CIRCUIT DESCRIPTION

UHF RECEIVER DESCRIPTION

 

 

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Limiter

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Mixer

 

 

 

 

 

 

 

RSSI

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

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Figure 3-6 Limiter/Detector U201 BlockDiagram

The 44.550 MHz injection frequency on pin 4 is produced by tripling the 14.850 MHz frequency of reference oscillator U806. To do this, a portion of the reference oscillator signal is applied to Q207 which is an amplifier with the output tuned for the third harmonic of the reference oscillator frequency. This output tuning is provided by a two-pole bandpass filter formed by L219, C287, C288, L217, and C290. The output level of this filter is approximately 0.25 V rms.

Ceramic Filters (Z202/Z206, Z205)

The 450 kHz output of the internal mixer is fed out of U201 on pin 20. It is then routed through ceramic filter Z202 for narrow-band (12.5 kHz) channels and through Z206 for wideband (25 kHz) chan- nels. Z202 has a nominal bandwidth at the –3 dB points of 8 kHz, and Z206 has a nominal bandwidth of 15 kHz. The function of this filter is to attenuate wideband noise present in the IF signal.

Routing of the IF signal to the appropriate filter is provided by Q205 and Q206, PIN diodes CR207- CR210, and several resistors and capacitors. It is controlled by the microcontroller through the Q4 output of shift register U800. This output is low for narrow-band channels and high for wideband channels.

If a narrow-band channel is selected, a low signal is applied to the base of Q205. That transistor then turns off and inverter Q206 turns on. CR209/CR210

are then forward biased and CR207/CR208 reverse biased. This routes the 450 kHz IF signal through Z202 and blocks it from Z206. If a wideband channel is selected, the opposite occurs. For more information on the operation of PIN diodes, refer to Section 3.8.1.

The filtered 450 MHz IF signal is then applied to pin 18, amplified by an internal amplifier, and then fed back out on pin 16 and applied to ceramic filter Z205. This filter is identical to Z206 and provides additional attenuation of wideband noise. The loss introduced by each ceramic filter is approximately 12 dB.

Limiter/Detector

The signal from Z205 is applied to an internal limiter connected to pin 14. The limiter amplifies the 450 kHz signal and then limits it to a specific value to remove amplitude variations. From the limiter, the signal is fed internally to the quadrature detector. An external phase shift network connected to pin 10 shifts the phase of one of the detector input signals 90° at 450 kHz (the other input is unshifted in phase). When modulation occurs, the frequency of the IF signal changes at an audio rate as does the phase of the shifted signal. The detector, which has no output with a 90° phase shift, converts this phase shift into an audio signal. Inductor L219 is tuned to provide maximum undistorted output from the detector.

The audio signal is then fed internally to an audio amplifier. The gain of this stage is set at about three by R255 and R256. The audio output signal on pin 8 is then fed to the audio/logic board.

Also in U201 is an RSSI detector which provides a temperature compensated RSSI (Receive Signal Strength Indicator) signal on pin 5. This is a low impedance (2k ohm) output with a dynamic range of 70 dB. It provides an indication of IF signal strength which changes in proportion to changes in signal strength. It is routed to an A/D input of the microcon- troller (pin 59) and used along with the squelch signal to determine receive signal strength. R259/C304 and R258/C303 provide low pass filtering, and C305 and C306 decouple RF on the audio and RSSI output lines.

 

February 2001

3-18

Part No. 001-9800-001