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

(10 mV)

(Pin 20)

(Pin 11)

(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

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

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-

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