Tait TM8000 Service Manual

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Tait TM8000 Service Manual Manual pdf 31 page image

TM8100 Mobile Radio Service Manual General Description 31October 2003 © Tait Electronics LimitedConnectors and circuit boards There is an 18-way electrical interface between the control head andradio body. The physical connection is via an 18-way loom. The controlhead normally contains a single PCB assembly called the control-headboard. If a dynamic microphone is used or a concealed microphonefitted, a small circuit board must be mounted on the control-head board.The added board has the necessary amplification, filtering and switchingfunctions. The internal speaker is connected to the control-head boardvia a lead with a mating connector so that it can be easily disconnected.Control-head board The control-head board does not include a microprocessor. Asynchronous bi-directional serial interface provides communication ofkey status, LCD and LED-indicator data between the radio body and thecontrol head. On the control-head board the serial data are converted toor from parallel form by a number of shift registers for the keys andindicators. For the LCD, the serial data are fed to a driver IC thatconverts the serial data to a form suitable for the LCD itself. The keysare scanned and the LCD and LED indicators updated approximatelyevery 50 ms.2.3 Operation in Receive ModeReceive path In this subsection the functioning of the radio in the receive mode isdescribed. As shown in Figure 2.2, the receive path exists in threedifferent domains:■ hardware■ custom logic■ DSPThe front-end hardware amplifies and image-filters the received RFspectrum, then down-converts the desired channel frequency to a firstintermediate frequency IF1 of 45.1 MHz (UHF) or 21.4 MHz (VHF)where coarse channel filtering is performed. The first LO signal isobtained from the frequency synthesizer. The output of the first IF isthen down-converted using an image-reject mixer to a low IF of 64 kHz.Quadrature demodulator The LO for the image-reject mixer (quadrature demodulator) issynthesized and uses the TCXO as a reference. This ensures goodcentring of the IF filters and more consistent group-delay performance.The quadrature demodulator device has an internal frequency division of2 so the second LO operates at 2 x (IF1+ 64 kHz). The quadratureoutput from this mixer is fed to a pair of ADCs with high dynamic rangewhere it is oversampled at 256 kHz and fed to the custom logic device.