DTT250M – 250W Digital Television Transmitter GENERAL DESCRIPTION INTRODUCTION This manual describes the LARCAN model DTT250M VHF Digital Television Transmitter. LARCAN all‐solid‐state 250W VHF transmitters are designed to operate conservatively at 250W average DTV power with superb performance, reliability and operating economy. The transmitter and exciter or translator chassis are packaged in a single 19" cabinet.
DTT250M – 250W Digital Television Transmitter GENERAL DESCRIPTION TRANSMITTER CONTROL The control circuitry in this solid state transmitter is simple. Interlocking consists of the enabling circuitry necessary to ensure that any external patch panel link operation, or RF switching, can only be done with RF turned off.
CONTENTS 1 BANDPASS FILTER ........................................................................................................................................................ 2 2 RF DIRECTIONAL COUPLER ......................................................................................................................................
1 BANDPASS FILTER Drawing References: Figure 1 and Figure 4. The LARCAN bandpass filter implementation consists of a cascaded series of coupled helical resonators. A helical resonator is essentially a self supporting high Q coil (the helix) mounted inside a metallic shield enclosure. One end of the coil is solidly connected to the shield enclosure and the other end is open circuited except for a small trimmer capacitance to ground.
Figure 1 5-Pole Bandpass Filter Curves Figure 2 5-Pole Bandpass Filter Used in the TTS1000B PUB96-26 Rev 1 September 13, 2005 26-3 RF Output: BP Filter & Directional Coupler
2 RF DIRECTIONAL COUPLER A directional coupler is based on the principles of inductive (magnetic) coupling and capacitive coupling. In the LARCAN quad directional coupler implementation as shown in Figure 3 (schematic equivalent) and Figure 5 (assembly), the RF to be sampled passes through a microstrip transmission line that is connected between the transmitter output filter at J3 and the antenna system at J4.
Figure 3 Quad Directional Coupler Equivalent Schematic PUB96-26 Rev 1 September 13, 2005 26-5 RF Output: BP Filter & Directional Coupler
POWER AMPLIFIER LOW BAND CONTENTS FUNCTIONAL DESCRIPTION.....................................................................................................................................1 6-WAY SPLITTER/INPUT BOARD .............................................................................................................................1 FET RF AMPLIFIERS ...............................................................................................................................................
POWER AMPLIFIER LOW BAND Functional Description The Power Amplifier module consists of a six‐way power splitter, six FET amplifiers, a six‐way power combiner, a VSWR protection board, and power & I/O connectors. Two full‐size heatsinks provide the cooling for the active devices. It is designed for 1.5 kW sync peak power output in Low Band 54 ‐ 88 MHz Analog television systems, and provides power gain of approximately 20 dB, with 1.5 kW peak sync visual or 900 W aural output.
POWER AMPLIFIER LOW BAND R3 and R6 provide a DC path for bias, and provide loading at lower frequencies where gate impedance is high, in order to assist in maintaining amplifier stability. The choice of C6 and C7 values, and the series inductance of board traces, also ensures effective bypassing at critical frequencies of interest. The output matching π‐network, consisting of inductors L3 thru L8, and capacitances C13 thru C16, transforms the very low output impedance of the FET, to 12.5 Ω.
POWER AMPLIFIER LOW BAND operating transmitter, it provides protection to the FETs against over‐dissipation due to high VSWR, and it monitors the module RF power gain. If the module is plugged into a powered transmitter using several modules running in parallel, the power supply connections are first made through the longer contacts of the module’s DC power connector and into VSWR board J1 pin 8.
POWER AMPLIFIER LOW BAND • • • • • • • • • Remove all fuses from the module to be tested. (There are 12 fuses in total). Adjust all bias pots to maximum resistance, for minimum bias voltage. (Again, there are 12). Use a clip lead to short the junction of C7, R6, R7 and R10 to ground. This shuts off side B of the amplifier so it will not interfere with measurement of bias current from side A. Terminate the RF input and output into a 50 Ω load.
POWER AMPLIFIER LOW BAND • Figure 1 Module sweep Setup Green LED Sensitivity Adjustment One of the functions of the VSWR board is to monitor the overall gain of the PA module. This VSWR board is located at the rear of the module, adjacent to the output RF connector. For the locations of the components on the board, please refer to Figure 7.
POWER AMPLIFIER LOW BAND It is recommended that R4 be adjusted one half turn at a time, to establish a known reference point. Place the AGC/MANUAL switch in the AGC position, and with the RAISE/LOWER switch, readjust the transmitter output power to 100%. Similarly, the aural amplifier may be adjusted in the same manner, but being an FM signal the modulation of the carrier is not critical. o • • PUB96‐28 Rev 2 Aug.
POWER AMPLIFIER 1.5KW HIGH BAND 40D1493G3 CONTENTS FUNCTIONAL DESCRIPTION.....................................................................................................................................1 6-WAY SPLITTER/INPUT BOARD .............................................................................................................................1 FET RF AMPLIFIERS ....................................................................................................................................
POWER AMPLIFIER 1.5KW HIGH BAND 40D1493G3 Functional Description Drawing references: Figure 2 through Figure 7 The Power Amplifier module consists of a six‐way power splitter, six 250 W FET amplifiers, a six‐way power combiner, a VSWR protection board, and power & I/O connectors. Two full‐size heatsinks provide the cooling for the active devices. It is designed for 1.
POWER AMPLIFIER 1.5KW HIGH BAND 40D1493G3 R3 and R6 provide a DC path for bias, and provide loading at lower frequencies in order to assist in maintaining amplifier stability. The choice of C4 and C5 values, and their internal equivalent series inductances, also ensures effective bypassing at all frequencies.
POWER AMPLIFIER 1.5KW HIGH BAND 40D1493G3 operating transmitter, it provides protection to the FETs against over‐dissipation due to high VSWR, and it monitors the module RF power gain. If the module is plugged into a powered transmitter using several modules running in parallel, the power supply connections are first made through the longer contacts of the module’s DC power connector and into VSWR board J1 pin 8.
POWER AMPLIFIER 1.5KW HIGH BAND 40D1493G3 Adjustment of bias voltage to establish proper quiescent FET bias current Important: 50 Ω input and output terminations are necessary to achieve consistent results and prevent damage to the devices when testing modules. Supplemental cooling is not required when performing bias adjustments or low power sweep of the PA modules. • • • • • • • • • Remove all fuses from the module to be tested. (There are 12 fuses in total).
POWER AMPLIFIER 1.5KW HIGH BAND 40D1493G3 Figure 1 Module Sweep Setup Green LED Sensitivity Adjustment One of the functions of the VSWR board is to monitor the overall gain of the PA module. This VSWR board is located at the rear of the module, adjacent to the output RF connector.
POWER AMPLIFIER 1.5KW HIGH BAND 40D1493G3 It is recommended that R4 be adjusted one half turn at a time, to establish a known reference point. Place the AGC/MANUAL switch in the AGC position, and with the RAISE/LOWER switch, readjust the transmitter output power to 100%. Similarly, the aural amplifier may be adjusted in the same manner, but being an FM signal the modulation of the carrier is not critical.
Intermediate Power Amplifier 30C1892G1 ‐ G2 ‐ G3: Figures 1, 2, and 7. The 30C1892 Intermediate Power Amplifier basically consists of a fan‐cooled heatsink and three circuit boards. These boards are the Preamplifier board, the Amplifier Input board, and the Amplifier Output board. This subassembly is equipped with shielding covers and is mounted on a standard 19" panel.
1 kW TTS1000B TRANSMITTER IPA ASSEMBLY In the High Band unit, the output of the hybrid is also fed via an attenuator R5 (GAIN) but this time to an additional preamplifier stage U4, whose output appears at the input of U2, which feeds terminal J2. The spec'd gain of type MWA330 in the U4 position is 6 dB, and type MHW6185 or CA2885 (U2) is 18 dB. A few dB of losses exist on the board, so the effective gain of the High Band preamp board 10A1453G3 with R5 at maximum is about 18 to 20 dB.
1 kW TTS1000B TRANSMITTER IPA ASSEMBLY MRF‐151‐G could be used as a replacement in case of dire emergency, but there are no guarantees as to its performance. Because these FETs are "enhancement mode N‐channel" devices, they require positive gate‐to‐source bias voltage on each gate to cause source‐drain conduction. The quiescent Class AB idling bias current is set at 0.6 ampere for each half.
1 kW TTS1000B TRANSMITTER IPA ASSEMBLY 1.Set up a 50 V power supply, current limited to a little more than 1.2 amps. 2.Turn both bias potentiometers to their maximum resistance position. 3. SRF 3943‐2 Intermediate Power Amplifier: Remove both fuses. LB Setup, continued. 3.Apply the 50 V supply to one transistor at a time (one half package) and adjust the corresponding bias resistor for 600 mA drain current.
1 kW TTS1000B TRANSMITTER IPA ASSEMBLY 1 Sweep setup and response for IPA alone, without preamp.
1 kW TTS1000B TRANSMITTER IPA ASSEMBLY High Band IPA Circuit Description The IPA consists of two, source grounded N‐channel, insulated gate Field Effect Transistors (FETs) packaged in a single case, and operating in a push‐pull configuration in class AB. These N‐channel FETs are "enhancement mode" devices, so require a positive gate‐to‐source bias voltage on each gate to cause source‐drain conduction. Quiescent Class AB idling bias current is set at 0.6 ampere for each half.
1 kW TTS1000B TRANSMITTER IPA ASSEMBLY 2.Turn both bias potentiometers to their maximum resistance for minimum bias. Short C6 with a clip lead. This zero‐biases the "B" half so it does not interfere (via L9) with the "A" half being adjusted. 3.Apply the supply to the B+ terminals and adjust R1 bias‐adjust potentiometer for 600 mA drain current on side "A".
CONTENTS CONTROL AND METERING PANEL...........................................................................................................................1 TRANSMITTER CONTROL CIRCUIT BOARD ASSEMBLY ...........................................................................................1 Control and Metering Panel 40D1985G1 Transmitter control and monitoring is performed by the Control and Metering Panel.
• • J6 interconnects with J5 of the Metering board for the AGC, VSWR cutback, and VSWR shutdown signals. J7 is the connection to the AGC feedback input of the exciter. The transmitter interlock chain begins with the +12V at K1‐7. When K1 is set ON by energizing its coil K1‐1, contacts 7 and 12 close and contacts 7 and 10 open, turning off the LED inside the OFF button S4.
output rises, the DC voltage increases, and this increases the amount of attenuation, thus the output is maintained at a constant level. The AGC processing is done by analog op‐amp circuits in the Metering Board, but the initial threshold setting is done in the Control board from AGC switch S5 and AGC potentiometer R9. These simply provide an adjustable reference bias voltage to the AGC circuit, which adjusts the power output inversely according to this bias voltage.
VHF OUTPUT RF METERING & AGC CIRCUIT BOARD Contents: Sec 1 2 Topic Page RF Metering & AGC Board Description 1 RF Metering Board Test and Calibration 3 RF Metering & AGC Board 20B1299G3: Figures 1 and 2. This board serves several functions: AGC, VSWR supervision, forward & reflected power metering, and telemetry. Except for their functions and input names, metering boards have identical RF detectors.
VHF OUTPUT RF METERING & AGC CIRCUIT BOARD The output of U1‐7 (U2‐7) drives the RF power meter through R32 (R30) which set the meter deflection with a known RF signal. U1‐7 (U2‐7) drives the telemetry buffer U4 through R29 (R47) which are adjusted to calibrate the telemetry to a standard voltage with a known RF signal. Forward calibration is done with full rated power and a forward RF sample from the probe section applied to J1. R29 is adjusted for 3.
VHF OUTPUT RF METERING & AGC CIRCUIT BOARD after one or two occurrences. cause lockout. The third occurrence within a predetermined time (C20, R51) should If enough VSWR events within a short time, or one sustained occurrence, causes U5 to produce three pulses in rapid succession, C20 acquires a sufficient charge thru R51 to raise the voltage of pin 5 of comparator U6 higher than its reference voltage on pin 6, then Q5 will be driven HIGH which energizes relay K2, thus locking out the transmitter.
VHF OUTPUT RF METERING & AGC CIRCUIT BOARD Adjust R36 to cut back the output of the transmitter until the reflected power meter upper scale now reads 20 (2.0%). This is about 17 to 18 dB below the full forward power output of the transmitter. With the 16 dB pad still in circuit, adjust R42 until K1 energizes, and the "VSWR C/B" indicator LED on the Control Panel lights up. Replace the 16 dB pad with a 10 dB pad, and adjust R49 slowly until U3‐7 goes LOW, causing U5 to pulse.
