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 GENERAL 

DESCRIPTION

 

 

 

TSM 20-275D rev 0:    Jul 1, 2010

 

2

 DTT250M

 

 DTT250M 

– 250W Digital Television Transmitter

 

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.

 

 

 

nable

 

J5

‐

5.

 

 

 

All

 

control

 

wiring

 

of

 

the

 

transmitter

 

passes

 

through

 

a

 

control

 

circuit

 

board

 

(prefix

 

5B),

 

and

 

facilities

 

are

 

provided

 

on

 

this

 

board

 

for

 

telemetry,

 

status,

 

and

 

control

 

connections

 

to

 

and

 

from

 

a

 

remote

 

control

 

system.

   

These

 

are

 

available

 

on

 

15

 

contact

 

D

‐

shell

 

connector

 

J5.

 

For

 

local

 

operation,

 

simply

 

place

 

the

 

LOC

‐

REM

 

switch

 

in

 

the

 

LOC

 

position.

   

For

 

remote

 

control

 

operation

 

the

 

LOC

‐

REM

 

switch

 

must

 

be

 

in

 

the

 

REM

 

position.

   

This

 

places

 

+12V

 

on

 

Remote

 

E

 

The

 

Remote

 

Enable

 

+12V

 

appears

 

as

 

an

 

arming

 

signal

 

at

 

J5

‐

5,

 

and

 

the

 

momentary

 

connection

 

of

 

this

 

+12V

 

to

 

J5

‐

13

 

turns

 

the

 

transmitter

 

ON,

 

and

 

momentary

 

connection

 

of

 

the

 

+12V

 

to

 

J5

‐

8

 

turns

 

the

 

transmitter

 

OFF.

 

 

The

 

transmitter

 

control

 

and

 

interlock

 

wiring

 

is

 

also

 

brought

 

out

 

on

 

J3,

 

which

 

is

 

provided

 

with

 

a

 

terminal

 

block

 

style

 

of

 

connector

 

interface.

   

Remote

 

Enable,

 

Remote

 

On,

 

Remote

 

Off,

 

and

 

External

 

Interlocks

 

1

and

 

2

 

are

 

all

 

brought

 

out

 

on

 

J3

 

for

 

connection

 

as

 

required.

 

 

A

 

thermostat

 

is

 

provided

 

in

 

the

 

PA

 

heatsink

 

to

 

open

 

the

 

interlock

 

chain

 

should

 

an

 

unlikely

 

overheating

 

condition

 

occur.

 

Summary of Contents for DTT250M

Page 1: ...lid state amplifier This amplifier requires no tuning or adjustment within its band of operation Simplicity of operation reduced maintenance costs and increased reliability are a few of the major benefits derived from this modular amplifier This module is operated well below its maximum ratings The amplifier chain consists of three stages of amplification The preamplifier stage is a high gain broa...

Page 2: ... on 15 contact D shell connector J5 For local operation simply place the LOC REM switch in the LOC position For remote control operation the LOC REM switch must be in the REM position This places 12V on Remote E The Remote Enable 12V appears as an arming signal at J5 5 and the momentary connection of this 12V to J5 13 turns the transmitter ON and momentary connection of the 12V to J5 8 turns the t...

Page 3: ...CONTENTS 1 BANDPASS FILTER 2 2 RF DIRECTIONAL COUPLER 4 PUB96 26 Rev 1 September 13 2005 26 1 RF Output BP Filter Directional Coupler ...

Page 4: ...the trimmer produces a change of capacitance and the trimmer s moveable slug is shaped to appear as a shorted turn which alters the inductance of the helix Matching from and to 50 ohm transmission lines is accomplished with taps on the input and output helixes Coupling between sections is electrically a bridged T network of capacitors and is made up of the small capacitance between the free ends o...

Page 5: ...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 ...

Page 6: ...ignated L1 through L4 They are in reality short pieces of Teflon sleeved magnet wire which although they may possess a fraction of a nanohenry of inductance are mainly small capacitors which are factory adjusted by bending the wire to control the amount of coupling capacitance between the transmission line and the sampling loop concerned The position of the capacitor along the loop does not seem t...

Page 7: ...Figure 3 Quad Directional Coupler Equivalent Schematic PUB96 26 Rev 1 September 13 2005 26 5 RF Output BP Filter Directional Coupler ...

Page 8: ...ER LOW BAND PUB96 28 Rev 2 Aug 2007 PA Module CONTENTS FUNCTIONAL DESCRIPTION 1 6 WAY SPLITTER INPUT BOARD 1 FET RF AMPLIFIERS 1 6 WAY COMBINER OUTPUT BOARD 2 VSWR CONTROL BOARD 2 GREEN LED SENSITIVITY ADJUSTMENT 5 ...

Page 9: ... provides input matching for the transition from the input connector to the input transmission line A built in detector CR102 and C147 is fed from a directional coupler on the input transmission line to provide a sample of the input signal for module gain monitoring R117 and R118 terminate the directional coupler FET RF Amplifiers References Figure 3 Figure 4 Figure 5 Figure 6 Each of the six ampl...

Page 10: ...se will blow in this case isolating the defective device from the rest of the module and transmitter power supply allowing the remaining devices to keep operating normally A blown fuse can serve as a valuable troubleshooting aid when trying to identify failed devices 6 Way Combiner Output Board Part number 40D1472G1 40D1472G2 References Figure 6 and Figure 3 The six amplifier outputs are applied t...

Page 11: ...s the control voltage set on pin 6 the output on pin 7 will go high R22 C10 and CR1 provide a fast attack slow release control voltage to Q2 when a high VSWR condition suddenly occurs This will turn on Q2 which turns on Q4 which quickly reduces the bias applied to the power amplifier FETs this reduced bias also reduces their gain and therefore their RF output and keeps the amplifier at safe levels...

Page 12: ...each companion bias potentiometer in the same manner for the proper bias current Install remaining fuses and remove the bias short after all bias adjustments have been made Low power sweep of amplifiers Note Low power sweep of PA modules should not be required under normal circumstances even when replacing FET devices There are no tuning adjustments on these modules Ensure that terminations are in...

Page 13: ...sition and adjust the exciter output power until the transmitter output power reads 110 Remove the module to be set up and remove the two front fuses from it in order to simulate a single FET package failure Replace this crippled module in the transmitter and apply a nominal 50 APL staircase video signal to the transmitter analog transmitters The green LED should now be extinguished if it is not r...

Page 14: ...f 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 ...

Page 15: ...W HIGH BAND 40D1493G3 PUB96 29 Rev 2 i 24 Oct 2008 CONTENTS FUNCTIONAL DESCRIPTION 1 6 WAY SPLITTER INPUT BOARD 1 FET RF AMPLIFIERS 1 6 WAY COMBINER OUTPUT BOARD 2 VSWR CONTROL BOARD G1 2 GREEN LED SENSITIVITY ADJUSTMENT 5 ...

Page 16: ...mission line A built in detector CR102 and C147 is fed from a directional coupler on the input transmission line to provide a sample of the input signal for module gain monitoring R113 and R117 terminate the directional coupler FET RF Amplifiers Drawing References Figure 3 through Figure 6 Each of the six amplifiers in the module consists of two source grounded N channel insulated gate Field Effec...

Page 17: ...of active devices such as these is short circuit and the fuse will blow in this case isolating the defective device from the rest of the module and transmitter power supply allowing the remaining devices to keep operating normally A blown fuse can serve as a valuable troubleshooting aid when trying to identify failed devices 6 Way Combiner Output Board Part number 40D1468G1 Drawing References Figu...

Page 18: ...ected and sensed at pin 11 of J1 to comparator circuit U2B R21 sets the level at which VSWR protection begins If the level of detected reflected power on pin 5 of U2B exceeds the control voltage set on pin 6 the output on pin 7 will go high R22 C10 and CR1 provide a fast attack slow release control voltage to Q2 when a high VSWR condition suddenly occurs This will turn on Q2 which turns on Q4 whic...

Page 19: ...mA for digital operation Remove the fuse Remove the side B bias short and place it on side A at the junction of C6 R2 R3 and R9 Place the fuse in side B Adjust the side B bias pot for the proper current Move the fuse to the remaining fuse holders one at a time and adjust each companion bias potentiometer in the same manner for the proper bias current Install remaining fuses and remove the bias sho...

Page 20: ...er output power until the transmitter output power reads 110 Remove the module to be set up and remove the two front fuses from it in order to simulate a single FET package failure Replace this crippled module in the transmitter and apply a nominal 50 APL staircase video signal to the transmitter analog transmitters The green LED should now be extinguished if it is not remove the module and adjust...

Page 21: ...ed 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 ...

Page 22: ... gain control of the input to each chain It therefore has components in place for adjustment of RF gain and phase to enable setting up these paralleled transmitters In a single chain transmitter such as the TTS1000B no requirement exists for control of RF phase nor consequently its components but our design standardization results in lower overall expense being incurred by simply leaving the compo...

Page 23: ...e overall system gain is sufficient to result in overdrive of later stages of the transmitter The transmitter or translator lineup may therefore include an in line attenuator in the RF chain ahead of the IPA module in order to prevent overdrive from certain models of exciter modulator U3 is a voltage regulator providing B to the amplifier and biasing for the varactor diodes RF isolation is provide...

Page 24: ... the values of R3 and R6 which have little or no effect at RF 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 C7 C20 and C21 values their series inductances and that of board traces also ensures effective bypassing at critical frequencies The output matching π network c...

Page 25: ...readjusted during system test for minimum intermods and FM noise 4 Connect a 30 dB 20 W attenuator to the output of the amplifier This will absorb amplifier output and protect the sweep detector Use it instead of the 20 dB pad shown in the diagram below 5 Reinstall fuses and apply B to both supply connections of the amplifier module 6 Apply a low level sweep to the amplifier and measure the DC inp...

Page 26: ...PUB96 30 rev 1 Jul 1 2010 30 5 1 kW IPA Assembly VHF 1 kW TTS1000B TRANSMITTER IPA ASSEMBLY 1 Sweep setup and response for IPA alone without preamp ...

Page 27: ...n applied to the gates The gate impedance at the operating frequency is much lower than R3 and R5 so these resistors have little or no effect at RF R3 and R5 provide a DC path for bias and provide loading at lower frequencies in order to assist in maintaining amplifier stability The choice of C2 and C6 values and their internal equivalent series inductances also ensures effective bypassing at crit...

Page 28: ... These will be the starting points the bias current settings and L9 will be readjusted during system test for minimum intermods and FM noise Disconnect the clip lead after adjusting bias currents 4 Connect a 30 dB 20 W attenuator to the output of the amplifier 5 Apply B to both supply connections of the amplifier 6 Apply a low level sweep to the module and measure the DC input current about 1 2 am...

Page 29: ...ent to cause the transmitter power output to decrease or even to momentarily turn off the transmitter The transmitter is provided with a VSWR cutback function that either reduces its power output to save it from harm in the event of gradual occurrences such as antenna ice build up or momentarily takes it off the air from random events Three VSWR events occurring rapidly in a short time will cause ...

Page 30: ...emote control of this fact through J5 7 Next stop for the 12V is a normally closed VSWR lockout relay contact connected via J1 5 and J1 9 from the Metering Board Prefix 5A K2 This relay operates and the interlock chain is opened if for some reason the transmitter has seen a large amount of reflected power and the Metering Board VSWR supervisory circuit has repeatedly tried and retried to keep the ...

Page 31: ...f a VSWR that exceeds a preset amount the AGC voltage becomes modified a little to reduce the transmitter output by an amount proportional to the reflected signal This VSWR Cutback permits the transmitter to remain on the air at reduced power if the antenna should gradually accumulate a layer of ice If the reflected power should exceed a much larger amount causing repeated momentary tripping off a...

Page 32: ...nificantly attenuates 3 58 MHz NTSC color subcarrier as well as any 4 5 MHz intercarrier that may be generated in CR1 or CR2 due to the presence of visual and aural RF signals together in the system Removal of these subcarrier components before the signal is peak detected enables the circuit to be responsive to sync peak pow only for visual or just CW aural power and relatively immune to undesired...

Page 33: ...tback mode which is what will need to be set up because it affects the exciter AGC and VSWR supervision The output from U1 U2 pin 7 is also applied to a second op amp U1 U2 pin 3 If the detected level at U1 pin 3 rises above the level set by the AGC pot on the Control board at U1 pin 2 the output on pin 1 will rise This AGC output is applied via J5 1 to a final buffer amplifier U2A on the Control ...

Page 34: ... TP1 drops to a minimum approximately 10 to 20 mVDC A DC coupled scope will make the adjustment easier to see the objective is to place the U1 output as near the op amp ground rail as possible without the op amp going into saturation Turning the pot farther will decrease the sensitivity of the system for small signals Once this minimum voltage has been reached do not re Reflect Power Meter Calibra...

Page 35: ...nergizes and the VSWR C B indicator LED on ulse After three pulses visible on the meter lock out and a red VSWR L O indication on the ontrol Panel should occur Check that RESET is possible using the RESET button S6 on the Control Panel A reads 20 2 0 This is about 17 to 18 dB below the full forward power output of the transmitter W the Control Panel lights up Replace the 16 dB pad with a 10 dB pad...

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