TX Techniques TX FM1, Техническое руководство

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TX-FM1/S/R/A Technical Manual www.txtechniques.co.uk
6. Circuitry
6.1 Switch Mode PSU
The Switch Mode PSU provide a 28VDC 6.5A output from a wide range of
input voltage supplies (90 – 264VAC 47-63Hz, or 120 – 370VDC). It features power
factor correction and compliance to generic EMC standards. A version capable of
Level 2 surge immunity (as defined by ETS 300 447 standard) can be supplied if
specifically requested at the time of ordering. The SMPSU is over voltage and short
circuit protected, and is a self-contained unit for fast, easy replacement.
6.2 ALC
The left and right circuits are identical. The following component references relate to
the left audio channel only.
This board also incorporates a switching regulator, which provides a +/- 12V supply
for this, and other boards, via PL4 (which also includes a +24V supply rail).
XLR balanced audio passes through an RF filter (FB1, FB2 and C1,C2) and is
converted to unbalanced audio by IC1 (Balanced Line Receiver).
IC2 buffers the Left and Right audio channels which are then bridged and presented
to the LCD meter and the DTMF TA controller as a mono source, via socket PL5.
The audio passes through IC3(A), the gain of which is controlled by OR1 in its
feedback path. The output of IC4 (precision rectifier) and IC5 (mute control) feeds
OR1. High audio levels are gradually reduced, or long term low levels boosted.
Extended periods of silence result in the ALC slowly returning to an overall 0dB gain
level, thus reducing noise on landlines or STL feeds.
Op-amp IC6 is configured as an amplifier enabling the ALC output level to be
adjusted (by changing the value of R31) . This will increase/decrease the input level
for the audio processor.
6.3 Audio Processor
Dual op-amps IC1, IC3 and IC5 dual op-amps form the band filtering. IC2, IC4 and
IC6 are dual VCAs set to perform a limiting/leveling action on each band. Their
outputs are combined by IC5b. LED3 and 4 provide ‘brick wall’ limiting. IC6b
performs an overall ‘sliding pre-emphasis’ limiting action. LED1 and LED2 further clip
the waveform ahead of FILTER 1, which is a sharp 15kHz low pass filter. Since the
input and output impedance have to be carefully matched for this filter to work
correctly, R47, R48, R49, R44 are used to ensure the correct drive and load
impedances.