TSH690 datasheet(12/14 Pages) STMICROELECTRONICS

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TSH690

12/14

ASK TRANSMITTER USING THE TSH690

Application purpose

The purpose is to use the TSH690 as a ASK trans-

mitter for remote control applications taking bene-

fits of the 2 stages architecture, the bias control

pin and output power capability.

The first transistor stage is devoted to the oscilla-

tor by the meaning of a Surface Acoustic Wave

(SAW) resonator while the second stage realizes

the power amplification to drive antennas includ-

ing the impedance matching.

Modulation is insured by applying the modulating

signal onto the bias pin of TSH690 to get an ampli-

tude modulation.

Figure 7 : Saw transmitter schematic

Oscillator Considerations

The oscillator frequency is given by the SAW res-

onator which is connected between pins 5 & 7 of

the TSH690 to ensure a well-known Colpitts archi-

tecture with 2 capacitors C1 and C2. Capacitor C2

is a small value one and, depending on PCB,

could be directly obtained from parasitics of mi-

crostrip lines. Center frequency is tuned with the

trimmer capacitor C1.

Note that the pin 7 is internally connected to an in-

tegrated self inductor L1 which is wired to the col-

lector of the first stage transistor. A resistor R1 is

placed to avoid resonance between L1 and C1 but

also to adjust the current consumption of the oscil-

lator.

Moreover, start-up conditions and output harmon-

ics levels are related to the R1 value, so that its

recommended to use a potentiometer for R1.

By tuning R1 and C1, stability of the circuit can be

guaranted disregarding the load impedance of the

output stage.

Output Stage

The output matching is defined for a 50

Ω load im-

pedance by adding a 100nH self-inductor L2 as

load to the open collector of the output stage while

capacitor C3 is just placed as a DC block with the

antenna.

In such described conditions, output power on fun-

damental reaches more than +13dBm under 3V

with an average current consumption of 50mA

featuring 2nd & 3rd harmonics levels respectively

of -18dBc et -27dBc.

Modulation

As a result of applying a modulating signal to the

bias pin (pin 8), the TSH690 features an amplitude

modulation up to the On-Off-Keying when the

modulating signal is digital.

A series resistor R2 can be added on pin 8 to

change biasing conditions but also oscillating con-

ditions and finally the available output power. In

most of applications, this resistor can be omitted.

In the case of digital modulation control, when a ‘0’

logic level is applied on pin 8, the circuit in set in

Standby mode during which the oscillator is

stopped whereas during a ‘1’ logic level, the circuit

radiates RF wave due to oscillator running

mode.The maximum data rate of modulating sig-

nal is given by the oscillator turn-on time which is

typically of 200

µs to reach the nominal operating

frequency and amplitude. Note that the turn-off

time is negligeable. Thus, it is recommended to

use a data rate of 2400b/s to keep a duty cycle of

25% (T_ON~200

µs, T_OFF~(400+200)µs). For a

higher data rate (maximum of 4800b/s), duty cycle

on transmission side decreases drastically so that

it is recommended to use a monostable on recep-

tion side to recover a correct 50% duty cycle. In or-

der to decrease the spectrum shape of transmis-

sion, a simple low-pass filter can be added in front

of pin 8 to attenuate high level harmonics of the

modulating signal.

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