RDL ACM-3 Datasheet

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RACK-UP

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SERIES

Model ACM-3
Synchronous AM Noise Monitor

ANYWHERE YOU NEED...

• Maintained Clarity

• Maximized Loudness

• Maximized Stereo Separation

• Reduced Subcarrier Crosstalk

• Positive Control Over Multipath Artifacts

• Assured Consistency in Signal Coverage

• Bright 20-LED Metering String

• Switch Selectable Signal Filtering

• Wideband, 75 µs, and High-Pass Filtering

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Programmable Alarm with Remote Output

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Continuous-Reading DC Output of AM Noise

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Transmitter Power DC Remote Metering

Introduction

The ACM-3 is a test instrument which monitors the amplitude component present on frequency modulated carriers. The AM component is
representative of bandwidth characteristics in the transmission system. The technology of significant synchronous AM noise monitoring was pioneered
by Radio Design Labs, the originator of AM noise monitors for FM radio and television aural transmission.

In theory, FM carriers are of a constant amplitude over the range of frequencies they swing above and below carrier frequency. In practice, however, the
amplitude is compromised by a variety of factors. These amplitude modulations of the carrier comprise AM noise. Two general varieties of AM noise are
considered: 1] Noise induced by power supplies and blower vibrations (noise not synchronized to the applied modulation); 2] AM Noise resulting from
frequency modulation of the carrier (noise synchronized to the applied modulation, hence “synchronous” AM noise). This noise is produced by passband
amplitude and phase non-linearities in the rf transmission system. Consequently interstage matching is a significant contributor to AM noise as is the
phase delay inherent in high gain amplifier stages. These factors combine to produce an operating passband for the transmission system.

As the carrier frequency shifts with modulation, the carrier frequency (or actually resultant sidebands) at a given instant will fall on some point of the
skirts of a tuned circuit. If the bandwidth of the tuned circuit is sufficient, the amplitude of the FM carrier will remain constant, or so nearly constant that
the amplitude variation is not significant. If there is any roll-off to the passband of the tuned circuit, the carrier amplitude will change with shifting
frequency resulting in amplitude modulation. The FM transmission system considered as a whole will exhibit different passband characteristics above
the carrier frequency as it does below that frequency. Therefore the sidebands, both upper and lower, generated by a single cycle of modulation will be
unequally attenuated. The resulting disparity produces a difference in carrier amplitude which can be demodulated in an FM receiver. The phase
relationship between this demodulated signal and the recovered frequency modulation produces an audible mix which, in stereo systems, is then further
demodulated into left and right. It is easily seen how these effects will degrade both stereo and audio receiver performance. The incoming amplitude
modulations are often further affected by automatic gain control circuits in the receiver.

EFFECTS OF AM NOISE

In the FM receiver, signal integrity is dependent on accurate demodulation of several signals, including both amplitude and often frequency modulated
subcarriers which are typically at least 20 dB below full carrier. Shifts in amplitude can therefore produce marked effects on the subcarrier performance,
as intermodulation distortion produces substantial baseband noise in the receiver. This is first usually noticed in degraded stereophonic performance
resulting from multipath distortion of the received signal. Observations of the station signal on analog tuners frequently yields an actual narrowing of the
occupied width on the dial, even with fairly minor increases in AM noise. AM noise levels in the transmission system can produce noise in the receiver
similar to the AM noise actually generated by multipath itself. These effects are detrimental to SCA operation, as crosstalk from the main channel
becomes objectionable. These same effects degenerate stereo performance. Even moderate levels of multipath which might otherwise not be
objectionable, have been observed to become severe when compounded with transmission system AM noise. Although keeping close monitoring and
control over variations in AM noise in the transmitter cannot eliminate multipath distortion which occurs after the signal leaves the antenna, minimizing
transmitter AM noise keeps the overall multipath artifacts at the very minimum possible in the receiver. Close control of transmitter AM noise makes
possible optimum performance of both SCA and stereophonic transmission.

SIGNIFICANT SYNCHRONOUS AM NOISE

Amplitude modulations which are of both sufficient amplitude and pulse width to produce any receiver artifacts are revealed by the various sampling
bandwidth and time constants of analysis circuits in the ACM-3. This significant synchronous noise component is displayed on a
damped-decay string display.

fast-rise,

AM NOISE MONITORING

As tubes slowly age, and temperatures change, the significant synchronous AM noise level in the transmission system changes. Very low AM noise
levels will not materially affect receivers. However, as AM noise increases, it will exceed the threshold above which performance degradation is noticed
in SCA or stereo operation. Good engineering of consistent FM transmission requires close control of AM noise, as well as the facility to distinguish
between signal inconsistencies produced by AM noise and those resulting from propagation anomalies. To accomplish this, the engineer must have
immediate access to AM noise readings. The ACM-3 constantly monitors the AM noise levels and provides an alarm output which can be used to alert
the duty operator when the AM noise has exceeded the value set by the engine er. B y setting this alarm threshold se veral dB belo w t he point wher e A M
effects are determined to be objectionable, a transmitter trip can be scheduled and the AM level can be controlled prior to the statio n signal suffering
any adverse effects. The ACM-3 also provides a calibrated remote control output permitting the AM noise level to be read from the studio location at all
times.

SAMPLING

Samples of FM carriers are often available at different points in the transmission system. The only place to obtain an rf sample appropriate for AM noise
measurements is AFTER THE LOW-PASS (or other bandwidth-limiting) FILTER. Monitor jacks in the PA cavity area of a transmitter are totally useless
for AM noise measurements. These jacks provide rf containing harmonic content which renders the AM noise readings totally erroneous. The
DCF-100MB supplied with the ACM-3 is to be connected to a DIRECTIONAL sampler situated in a metering line section at the point nearest the
antenna feed line. Samplers are available in two general types: Capacitive samplers and Directional samplers. It is a sampler of the directional type
which must be used for AM noise readings. Capacitive samplers typically have a screw adjustment on them to set rf pickup, and they sample both
forward and reflected waves. For AM noise measurements, only the forward wave must be used. It is imperative that the DCF-100MB be connected
physically RIGHT AT THE SAMPLER JACK. Do not connect a length of cable between the sampler and the DCF-100MB input, as even the slightest
VSWR in this cable can substantially impair the accuracy of your readings. Often a station will have a line section already installed just prior to the
antenna feed line for power monitoring purposes. One of these sample ports is suitable for installing a directional sampler to feed the DCF-100MB.
Often, however, one or both of these ports is being used to feed either remote cont rol readings or to detect transmitter power for automatic switchover
or alerts. If the sampler for the DCF-100MB is displacing a metering slug normally used to supply a forward power indication, note that the ACM-3 has a
DC power metering output. This output can be used for remote power metering, or can be resistively divided down to a usable sample level for other
switching purposes. This ACM-3 output is independent of the front-panel calibrate control, and is buffered against external load effects.

Installation: READ AND UNDERSTAND THIS BEFORE INSTALLING!!!

Many hookups within a broadcast facility can be done merely to make it work, even though they may not be exactly done right. THIS IS NOT THE CASE
WITH AM NOISE SAMPLES. If you do not begin with an accurate sample, your ACM-3 may still register readings, ho wever, those readings can be
grossly inaccurate, leading you to make corresponding adjustments to your transmitter that have an adverse effect on your signal or efficiency.
Obtaining a correct sample is not difficult, but the importance of doing it right cannot be emphasized enough. Once a correct sample is established, you
can have years of optimum service from your transmitter without giving the sample port a second thought. If you have not yet read the preceding
section, SAMPLING, read it now, as it details all the pertinent considerations in establishing an accurate sample point. Plugging the DCF-100MB into
just any BNC rf sample may not be appropriate. If the sample contains harmonic energy (as in PA rf loop samples, or any pre-harmonic-filter samples),
or if it contains reflected power, this additional material can add or subtract from your fundamental carrier rf voltage, thereby generating AM noise in
your sample. This problem is avoided by the use of a DIRECTIONAL (forward) sample. Many, if not most, FM stat ions have a line section with slug
ports for power metering. An excellent method of obtaining a forward sample suitable for AM noise metering is to replace a slug with a directional rf
sampler, as shown in the drawing.

The attenuation of the sampler must produce an rf sample voltage within the operating input range of the DCF-100MB. The following table lists the
appropriate attenuation levels for common power ranges. Part numbers are shown for samplers produced by Coaxial Dynamics (Cleveland, Ohio).
These samplers are specifically designed for use with the ACM-3.

Transmitter Power Line Section Attenuation Sampler
440 W to 5 kW, 1-5/8”, 40 dB, 87024H

4.4 kW to 35 kW, 3-1/8”, 50 dB, 87035H

The ACM-3 installation consists of two units plus any optional rack mounting accessories. The DCF-100MB is plugged directly into a directional
sampler, as described above. You must connect a coaxial cable with a BNC plug on each end between the DCF-100MB and the ACM-3. The ACM-3 is
designed to mount in an RDL RU-RA3HD rack adapter that fits a standard 19” equipment rack. The RU-RA3HD provides two additional Rack-Up
Series mounting locations, conveniently used for related products such as the RDL RU-SP1 speaker (used for listening to AM noise during tuning).
Slots are provided in the ACM-3 chassis for ventilation. Do not mount the ACM-3 such that these slots are obstructed. As supplied from the factory, the
ACM-3 is set up for ACM Standard Wideband signal analysis, with no high-pass filtering. This is the correct setting for nearl y every installation. Two
options are available, however, and should be considered. Two filtering switches are provided on the ACM-3 rear panel. One selects 75 µS de­emphasis for all AM noise readings. This may be selected if the ACM-3 is being used to analyze asynchronous noise, or is used in a country or for
measurements where de-emphasis is required. The second switch selects the high-pass filter. This is used in the rare circumstance where the
asynchronous AM noise (power supply and/or blower vibrations) are a fe w dB greater than the synchronous AM noise. This filter rolls off the low
frequencies to permit accurate synchronous AM noise nulling on such transmitters. In most instances where asynchronous noise is greater than
synchronous noise, this indicates a need for power supply repair rather than a need to select this filter! It is recommended that the ACM-3 be first
installed using the factory settings.

LOW PASS

FILTER

TO ANTENNA

DIRECTIONAL SAMPLER

DCF

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CAUTION

DO NOT CONNECT THE RF SAMPLE FROM YOUR TRANSMISSION LINE SAMPLER DIRECTLY TO THE INPUT ON THE ACM-3. THE
DCF-100MB MUST BE CONNECTED BETWEEN THE SAMPLER AND THE ACM-3. INSURE THAT THE RF SAMPLE DOES NOT PRODUCE A
VOLTAGE WHICH EXCEEDS THE INPUT RATING OF THE DCF-100MB. A MAXIMUM SIGNAL INPUT OF 20 V P-P INTO 50 OHMS IS PERMITTED.
NOMINAL CORRECT INPUT IS 8 V P-P INTO 50 OHMS. Before connecting the DCF-100MB to the sample port, first connect it to the input of t he
ACM-3, with power applied to the ACM-3. Connect a DC voltmeter between terminals 3 (+) and 4 (Gnd) on the rear terminal board. Set the voltmeter t o
a scale appropriate to read from 0 to 20 Vdc. The coarse input level trimmer is located below the input jack on the rear of the ACM-3. Set the rear-panel
input LEVEL trimmer approximately 1/8 turn from the off (counter-clockwise) stop. With the transmitter on, connect the DCF-100MB to the sample port
and observe the voltage on your DC voltmeter. If the voltage exceeds 10 Vdc, then your sample is likely to exceed the input limits of the DCF-100MB.