illrrrrrrW

INPUT
)1
c
I SO MHz
1N4381
IOO~
450MHz
of freq ue ncy m odul ation via the va racto r.
Also . by prope rly proporti oni ng the fluctuati
ng au dio vol tage goi ng into th e varactor
with respec t to the osci lla to r. eithe r narrowban
d or wideba nd Ff\.l m ay be ob ta ined . as
shown in Fig. 4.
Fig. 3. Vere ct o r tripler circuit.
18 watts with an input of 30 watts) . T he
idler circ uit is tuned to one harmonic
below the ou tput frequency . In thi s case
th e idler should be tuned to resonate at
300 :\I Hz. T he bias resistor is chosen as
l OOK so the circuit will operate linearly.
T he out pu t circ uit is then tuned to resonate
at the desired output fre quency (450 ~I H z) .
After alignment . it is a goo d ide a to repeat
the tu ning pr ocedure because there is almost
always some interac tion bet ween stages.
An other use for the va ructo r is in the
developme nt o f an f.\1 signal. By rectifying
a modulat ed signal and ap plying tha t fluctuatin
g voltage to the ter mi na ls of a va ructor.
we co uld lise the cha nging ca pacitance of
the vuructor to cause freq ue ncy de viatio n
of a n oscillator. Hen ce. the development
I n the circ uit of F ig. 4. a rectified a ud io
vol ta ge is in troduced a t the potentiome ter
whi ch ca n be adjusted to allow the required
freq uency de viatio n whether it be wide ­
ba nd or nar rowband. The ch arging and
di sch ar gin g of ca pacitor C 1 thro ugh resistor
R 1 a pplies a fluctu at ing voltage on the
a node of th e varactor. T his fluctuating volt ­
age will
cau se the ca pacita nce capabilities
of the va rac to r to vary. th ereb y pulling the
oscillator olf irs ce nter frequency .
As mentio ned earlier. the prope rt y of
being a ble to va ry the ca pacita nce of a
va racto r by varying the inp ut voltage en ­
a bles it 10 do [he wo rk of a co nve ntio nal
variab le capacitor. O ne great adv a ntage as
opposed to conventional tuning is min iatu
rization.
A typical va ractor for th is type
of service in mo st . cases is about th e size
of a small signal d iod e (l N34, for example)
a nd this is m a ny times smaller th an even
D[V
f M
Rf OUT
AUDIO IN
+
Fig. 4. Frequency Modulation using the verec ror.
18
73 MAGAZINE