10

LT1711/LT1712

Figure 4. Performance of Figure 3’s Circuit When

Operated Unidirectionally. Eye is Wide Open

171112 F04

TYPICAL APPLICATIO S

Figure 5. Performance When Operated Simultaneous

Bidirectionally (Full Duplex). Crosstalk Appears as Noise.

Eye is Slightly Shut But Performance is Still Excellent

171112 F05

This comes out to 120

Ω for the values shown. The

Thevenin equivalent source voltage is given by:

VV

RR

R

RRR

R

RR

R

R

TH

S

O

O

=

+

++

++

[]

•

(–

)

()

•

•||(

)

23

1

23

1

21

2

3

This amounts to an attenuation factor of 0.0978 with the

values shown. (The actual voltage on the lines will be cut

in half again due to the 120

attenuation factor is important is that it is the key to

deciding the ratio between the R2-R3 resistor divider in

the receiver path. This divider allows the receiver to reject

the large signal of the local transmitter and instead sense

the attenuated signal of the remote transmitter. Note that

in the above equations, R2 and R3 are not yet fully

determined because they only appear as a sum. This

allows the designer to now place an additional constraint

on their values. The R2-R3 divide ratio should be set to

equal half the attenuation factor mentioned above or:

Having already designed R2 + R3 to be 2.653k (by allocat-

requisite 120

Ω), R2 and R3 then become 2529Ω and

123.5

Ω respectively. The nearest 1% value for R2 is 2.55k

and that for R3 is 124

Ω.

Voltage-Tunable Crystal Oscillator

The front page application is a variant of a basic crystal

oscillator that permits voltage tuning of the output fre-

quency. Such voltage-controlled crystal oscillators (VCXO)

are often employed where slight variation of a stable

carrier is required. This example is specifically intended to

provide a 4

× NTSC sub-carrier tunable oscillator suitable

for phase locking.

The LT1711 is set up as a crystal oscillator. The varactor

diode is biased from the tuning input. The tuning network

is arranged so a 0V to 5V drive provides a reasonably

symmetric, broad tuning range around the 14.31818MHz

center frequency. The indicated selected capacitor sets

tuning bandwidth. It should be picked to complement loop

response in phase locking applications. Figure 6 is a plot

of tuning input voltage versus frequency deviation. Tuning

deviation from the 4

× NTSC 14.31818MHz center fre-

quency exceeds

±240ppm for a 0V to 5V input.

INPUT VOLTAGE (V)

0

2

4

5

9

8

7

6

5

4

3

2

1

0

171112 F06

13

14.3140MHz

14.31818MHz

14.3217MHz

Figure 6. Control Voltage vs Output Frequency for the

Front Page Application Circuit. Tuning Deviation from

Center Frequency Exceeds

±240ppm

124

Ω = 2.674k.