Tait TB7100 Installation And Operation Manual

Also see for TB7100: Installation guide Installation guide Operation manual Service manual FITTING INSTRUCTION

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Tait TB7100 Installation And Operation Manual Manual pdf 24 page image

24 Functional Description TB7100 Installation and Operation Manual© Tait Electronics Limited December 2005QuadratureDemodulator The LO for the image-reject mixer (quadrature demodulator) is synthesizedand uses the TCXO (temperature-compensated crystal oscillator) as areference. This ensures good centring of the IF filters and more consistentgroup-delay performance. The quadrature demodulator device has aninternal frequency division of 2 so the second LO operates at2 x (IF1 + 64kHz). The quadrature output from this mixer is fed to a pairof ADCs (analog-to-digital converters) with high dynamic range where it isoversampled at 256kHz and fed to the custom logic device.Automatic GainControl The AGC (automatic gain control) is used to limit the maximum signal levelapplied to the image-reject mixer and ADCs in order to meet therequirements for intermodulation and selectivity performance. Hardwaregain control is performed by a variable-gain amplifier within the quadraturedemodulator device driven by a 10-bit DAC (digital-to-analog converter).Information about the signal level is obtained from the IQ (in-phase andquadrature) data output stream from the ADCs. The control loop iscompleted within custom logic. The AGC will begin to reduce gain whenthe combined signal power of the wanted signal and first adjacent channelsis greater than about –70dBm. In the presence of a strong adjacent-channelsignal it is therefore possible that the AGC may start acting when the wantedsignal is well below –70dBm.3.1.2 Digital Baseband ProcessingCustom Logic The remainder of the receiver processing up to demodulation is performedby custom logic. The digitised quadrature signal from the RF hardware isdigitally down-converted to a zero IF, and channel filtering is performed atbase-band. Different filter shapes are possible to accommodate the variouschannel spacings and data requirements. These filters provide the bulk ofadjacent channel selectivity for narrow-band operation. The filters havelinear phase response so that good group-delay performance for data isachieved. The filters also decimate the sample rate down to 48kHz. Customlogic also performs demodulation, which is multiplexed along with AGCand amplitude data, and fed via a single synchronous serial port to the DSP.The stream is demultiplexed and the demodulation data used as an input forfurther audio processing.Noise Squelch The noise squelch process resides in the DSP. The noise content above andadjacent to the voice band is measured and compared with a presetthreshold. When a wanted signal is present, out-of-band noise content isreduced and, if below the preset threshold, is indicated as a valid wantedsignal.Received SignalStrength IndicationReceived signal strength is measured by a process resident in the DSP.This process obtains its input from the demodulator (value of RF signalmagnitude) and from the AGC (value of present gain). With these twoinputs and a calibration factor, the RF signal strength at the antenna can beaccurately calculated.