BK Technologies GMH599 VHF Mobile Transceiver User Manual Section 4

RELM WIRELESS CORP. - BK RADIO VHF Mobile Transceiver Users Manual Section 4

Users Manual Section 4

BK RADIOPage 4-1SECTION IVTHEORY OF OPERATION4.1 INTRODUCTIONThis section contains a description of equipment and a theory of operation for the BK Radio GMH Seriesmobile radio. To aid in understanding the operation of the equipment, schematic diagrams are found inSection VI of this manual.4.2 EQUIPMENT DESCRIPTIONThe BK Radio GMH Series radio comprises the following sub-assemblies:4.2.1 System BoardThis sub-assembly consists of core microprocessor, synthesizer, regulation, and switching circuitry. Acasting is used to shield the synthesizer area.4.2.2 RX/TX BoardThis sub-assembly consists of the RX/TX antenna switch, the receiver circuitry from the front-end throughthe discriminator, and the low level transmitter line up.4.2.3 High Level Power Amplifier BoardThis sub-assembly consists of the final two stages of the transmitter power amplifier, harmonic filter, anddirectional coupler.4.2.4 Options BoardThis sub-assembly consists of receive and transmit audio chains, and interfaces with both the systemboard and the control board.4.2.5 Control boardThis sub-assembly consists of a control head microprocessor, switching regulator, display driver, andaudio power amplifier. It interfaces with the front panel and accessory connector.4.2.6 VCO BoardThe VCO board is a separate assembly that resides in its own shielded enclosure and interconnects withthe system board and RX/TX board.
Theory of Operation GMH Series VHF RadioPage 4-2BK RADIO4.3 THEORY OF OPERATION4.3.1 System boardSystem board functions include:1. Core Microprocessor2. Regulation: 5.0, 8.6, 9.6, and 20.0 Volts3. Synthesizer4. Squelch Detection5. CTCSS/CDCSS Decode6. Power Control4.3.1.1 Microprocessor ControlThe core microprocessor (U507) communicates with the control head microprocessor and controls radiofunctions such as loading the synthesizer, adjusting the deviation and receiver tuning, and CTCSS/CDCSSdetection. An EEPROM is used to store calibration and tuning data unique to each radio. A 32.768 kHzcrystal is used as a clock for the core microprocessor.Various transistors provide level interfaces and current capability.4.3.1.2 Regulation: 5.0, 8.6, 9.6, and 20.0 VoltsU502 and associated circuitry comprise a 9.6 volt low noise feedback regulator for use by noise-sensitiveanalog circuitry on the system and RX/TX boards. A low battery indication is derived by sensing the errorcontrol voltage of this regulator.U601 and associated circuitry provide a second level of regulation to supply the VCO and synthesizer with8.6 volts and 5.0 volts.U504 is the 5 volt supply for the core of the radio.The microprocessor drives a FET amplifier (Q503) tied to a voltage doubler. This forms a 20 volt switchingregulator which is regulated by error amplifier U502 which controls the gain on the FET.4.3.1.3 SynthesizerSynthesizer IC - U604 forms the main synthesizer IC which contains three programmable CMOS dividersand a sample-and–hold phase detector. The first divider (divide-by-R) divides the reference oscillatordown to a frequency which is used as a reference by the sample-and-hold phase detector. The seconddivider (divide-by-N) divides the output of the Prescaler down to a frequency which is equal to the divideddown reference frequency when the loop is locked. The third divider (divide-by-A) controls the moduluscontrol line of the Prescaler. The sample-and-hold phase detector provides a DC voltage that isproportional to the phase error between the divided down reference frequency and the divided downcarrier frequency. This voltage is fed through the loop filter to the VCO and adjusts the VCO frequency tomaintain phase lock between the divided down frequencies.
GMH Series VHF Radio General InformationBK RADIOPage 4-3Prescaler - The prescaler U602 is the first divider in the feedback path of the synthesizer. It divides the RFsignal to a frequency which can be processed by the following CMOS dividers. The prescaler is a dualmodulus type which allows the division ratio to be set by the synthesizer to either divide-by-128 (moduluscontrol line high) or divide-by-129 (modulus control line low). This capability allows the channel spacing tobe determined by the divided down reference frequency and not a multiple thereof.Reference Oscillator - The Reference Oscillator provides the frequency reference from which the receiverand transmitter injection signals are synthesized. The oscillator frequency is controlled by the crystal Y601which operates in the parallel resonant mode. The core microprocessor uses temperature sensor U607 tomeasure the temperature of the crystal and changes the bias on varactor CR602 accordingly, tocompensate the crystal to less than ±2.5 ppm tolerance. The varactor provides a means for modulatingthe reference oscillator to improve the synthesizer frequency response for low frequency modulation.Loop Filter - The Loop Filter removes noise and unwanted frequency components from the output of thesample-and-hold phase detector which otherwise would modulate the VCO. In addition, it employs amultiple filter bandwidth design which allows fast response during frequency changes (such as in channelscan) without degrading the noise and spurious performance of the receiver during steady state receiveand transmit conditions. The filter bandwidth is switched to a wide condition when the LATCH line pulseshigh for approximately 6 msec during a frequency change. This allows the new frequency to be reachedquickly. When the LATCH line to returns to a low state, the filter bandwidth changes to a narrow conditionand provides for good noise and spurious performance. Different filter bandwidths are used for transmitand receive to provide better hum and noise performance in transmit and faster response time in receive.This is accomplished by changing the filter bandwidth to a narrower value when the RX/-TX line goes lowduring transmit.Offset D/A – U603 and associated resistors form a serial latching D/A which is loaded by themicroprocessor when it loads the synthesizer IC. This provides an offset voltage which is summed into theloop filter op amp U605 to shift up the 0 to 4 volt output of the phase detector. Calibration of the VCO isprovided for in the software by altering the loaded values of this IC.Deviation Compensation – U508 is a digital pot used to control the amplitude of the transmit modulationsignal. As the transmit frequency increases, less voltage is needed at the VCO, so the modulation signal isattenuated.4.3.1.4 Squelch DetectionU505 and associated circuitry form a bandpass filter that selects and amplifies the noise on thediscriminator output near 20 kHz. The core microprocessor samples the output of the filter, averages anumber of samples, and compares the result to a threshold number to determine if carrier is present.4.3.1.5 CTCSS/CDCSS DecodeU506 and associated circuitry act as a filter and limiter for CTCSS/CDCSS decoding interface to the coremicroprocessor.4.3.1.6 Power ControlThe core microprocessor controls the output power of the transmitter by setting the reference voltage of afeedback control loop. U509 and associated circuitry integrate the difference between the referencevoltage and the forward detected voltage from the directional coupler, and drives the amplifier/driver Q507.Q507 supplies bias to the low level transmitter driver stage collector.
Theory of Operation GMH Series VHF RadioPage 4-4BK RADIO4.3.2 RX/TX Board4.3.2.1 ReceiverBuffer - Q101 is an L.O. buffer which provides approximately 15 dB of gain to supply the mixer andtransmitter line-up.Front End - The preselectors are varactor-tuned direct coupled filters with impedance transformations builtinto the configuration. The bipolar preamp provides approximately 18 dB of gain to overcome filter lossesand provides a low noise figure.Mixer/IF - The active singly-balanced JFET mixer converts the signal to an IF of 16.9 MHz where it isfiltered by the crystal filters and amplified by the IF amplifier.Demodulator IC - I1 is a multi-function IC which provides a second mixer, second IF oscillator, amplifier,and quadrature detection.Antenna Switch- The antenna switch provides RX/TX isolation and switching using a TX series PIN diodepair (which is located on the High Level PA Board) and RX shunt PIN diode, with the coaxial cable runningfrom the High Level PA Board to the RX/TX Board being one quarter wave-length at mid-band.Front End Tuning- I2 is a Hex DAC that provides an independent tuning capability for each of the fivevaractor elements in the two pre-selectors.4.3.2.2 Low Level Power AmplifierLine-up- The low level power amplifier consists of the following stages and power levels:Low Level Amp to 0.02 wattDriver to 0.2 wattFinal to 2.0 wattsBroadband matching networks are used throughout the line-up. The Low Level Amp runs class A, and thedriver and final are operated in class C mode. The Power control voltage is applied to the driver stagecollector.
GMH Series VHF Radio General InformationBK RADIOPage 4-54.3.3 High Level PA Board4.3.3.1 Power AmplifierThe high level power amplifier consists of a driver stage with a nominal output power of 15 watts and afinal stage with sufficient output power to provide 50 watts at the antenna port. Both stages utilizebroadband matching networks and are operated in a class C mode.4.3.3.2 Harmonic FilterA 9th order elliptical filter is used to attenuate harmonic components before they reach the antenna port.4.3.3.3 Directional CouplerA stripline directional coupler and associated detectors provide DC voltages proportional to the forwardand reflected power at the antenna connector. The forward detected voltage is used for feedback to thepower control loop and the reverse detected voltage is used to detect high VSWR conditions.4.3.4 Options Board4.3.4.1 Control FunctionsThe routing of audio signals through the options board is determined by analog switch I1. The audio pathis controlled by outputs from the control board.4.3.4.2 Receiver AudioLow frequency tones are removed from the audio by a high-pass filter of I2A, I2B, I2C, and associatedcircuitry. De-emphasis is set by C23.4.3.4.3 Transmitter AudioI3D performs pre – emphasis, and I3C performs limiting. I3B is used as a buffer/amplifier and drives athree pole low pass filter consisting of I3D and associated circuitry. This low pass filter has a cutoff at 3kHz and provides modulation filtering to within FCC requirements. The output is routed to the systemsboard. Signaling tones are generated on the control boar and summed with the transmitter audio at I3B.
Theory of Operation GMH Series VHF RadioPage 4-6BK RADIO4.3.5 Control BoardControl Board functions include:1. Microprocessor Control2. EEPROM storage3. Regulation4. Display Driver5. Signaling6. Audio Power Amplifier4.3.5.1 Microprocessor ControlU102 controls the interface between the radio and the user. During normal operation, U102 monitors thefront panel and push-to-talk switches, and provides data to the display. In the radio programming mode,U102 interprets commands from the serial bus and provides a transparent interface to the externalprogramming source.4.3.5.2 EEPROM StorageEEPROM U105 is used to store the radio configuration and channel information, i.e., receive and transmitfrequencies and code guard values for each channel.4.3.5.3 RegulationA switching regulator, U103, is used to generate +32 volts for the vacuum fluorescent display and driver. A+5 volt regulator, U108, is used to supply power requirements for the control and options boards.4.3.5.4 Display DriverDisplay driver U100 receives data to be displayed from U102 and drives the vacuum fluorescent display.4.3.5.5 SignalingU206 generates audible and subaudible tones (such as CTCSS).4.3.5.6 Audio Power AmplifierThe audio amplifier, U107, can deliver a maximum of 4 watts into a 3.2 ohm load. Audio muting is providedby U109.
GMH Series VHF Radio General InformationBK RADIOPage 4-74.3.6 VCO BoardThe VCO is a varactor tuned feedback oscillator using Q1 with feedback from tapped auto transformer T1.A steering voltage is used to set the capacitance of CR1, CR10, CR11, and CR12, which in turn controlsthe frequency of oscillation. CR3 provides a low sensitivity input for modulating the carrier frequency.The cascode amplifier comprised of Q6 isolates the VCO from its load and provides a nominal drive levelof 0 dBm in receive and transmit.

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