AMX TXC16 Plus Specifications download pdf (Page 153)

SG-2000 MANUAL 146

The SGC Building, 13737 S.E. 26th St. Bellevue, WA. 98005 USA

mainly drivers and a multiplexer (IC510) for addressing each VCO. The

oscillators are on sheet 4 of 6 and each has an input tuning voltage range

between 0 and 8 volts. As the voltage increases, the capacitance on the varicaps

goes down, thus increasing the oscillator frequency. The PLLs control this

voltage by doing a comparison between a division of the reference oscillator

and the output frequency.

The receiver section of the radio involves three LOs to create two IF

frequencies. Starting on sheet 1 of 6, drawing J30100920, the signal comes in via

J101 from the Linear Power Amplifier (LPA), then through a switchable 20 dB

pad, then through the 10 dB amplifier made with IC103 and then through a 30

MHz low pass filter to the first mixer. The mixer flips the frequency up to an IF

frequency between 82 and 86 MHz. This frequency span is cycled through

every 20 KHz of operating frequency due to the mixing scheme of the radio.

The 82 MHz RF amplifier adds about 25 dB to the signal before going through

the 82 MHz band pass filter. The 82 MHz filter is a very tight filter used in the

IF stage to increase the selectivity of the receiver and to attenuate the LO signal

coming through Mixer A102. It is then mixed down to 10.7 MHz.

Sheet 2 of 6 takes the signal and passes it through the side band filter or an

optional filter depending on which mode or option the radio is configured for.

The IF amplifier transfers the signal to the demodulator, IC202, to create the

audio signal at TP204. The IF amplifier has a gain control on pin 5 which is a

feedback from the AM detector on sheet 3 of 6. This is used to limit the signal

level and prevent the demodulator from being over driven.

The audio signal is then filtered by either the voice filter or the telex filter

depending on which mode of operation the radio is in. The signal is then

routed to an Automatic Gain Control (AGC) circuit. The AGC circuit tries to

maintain a certain audio level going into the input / output buffer on sheet 3 of

6. The receiving path is divided into two paths, one which is the 600 Ohm

output buffer and the other is the balanced head audio output.

In general, the transmit path follows the receiver path backwards. The audio

signal is brought in by either the back connector J301 or through the balanced

audio input (J503 and J504) from the heads. The signal is then brought into the

Automatic Level Control (ALC) circuit, IC208, on sheet 2 of 6 drawing

J30100920 where the proper audio level is established and is sent to the

modulator (IC202).

The audio is then modulated to 10.7 MHz and sent to the clipper circuit. The

clipper circuit is used for compression of the audio signal on the RF level. This

is done to produce a higher RMS power level and to produce a more

intelligible signal when the signal is received on the other end. This signal is

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AMX TXC16 Plus Specifications Page 153