Simplified Diagram
TUH/7509—1 7
SSOZW'I/SQGL W'l/SQQlW'I
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Datasheets for electronic components.
National Semiconductor was acquired by Texas Instruments.
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This file is the datasheet for the following electronic components:
LM2065N - product/lm2065n?HQS=TI-nu|I-nu|I-dscatalog-df-pf-null-wwe
LM1965N - productllm1965n?HQS=T|-nu|I-nu|I-dscatalog-df-pf-null-wwe
