to the center frequency of the incoming station Both the

signal strength threshold and the center tune window are

externally adjustable

The signal strength threshold is set by resistor R1 Increas-

ing the value of this resistor will reduce the signal level

threshold There is no difficulty in setting the signal strength

threshold either above or below the AGC threshold

Resistor R2 sets the center tune window The incoming sta-

tion is considered to be center tuned whenever the frequen-

cy of the signal at the IF output falls within the center tune

window Increasing the value of R2 will narrow the window

while decreasing R2 will widen the window Since there is

some interaction between R2 and R1 R2 should be chosen

before R1 In the United States stations within the AM band

are spaced no closer than 10 kHz apart Consequently the

controller should be set up to stop every 10 kHz within the

AM band when the ETR is in scan mode A center tune

in determining the center tune condition though a narrower

stop window is desirable in order to minimize the chance

that side bands from a strong adjacent channel will fall with-

in the stop window

Because of asymmetry in the resonator amplitude charac-

teristic the center tune stop window will not be symmetric

about the center frequency of the resonator This is not a

problem as long as the stop window brackets the center

frequency of the IF and does not extend into the next chan-

nel However in order to avoid any problems in this regard it

is recommended that the resonator center frequency devi-

IF

The stop output Pin 9 is an open collector NPN transistor

This output must be taken to a positive voltage through a

load resistor R3 A valid stop condition is indicated by a

high output level on Pin 9 (ie the NPN is turned off) The

voltage on this pin should not exceed 16 volts

STOP DETECTOR STOP TIME

The amount of time required for the LM1863 to output an

accurate stop indication on Pin 9 is defined as the stop time

The stop time determines how quickly the ETR can scan

across the AM band There are several factors that influ-

ence the stop time Since the signal level stop function op-

erates in conjunction with the Automatic Gain Control

(AGC) the AGC settling time is a critical factor This settling

time is dominated by the low frequency AGC pole which is

set by C1 and internal IC resistances Decreasing C1 will

decrease the AGC settling time but increase total harmonic

distortion THD of the recovered audio A good compromise

between AGC settling time and THD is very difficult to reach

with a single pole AGC system Consequently the LM1863

has been designed with a second higher frequency AGC

pole This non-dominant pole is externally set by capacitor

C2 As a result C1 can be made much smaller than it other-

wise could for an equivalent amount of THD Reducing C1

will reduce the stop time The combination of C1 and C2 as

shown in the applications circuit results in a stop time of less

than 50 ms for most input conditions while at the same time

the circuit achieves 9% THD at 80% modulation with 400

Hz modulation frequency at 10 mV input signal strength

Had C2 not been present the stop time would still be 50 ms

but the THD for similar input conditions would be 8% By

decreasing both C1 and C2 (keeping the ratio of C1C2

constant) the stop time can be reduced at the expense of

THD while the converse is also true

The addition of a second pole to the AGC response does

add some ringing to the AGC voltage following signal tran-

sients The frequency duration and amount of ringing are

dependent on where both AGC poles are placed and to

some extent the input signal conditions The amount of ring-

ing should be kept to a minimum in order to insure proper

stop indications The amount of ringing can be reduced by

either reducing C2 (this will increase THD) or by increasing

C1 (this will improve THD but increase stop time)

If the ratio of C1C2 is made too small an increase in low

frequency noise may be noticed resulting from the peaking

that a closed loop two pole system exhibits near the unity

gain frequency The extent of this peaking can be observed

by examining the amount of recovered audio at various low

frequency modulations In general the values shown reach

a good compromise between THD stop time ringing and

low frequency noise

The center tuning detector on the LM1863 passes the sig-

nal at the IF output through a limiting amplifier which re-

moves most of the modulation from the IF waveform The

output of this limiter is then applied to the resonator on Pin

7 Unfortunately large modulation peaks are not completely

removed by the limiting amplifier Without C3 these large

modulation peaks would cause glitches on the stop output

when the LM1863 was tuned to a valid station C3 acts to

reduce these glitches by filtering the output of the center

tune circuit C3 however also affects the stop time and

cannot be made arbitrarily large A time constant of about

30 ms on Pin 5 gives the best compromise R21 biases Pin

5 to about 4 volts which is below the stop threshold at this

point This biasing results in a shorter stop time

Extra precaution can be taken within the software of the

controller IC to further insure accurate stop detector per-

formance over a wide variety of input signal conditions A

typical controller IC stop algorithm is as follows

The controller waits the first 10 ms after the LM1863 is

tuned to the next channel The controller then samples

the LM1863 stop output 10 times within the next 40 ms

If no high output is sensed within that time the control-

ler concludes there is no valid station at the frequency

and moves to the next channel If however at least

one high output is detected within the first 50 ms the

controller waits an additional 200 ms and at the end of

that time re-samples the stop output in order to make

its final stop determination

RF AGC

The RF AGC detector is designed to control the gain of an

external RF amplifier which is placed between the antenna

and the mixer input The RF AGC operates by detecting

when the input signal to the mixer reaches 6 mVrms the RF

AGC threshold When the mixer input signal reaches this

level the RF AGC is activated and will hold the mixer input

level relatively constant at the level of the RF AGC thresh-

old The gain of the RF AGC determines how constant the

RF AGC can control the RF output The LM1863 RF AGC is

high gain and consequently the RF AGC output Pin 3 will

transition from high to low over a very narrow input range to

the mixer when the LM1863 is examined in an OPEN LOOP

condition However in a radio where the RF AGC controls

the RF gain a CLOSED LOOP negative feedback system is

established In this application the RF AGC output will tran-

sition from high to low over a large range of signal levels to

the input of the RF stage

9