Spectra Engineering MX800FF RF Base Station User Manual 4 Tech desc

Spectra Engineering PTY LTD RF Base Station 4 Tech desc

ChapterFour

4. Technical DescriptionThe internal design of the MX800 is of a modular nature allowing for simpleconfiguration and maintenance while ensuring minimal downtime. For referencepurposes, the top view of a typical MX800 with top cover removed is shown below. Thestandard unit without speaker and microphone is illustrated.MX800 top view.PA ModuleExciterModuleMicro ControllerBoardReceiverModule
4.1. Exciter ModuleRefer.Exciter Block Diagram: BD002Exciter Circuit Diagram: CS002-1TX VCO Circuit Diagram: CS004-1 (Bands A to Q)RF from the VCO on SKU-1 at a nominal level of+3dBm is applied to the fractional-Nsynthesizer ICI0 main divider input. This signal is compared with the reference oscillatorfrequency and the correction voltage from the synthesizers charge pump output is filteredthen amplified by the non inverting low noise op amp IC9A. This correction voltage isfed back to the VCO to maintain loop lock as well as being fed to the Micro Controllervia SKD-14. A lock detect signal from IC10 is also fed to the Micro Controller via SKD-16.The op amp uses a 25 volt power supply (generated on the Micro Controller) so as toprovide a wide tuning range voltage to the frequency control varicaps located on theVCOboard. Frequency programming data for the exciter is sent to the synthesizer chip fromthe Micro Controller via a serial data line on SKD-18 under the control of the Clock(SKD-15) and Strobe (SKD-17) lines.Provision is made for the optional injection of an external reference frequency. If thisoption is selected CN3 is fitted and X1 is not fitted. Components R30 and TR7 are alsoomitted.A second RF output from the VCO on SKT-6 also at +3dBm is used as the main transmitRF amplifier signal source. This main signal is first buffered by a very high isolationcircuit consisting ofa 10dB pad and a MMIC amplifier (IC1). The signal is furtheramplified by IC2 and a variable gain wide band amplifier with 40dB control range andpower output of 300mW. The drive power of this stage is used to set the output power tothe main power amplifier under the control of the DC voltage on SKD-4 from the MicroController board.The VCO boards and synthesizer circuits are the same for the exciter and receivermodules. The VCO consists ofa 10mm ceramic coaxial resonator with common baseoscillator for low phase noise for bands 805-960 MHz. Frequencies below 520 MHz usean LC tank circuit. The power supply to the VCO consists of an 8 volt regulator andactive filter for maximum noise rejection. It is controlled by the Micro Controller throughSKD-7 which connects to switch TR6. For standard modulation, transmit audio is fed onSKD-8 to the conventional point of the VCO varactor. For 2 point modulation, audio isalso fed via SKD-19 to the voltage control pin of the VC-TXCO, this in effect cancels outthe PLL error that would otherwise have occurred for low audio frequencies, hence
resulting in a flat VF response.4.2. Receiver ModuleRefer:Receiver Block Diagram: BD001Receiver Front End Circuit Diagram: CS003-2Receiver IF Circuit Diagram: CS003-1RX VCO Circuit Diagram: CS005-1 (Band A to Q)CS012-1 (Band R to X)The receive signal from the antenna enters on CN1 a 3 section Bandpass filter whichprovides the initial filtering for the front end amplifier. The front end amplifier IC12 is abroad band high performance MMIC with a gain of 18dB, noise figure of 4dB and 3rdorder intermodulation intercept of+36dB. This is followed by a 4dB pad and a second 3section BPF, and a high level double balanced mixer M1.The receiver uses high side local oscillator injection for bands A to I and low sideinjection for all other bands. RF from the VCO main output on SKT-6 is buffered andamplified to + 17dBm by IC4 and injected in the high level mixer which down convertsthe signal to the first IF frequency of 90 MHz (45MHz for bands A to D3). This IF signalfrom the mixer is terminated by a bi-directional constant impedance network and is thenamplified by a hipolar amplifier TR2 with a gain of 15dB and 3rd order intermodulationintercept +35dB. This provides a high degree of intermodulation rejection for thereceiver. This stage is followed by a 4 pole 90 MHz crystal filter FL3A/B with itsassociated matching networks. The signal is further amplified and filtered by a transistoramplifier TR3 and its associated 2 pole crystal filter FL4 before being fed into the mainIF demodulator chip IC 1 with a second IF frequency of 455kHz. The resulting audio ispassed out to the Micro Controller board on SKD-3. The Received Signal StrengthIndication (RSSI) from IC1 is buffered by IC5A and connected to the Micro Controllerboard via SKD-4.RF from the VCO on SKU-1 at a nominal level of+3dBm, is applied to the fractional-Nsynthesizer (IC10) main divider input. This signal is compared with the referenceoscillator frequency and the correction voltage from the synthesizer's charge pump outputis filtered then amplified by the non inverting low noise op amp (ICI 1A). This correctionvoltage is fed back to the VCO to maintain loop lock as well as being fed to the MicroController via SKD44. A lock detect signal from IC10 is also fed to the Micro Controllervia SKD-I6. The op amp uses a 25 volt power supply (generated on the Micro Controller)
so as to provide a wide tuning range voltage to the frequency control varicaps located onthe VCO board. Frequency programming data for the receiver is sent to the synthesizerchip from the Micro Controller via the a serial data line on SKD-18 under the control ofthe Clock (SKD- 15) and Strobe (SKD- 17) linesThe local oscillator signal to the mixer is controlled by the Micro Controller through anenable signal on SKD-8. This signal switches the supply to the local oscillator amplifierand is used to enable or disable the receiver.Provision is made for the optional injection of an external reference frequency. If thisoption is selected CN3 is fitted.4.3. Power Amplifier ModuleRefer:PA Block Diagram BD004PA Circuit Diagram: CSO16-1 (Band A to B)CS006-1 (Band C to D3 )CS007-1 (Band E to F)CS013-1 (Band G to I)CS008-1 (Band J to M)CS009-1 (Band N to Q)CS0010-1 (Band R to X)CS014-1 (Band Nto Q 5W)RF from the Exciter on CN1 is first attenuated by a 50 ohm pad which is used to providea good 50 ohm source impedance for the high power hybrid amplifier IC1. The RF isamplified to between 5 and 13 watts at the hybrid 50 ohm output. The signal from thehybrid is then matched by a broad band network to drive the low input impedanceassociated with the final transmit power amplifier transistor TR1. The transistors lowcollector impedance is then also matched back to 50 ohms by a broadband matchingnetwork. Trimmer capacitors enable adjustment of the power amplifier over a widebandwidth so as to maintain good conversion efficiency. Prior to transmission a low loss13 element elliptical low pass filter, filters out the unwanted harmonics to less than-90dBc.A dual directional coupler consists of coupled microstrip transmission lines S5, S6 andS7 fabricated on the PCB artwork. The sampled RF energy is rectified to provide aproportional DC voltage output on CN4-8 (FWD) and CN4-5 (REFL).TR2 serves to switch the DC supply to the Hybrid under control of the PTT line from theMicro Controller on CN4-2. A thermistor TS1 physically located on the PA heatsink isconnected to the Micro Controller via CN4-4/6
4.4. Micro Controller BoardRefer:Block Diagram BD003 ( Rev A to I-DBD005 (Rev I upwards)Circuit Diagram CS001-1/6The MX800 Micro Controller Board has 4 main functionsOverall radio managementTX and RX signal processingRF power controlUser interface4.4.1. Overall Radio ManagementIn addition to analogue signal processing circuitry the Micro Controller boardaccommodates an 80C552 microprocessor IC1, a 64kbyte EPROM IC3, 32kbyte RAMIC4, a 16kbyte EEPROM as well as address decoding, I/O latches and othermiscellaneous circuitry. The Micro Controller is responsible for ensuring that the radioacts as programmed by the user. It stores the user-entered parameters for each channel inEEPROM. This information includes RX and TX RF frequencies, RX and TX CTCSSfrequencies as weIl as RF output power and operating mode. An 8 channel analog todigital converter allows the microprocessor to read 8 analogue values internal to the radiowhich in conjunction with the digital inputs to the microprocessor allow the operatingstatus of the radio to be monitored and controlled. The following analogue items are readPort ParameterADC0 Received Signal Strength Indication(RSSl)ADC 1 Detected Discriminator output levelADC2 DC operating voitageADC3 RX VCO tuning VoltageADC4 TX VCO tuning VoltageADC5 PA temperatureADC6 PA forward powerADC7 PA reflected powerThe Micro Controller sends programming data to the synthesizer ICs on the Receiver andExciter modules each time the channel is changed as well as on PTT. This information is
communicated to the Receiver and Exciter modules by way of bussed data and clocklines on SKC/D-18 and SKC/D-15 (Exciter/Receiver) and an individual module strobe onSKC/D-17. A lock detect signal from each module on SKC/D-16 is read by the microcontroller.In addition to the synthesizer programming bus an I2C bus goes to each of the modules.This bus is currently unused.IC25 is a non predictive, full duplex CTCSS encoder/decoder which is under the controlof the microprocessor.4.4.2. TX Signal ProcessingTX audio may be sourced from a number of different paths. These include VF from line,Talk through audio, the microphone, DC FM/Wideband input, Tone generator frommicrocontroller, Test TX VF inject from the Monitor port, CTCSS generator and theinternal RF modem.The TX VF path is readily user configurable with most major functional blocks beingpossible to either select or bypass by means of links ( refer to 2.2.2.1 for details of linksettings). The VF from line enters the board on SKH-14 and RV4 (TX VF gain) providesgain adjustment to accommodate different line levels. This is followed by a selectablecompressor (IC34A) with 30rib dynamic range. The output of the compressor apart frombeing fed to the main TX audio path is also connected to the input side of the 'Line' FFSKmodem and via a gain control pot the speaker amp (IC60). Following the main TX audiopath, IC36B serves to switch the VF further on the TX audio path or, under the control ofthe external input on SKH- 13, to loop it back to the line output via RV7 which provideslevel adjustment for the looped back signal. From this switch the VF passes though asecond switch IC32B under the control of the Micro Controller and the external TX talkline on SKH-10. This switch is used to disable the line TX VF path. From here the VFpasses through a pre-emphasis stage C66, R67 and a 300Hz high pass filter (IC30B andIC29A) each of which is selectable through links. The output of this filter is fed to IC30Aa summer/limiter amp.The summing point of this amp also serves to combine the audio from the talkthroughpath which is level adjusted by RV3, the microphone which is processed through asimilar compression, pre-emphasis and filtering chain as the line VF, the tone ( Morsecode) signal from the microprocessor, the DC FM input which enters the board on SKH-6and the Test TX VF injection from SKE-2. The latter three inputs are all at fixed levels.RV2 on IC30A provides overall gain adjustment. Following the limiter amp is the TX VFlow pass filter. The standard Bessel filter has a 3.4kHz cut off frequency but mayoptionally be ordered as a 3kHz Chebychev filter. A second summing amplifier iC24Bfollows which combines the CTCSS, optional modem (IC52 and IC54) and Widebandaudio inputs with the TX VF. Level adjustment of the CTCSS signal is in three 0.85dBsteps either above or below the nominal level ( 10% of max dev ). These level steps are
set up using the Configure screen of MXTOOLS. The output of this final stage is fed totwo microprocessor controlled digipots which serve to adjust the modulating signal levelto the VCO and the reference oscillator. A third digipot is used to provide an offset nullfor the reference oscillator center frequency. Adjustment of these levels is also by way ofMXTOOLS using the Channel Edit screen.4.4.3. RX Signal Processing.In a similar fashion to the TX audio path the major functional elements of the RX audiopath are capable of being selected or bypassed by means of links.Discriminator audio enters the board on SKD-3 and is fed to a selectable 300Hz highpassfilter comprising IC37 and IC38B. This output or an unfiltered version of thediscriminator audio is fed via a switch IC32A to the TTR path. The switch is used todisable talk through audio under the control of the microprocessor and the external TXtalk line on SKH-10. Following the HPF a selectable 3.4kHz cutoff low pass filter IC38Aand IC39B connects to the de-emphasis circuit IC39A. From here the audio passesthrough the mute switch IC32C which is under control of the microprocessor and on tothe RX talk switch ICIC36C which is controlled externally from SKH-3. This secondswitch is used to disable RX audio to line. A final switch IC36A selects between RXaudio and TX looped back audio for output to line via amplifiers IC40A and IC40B.Secondary inputs to these amplifiers are the TX audio from the line modem andmicrophone audio to line. The output on SKH-2 forms an unbalanced 600• VF output toline and, alternatively, outputs SKH-2 and SKH-11 form a differential output.Discriminator audio is also fed via a low pass filter IC31 to the CTCSS decoder as wellas to a level detector D9 the output of which is connected to ADC 1 on themicroprocessor.Discriminator audio is also fed to the mute detection circuit. This comprises a high passfilter IC41 followed by RV4 which sets the mute threshold. Following amplification byIC42A and rectification by IC42B and IC43A a comparator, IC43B, determines when thedetected signal passes a fixed threshold. The output of the comparator is fed to themicroprocessor and via a selectable inverter to SKH-4.Discriminator audio is also fed to RX port of the RF modem IC54.4.4.4. RF Power ControlForward power is controlled by the microprocessor through two mechanisms. Based onpreprogrammed per channel adjustments the microprocessor sets the digipot IC 10 to areference setting. IC23A serves as a comparator and, with the non-inverting input
connected to the wiper of the digipot, is set up with a reference voltage. The detectedactual PA forward power is fed to the inverting input oflC23A. The error voltage at theoutput of IC23A is fed to the exciter output power control circuit via SKC-4 and theactionof the control loop is to set the RF power such that the actual detected volts equals thereference volts. The digipot setting is static for each channel unless the required forwardpower is changed.The voltage to the top of the digipot is set up by the microprocessor through the PulseWidth Modulator output PWM1. On PTT the ON duty cycle of the PWMI output isprogressively increased and the filtered result of this forms a ramp to the top of the powercontrol digipot. Once 100% duty cycle is reached full power is produced. This results ina fast but controlled RF power rise characteristic.4.4.5. User InterfaceAll user interface to the MX800 except the RF connections is made by way of the MicroController board.§ Output latch IC18 drives the six LED indicators on the front panel. The function isdescribed in section 1.1.1.§ Input latch IC14 serves to accept the programmed status of the 8 bit binarychannel select DIP switch SW1. Refer section 6.3 for programming instructions.§ Input latch IC 13 serves to accept the programmed status of the 6 bit binary modeselect DIP switch SW2. Refer section 2.2.2.2 for mode selection instructions.§ Input latches IC48 and IC47 provide two eight bit +5V CMOS compatible digitalinput ports (ports A and B respectively). The inputs may be either internallypulled high and active low or internally pulled low and active high. Port B inputsare configured as external channel select inputs. The inputs are buffered by 10k.Q§ Output latch IC46 provides 8 uncommitted +5V CMOS compatible digital outputs.Each output is buffered by a 1kohm resistor.§ IC60 is a 1 watt speaker amplifier, which drives the internal speaker. This ICincludes the volume control function connection to the front panel toggle switch.§ Microphone audio is fed via amplifier IC61A to a compressor (IC34B). This VF ispre-emphasized and filtered before being fed to the main TX voice path.§ IC22 provides CMOS to RS-232 conversion for the serial port.

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