Assembly Instructions Chapter 4

ManualsBrandsUBS-Axcera ManualsElectronics10,000-watt UHF solid state television transmitter

10-kW UHF Transmitter with Chapter 4, Circuit

Feedforward Drive Descriptions

840A, Rev. 0 4-1

Chapter 4

Circuit Descriptions

4.1. (A1) Dual 250-Watt

Driver/Amplifier Assembly

(1094334; Appendix A)

4.1.1 (A1-A4) UHF Exciter Tray

(1094019; Appendix A)

4.1.1.1 (A4) Aural IF Synthesizer Board,

4.5 MHz (1265-1303; Appendix B)

The aural IF synthesizer board amplifies

each of the three possible audio inputs

and the amplifier circuits that supply the

single audio output. The balanced audio

or the composite audio input is connected

to the board while the subcarrier audio

(SCA) input can be connected at the

same time as either of the other two

inputs. The board has the 4.5-MHz

voltage-controlled oscillator (VCO) and

the aural modulation circuitry that

produces the modulated 4.5-MHz output.

The board also contains a phase lock loop

(PLL) circuit that maintains the precise

4.5-MHz separation between the aural

(41.25 MHz) and the visual (45.75 MHz)

IF frequencies.

Balanced Audio Input

The first of the three possible baseband

inputs to the board is a 600Ω-balanced

audio input (+10 dBm) that enters

through jack J2, pins 1 (+), 2 (GND), and

3 (-), and is buffered by U1B and U1C.

Diodes CR1 to CR4 protect the input

stages of U1B and U1C if an excessive

signal level is present on the input leads

of jack J2. The outputs of U1B and U1C

are applied to differential amplifier U1A;

U1A eliminates the common mode

signals (hum) on its input leads. A

pre-emphasis of 75 ms is provided by

R11, C11, and R10 and can be eliminated

by removing jumper W5 on J5. The signal

is then applied to amplifier U1D whose

gain is controlled by jumper W3 on J11.

Jumper W3 on jack J11 is positioned

according to the input level of the audio

signal (0 or +10 dBm). If the input level

is approximately 0 dBm, the mini-jumper

should be in the high gain position

between pins 1 and 2 of jack J11. If the

input level is approximately +10 dBm,

the mini-jumper should be in low gain

position between pins 2 and 3 of jack

J11. The balanced audio is then

connected to buffer amplifier U2A whose

input level is determined by the setting of

balanced audio gain pot R13. The output

of the amplifier stage is wired to the

summing point at U2D, pin 13.

Composite Audio Input

The second possible audio input to the

board is the composite audio (stereo)

input at BNC jacks J3 and J13. The two

jacks are loop-through connected; as a

result, the audio can be used in another

application by connecting the unused

jack and removing W4 from J12. Jumper

W4 on jack J12 provides a 75Ω-input

impedance when the jumper is between

pins 1 and 2 of jack J12 and a high

impedance when it is between pins 2 and

3. Diodes CR9 to CR12 protect the input

stages of U6A and U6B if an excessive

signal level is applied to the board. The

outputs of U6A and U6B are applied to

differential amplifier U2C, which

eliminates common mode signals (hum)

on its input leads. The composite input

signal is then applied to amplifier U2B;

the gain of this amplifier is controlled by

composite audio gain pot R17. The

composite audio signal is connected to

the summing point at U2D, pin 13.

Subcarrier Audio Input

The third possible input to the board is

the SCA input at BNC jack J4. The SCA

input has an input impedance of 75Ω that

can be eliminated by removing jumper

W2 from pins 1 and 2 of J14. The SCA

input is bandpass filtered by C66, C14,

R22, C15, C67, and R23 and is fed to

Summary of content (42 pages)

PAGE 1
0-kW UHF Transmitter with Feedforward Drive Chapter 4, Circuit Descriptions Chapter 4 Circuit Descriptions 4.1. (A1) Dual 250-Watt Driver/Amplifier Assembly (1094334; Appendix A) signal (0 or +10 dBm). If the input level is approximately 0 dBm, the mini-jumper should be in the high gain position between pins 1 and 2 of jack J11. If the input level is approximately +10 dBm, the mini-jumper should be in low gain position between pins 2 and 3 of jack J11.
PAGE 2
10-kW UHF Transmitter with Feedforward Drive Chapter 4, Circuit Descriptions buffer amplifier U3A. The amplified signal is then applied though SCA gain pot R24 to the summing point at pin 13 of U2D. CR17 to the 4.5-MHz VCO IC U10; this sets up a PLL circuit. The 4.5-MHz VCO will maintain the extremely accurate 4.5MHz separation between the visual and aural IF signals; any change in frequency will be corrected by the AFC error voltage.
PAGE 3
10-kW UHF Transmitter with Feedforward Drive Chapter 4, Circuit Descriptions maintains a constant peak of sync level over varying average picture levels (APL). The modulator portion of the board contains the circuitry that generates an amplitude-modulated vestigial sideband visual IF signal output that is made up of the baseband video input signal (1 Vpk-pk) modulated onto an externally generated 45.75-MHz IF carrier frequency.
PAGE 4
10-kW UHF Transmitter with Feedforward Drive Chapter 4, Circuit Descriptions to Q2. In the Manual position, jumper W7 on J4 is in the Clamp-Off position, between pins 1 and 2, and adjustable resistor R41 provides the manual clamp bias adjustment for the video that connects to Q2. Visual Modulator Circuit The clamped video signal from U2A is split.
PAGE 5
10-kW UHF Transmitter with Feedforward Drive Chapter 4, Circuit Descriptions consisting of T1, FL1, and T2, and produces a vestigial sideband visual IF signal output. The filtered vestigial sideband visual IF is amplified by U7 and connected to a T-type attenuator. R62 can be adjusted to set the visual IF gain; this is the amount of visual IF signal that is coupled to amplifier IC U8. R63 and C30 are adjusted for the best VSBF frequency response.
PAGE 6
10-kW UHF Transmitter with Feedforward Drive Chapter 4, Circuit Descriptions the frequency response, and amplification of the video signal. The +12 VDC is applied through jack J10 to crystal oven HR1; HR1 is preset to operate at 60° C. The oven encloses crystal Y1 and stabilizes the crystal temperature. The crystal is the primary device that determines the operating frequency and is the most sensitive in terms of temperature stability.
PAGE 7
10-kW UHF Transmitter with Feedforward Drive Chapter 4, Circuit Descriptions Q4 drives two different output circuits. One output is directed through voltage divider R14 and R15 to jack J2 and is fed to a frequency counter. While monitoring J2, the oscillator can be set exactly on the operating frequency (45.75 MHz) by adjusting C17. The output at J2 is at a power level of approximately -2 dBm, which is sufficient to drive most frequency counters.
PAGE 8
10-kW UHF Transmitter with Feedforward Drive Chapter 4, Circuit Descriptions video loss cutback, and the threshold detector circuits will only operate when jumper W3 on jack J6 is in the Auto position, between pins 1 and 2. Without the threshold detector, and with the pindiode attenuator at minimum, when the signal is restored it will overdrive the stages following this board.
PAGE 9
10-kW UHF Transmitter with Feedforward Drive Chapter 4, Circuit Descriptions input signal level to the detector at TP3 falls below this reference threshold, which acts as a loss of sync detector circuit, the output of U9C and U9D goes towards the -12 VDC rail and is split, with one part biasing on transistor Q5. A current path is then established from the +12 VDC line through Q5, the resistors R69, R137, and the red LED DS3 (video loss indicator), which becomes lit.
PAGE 10
-kW UHF Transmitter with Feedforward Drive The output of U1 is available, as a sample of the pre-correction IF for troubleshooting purposes and system setup, at jack J2. The IF signal is then connected to the linearity corrector portion of the board. Linearity Corrector Circuits The linearity corrector circuits use three stages of correction to correct for any amplitude non-linearities of the IF signal.
PAGE 11
10-kW UHF Transmitter with Feedforward Drive Following the bandpass filter, the signal is split using L24, L25, and R89. The signal passing through L24 is the main IF path through the board. A sample of the corrected IF signal is split off and connected to J10, the IF sample jack. The IF connects to jacks J27 and J28. These jacks control whether a 6-dB pad is included in the circuit by the positioning of jumpers W9 and W10.
PAGE 12
10-kW UHF Transmitter with Feedforward Drive the ALC detector is offset, or complemented, by current taken away from the summing junction. In normal operation, U10A, pin 2, is at 0 VDC when the loop is satisfied. If the recovered or peak-detected IF signal at IF input jack J7 of this board should drop in level, which normally means that the output power is decreasing, the null condition would no longer occur at U10A, pin 2. When the level drops, the output of U10A, pin 1, will go more positive.
PAGE 13
10-kW UHF Transmitter with Feedforward Drive action, by reverse biasing CR3, in the event of an external VSWR fault. ±12 VDC Needed to Operate the Board The ±12 VDC connects to the board at J14. The +12 VDC connects to J14-3 and is filtered by L30, L41, and C80 before it is applied to the rest of the board. The -12 VDC connects to J14-5 and is filtered by L31 and C81 before it is applied to the rest of the board.
PAGE 14
10-kW UHF Transmitter with Feedforward Drive through L4 and R10 from the +12 VDC line. After the signal is amplified by U2, it is applied to the second corrector stage through T2 and then to a third corrector stage through T3. The other two corrector stages operate in the same manner as the first; they are independent and do not interact with each other. When jumper W1 on J8 is connected from center to ground, R15 is put in series with ground.
PAGE 15
10-kW UHF Transmitter with Feedforward Drive third stage through T6. The transformer doubles the voltage swing by means of a 1:4 impedance transformation. Resistors R42, R59, R60, and R43 form an L-pad to lower the level of the signal. The signal is applied to amplifier U7 to compensate for the loss in level through the L-pad. TP1 is a test point that gives the operator a place to measure the level of the inphase IF signal that is connected to mixer stage Z2.
PAGE 16
10-kW UHF Transmitter with Feedforward Drive The RF input signal from the external filter re-enters the board at J4 (-11 to 14 dBm) and is capacitively coupled to the pin-diode attenuator circuit that consists of CR1, CR2, and CR5. The pindiode attenuator acts as a voltagevariable attenuator in which each pin diode functions as a voltage-variable resistor and depends on the DC bias supplied to the diode for the resistance value.
PAGE 17
10-kW UHF Transmitter with Feedforward Drive harmonic is then amplified by U3 and fed through another bandpass filter tuned to the fourth harmonic consisting of L11 and C18 (348 to 456 MHz). The voltage measured at TP3 is typically +2.0 VDC. The final doubler stage consists of Z3 with bandpass filter C20 and L12 tuned to the eighth harmonic of the fundamental frequency (696 to 912 MHz). The signal is amplified by U4, U5, and U6 to a typical value of from +2 to +4 VDC as measured at test point TP4.
PAGE 18
10-kW UHF Transmitter with Feedforward Drive a low to Q2, which turns off and extinguishes the green Operate LED DS2. In addition, Q12 is biased on; it connects to U2C, whose output goes low and pulls the ALC voltages at J1 low, lowering the gain of the external amplifier trays. The high from U4B-13 is also applied to Q4 and Q24, which are biased on and apply disables at J1-4 and J18-1. The high from U4B-13 connects to Q10, which is biased off.
PAGE 19
10-kW UHF Transmitter with Feedforward Drive 11, biases on Q19, which applies a low enable to the Standby switch; this places the transmitter in the Standby mode. When the video is returned, J7-5 goes high. The high is applied to Q16, which is biased on, and to the red Video Fault LED DS9, which is extinguished. The output of Q16 goes low and connects to U5B, pin 5. If there is no receiver ALC fault, U5B, pin 6, is also low, which causes the output at pin 4 to go high.
PAGE 20
10-kW UHF Transmitter with Feedforward Drive output of U5D to go low and reverse bias Q19. The drain of Q19 goes high and removes the low from the standby coil in relay K1. VSWR Cutback Fault. The reflected power sample of the RF output of the diacrode transmitter is connected to J2, pin 9, of the board. The sample connects to op-amp U1B, pin 5, which buffers the signal. The remote reflected sample connects to U2B, pin 5.
PAGE 21
10-kW UHF Transmitter with Feedforward Drive Chapter 4, Circuit Descriptions Operational Voltages Aural Level Circuit The +12 VDC needed for the operation of the transmitter control board enters the board at jack J4, pin 3. C28, L1, and L3 are for the filtering and isolation of the +12 VDC before it is split and applied to the rest of the board. The -12 VDC needed for the operation of the board enters the board at jack J4, pin 5.
PAGE 22
10-kW UHF Transmitter with Feedforward Drive connects to J6, pins 2 and 3, which supply the peak of sync visual level output to the front panel meter for monitoring. If this board is operated with scrambling, using suppressed sync, the visual level circuit operates differently than described above because there is no peak of sync present on the forward sample input. For the board to operate properly, a timing pulse from the scrambling encoder must connect to the board at J4.
PAGE 23
10-kW UHF Transmitter with Feedforward Drive between J7-1 and J7-2 for the operation of the diacrode (external reference). The (A25) AGC control board (11371201) compares a sample of the output of the diacrode with a sample of the 3watt tray. The front panel gain control R2 adjusts the overall gain of the tray.
PAGE 24
10-kW UHF Transmitter with Feedforward Drive is switched on, AC is applied to fans A19 and A20 and also to (A21) the step-down transformer for the ±12V power supply. The stepped-down AC is applied to (A22) the ±12V power supply board (10621013) that supplies the regulated ±12V to the rest of the boards in the tray. LEDs DS1 for +12V and DS2 for -12V on the board will light to indicate that the power supply is operational.
PAGE 25
10-kW UHF Transmitter with Feedforward Drive cancel any remaining non-linear distortion at the output. The reflected port is terminated into a 1watt, 50Ω load (A19-A10). After the linear and non-linear samples have been matched for gain and delay, and their phases have been adjusted to a 180° difference, they are summed together in the (A5) dual stripline coupler board. From A5-J2, the mixed signals are fed to the (A20) correction phase/gain enclosure at J1.
PAGE 26
10-kW UHF Transmitter with Feedforward Drive that provides the +26.5 VDC to the two cooling fans and, when the tray is enabled, through the amplifier protection board to the rest of the boards in the tray. The enable is applied to the tray at J4-5 on the back of the tray. The output of the switching power supply is connected to (A15) the amplifier protection board and also to (A13 and A14), which are two fans used for cooling the tray.
PAGE 27
10-kW UHF Transmitter with Feedforward Drive meter on the metering control panel. R6 calibrates the bias (control grid) current meter reading at J2, pins 1 and 2, that is applied to the current meter on the metering control panel. These meter calibration adjustments were set at the factory for typical operation and should not need adjustments at this time. 4.2.2 Control and Bias Power Supply Assembly The control and bias power supply assembly (1181-1402) contains two separate power supplies.
PAGE 28
10-kW UHF Transmitter with Feedforward Drive Chapter 4, Circuit Descriptions The 220 VAC is applied through the closed contacts of (A7) the isolation relay board, to (A8) the Sola regulator assembly, and to transformer T1 as long as the screen circuit breaker on the AC control assembly is switched on. The screen-on command is applied to TB1-6 and the control power supply is on to produce the +12 VDC that is applied to TB1-5. voltage protection.
PAGE 29
10-kW UHF Transmitter with Feedforward Drive 4.2.4 Filament Power Supply Assembly The filament power supply assembly (1299-1107) consists of input terminal block TB2, status and control terminal block TB1, the (A1) filament power supply control board (1293-1304; Appendix B), and the (A2) programmable filament power supply (1293-6302). The 220 VAC is supplied at TB2-1A and TB2-2A and ground at TB2-3A.
PAGE 30
10-kW UHF Transmitter with Feedforward Drive Chapter 4, Circuit Descriptions control for DS1 is from U4, PLMA, pin 20, to FET Q2. activated through pin 35 of U4 and FET Q3. Table 4-1 below lists the state, condition and flash-rate of the LED. DS2 is the Output Verified LED. It is used to indicate system status and is Table 4-1. State, Condition, and Flash Rate of LED DS2 STATE CONDITION Normal Condition -- AC Fault J4-1 <4 VDC DC Fault J1-3 <9 VDC DAC Fault DAC = 4.625 Vout DAC = 0.
PAGE 31
10-kW UHF Transmitter with Feedforward Drive enables the tube hour counter that is connected through connector J7-1 and J7-5. Operational Voltages The +12 VDC needed to operate the board enters at J1, pin 3, and is filtered by L1, C1, and C4 before it is fed to the rest of the board. The -12 VDC needed to operate the board enters at J1, pin 5, and is filtered Chapter 4, Circuit Descriptions by L2 and C21 and C22 before being fed to the rest of the board.
PAGE 32
10-kW UHF Transmitter with Feedforward Drive (S10) the Driver Mode Normal/Test switch, (S9) the Fault Reset switch, and (S8) the Power Output Raise/Lower switch. The ±12 VDC needed to operate the boards in the metering control panel are provided to J2-1 (+12 VDC), J2-2 (±12 VDC Return), and J1-50 (-12 VDC) by the control section of (A5) the control and bias power supply assembly. The ±12 VDC is applied to the metering control panel when the control circuit breaker is switched on.
PAGE 33
10-kW UHF Transmitter with Feedforward Drive pin 4 goes high. The high forward biases Q2, whose collector goes low and applies the Low Filament-On command through the front panel-mounted Command Status Filament LED DS9, which will light, to the Filament Control Relay. The low energizes the relay and turns On the filament power supply. The output voltage from the filament power supply is monitored and produces a low, filamenton sense input to the board at J2-7.
PAGE 34
10-kW UHF Transmitter with Feedforward Drive High Voltage-On command circuit. U211, which connects to U21-10, will be a low if both the high voltage is enabled, U21-8 low, and the screen interlock is closed, U21-9 low. When U21-1 and U212 are both low, this causes a low at U213, which is connected to inverter IC U135, and whose output at U13-4 goes high. The high forward biases Q7, causing its collector at J9-11 to go low and apply the RF-On command to the exciter.
PAGE 35
10-kW UHF Transmitter with Feedforward Drive U3-12, whose output at pin 10 goes high. The high connects to U15-1, the three fault counter IC, and also to U19-8 and U19-9. U19-10 goes high and forward biases Q9, whose collector goes low and lights the Fault Indicator LED DS7 on the front panel. The fault high at J5-7 also connects to the set S1 input at pin 6, to one of four quad R-S latches that make up U20, and to U11-4. The output of the latch at U20-9 goes high and is inverted by U18.
PAGE 36
10-kW UHF Transmitter with Feedforward Drive this is applied to DS15, the Cathode I Fault LED on the board, causing it to light. Because of the high on its input at U11-3, the output of U11 at pin 1 goes high and resets clock IC U9. The Q1 and Q0 outputs of U9 go low during reset, then back to high, and are applied to U14, whose output will be high when both inputs are high.
PAGE 37
10-kW UHF Transmitter with Feedforward Drive Chapter 4, Circuit Descriptions When the Exciter Test switch is in Normal, with K4 energized, it applies a low to CR13 and then to inverter U8E, pin 11, whose output at pin 12 then goes high. The high also forward biases Q26 and Q27 and lights DS14, the front panel-mounted Normal LED connected to J14, pins 8 and 6. The high input to inverter U8B, pin 5, causes the output at pin 4 to go low.
PAGE 38
10-kW UHF Transmitter with Feedforward Drive If the video input to the modulator is removed, a video fault low is applied to J2, pin 11. The low connects to inverter U7B, pin 5, whose output at pin 4 goes high. The high lights the red Video Fault LED DS3. The low connects to timer U1A, pin 5, and also connects directly to NOR gate U5C, pin 9. After 5 seconds, enough time for the loss of video to be determined as permanent, the output of U1A at pin 8 will go low; the low is applied to U5C, pin 8.
PAGE 39
10-kW UHF Transmitter with Feedforward Drive A forward power sample of the RF output of the transmitter is connected to J15, pin 7, of the board. The sample connects to op-amp U3A, pin 3. If the input sample level stays above the level set by R38, the output of U3A at pin 1 will be high. The high forward biases Q15 and Q16 and causes their drains to go low. J14, pin 2, is low and connects to the front panel RF Present LED DS21, causing it to light.
PAGE 40
10-kW UHF Transmitter with Feedforward Drive board removes the filament-on sense to the automatic on/off sequence. The fault sense board is supplied by a +12VDC source at J3-3. The capacitors C1 and C2 provide filtering. 4.2.5.5 Inverting Fault Sense Board (1016-1401; Appendix B) The inverting fault sense board provides an inverted output for a sample input, generally from the differential buffer board. For example, a filament-on sense sample is applied to J1-8 of the board.
PAGE 41
10-kW UHF Transmitter with Feedforward Drive remains below this reference level, U1, pin 2, will go low and the low is applied to NAND gate U2, pins 1 and 2. U2, pin 3, will go high and the high is applied to NAND gate U2, pin 5. Because J4-1 is jumpered to J4-4, the input sample of the low screen current is also applied to U1, pin 7. R12 sets the lower limit trip point for this section of the IC.
PAGE 42
10-kW UHF Transmitter with Feedforward Drive VR1 and VR2 are back-to-back 10 VDC zener diodes that protect the external metering circuits from a high-voltage problem by shunting out the higher voltage and allowing only a maximum of ±10 VDC to be present at the output. C1 is an AC bypass capacitor for the reduction of any AC ripple at the output. The external current metering circuit connects to J2, pins 1 and 3. 4.3.