check the current page or check for previous versions at the Internet Archive. Yahoo! is not affiliated with the authors of this page or responsible for its content.
Untitled

2
10% aural in NTSC service and 33kW
average/130kW peak in DTV service)
than supported by traditional, early
designs. In particular, this higher power
capability leads to better linearity and
more stable long-term performance.

Not only do these new IOT products
provide powers greatly exceeding the
earlier variety, but are much simpler by
design to handle and maintain. These
units are true plug-in designs that can
be easily serviced and changed by one
person, a feature taken full advantage of
in the Quantum series by the providing
of a built-in hoist mechanism for the
IOT. The basic elements of the EEV
plug-in IOT assembly are shown below.

To further support the wishes for a
reliable and maintainable solution, a
new approach to the general
organization of the HPA cabinet was
employed. In addition to placing the
many support elements (power supplies,
control units, etc.) in close proximity to
the supported device, the cooling for the
entire HPA was centralized. Specifically,
the use of multiple localized muffin fan
air distribution devices were eliminated.
In its place, a system of ducted, directed
air was implemented to provide
generous amounts of cooling for
individual elements (such as power
supplies) while allowing for the general
needs of flushing the entire HPA with
monitored filtered air in a pressurized
cabinet design. This positive pressure
technique is essential to minimize or
eliminate the ingress of dust and dirt.

This unconventional approach yields
greatly increased system reliability, and
overcomes the often-troubling random
failures of discreet cooling devices that
can lead to
major transmitter failures
.

This greatly simplifies the maintenance
requirements of the complete system,
and relegates most of the HPA cabinet
cooling servicing to
regularly scheduled
activity. Additio
nally, with differential
pressure monitoring of the HPA central
air system filtering and
flow, the general
well being of the HPAs cooling (and
other) support systems are continuously
monitored and displayed. This is done i
n
conjunction with the advanced
monitoring capabilities providing
trending analysis of parameters such as
blower motor bearing operating
temperature.



High Performance LDMOS IPAs

The drive requirements for this new
series of plug-in IOTs are somewhat
higher than the earlier class of tubes.
This is driven by the internal physics of
tube technologies that affect the
overall
tube gain and stability. This requirement
was easily addressed by advanced
L
DMOS technologies. It was desirable
to have a single class of driver amplifier
devices to cover the entire UHF band.
By looking at the combined needs of
NTSC and DTV, an integrated high gain
driver amplifier stage was chosen that

could be paralleled for the required drive
power levels. This single stage meets
the requirements of providing the peak
powers of NTSC common-amplification
(visual and aural combined) and the
high peak to average powers of DTV,
Input Cavity
Magnet
Primary
Output Cavity
RF Input
Secondary
Output Cavity
RF Output
IOT
3
while capable of delivering the stable
and continuous aver
age powers of both.

Past experience with IOT transmitters
dictated the need to provide effective
AGC for the entire gain block inclusive
of the IOT, and such full loop
provisioning is accomplished within the
LDMOS driver amplifier AGC circuit.
The stability provided by this loop allows
for efficient control of correction and
long-term in spec operational
characteristics.


Packaging of the driver(s) close to the
IOT
minimizes cable losses
, and allows
the cooling to be easily provided by the
centralized blower system. It also makes
the unit(s) fully accessible for simple
hot-swappable replacement and/or
maintenance.
Redundant supplies for
each driver amplifier add to the general
maintainability
of the system.



Central Access Control/Monitoring

Another concern raised by many
Broadcasters,
was the need for a
minimum amount of specialized
measurement tools and internal cabinet
level access f
or operator intervention
during typical maintenance and
operating situations.

This is facilitated by providing an
advanced
central interface for
adjustments, calibrations and
measurements. A digital interface is
provided whi
ch allows users access to a
set of digital screwdrivers for effecting
HPA parameter and monitoring
adjustments through a flexible industry

proven front-panel controller (below).
This alpha/numeric control unit is menu
driven, and provides various levels of
access to operating parameters and
setup. It is capable of providing in-depth
information to compliment trouble-
shooting and other activities.

The same front panel controller, with
analog meters, provides a rapid
indication of critical transmitter operating
parameters such as power, beam
current and high voltage.



Rugged, Robust IOT support
System

A reliable, high power IOT system is of
no use unless the direct support
systems provide for the same level of
service. For this reason, careful
selection of these elements has
provided field proven solutions.

While so
lid-state switching low voltage
DC supplies ideally suit specific
requirements for stable long term,
service,
this does not hold true for
critical IOT suppo
rt devices.
Given the
potential for extremely harsh AC
conditions to exist because of both
normal and abnormal IOT operating
conditions, it was deemed imperative to
make maximum use of linear power
supplies in this critical path. While there
are cost and size constraints imposed
by such reasoning, the result is a proper
balance of technologies required to
provide long-term reliability in most
every conceivable operating condition
and fault.

High MTBF
(Mean Time Between
Failures) unitized
beam supplies have
been chosen to provide the
primary beam power. These oil-
filled units are ideally suited for
the variety of operative
environmental conditions of
most every transmitter
installation. While other choices
4
exist for special
cases, these
units have a
proven history of
providing long-
running reliable
outdoor service
in this and other
industries.

To provide an enhanced level of
protection for both the IOT and its
support systems,
a unique primary AC
fast disconnect solid-state SCR step-
start is employed exclusively designed
by Acrodyne for the Quantum series.
This provides for both
the stepped
application of AC to minimize the inrush
current at turn-on, and rapid removal of
the beam supply primary voltage. Its
rapid removal of primary AC under
crowbar and/or related HV fault
conditions eliminates the need for
mechanical breaker resets, simplifies
the support system, and otherwise limits
the number of critical elements in the
beam supply voltage path.

Use of linear supplies for the heater/bias
requirements was essential to be able to
survive the sometimes otherwise
catastrophic HV events of every high
power tube system.

To
enhance
the overall safety of the
system, a high reliability mechanical AC
isolation and HV grounding (IEC 215
compliant) system is provided. Such a
system provides for improved safety, as
well as being an aid in system level
trouble-shooting.


State-of-the-Art Digital Signal
Processing and Correction

Acrodyne understands the industries
need for complete All Digital solutions
in the DTV era. Using Industry
Standard digital implementations, with
a mix of proprietary complimentary
fixed correction techniques, a solution
with unsurpassed performance and
value has been derived.

A proprietary fixed complimentary
corrector is used to provide for nominal
correction requirements of the complete
DTV RF system (active and passive). It
also allows for nominal correction in
special applications for adjacent channel
combiners, sharp tuned filters, etc. The
inclusion of this proprietary approach
allows the active digital heart of the
system to apply more filter tap energy
for optimum system
correction, thus
resulting in very high SNR performance.

The digital heart of
this ATSC compliant
DTV system is the
Zenith Digital Adaptive modulator. This
is a 6MHz ATSC Trellis coded
modulator with a standardized SMPTE-
310 input. This
active digital
DTV pre-
corrector
provides for
non-linear and linear (amplitude and
group delay) compensation of the entire
system. It is an active system designed
to provide for automatic or instructed
s