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TRANSMITTER OPERATION

The transceiver’s transmitter section is a wide-band FM transmitter capable of

generating -10dBm of output power into a 50-ohm load.

The transmitter is comprised of a VCO and a crystal-controlled frequency

synthesizer. The frequency synthesizer, referenced to a precision crystal, locks

the VCO to achieve a high-Q, low phase-noise oscillator.

FM modulation is achieved by direct modulation of the reference crystal. By

modulating the transmitter in this manner, a wide modulation bandwidth is

achieved. If the transmitter LO were modulated, the frequency synthesizer would

track out modulation within the bandwidth of the loop filter (this is a common

limitation of most synthesized FM transmitters).

When the transmitter will be used to transmit digital data, the TXDATA is best

driven from a 5-volt source. The transmitter is designed to give an average

deviation of 110kHz with a 5V square wave input. It is possible to successfully

modulate the transceiver with a level as low as 3 volts; however, the reduced

carrier deviation will cause a slight loss in receiver sensitivity and a 10 to 20%

increase in settling times.

Analog sources such as audio should provide a 3V P-P maximum waveform and

should be AC-coupled into the TXDATA pin. The size of the coupling capacitor

should be large enough to insure the passage of all desired frequencies. Since

the modulation voltage applied to the TXDATA pin determines the carrier

deviation, distortion can occur if the TXDATA pin is over-driven. The actual level

of the input waveform should be adjusted to achieve optimum in-circuit results.

The transmitter’s output power is not affected by supply voltage but it can be

externally adjusted. To ensure complete FCC compliance under every condition

of operation, the SC-PA module is internally attenuated to a lower level than that

of a standard SC module. If the user wishes to reduce the output power further,

a resistor may be placed from the LVLADJ pin to ground. Depending on the value

of resistor selected up to 5dB of additional attenuation may be realized. Under no

circumstance should the pin be held high as the pre-certified status would be

compromised and internal damage could result.

RECEIVER OPERATION

The receiver is configured as a single conversion superhet FM receiver with a

baseband analog bandwidth of 17kHz.

Low-level RF signals are filtered at the front end by a SAW band-pass filter. This

filter reduces the signal levels of interfering transmitters such as pager towers

and cell phones.

The filtered signal is amplified and down-converted to the 10.7 MHz IF by mixing

it with a LO frequency generated by a PLL locked differential VCO. Then the 10.7

MHz IF is amplified, filtered, and finally demodulated to recover the analog

baseband signal originally transmitted. The analog baseband output is low-pass

filtered and then made available externally via the audio pin. The signal is also

squared by a proprietary data slicer for output of digital information at the RX

Data pin. The data slicer is capable of recreating squared waveforms in the

150Hz to 17kHz band, giving a data-rate bandwidth of 300bps to 33.6Kbps.