teristics of the radio are determined by the design of this

stage Generally speaking it is very difficult to design an

integrated RF stage in bipolar as bipolar transistors do not

have good overload characteristics Thus the RF stage is

usually designed using discrete components Because of

this there is a great deal of concern with minimizing the

number of discrete components without severely sacrificing

performance The applications circuit RF stage does just

this

The circuit consists of only two active devices an N-chan-

nel JFET Q1 which is connected in a cascode type of con-

figuration with an NPN BJT Q2 Both Q1 and Q2 are varac-

tor tuned gain stages Q2 also serves to gain reduce Q1

when Q2’s base is pulled low by the RF AGC circuit on the

LM1863 The gain reduction occurs because Q1 is driven

into a low gain resistive region as its drain voltage is re-

duced R10 and C15 set the gain of the 1’st RF stage which

is kept high (about 19 dB) for good low signal signalnoise

performance The gain of the front end to the mixer input

T2 in conjunction with D1 C21 and C26 form the 1’st tuned

circuit C26 does not completely de-couple the RF signal at

the cathode of the varactor In fact the combination of C26

and C19 act to keep the gain of the whole RF stage con-

stant over the entire AM band Without special care in this

regard the gain variation could be as high as 14 dB This gain

variation would result from the increase in impedance at the

secondary’s of T2 and T1 as the tuned frequency is in-

creased The increased impedance results from a constant

C19 the gain is held constant to within 6 dB (including the

tracking error) over the entire AM band

C27 de-couples RF signal from the top of T2’s primary and

allows Q2 to operate properly C18 is a coupling capacitor

which in conjunction with C19 couples the signal from the

1’st RF stage to the 2’nd RF stage R20 acts to isolate this

signal from AC ground at C11 R19 acts in conjunction with

C12 to set a high frequency (ie non-dominant) RF AGC

pole which is important for low distortion when the RF AGC

is active The dominant RF AGC pole is set by R8 and C11

Q2 is a high beta transistor allowing for little voltage drop

across R20 and R8 due to base current This keeps the

emitter of Q2 sufficiently high (in the absence of RF AGC) to

bias Q1 in its square law region

R13 acts to reduce the 2’nd stage gain and increase Q2’s

signal handling R13 must not get too large however (ie

in conjunction with C20 C27 and D2 form the 2’nd RF tuned

circuit The output of Q2 is capacitively coupled through C28

to the mixer input The output of Q2 is loaded not only by

the reflected secondary impedance but also by R22 R22 is

carefully chosen to load the 2’nd stage tuned circuit and

broaden its bandwidth The increased bandwidth of the 2’nd

stage greatly improves the cross modulation performance of

the front end In the absence of this increased bandwidth

the relatively large AC signals across varactor D2 result in

cross modulation R22 also reduces the total gain of the

2’nd stage R22 does slightly degrade (by about 6 dB) the

image rejection especially at the high end of the AM band

However the image rejection of this front end is still excel-

lent and 6 dB is a small price to pay for the greatly increased

immunity to cross modulation

coupled into the RF stage This front end also offers superi-

or performance with respect to varactor overload by strong

adjacent channels This results because of the way that

gain has been distributed between the 1’st and 2’nd stages

In summary this front end offers two stages of RF gain with

the 2’nd stage acting to gain reduce the 1’st stage when RF

AGC is active Furthermore a unique coupling scheme is

employed from the output of the 1’st stage to the input of

the 2’nd stage This coupling scheme equalizes the gain

from one end of the AM band to the other Additional care

has been taken to insure that excellent cross modulation

performance image rejection signal to noise performance

overload performance and low distortion are achieved Per-

formance characteristics for this front end in conjunction

with the LM1863 are shown in the data sheet Also informa-

tion with regard to the bandwidth of the front end versus

tuned frequency are given below

TUNED FREQUENCY

b

3 dB BANDWIDTH

530 kHz

66 kHz

600 kHz

72 kHz

1200 kHz

206 kHz

1500 kHz

264 kHz

1630 kHz

36 kHz

VARACTOR ALIGNMENT PROCEDURE

The following is a procedure which will allow you to properly

align the RF and local oscillator trim capacitors and coils to

insure proper tracking across the AM band

2 Set the trimmers to 50%

3 Adjust the oscillator coil until the local oscillator is at 980

kHz

4 Increase the varactor voltage until the local oscillator

(L0) is at 2060 kHz and check to see if this voltage is less

than 95 volts but greater than 75 volts If it is then the

L0 is aligned If it is not then adjust the L0 coiltrimmer

until the varactor voltage falls in this range

5 Set the RF in to 600 kHz and adjust the tuning voltage

until the L0 is at 1050 kHz Peak all RF coils for maxi-

mum recovered audio at low input levels

6 Set RF in to 1500 kHz and adjust the tuning voltage until

the L0 is at 1950 kHz Peak all RF trim capacitors for

maximum recovered audio at low input levels

7 Go back to step 5 and iterate for best adjustment

8 Check the radio gain at 530 kHz and 750 kHz to make

sure that the gain is about the same at these two fre-

quencys If it is not then slightly adjust the RF coils until

it is

The above procedure will insure perfect tracking at 600 kHz

950 kHz and 1500 kHz The amount of gain variation across

the AM band using the above procedure should not exceed

6 dB

ADDITIONAL INFORMATION

R5 and C7 act as a low pass filter to remove most of the

residual 450 kHz IF signal from the audio output Some re-

sidual 450 kHz signal is still present however and may

need to be further removed prior to audio amplification This

need becomes more important when the LM1863 is used in

conjunction with a loopstick antenna which might pick up an

amplified 450 kHz signal An additional pole can be added

to the audio output after R5 and C7 prior to audio amplifica-

tion if further reduction of the 450 kHz component is re-

quired

11