8877 Linear Amplifier Control Circuitry
Logic, Motor, & Heater Supply --
This is a commercial power supply providing +5 volts, +/- 15 volts and 48 volts. A modification to the output stage of this supply provides a 30 volt and a 100 volt source. The +5 volts is used for the tube heater in addition to the control logic.
Keying circuit --
A power FET is used to switch from standby to transmit in the cathode line.
Bias Generator --
A TL431A, with a current boost transistor, is used to establish the bias voltage. This bias is applied to the grid circuit rather than in the typical cathode location. A "keep alive" current is supplied from the +15 volt supply to maintain the required minumum current through this regulator in standby. Operation of the circuit shown has been verified up to two amps; Far in excess of the maximum grid current.
A drawing showing the above sections in concert may be seen here.
Grid & Plate Current sensors --
The Grid and Plate currents are sensed across small value resistors by integrated 3 op-amp topology instrumentation amplifiers. The operation of the peak detectors is the same as the circuit described at the bottom of the page displayed here.
Fault sensors: Grid & Plate -- Simple transistor comparators, with levels set by trimpots, connected to the grid and plate current sense lines trigger small SCR's in the fault matrix.
Fault Matrix -- A bit of diode logic which shuts down the cathode key line on any grid or plate current fault and also drives the front panel fault indicators.
ALC circuit --
The ALC is derived from the grid current metering rather than the traditional RF drive level sense.
Antenna Relay Drive -- The antenna relays are energized by an optoisolated current source.
Fwd & Rev Power Detectors + Peak detection -- The sensor voltages are squared by analog multipliers. The forward and reverse power meters may be driven directly from this point or selectively from a delayed release peak detector.
The operation of these peak detectors is identical to that employed at the bottom of the page displayed here
Engaging the keyswitch powers the small pilot supply which enables the power switch and the remote start circuit.
Motor Drive --
Two speed servo controlled by the microprocessor.
Power Fail Sense --
The AC line is sampled to provide for an orderly shut down in the event of a power failure.
Preset Storage --
The microprocessor has 256 bytes of EEPROM which store the preset tuning values for each band as well as soft limits for the vacuum capacitors
Grid & Plate Current Peak Detectors -- The grid and plate current meters may be driven directly from this point or selectively from a delayed release peak detector.
The operation of the peak detectors is the same as the circuit described at the bottom of the page displayed here.
The meters are all 5 volt (1K ohm/volt) intrinsic movements
The lower two 3 digit displays show the current setting of the two quadrature encoded knobs.
The upper 3 digit display shows the current operation frequency.
An eight digit alphanumeric display at the top will provide mode information.
The two knobs are connected to quadrature encoders to set manual tuning and other functions.
The left knob is typically manual tune, the right manual load.
Power on-off -- Alternately initiates a power on or power off cycle.
Peak hold -- Selects the peak hold function for four of the meters.
Autotune -- Initiates an autotune cycle: Detect input frequency, preset reactive elements, enable the amplifier, and tweak final tuning.
An "Emergency Off" may be commanded by simultaneously pressing the power switch and any of the other three switches.
Pilot Supply --
Engaging the keyswitch powers a small pilot supply which enables the power switch.