Using Other References
As a quick review, to simulcast two or more transmitters requires the following two factors:
(a) identical equipment, stable enough to maintain transmitter frequency within less than 1 Hertz [cycle] and,
(b) modulate the transmitter with the exact same information at the exact same time.
achieve frequency stable transmitters, most systems rely upon GPS
Disciplined Oscillators that maintain their accuracy within 0.001parts
per billion [ppb] or effectively 0.001 Hz at 1,000 MHz continuously.
This is an expensive option as most decent GPS DOs cost $500 to
$1,000 each. A
Disciplined Oscillator is required as the GPS signal is not 100%
reliable and when the GPS cannot maintain a stable timing output, the
simulcast transmitters would lose stability and interfere with each
However, other references will work just as well,
depending upon the the type of modulation [AM and/or FM] and
whether it is analog or digital in nature. This will be covered in
greater detail below.
The information necessary to modulate
the transmitters can be transported to each site by a dedicated radio
channel that covers the system operating area, or by using a public or
private data network like the Internet. Often times, the
information transport method can heavily influence the stability Reference issue.
Before we list some practical examples, let us review some of
the choices we have available for transport and stability.
AM broadcast station
- With more than 60 "clear channel" stations operating with
10,000 to 50,000 watts 24 hours daily, one can be found in every
community. Their carrier would provide a good reference for a stable
transmitter, at any frequency. Using a tuned radio frequency [TRF]
receiver with a crystal filter, the carrier can be limited, buffered
and divided to a suitable reference frequency like 10 kHz, for phase
72-76 MHz Fixed Band
- With more than 68 single wide-band 20 kHz channels available in all
services, including common carrier, these channels can be licensed to
provide a stable reference as well as information transport. These
frequencies can be licensed at 50 watts or more, but rarely needs more
than a few watts to maintain a reliable signal over a wide operating
area. A TRF could work to derive a stable reference, but this is excellent for transport.
Point to Multi-point above 927 MHz in Part 101 - More than 16 single channels at 956 MHz. and 85 duplex pairs available
in all services. All channels are
narrow-band and some can be combined for greater
bandwidth. These work well for transport, but may only work well for a
stability reference using a modulated tone. When using a
modulated tone, use a frequency reference to generate the tone at least
10 times better than required at your operating radio frequency. A one or two second pulse can be added, if required to
time the information to be modulated.
Some Other Always Available Signals
- Other references may be available, such as an FM
station sub-carrier or WWV at 5, 10, 15 MHz or WWB at 60 kHz.
There are lots of other signals out there.
Wide-area Paging or Information Delivery System
- Private as well as common-carrier paging frequencies are available in
the 928-932 MHz range for delivery of information or alerts to
mobile receivers. This could include traditional pocket
pagers delivering voice or data, or large or small information
billboards, providing timely information to the public.
Campus-Wide FM Radio Station
- Using unlicensed low-power [AM or FM] transmitters in each building
throughout the campus. Distribute the audio using your LAN.
Recreation Area, Beach Town or Small Community AM Station
- Combine several city or county licensed Travelers Advisory stations
together to cover a wider area than available with a single
Revised May 15, 2018