home
***
CD-ROM
|
disk
|
FTP
|
other
***
search
/
HamCall (October 1991)
/
HamCall (Whitehall Publishing)(1991).bin
/
bcast
/
miscbcst
/
scabasic.txt
< prev
next >
Wrap
Text File
|
1990-10-14
|
12KB
|
228 lines
*********************************************************************
* NOTE: The following ASCII text file (without graphics) *
* is contained in a printed technical paper available *
* from Broadcast Electronics Inc. Unfortunately, it *
* was not possible to reproduce the graphics portions *
* of this paper within this text file. If you find the *
* information in this file of interest, you may request *
* a complimentary, printed, copy including figures and *
* graphics from: BROADCAST ELECTRONICS INC. *
* P.O. BOX 3606 *
* 4100 N. 24TH STREET *
* QUINCY, IL. 62305-3606 *
* ATTN: SALES DEPARTMENT *
* PH 217-224-9600 *
* FAX 217-224-9607 *
* *
* The contents of this technical paper are *
* Copyrighted (c) 1984, by Broadcast Electronics Inc. *
* All rights reserved. *
*********************************************************************
SCA BASICS
BY:
Geoffrey N. Mendenhall, P.E.
Vice President - Engineering
Broadcast Electronics Inc.
Quincy, Illinois
SCA OVERVIEW
The recent de-regulation of broadcast SCA usage has spurred a new interest
in using the SCA for data transmission and paging. SCA stands for "Subsidiary
Communications Authorization", which allows FM broadcast stations to
"piggy-back" up to two additional audio or data channels on the main carrier of
the station.
This article will explain the basic theory behind SCA transmission and will
also describe the equipment required to add an SCA to the FM broadcast signal
HOW SCA's ARE ADDED TO THE BROADCAST SIGNAL
SCA's are added to the broadcast FM carrier by a technique called frequency
domain multiplexing which allows the additional subchannels to be separated from
each other and from the main channel by use of specific frequency bands for each
subchannel. In the case of an FM station broadcasting in stereo with two SCA's,
the frequencies used are 67KHz and 92KHz for the two subcarriers. The main
channel and stereophonic information are transmitted in the frequency band
extending from 30Hz to 53KHz while the SCA information is transmitted above this
frequency range in a band extending from 54KHz to 99KHz. The SCA subcarriers
each modulate the main FM carrier by a maximum of 10% of the total modulation.
This means that the effective coverage will not be as great as the main channel
since the SCA does not have full use of the transmitted power. The sum of all
the different components being transmitted is called the composite baseband. It
includes the following individual components as shown in figure 1A.
(FIGURE 1A)
Figure 1B shows what the composite baseband looks like if viewed on an
oscilloscope with the peak-to-peak amplitude shown as a function of time. It is
difficult to identify the various components as a function of time.
Figure 1C shows the composite baseband as viewed on a low frequency spectrum
analyzer. This is a representation of amplitude as a function of frequency. It
is now easy to identify the various frequency components within the composite
baseband.
(FIGURE 1B AND FIGURE 1C)
The method of modulation used is frequency modulation of the SCA subcarrier
itself which in turn frequency modulates the main transmitter carrier. This
type of "FM on FM" system is difficult for many people to fully understand.
Think of it this way, the main carrier is deviated a constant plus and minus
7.5KHz (10% of the total) by the 67kHz subcarrier while the 67KHz subcarrier is
itself deviated up to plus and minus 6KHz by the audio or data being fed into
the SCA modulator. The key points to remember are that the deviation of the
main carrier is dependent only on the level of the subcarrier and is not
effected by the modulation applied to the subcarrier while the deviation of the
subcarrier is dependent only on the modulation applied to the subcarrier and is
not related to main channel deviation.
RECEIVING THE SCA
The SCA receiver must first FM demodulate the entire baseband, filter out
the desired subcarrier from the rest of the baseband components, and then FM
demodulate the SCA information. This requires two IF strips and two FM
demodulators. The first IF strip is usually operated at 10.7 MHz with a
bandwidth of about 250 KHz while the second IF strip is located at the SCA
frequency (67 KHz or 92 KHz) with a narrow bandwidth (typically 25KHz or less).
The first FM demodulator is usually a quadrature or pulse counting type while
the second FM demodulator is usually a Phase-Locked-Loop to minimize
interference from the main and stereo channels.
This dual demodulation process places some limitations on the performance of
the SCA receiver. The amplitude and phase response of the IF filters will have
a significant effect on the crosstalk into the SCA from the main and stereo
channels. This type of receiving system is also more susceptible to multipath
than a simple FM system.
The SCA subcarrier being received only has access to a maximum of 10% of the
transmitter power. The additional noise factors introduced by the "FM on FM"
nature of the system cause further degradation of the signal to noise ratio
which may yield a total penalty in coverage of more than 20db when compared to
main channel.
TRANSMITTING THE SCA
The SCA is added to the broadcast signal by connecting an SCA generator to
the FM exciter located in the FM broadcast transmitter.
The SCA generator is usually a self contained device that accepts audio
and/or DC coupled data inputs and provides a frequency modulated subcarrier (67
KHz or 92 KHz) at its output. The SCA generator may also have additional
features which allow automatic muting of the SCA transmission under certain
conditions, multiple inputs, active filtering of the inputs, and modulation
indication. Since the SCA generator is located at the broadcast transmitter, it
is usually supplied by the broadcaster.
The FM exciter must have a wideband SCA input to accept the subcarrier from
the SCA generator and add this signal to the rest of the baseband for FM
modulation onto the main radio frequency carrier.
In order to prevent interference among the main channel, the stereo
channel, and the SCA the exciter should be a state-of-the-art unit employing an
ultra-linear modulator. This type of modulator will minimize the generation of
intermodulation and harmonic products within the baseband. This is important
because non-linearity within the modulation process will alter the composition
of the baseband, which results in distortion of the demodulated signal at the
receiver.
Modulator linearization has reduced harmonic and intermodulation distortion
to less than 0.05% in newly-developed equipment.
Any distortion of the baseband signal caused by the modulated oscillator
will have secondary effects on stereo and SCA crosstalk, which are quite
noticeable at the receiver in spite of the rather small amounts of distortion to
the baseband. For example, if the harmonic distortion to the baseband is
increased from 0.05% to 1.0%, as much as 26dB additional crosstalk into the SCA
can be expected. This amount of crosstalk would seriously degrade the
performance and utility of the SCA.
Assuring that the composite baseband signal undergoes minimal distortion in
the modulation process will suppress undesired harmonic and intermodulation
products in the baseband, making the FM exciter transparent to the SCA signal
coupled into it.
The stereo generator's characteristics play an important role in preventing
interference to the SCA. The second harmonic components of the stereophonic
subcarrier fall directly on top of the SCA so it is important to use an stereo
generator that suppresses these spurious components.
The bandwidth of the FM transmitter and antenna system will also affect the
performance of the SCA. The distortion in any practical FM system will depend
on the amount of bandwidth available versus the modulation index being
transmitted. Relating the specific quantitative effect of bandwidth limitations
imposed by a particular transmitter to the actual interference to the SCA is a
complicated problem indeed. Some factors which will improve the situation are:
1. Maximize bandwidth by using a broadband exciter and a
broadband IPA stage.
2. Use a transmitter of single tube design or broadband
solid state design where feasible.
3. Optimize both the grid circuit and plate circuit tuning
for best possible bandwidth.
4. Use a broadband antenna system with low standing wave
ratio on the transmission line.
ADJUSTING THE TRANSMITTER FOR BEST SCA PERFORMANCE
Start out by remembering that all optimization should be done with the
transmitter connected to the normal antenna system rather than to a dummy load.
The transmitter is first tuned for normal output power and proper
efficiency. A simple method for centering the transmitter passband on the
carrier frequency involves adjustment for minimum synchronous AM. Synchronous
AM is AM modulation of the carrier caused by frequency modulation of the carrier
frequency. If the bandwidth is narrow or skewed, increasing amplitude
modulation of the carrier will result.
A typical adjustment procedure is to FM modulate 100% at 400 Hz in the
monaural mode and fine tune the transmitter for minimum 400 Hz amplitude
modulation as detected by a wideband envelope detector. At the optimum point,
the residual demodulated AM will double in frequency to 800 Hz. It should be
possible to minimize synchronous AM while maintaining output power and
efficiency.
Transmitter tuning becomes very critical when minimizing crosstalk into the
SCA. A more sensitive adjustment procedure involves modulating one channel only
on the stereo generator to 100% with a 4.5 KHz tone. This will place the lower
second order (L-R) stereo sideband on top of the 67 KHz SCA. Activate the SCA
at normal injection level without modulation on the SCA. Tune the transmitter
for minimum output from the SCA demodulator. A similar adjustment can also be
made by listening to the residual crosstalk into an unmodulated SCA while normal
stereo programming is being broadcast. In any of these tests, the grid tuning
is frequently more critical than the plate tuning. This is because the
impedance match into the input capacitance of the grid becomes the bandwidth
limiting factor.
BRINGING IT ALL TOGETHER
SCA's do offer an important new opportunity to transmit information over
existing FM broadcast facilities. Whether SCA's are used primarily for paging,
data transmission to fixed points, or other uses remains to be seen. In any
case, it is important that each subsystem be individually optimized before the
complete transmission and reception system can perform properly. The complexity
and technical limitations of the SCA system makes attention to details critical
to the success of the operation.