Larcan MXD1V Digital Television Broadcast translator User Manual part 1

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Date Submitted	2010-01-19 00:00:00
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1W VHF AMPLIFIER ASSEMBLY
Contents:
Part Topic Page,
1 Amplifier Chassis Description 94-1
2 PA Module ........ 94-2
3 Directional Couple 94-2
4 Bandpass Filters i 94-2
5 Metering Board 94~5
6 Power Supply 94-7
7 Basic Maintenance/Replacement Parts 94-7
8 Parts List....,...,..
List of Figures:
Figure Title Drawing Reference
944A Chassis Assembly Diagram (LOW BAND). .40D2180G1
94-18 Chassis Assembly Diagram (HIGH BAND). .400218062
94—2 Wiring and Block Diagram ,.,2ot32449
94-3 PA Module Assembly and Schematic Diagrams . 10A1453G8
10A1453sheet15
94-4 Directional Coupler Assembly and Schematic Diagrams ...... , 10A12SOG4
10A12523hee12
94-5 Bandpass Filter (HIGH BAND) 208704G1
94-6 Bandpass Filter (LOW BAND) 20871262
94-7 Metering Board Assembly, 208123565
94-8 Metering Board Schematic 2052448
1.0 Amplifier Chassis Assembly 40D2180G1 a. 2 Figure 944.
The Amplifier Chassis consists of a standard 19" rack mounted 3.5“, 2FlU enclosure containing a line filter,
a power supply, an amplifier module assembly, a directional coupler, metering board. bandpass filter and
a metering panel. its basic part number is 4002180. There are two groups in this assembly. Group 1
covers channels 7-13, while group 2 covers channels 2-6. The only difference between the two
assemblies is the output bandpass filter.
117VAC to the amplifier chassis comes in via a fused line filter, 2LF1. Pressing the ONIOFF switch to the
ON position (UP) applies AC to the primary of the power supplytranslormer, thus, applying DC power to
the amplifier stage and metering circuitry.
Metering is achieved using a directional coupler which samples the RF signal. It is fed to a peak detection
metering board which then displays the corresponding power level in percent on the front panel analog
meter.
The chassis is wired according to the wiring diagram, 2082449, shown on Figure 94-2.
Chassis parts lists are provided on the last pages of this manual. The circled numbers seen on the
assembly drawing correspond to the ‘symbol' item numbers on the parts list.
PU899-94 rev 0: June 14, 2000 94-1 1W VHF Amplifier
1W VHF AMPLIFIER ASSEMBLY
2.0 Power Amplifier Module Witt/15368: Figures 94-3
The Pie-amplifier module in it Iiullselvullvciy designed broadband amplifier which can operate on
frequencies between 54MH7 to 916MHz with a excellent gain OI about 18dB. It also has a very good return
loss and DC current stability with temperature
The RF output oi the Channel Processor or Exciter which is about +12dBm is fed to this amplifier which
then raises the level to 1Watt sync peak. This module requires no tuning nor bias adjustments. The
amplifier DC supply vottage is 24V, and it draws about 400mA (class A).
The amplifier module is broadband thus covering the entire VHF teIeViSion spectrum ranging from 54MHz
to 216MHz (channels 2-13).
3.0 VHF Directional Coupler Assembly 10A125064: Figure 94-4
The main tunction or the directional coupler is to supply FlF sample to the metering board such that the
lorward power can be monitored for the purpose ol metering. The RF sample level is about 14 dB below
1W which amounts to about AOmW. This signal is led to the metering board forward input. 5J‘l.
4.0 Bandpass Filters
4.1. GENERIC Helical Resonator Bandpass Fitter:
Many selective filter designs tor VHF applications are based on helical resonator tuned circuits to achieve the
required selectivity. As an introduction to the subject oi filters, we will examine helical resonators first.
The helical resonator was developed during the late 1950's and first described in "Proceedings of the IFtE"
magazine by W, Wt McAIpine and Ft. 0. Schildknecht, “Coaxial Resonators with Helical inner Conductor,“
Proceedings of the IRE, vol. 47, no. 12, pp. 2099-2105: December, 1959, The same authors later published
another magazine article “Helical Resonator Design Chart," Electronics, p, 140; 12 August 1960.
IRE stood for the “Institute of Radio Engineers“ which was responsible for some at the television transmission
standards that remain in use today. IRE later merged with the “American Institute at Electrical Engineers“ to
become the "Institute of Electrical and Electronic Engineers" which is known to us as the "l-triple-E" and which
continues publication of important electrical and electronic engineering research papers in the “Proceedings of
the IEEE“ and in the "IEEE Transactions" dealing with electrical and electronics interests.
We generally avoid such papers in our manuals except for the rare instance where critical inlormatton is involved,
as the content oi most ol these publications are considered to be excessively arcane and esoteric tor the
beleaguered technician vmose sole interest is to get the transmitter back on the air. Should you wish turther
inlcrmation, we refer you to the above cited publications; to "Reference Data for Fladio Engineers, sixth edition"
published by Howard W. Sams & Co.; to "The Radio Handbook" which is specific to amateur radio applications
and published by Editors and Engineers; and to the "ARFlL Fladio Amateufs Handbook"published annually by
the American Radio Relay League. The latter also publishes "CST" magazine every month, and back issues
at this publication may also be available in your public library system.
LAFtCAN bandpass filter implementations lor the VHF channels, generally consist ol a cascaded series of
coupled resonators. Some use helical resonators, 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 clrcuited except for a small trimmer capacitance to ground. The dimensions of the coil are critical
to the frequency of operation: the assembly behaves as though it were a quarter wave coaxial transmission line
resonator.
The desired response shape is presented as Figure 1 below, and the tilter electrical equivalents are presented
on the next page as Figure 2. when we examine the assembly, and take capacitances into account, the
Fuses-94 rev 0: June 14, 2000 944 1w VHF Amplifier
1W VHF AMPLIFIER ASSEMBLY
equivalent circuit at a helical resonator becomes simply a parallel resonant LC tank circuit having low (trimmer)
capacrtance and relatively high inductance. Adjustment ot the trimmer produces a change of capacitance. and
the trimmer‘s moveable slug is shaped to appear as a shorted turn, which is coupled to the helix and theretore
alters its inductance at the same time.
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 at capacitors, and is made up of the small
capacitance between the free ends of the coils, controllable by the amount of capacitance to ground that is
introduced by the coupling adiustment screws; the coupling is maximum when the screws are backed out iully
from the enclosure. Shielding partitions placed inside the enclosure between helixes. produce tixed area
apertures which affect the coupling capacitance between helixes. Helix #3 in the Figure 3 drawing has taller
partitions on both sides of it, giving lower capacitance and less coupling than the others.
For system use, the tuning and coupling is adiusted tor a flat topped response with sleep sides. and the desired
shape typically is such that fv - 4.5 MHz and iv + 9.0 MHz are both 30 dB down. but the carriers must be iv less
than 0.6 dB and hi less than 07 dB departure from flatness. Input and output return loss must be 20 dB or better
over the mil 6 MHz bandwidth. These idyllic sweep curves are shown below.
fv94 rev 0: June 14, 2000 94—4 1W VHF Amplifier
1W VHF AMPLIFIER ASSEMBLY
it is possible to use a sweep generator and detector for setting the response of any tiller. but an accurate 50 ohm
return—loss bridge must he used with the sweep generator for setting input and output matching.
4.3 Bandpass Filler Assembly for Low Band 20871262: Figure 94-6
The schematic indicates this filter is a little different from the helical resonator types discussed above. In those,
the resonant elements are between Signal nodes and ground. and coupled each to the next with small
capacitances; the low band filter resonant elements are in series with the signal and coupling is done through
currents flowing in common through the shunt capacitors to ground.
For the past fifty years at least, filters have been designed using so-called "image parameter theory". and each
section of this one can be thought of as "m-derived series bandpass type I" arranged three in a pi-coniiguretion.
with the ends terminated by shunt capacitors “Image parameters design" works well for applications in vimich
close approximations are acceptable, especially as in the present application where all filter elements can be
adiusted to precise values.
Oversimplifying the explanation, basically the three tuned circuits L1-CS, L206, and La-co are resonant to one
frequency in the center of the passband, The coupling between them is provided by 04-05 and 07-03 This
coupling is tighter than so—called “critical" coupling, and the end result is a bandpass curve that looks similar to
that for the High Band helical resonator filter discussed earlier in this section.
L4
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