home
***
CD-ROM
|
disk
|
FTP
|
other
***
search
/
HAM Radio 1
/
HamRadio.cdr
/
tech
/
amnoise
/
amnoise.txt
Wrap
Text File
|
1987-09-18
|
15KB
|
256 lines
Simple Method used at KCRW
to minimize Synchronous AM Noise in FM Transmitters.
Over the past several years there have been a number of articles
and papers on minimizing the effects of AM noise in FM
transmitters. I am sure that by this point the majority of
working Broadcast Engineers are aware of the effects of
mistuning or narrowband tuning of FM transmitters. Increased
crosstalk and/or multipath are the resultant problems.
The one common thread that the authors of these articles have
stressed is to measure the Amplitude Modulated portion of the
signal using a "precision envelope detector". There have been
few suggestions as to whose unit to use or even possible sources
for these devices. They are not commonly found in Broadcast
facilities or through the common Broadcast Equipment vendors.
At the 1986 SBE National Convention in St Louis, this subject
was discussed again by Geoff Mendenhall of Broadcast
Electronics. He suggested that such a device should introduce
a VSWR of less than 1.06:1. He further stated that the
Modulation Monitors in general use are not accurate enough for
proper measurement of Synchronous AM Noise in FM transmitters.
At the same conference, this subject came up again during a
panel discussion. A suggestion was made from the audience by
John (J.D.) Strahler of KTMS, Santa Barbara CA. He felt that
the simplest means to tune for minimum incidental (Bandwidth
related) AM noise in an FM transmitter was to monitor the output
of the forward sample diode on the transmitter output
directional coupler with an audio amplifier and speaker.
The physics of the subject are simple. Any variations in the
amplitude of the carrier detected by the sampling diodes will be
carried on the rectified RF (which is to all practical purposes
DC) and can easily be used to determine the "relative level" of
the Synchronous AM Noise in an FM transmitter.
The suggestion struck me as extermely sensible and I decided to
try this method. I will detail the results of this trial.
AM noise pg 2.
KCRW operates with an ERP of 6900 Watts at 1100ft AHAAT. The
antenna is a HARRIS (ERI) FMH-2AE 2 bay fed using 3" air
dielectric co-axial cable. The FM transmitter is a CSI
FM12000E operated with a TPO of 7410 Watts. This transmitter
is a two tube transmitter with a grounded-grid final tube. The
tubes are a 3CX10,000A7 final and a 5CX1500B driver. The
exciter is use is a HARRIS MX-15. This exciter is backed up by
a HARRIS MS-15 on hot standby. KCRW operates in Stereo with
subcarrier tenants on both 67KHz (voice & music) and 92KHz
(9600bps data.)
At KCRW we have been tuning for minimum AM noise for quite some
time. The first step in the tuning is to set the tuning for
minimum observed reflected between the exciter and the
transmitter input. We know from long experience that the most
critical tuning is the input tuning of the transmitter (input of
the 5CX1500B). The normal method that we use at KCRW for
monitoring AM noise out of the transmitter is the RCA BW75 Main
chan and BW85 Stereo/test set monitors. Though these monitors
say RCA on the nameplate they are also known as the FMM-1 and
FMS-1 by BELAR. It is fair to say that they are in common use.
The RF input to the FM Modulation Monitor/Test Set is taken from
an RF tap on the PHELPS-DODGE Directional coupler that was
provided with the transmitter. The RF tap is variable and is
set to the minimum point. The coupler is installed after the
transmitter output low pass filter. This filter is considered
by the FCC to be a part of the transmitter. Sampling at the
output of the filter prevents the harmonics normally found in
the PA from causing inaccurate readings of AM Noise.
Connecting the Modulation Monitor to the PA sample point may
cause inaccuracies in AM Noise readings as well as Modulation
percenage. The placement of the sample coupler prior to the
low pass filter will cause similar inaccuracies.
We measured the output of the tap (at minimum setting) at 0.48
Watts into 50ohms resistive. The feed cable to the monitors is
RG58-U in two pieces of 12ft each with a "BNC" "tee" between
them to allow us to connect a Spectrum Analyzer for measurements
of transmitted spectra. This cable also drives (through
isolation/loss pads) test or monitor receivers belonging to our
subcarrier tenants.
AM Noise pg.3
The design of the Phelps-Dodge directional coupler is
particularly well suited to easy connection of a speaker amp to
either directional coupler output. The directional coupler
diodes are contained in an assembly that is "N" male at the end
where it connects to the sample line on the "inline coupler" and
"BNC" female at the end used to drive the meter (output of the
diode).
I simply installed a "BNC" "tee" between the output of the diode
assembly and the cable to the transmitter forward power meter.
I then connected another length of RG58-U to the tee and at the
opposite end I used an adapter from "BNC" to 1/4in male phone to
connect it to the input of a FOSTEX 6301 speaker/amp assembly.
The simple method of tuning for minimum Incidental AM noise
described above is ideal for the "working engineer". It does
not require any unusual or elaborate test equipment. It can be
done without any interruption of normal station operations.
The tests we made at KCRW to verify this method of tuning for
minimum Incidental AM Noise were empirical in nature. The
tests were done with the normal KCRW programming on the air.
They are described in greater detail in the following text.
KCRW FM 89.9MHz is licensed to Santa Monica CA with a
transmitter located in the "Santa Monica Mountains" above West
Hollywood CA. One of the translators operated by KCRW is
licensed to Ventura California. This translator is located
about 50 miles west of the KCRW transmitter site. W. "Dow"
Jones is an engineer for one of the Ventura CA stations and is
under contract to KCRW to maintain the Ventura translator. He
has an SCA/Main receiver at his residence (15 miles from the
translator) where he receives an adequate signal.
We co-ordinated these tests so that he listened to the level of
crosstalk from the KCRW programming into the (67KHz) Subcarrier
while the transmitter tuning was varied. The subcarrier
receiver used for these tests was a Johnson DTR-6 tuned for the
translator output frequency of 89.1 and receiving a subcarrier
of 67KHz. The programming on the subcarrier is simultaneous
real-time translation into Russian of various Network TV
programs. It is a subscription service operated by expatriate
Soviet Jews. The nature of the subcarrier programming
was ideal for the listening required for these tests.
AM Noise pg. 4
The point of minimum program audio level out of the test speaker
was also at the point of minimum crosstalk of KCRW program into
the Subcarrier on the receiver in Ventura CA. It was
simultaneously the point of minimum crosstalk from the 92KHz
data bursts into the 67KHz receiver. The point at which the
level of the program audio out of the speaker was minimum
differed from the point of minimum AM indicated on the RCA/Belar
FM Monitor(s). At the point of minimum level from the speaker,
both the incidental and continuous AM noise, as indicated on the
BW85 (-48dB), were about 8db higher than the minimum reading
when the transmitter was tuned based on the indications from the
RCA/BELAR Modulation Monitors (-56dB).
KCRW also has a QEI 691-02 modulation monitor. We do not use
the QEI for AM noise measurements. The QEI AM Noise readings
are about 10dB worse than those on the RCA/BELAR. I believe
that the readings on the QEI are that much less accurate.
At the point of minimum program audio on the speaker, the
continuous components of the AM noise remain. The residual hum
from ripple components in the transmitter power supplies are
clearly audible. Given the proper instruments, the individual
hum components may be identified and, working with the
transmitter manufacturer, decreased. As John Strahler of KTMS
stated to me in a letter, "..it also makes it very easy to
locate a rectifier stack gone sour."
I chose to perform this test through the translator as a "worst
case" test and was gratified to discover that, in Ventura, the
level of crosstalk from KCRW programming (main and stereo
subchannel) into our 67KHz subcarrier was not a function of the
translator. The TTC XL10FM series translator passed on the
KCRW signal unchanged, including the main transmitter generated
crosstalk components.
I have taken the time to verify similar crosstalk performance on
the main KCRW signal using the same model SCA receiver at my
residence. I am lucky enough to have excellent reception at my
home. The reception is so good that I am able to receive a
KCRW signal (albiet a noisy one) from exciter leakage through
the transmitter when the transmitter fails. (An outside
antenna does help!) This is over a distance of 8 miles.
AM Noise pg. 5
The further proof was in observations that I made of station
reception on the radio in my personal automobile. There is a
section of Interstate highway that I drive daily where I had
observed a "problem area" of about 3 miles length. For those
of you familiar with Los Angeles it is I405 between I10 and
Sunset Blvd. While it is about 5 miles from the transmitter,
there are high buildings in the near vicinity. In this area I
heard a whistle (high frequency tone) directly related to the
presence of either subcarrier (worse with both subcarriers
present) on the radio. It had been evident in this area ever
since the subcarriers were installed and tested.
The problem is due in part to multipath distortion. If you are
unfamiliar with multipath, it is simply the summation of two (or
more) received signals, one delayed in time from another
(others.) One of the signals is direct and the other
reflected. The direct may not be the stronger signal. The
analogy can be drawn to "TV ghosting" or standing wave problems.
It can be evidenced by a "tearing" or "picket fencing" of the
signal as you drive through an area. In some cases the result
can be nearly complete loss of signal (picket fencing.) In
others it is a loss of carrier but not sidebands (tearing.) In
some cases, particularly with clock radios in apartment
buildings, the opening or closing of a nearby refrigerator door
may result in a complete loss of the desired signal.
As is typical of problems related to multipath distortion, the
problem came and went as I drove along this stretch of freeway.
After the alternate method of tuning the transmitter was used,
was very pleasantly surprised to find that the problem area was
nearly gone. There are now only two short stretches of about
100ft each where the problem with the whistle is still evident.
now use that section of road as a daily check on the operation
of the station. Whenever the problem is present, we now check
for some transmitter deficiency or, at the minimum, improper
tuning.
AM Noise pg. 6
With this in mind, I will speculate on the mechanism that caused
the problems with reception on the radio in my automobile. The
mistuning of the transmitter caused the transmitter bandpass (or
the bandpass of the mistuned stage) not to be centered on the
KCRW carrier. When modulation was applied, the result was non-
symmetrical sidebands. The absence of sidebands on one side of
an fm carrier is received as distortions of the original signal.
The mistuning of the transmitter had a more severe effect on the
subcarriers as the sidebands generated by the subcarriers fall
further from carrier center. The result of any mistuning of
the transmitter appears at the receiver as an insidious type of
intermodulation distortion. The presence of additional
distortions in the form of multipath bring the first distortion
into clearer focus. (a multiplicative effect) Correct tuning
of the transmitter bandpass alleviates much of the perceived
problem with reception.
These results fit the conclusions of Geoff Mendenhall of B.E.
that the point of minimum AM noise may not be at the point
indicated by the Modulation Monitor/test set. They further
suggest a simple means to verify the proper tuning of an FM
transmitter. It makes use of items commonly found at a
transmitter site or readily available at most FM Broadcast
stations.
This method of tuning for minimum Incidental AM noise is ideal
for the "working engineer". It can be done without any
interruption of normal station operations. It requires no
expensive or unusual test equipment and makes use of nearly
every Broadcast Engineer's most useful tools - hearing and
judgement.
We are continuing to operate the transmitter tuned as per the
above method for minimum AM noise based on the tests above.
have sent a copy of this letter to a few others for review and
comments. Should you have any contradictory findings let me
now. ì
PN