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*********************************************************************
* 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. *
*********************************************************************
SECOND GENERATION TECHNIQUES
FOR AM STEREO EXCITER DESIGN
Edward J. Anthony
Broadcast Electronics, Inc.
Quincy, Illinois
Published: 9-13-84
I. INTRODUCTION.
With the introduction of AM stereo to the broadcast industry, a new trans-
mission mode has been defined with a new set of complex and unique problems to
be overcome. Not only are there multiple systems available to transmit stereo
due to the FCC marketplace decision, but within each system it is possible to
improve the design quality and stereo performance.
During the infancy of AM stereo, there were only the system proponents man-
ufacturing their own equipment. This first generation hardware was not
broadcast quality and often fell short in producing the best possible
performance for its particular system. Controls for alignment and operation
were frequently inaccessible. Today there is a great need for more flexible and
reliable hardware.
Motorola, Inc., the inventor of the C-QUAM* system, has licensed several
experienced broadcast equipment manufacturers to fill this need. This
presentation will review some of the improvements and new approaches developed
by Broadcast Electronics, Inc. during the design effort for the AX-10 exciter:
an all new, second generation C-QUAM AM stereo system. (* C-QUAM is a
registered trademark of Motorola, Inc.)
II. SECOND GENERATION REFINEMENTS.
After reviewing the currently available hardware for AM stereo, several
areas for improvement were discovered. Many of these improvements were based on
state of the art design techniques employed in the BE FX-30 FM exciter and the
BE FS-30 FM Stereo Generator. Others were new innovations developed to improve
the C-QUAM system performance. These refinements include:
1. Digital, independent IF modulation technique.
2. Simplified transmitter interfacing.
3. Extended RF output power range.
4. AM SCA capability.
5. Transmitter protection circuitry.
6. Balanced, transformerless audio inputs and outputs.
7. External reference capability to eliminate "platform motion".
8. Human engineering for easy accessibility.
9. Remote control and status.
2.1 Digital, Independent IF Modulation.
Digital, independent IF modulation is the heart of the BE AX-10 AM stereo
exciter. All clock signals are derived from a single 10 MHz temperature compen-
sated crystal oscillator (TCX0). This highly stable reference improves overall
stability. The stereo signal is generated at an intermediate frequency of 250
kHz for all station frequencies assuring equal stereo performance across the en-
tire AM band. Each audio channel is modulated separately, then summed to L+R
and L-R, hence the term independent IF modulation. This scheme provides inde-
pendent equalization of left and right channels for best separation, distortion
and frequency response. The total system provides a stable carrier frequency
output across the AM band without successive re-tuning or nulling, and with re-
peatable stereo performance at all frequencies.
Figure 1 shows the conventional matrix modulation approach to C-QUAM stereo
generation. The summed L+R information, together with a DC offset to produce a
carrier [1+M, (S(t))] is modulated with a 0 degree RF signal [Ac (COS ct)].
The difference L-R information [M2(D(t))] is modulated with an RF signal phase
shifted by 90 degrees [Ac (SIN ct)]. These two signals are summed, providing
a quadrature modulation signal [Xc(t)]. At this point, stereo information is
fully present and can be decoded by a synchronous detector. However, this sig-
nal is not mono compatible on an envelope detector. Therefore, it is amplitude
limited to produce a quadrature phase-only signal [Yc(t)]. This phase modulated
RF signal is then amplitude modulated with the 1+M, (S(t)) signal in the AM
transmitter to produce the mono compatible C-QUAM signal [Zc(t)].
Figure 2 describes a fully independent modulation C-QUAM stereo system de-
veloped by Broadcast Electronics. In this configuration, the left channel
[M3(L(t))] is modulated with a 90 degree phase referenced RF signal [Ac (SIN
ct)] to produce Qc(t). The right channel [M4(R(t))] is modulated with a 270 de-
gree phase referenced RF signal [-Ac (SIN ct)] to produce Tc(t). These two
signals are summed to produce Wc(t).
M3(L(t)) is also summed with a DC offset to produce a carrier signal of
half-magnitude, then modulated with a 0 degree phase referenced RF signal [Ac
(COS ct)] to produce Pc(t). M4(R(t)) is summed with a DC offset of half-mag-
nitude, inverted, then modulated with a 180 degree phase referenced RF signal
[-Ac (COS ct)] to produce Nc(t). These signals are summed to produce Vc(t).
FIGURE 1. CONVENTIONAL MATRIX C-QUAM STEREO GENERATION
Vc(t) and Wc(t) are summed to produce the identical quadrature modulation
as in Figure 1 [Xc(t)]. This signal is amplitude limited producing two quad-
rature phase-only RF signals 180 degrees out-of-phase [Yc(t) and -Yc(t)]. These
RF signals are amplitude modulated independently by left channel plus half car-
rier [0.5+M3(L(t))] producing Y'c(t) and -right channel plus -half carrier
[-0.5-M4(R(t))] producing -Y'c(t). These signals are summed to produce the
identical C-QUAM signal [Zc(t)].
FIGURE 2. B.E. INDEPENDENT MODULATOR C-QUAM STEREO METHOD
This configuration has the potential of having completely independent equal-
ization of left and right channels, but has one serious problem. Currently,
there are no AM transmitters available that can accept a differential RF signal
and have capabilities for independent modulation. Therefore, a modified inde-
pendent modulation system was developed for use in the BE AX-10 exciter. This
is shown in Figure 3.
FIGURE 3. MODIFIED INDEPENDENT MODULATOR SYSTEM
FOR USE WITH EXISTING AM TRANSMITTERS
In this configuration, the left channel [M3(L(t))] is modulated with a 90
degree phase referenced RF signal [Ac (SIN ct)] to produce Qc(t). The right
channel [M4(R(t))] is modulated with a 270 degree phase referenced RF signal
[-Ac (SIN ct)] to produce Tc(t). These signals are summed to produce Wc(t).
M3(L(t)) is also summed with a DC offset to produce a carrier signal of
half-magnitude, then modulated with a 0 degree phase referenced RF signal [Ac
(COS ct)] to produce Pc(t). M4(R(t)) is summed with a DC offset of half-
magnitude, inverted, and modulated with a 180 degree phase referenced RF signal
[-Ac (COS ct)] to produce Nc(t). These signals are summed to produce Vc(t).
Vc(t) and Wc(t) are summed to produce the identical quadrature modulation as
shown in Figure 1 and Figure 2 [Xc(t)].
From this point, the system is identical to that of Figure 1. It is ampli-
tude limited to produce a quadrature phase-only signal [Yc(t)]. This phase mod-
ulated RF signal is then amplitude modulated with the 1+M,(S(t)) signal in the
AM transmitter to produce the mono compatible C-QUAM signal [Zc(t)].
While the end result in the transmitter is a matrix type modulation, the
phase modulated RF signal is derived through independent modulators, thereby
providing much less interaction of left and right channels. Although the
derivations and diagrams for the fully independent and the modified independent
modulation techniques appear much more complex than their matrixed counterpart,
in reality the circuitry remains virtually the same.
2.2 Transmitter Interfacing Requirements.
In any AM stereo system employing some form of phase modulated RF signal
combined with conventional audio amplitude modulation of that signal, equaliza-
tion must be used in the phase modulated signal and/or the mono audio signal to
the transmitter. This is necessary to match the time delay characteristics of
the two paths to construct the proper C-QUAM sideband distribution at the trans-
mitter output, thereby insuring correct de-matrixing to left and right channels
in the receiver. Figure 4 shows the relationship of amplitude and phase
matching required between the mono L+R signal and the phase modulated L-R signal
to achieve any given separation when de-matrixed.
Due to the wide variety of AM transmitters in use today, the task of equal-
izing these paths to transmit accurate stereo becomes complex. Equalization re-
quirements differ greatly from one transmitter to another, but in most cases the
required equalization can be divided into three sections:
1. Group delay in either the RF or transmitter audio path to match the pro-
pagation differences between the two.
2. Some form of phase and amplitude correction for higher frequencies due
to transmitter and antenna bandwidth/phase characteristics.
3. Low frequency phase correction in some cases.
FIGURE 4. STEREO SEPARATION AS A FUNCTION OF AMPLITUDE AND PHASE RESPONSE
Determining the exact requirements for a particular transmitter can be a
confusing task, and often a series of trial-and-error experimentation results.
This process involves trying various equalizers in the RF and audio paths,
thereby testing their effect on stereo separation and distortion. Until
specific transmitter equalization requirements are documented, alignment will
prove to be an involved undertaking.
Because of the need to route any or all of the equalization circuits to
either the RF or transmitter audio paths, some form of "patch bay" setup would
be advantageous for block equalization selection. In the BE AX-10, this patch
bay approach is accomplished by miniature matrix switches accessible under the
top cover (see Figure 5). Once the basic layout of the switch is understood,
it becomes extremely fast and easy to select any equalization and to route its
output to either the input of another equalization block, or directly to the
required path.
The type and amount of available equalization varies from exciter to ex-
citer, but in general it can be said the greater the available range of
equalization, the more transmitters can be easily converted to transmit AM
stereo. As an example of one set of equalization, the next few paragraphs
will discuss in detail the equalization circuitry used in the BE AX-10 exciter.
The amount of available equalization was chosen after researching AM stereo
consultants and users.
FIGURE 5. MATRIX SWITCHING
Two identical and independent sets of equalization are provided for day and
night correction of changing antenna patterns, low power setting, or for a
standby transmitter. Day/night equalization selection and status are remote
controllable with either momentary ground closure, or by constant ground closure
which may be initiated by antenna selection.
It was found that some AM stereo exciters did not contain enough available
group delay. Systems may require more than 40 microseconds of delay. For that
reason, any amount of constant group delay from 0 to 66 microseconds can be se-
lected. This is accomplished by a miniature rotary switch selecting coarse
delay in 4 microsecond increments followed by a 0-6 microsecond fine delay
adjustment (see Figure 6). In addition, by routing day equalization through
night equalization via the matrix switching, a single equalization of 0-132
microseconds of group delay is possible. So far, only the RCA BTA-5SS solid
state transmitter has required more than 60 microseconds. This installation
required over 100 microseconds of group delay equalization. The amplitude
response remains constant in this equalizer from 20-20,000 Hz.
The low frequency phase equalizer is primarily required by plate modulated
AM transmitters to correct for time delays introduced at low frequencies by the
plate transformer. Figure 7 shows that this first order all-pass network pro-
vides a 0 to 90 degree phase shift maximum at 100 Hz while maintaining flat fre-
quency response.
FIGURE 6. GROUP DELAY FIGURE 7. LOW FREQUENCY EQUALIZATION
FIGURE 8. HIGH FREQUENCY EQUALIZATION
Due to the different phase and amplitude responses of the RF and transmitter
audio chain at higher frequencies, correction must be made to provide good sep-
aration and distortion. Figure 8 shows some typical responses used to equalize
the system at high modulating frequencies. Because of the separate turnover and
peaking available, the responses can be tailored to fit the particular
installation.
Another interfacing requirement for the exciter is the available range of RF
output power. It is generally better to inject the RF signal as far as possible
into the transmitter to diminish the effect of bandpass or lowpass filters in
low level stages. These filters will degrade overall stereo performance and
must be corrected with equalization circuitry in the exciter. By providing 150
milliwatts to 10 watts RMS, a suitable insertion point should be found for good
stereo performance. 150 milliwatts into 50 Ohms corresponds to standard TTL
signal level to drive transmitters with digital inputs. Some digital
transmitters require an asymmetrical duty cycle square wave for best
performance. In this case, an optional TTL interface provides from 25% to 75%
continuously adjustable duty cycle.
In an effort to remove any phase and amplitude mismatches between the RF and
transmitter audio chain due to audio transformers, the mono envelope signal from
the exciter is actively balanced. A high output level of +20 dBm provides
additional headroom and permits the use of lossy modulation enhancement devices.
This output is continuously variable to accommodate transmitters with different
input level requirements and more importantly, to exactly match corresponding
L+R to L-R levels for good separation and crosstalk. For those transmitters
requiring different daytime and nighttime audio levels, separate output level
adjustments are included.
2.3 AM SCA.
With the recent FCC deregulation of AM SCA, AM stations are now able to use
subsonic phase modulation for services such as load management. For
conventional mono AM stations, this requires additional equipment. For AM
stereo stations, however, the addition of AM SCA can be extremely simple.
Figure 9 shows the block diagram for the AX-10 pilot tone and AM SCA inser-
tion method. The information to be phase modulated as AM SCA is inserted via
the rear panel "Auxiliary Pilot Input". This signal is lowpass filtered to
insure higher frequency components are not transmitted, then summed with the
digitally derived and filtered 25 Hz pilot tone. This signal is then summed
differentially with the left [M3(L(t))] and right [M4(R(t))] channel information
for modulation (refer to Figure 3).
FIGURE 9. PILOT TONE AND AM SCA INSERTION CIRCUITRY
2.4 Transmitter Protection.
In any transmission system using a frequency synthesizer to derive individ-
ual station frequencies, some form of muting signal should be included during
initial lockup time, or if phase lock is lost during normal operation. For
safety reasons, a muting signal must be available if the exciter fails to output
RF to the transmitter. Some transmitters can be seriously damaged if RF drive
is lost.
Figure 10 details the transmitter protection circuitry in the AX-10 exciter.
If the synthesizer loses phase lock or if a loss of RF presence is detected at
the output, an external open collector mute signal is initiated to drive a 40 mA
ground closure. This signal could be used to remove high voltage to protect
the transmitter. Internal to the AX-10, the mute signal will extinguish the
day or night LED on the front panel as a local indication.
FIGURE 10. B.E. AX-10 PHASE LOCK AND RF PRESENCE PROTECTION CIRCUITRY
2.5 Audio Circuitry.
The audio inputs of an exciter should be completely transparent to the ap-
plied program content. Some desired characteristics include:
1. Actively balanced inputs.
2. High common mode rejection ratio (CMRR).
3. Good transient response.
4. Low distortion and noise.
5. Flat frequency response from 1 Hz to 15 kHz.
6. Identical phase and amplitude characteristics for both
inputs.
Transformers are capable of balanced input, CMRR and acceptable noise and
distortion, but are lacking in transient response, frequency response, phase and
amplitude matching of inputs. Incorrect phase and amplitude matching will
result in poor main-to-sub and sub-to-main crosstalk, just as poor amplitude and
phase matching of main and sub channel response throughout the transmitter
results in poor separation (refer to Figure 4).
Fully balanced instrumentation inputs are capable of frequency response from
DC to well above 15 kHz, distortion below 0.005%, signal to noise of greater
than 100 dB, excellent CMRR, and superior transient response. They also provide
excellent phase and amplitude matching.
2.6 External Reference Capability.
For AM stereo systems employing phase modulation, nighttime co-channel in-
terference known as "platform motion" can occur under some conditions. While
this phenomenon occurs only in fringe areas where even mono reception is poor,
there has been some concern about its presence. Platform motion is caused by
cochannel stations having a slightly different station frequency due to the
timebase employed. In AM stereo, a rotational effect is created as the receiver
decodes the frequency difference from one channel to another at a rate equal to
the difference in frequency of the co-channel stations. Because the AX-10 is a
digital modulation system deriving all station frequencies from one 10 MHz
source, the master oscillator can be replaced with a reference source from WWV
or some other standard. If all co-channel stations become frequency locked,
platform motion is eliminated.
Figure 11 shows the frequency conversion technique used in the AX-10. The
10 MHz master clock is used to generate all other frequencies. If the internal
oscillator is used, this reference will be within 30 Hz from 0-50 degrees C.
The tolerances after the other clock frequencies are maximum deviations due to
the TCXO employed.
FIGURE 11. FREQUENCY CONVERSION TECHNIQUE
The 10 MHz is first divided by 2 to obtain 5 MHz. This is used in two
places. First, it is divided by 20 to obtain a 4 phase clock generator at 250
kHz to drive the IF C-QUAM modulator. After stereo generation, the 250 kHz
signal is mixed with the second 5 MHz signal, thereby up-converting to 5.25 MHz.
This second IF frequency passes through a linear phase bandpass filter to remove
other mixing products.
Phase 0 from the 250 kHz clock generator is divided to 10 kHz to provide a
reference frequency for the synthesizer. This synthesizer operates from 5.780
MHz to 6.870 MHz. This frequency is mixed with the second IF frequency of 5.25
MHz to produce a difference frequency from 530 kHz to 1620 kHz which is lowpass
filtered to remove higher order mixing products. The up-down conversion scheme
provides a frequency agile system without re-tuning and eliminates the need for
individual bandpass filters assigned to the station's carrier frequency. The
difference term from the last mixer is free of images in the AM band. Since
there are no bandpass filters or other tuning adjustments specific to the
station frequency, the AX-10 can be quickly moved to any channel assignment by
simply reprogramming the frequency synthesizer. This technique also guarantees
identical stereo performance across the AM band.
Because the synthesizer is phase locked to the master clock and high side
injection is used in the last mixer (i.e. the difference term is the one of in-
terest), the frequency errors due to crystal drift subtract, thereby increasing
frequency stability. This provides a total error of no more than 5 Hz at car-
rier frequency across the entire AM band over the 0-50 degree C rating of the
TCXO.
2.7 Human Engineering.
Due to the need for flexible interfacing capabilities, any AM stereo exciter
must contain a wide range of adjustments from audio, RF and pilot levels to
transmitter equalization controls. Accessibility to these controls is of prime
importance to the engineer who must align and maintain the exciter. It would be
most advantageous for these controls to be available without removal of the
unit.
Not all adjustments require immediate access, however there are some which
need to be readily available. These include:
1. Transmitter equalization controls (day/night).
2. Transmitter audio level (day/night).
3. RF output level control.
4. Pilot injection level.
Beyond these, any user helpful controls such as mode selection, pilot off
switch, single channel limiter defeat switch, manual day/night equalization
selection switch, or any required monitoring or diagnostic ports should also be
located on the front panel. Should access to internal circuitry be required,
the exciter is mounted on standard 19" slide rails for convenience. The top
cover can be easily removed. Care must be taken to insure good RFI shielding
for AM, FM, and TV frequencies.
Figure 12 shows the location of controls on the AX-10 exciter. All trans-
mitter equalization controls, together with transmitter audio levels, RF output
level, pilot injection level, pilot off switch, mode selection, single channel
limiter defeat, local day/night equalization selection, and an audio monitoring
point are located behind the front panel door.
FIGURE 12. AX-10 ALIGNMENT CONTROL LOCATION
2.8 Remote Control and Status Indication.
Under no circumstance should there be any loss in mono loudness to the mono
listener during a stereo broadcast, and in fact under normal operating
conditions there is not. It is possible, however, to lose up to 6 dB of mono
loudness if one audio input to the stereo exciter is lost. For this reason,
some form of alternate mode selection should be used. The exciter should be
capable of single channel operation with no loss in mono loudness.
The BE AX-10 can be run in one of four modes:
1. Mono Left
2. Mono Right
3. Mono L+R
4. Stereo
If one channel to the exciter should fail, the unit can be switched to the
opposite channel with no loss in mono loudness. In all mono modes, the 25 Hz
pilot tone is muted to return the C-QUAM only receivers to their mono state.
Because of this, the exciter can be run in the Mono L+R mode during long mono
transmissions.
All modes and equalization states are remote selectable with momentary
ground closures. Their status indications are also provided. The transmitter
mute signal is also provided on the same connector. All remote controls and in-
dications are optically isolated to reduce ground loops and RF contamination.
The inclusion of a standard remote system removes the need to add additional in-
terfacing equipment in the field. It also speeds changeover time to mono modes
in case of failure.
III CONCLUSION
Figure 13 shows the overall block diagram of the Broadcast Electronics AX-10
AM Stereo exciter. This products reflects 20 months of in-depth research and
development which has provided the forenamed improvements to the C-QUAM AM
Stereo system.
Such second generation design techniques offer improvements to look for in
the selection of an AM stereo exciter.
FIGURE 13. AX-10 OVERALL BLOCK DIAGRAM
IV. ACKNOWLEDGEMENTS
I would like to express my appreciation to Mr. Stanley Salek, formerly the
AM Stereo Project Engineer at Broadcast Electronics for his design work on the
AX-10 and his contributions to the content of this paper. I would also like to
thank the members of the engineering and drafting departments at Broadcast
Electronics who helped prepare the illustrations and manuscript.
V. BIOGRAPHICAL INFORMATION
Edward Anthony is an Audio Design Engineer in charge of the AM Stereo pro-
ject at Broadcast Electronics, Inc. in Quincy, Illinois. He has also
contributed to the TV Stereo design. Previously, he held the position of Test
Engineer at Broadcast Electronics where his main responsibility was
implementing test procedures and supporting the design quality of the FX-30
FM exciter, FS-30 FM Stereo generator and FC-30 SCA generator.
Ed received a BSEET degree in 1982 from Central Missouri State University
where he worked as an engineer at KMOS-TV. He is also a member of the Institute
of Electrical and Electronics Engineers and a member of Phi Kappa Phi honor
society.
VI. REFERENCES
(1) Chris Payne - "Introduction to the Motorola C-QUAM AM Stereo System",
Motorola, Inc., Copyright (c) 1982.
(2) Geoffrey N. Mendenhall - "The Composite Signal - Key to Quality FM
Broadcasting", Broadcast Electronics, Inc. Copyright (c) 1981.