Proper Power & Modulation Adjustments
Everyone seems to be looking for more power and modulation. The 40 channel radios have their modulation limited on
AM by the AMC (automatic modulation control). Most have a potentiometer (POT) that controls the amount of
maximum modulation the limiter allows the radio to put out. The reason it has been limited is to reduce the adjacent
channel splash on the band. The primary cause of splash is by over modulation distortion. But many radios are turned
down well below the point of distortion. Also the AMC may be slow and allow distortion during some syllables. The
radios without an adjustment pot are quickly modified by clipping out a resistor, diode, or transistor, but this eliminates
the AMC. This is OK if the output is constantly monitored and good sense used. The AMC constantly monitors the
output and automatically reduces the input sensitivity to maintain a constant maximum modulation.
Below are illustrations of an AM transmitter under dead key and audio test. These illustrations are representations of
what you would see on an oscilloscope if you were monitoring the RF output of the transmitter under test. Notice the
over modulated signal is clipped off at the top and bottom. There is much audio information missing. The ideal situation
is to get the maximum modulation out without loosing any part of what is going into the audio circuit.
Above is an un-modulated AM transmission.
Above is an under modulated AM transmission.
Above is an over modulated AM transmission.
Above is a 100% modulated transmission.
How many times have we heard or said, "You're breaking my speaker?" Over modulation distortion does sound like a
breaking or broken speaker. But everyone thinks that they're so powerful that they are overloading the other parties
receiver. This isn't true. Yes, you're loud, but the more you over modulate the more voice information is being lost.
The dead key power should be one third of the peak power. Say the radio is capable of 18 watts peak without distortion,
then the dead key should be 6 watts. Less if you're driving a linear amp. We'll cover amps in more detail later in this
issue. This ratio will make your signal loud and clear. Not only will that impress other operators, it will increase your
range of transmission.
If you don't have access to a scope, you can get an idea of what the peak level of modulation is available without
distortion. With the radio tuned for maximum whistle loudly into the n@dc and take a reading on your peak reading watt
meter. Usually reducing the peak by 15% with the AMC will give you clear audio. Then reduce the carrier to 1/3 of the
peak power. For example we have a radio the produces 20 watt peak modulation. Reduce it by 15% by adjusting the
AMC to read 17 watts peak with a whistle. Then adjust e carrier to 5.5 watts. Check the peak for original 17 watt setting.
Now you have approximately 100% clean modulation.
An ALC (automatic level control) is incorporated on sideband radios to limit the maximum PEP output. A radio with a
miss adjusted or defeated ALC can clip in a manner similar to the illustration of over modulation on an AM transmitter.
When the signal clips, information is being lost. It may look good on the meter, but it doesn't sound right at the
receiver's end. Sideband is a different animal from AM. It doesn't take as much audio from a mic to carry a great
distance. Therefore sideband is more susceptible to background noise and an overpowering mic gain adjustment is
In understanding how to adjust your audio on sideband you must first know what the receiver AGC (automatic gain
circuit ) does as it receives a sideband signal. The AGC has the job of varying the receivers gain to prevent overloading
from strong signals. Without it, signals from around S-5 and above would start or completely overload the receiver
rendering the signal unintelligible. In controlling the gain the AGC keeps the audio compressed to a somewhat constant
maximum level. The AGC has a recovery delay time that varies from radio to radio. This delay keeps the gain according
to last signal received. If a weak signal comes in after a strong one, the first word or two may be very low or completely
Now, understanding that to increase a signal by 3db the power must double and a calibrated "S" meter reads I- S unit for
every 6db. We can use the reverse for an example. If we cut the output power in half we loose 3db of signal. Receiving
an 18 watt signal at S-9 we would receive a S-8 at 4.5 watts, a S-7 at 1.125 watts, a S-6 at .14 watts, a S-5 at .07 watts
and a S-4 at .018 watts. That's 18 milliwatts to give a S-4 signal and 1000 times that (18 watts) to give a S-9 on the
meter. Background noise that wouldn't be close to registering on your RF meter would be received with a decent signal
strength. Imagine if the background was flicking the meter at around I watt, approximately 20% of full output. Your
background noise would be giving a S-7 and your peak signal only 2 "S" units more. The background is desensitizing
the receive with the AGC kicking in, making your peaks have less impact or punch. The background would get mixed in
with the voice and make it difficult to understand what is intended to be heard. On sideband the name of the game is
signal to noise ratio, the less noise the less distraction from the intended transmission. There's enough noise out there to
get through without creating your own to overcome.
In setting the ALC it is good practice, with a loud whistle, to open the ALC up all the way and then back it down until
you see a reduction of about 2 watts. This will give you peak performance without clipping in most cases. Some radios
won't distort with the ALC wide open while others will distort terribly once the mic is cranked up. If you control the mic
gain carefully you can leave the ALC open.
For true reproduction of your voice a good quality mic is essential. Using only the amount of gain needed to amplify
your voice to a respectable level, holding the hand mic or desk mic close to your mouth and talking into the side or
talking across the screen will give your transmission the most natural sound .
In This Issue
© CB World Informer 1996 - 2015 Worldwide Rights Reserved
Review Of The Chipswitch
How To Make The Best Solder Connections
Slick Tricks On Microphone Wiring
Proper Base Station & Mobile Grounding
Advertising Claims...Smoke And Mirrors?
Comparison of Cobra 2010 to 2000
Power & Modulation Adjustments
New DF 10,000 Low Pass Filter
Coax Types & Lengths
Linear Amp Selection &Setup
Mobile Radio Interference
New Product Review: Astatic's EchoMax 2000
Channel Kit For PC-122 & Cobra 146-GTL
Did You Know?
Increasing Interest In CB Rumored
What's Happened To Organized CB
Santa's Best CB Gift List
Bob's CB Reopens
New Product: Midland 79-290 AM/SSB Mobile
Swap & Sell
New Product: New Anttron 305 Base Antenna
What's Happening To CB?
CBWI Proposal To Improve 11 Meters
Cobra/Uniden SSB Chassis Mods.
Review Of Midland 79-290 AM/SSB Mobile
Cobra/Uniden SSB Chassis Mod UPDATE
President Jackson Unlocked Clarifier Mod.
Cobra 148 & Uniden GrantXL Clarifier Mod.
Cobra 142GTL & Uniden Washington Clarifier
Uniden Grant Unlocked Clarifier Mod.
Uniden PCI22 PRO SSB Clarifier Mod.
Review Of The Northstar DX880HL
Big Bust At The Consumer Electronics Show
Bob's CB Has Opened
The New Mongoose Model 450 Review
Wilson Antenna Tests The Trucker 5000
A Company With Interference Solutions
Solving Telephone RF Interference
Lowpass Filters: What, Where, And How
Using Highpass Filters For TVI
How To Conduct A Noise Audit
Modern Do-It-Yourself Grounding Techniques
Using Water Pipes For RF Grounding
Using Water Pipes For RF Grounding
The New Emperor TS-3010 Review
Grounding Coaxial Cable Shields
Modern Lightning Protection - RF Entry Ports
Modern Lightning Protection - AC Power Lines
Modern Lightning Protection - Control Lines
Modern Lightning Arrestors - Polyphaser VS I.C.E.
Modern Lightning Arrestors - Alpha Delta VS I.C.E.
Modern Lightning Arrestors - Cushcraft VS I.C.E.
Galaxy DX 2547 Reveiw
Inside The DX 2547
DX 2547 Channel Mod
DX 2547 Clarifier Mod
DX 2547 Photos
DX 2547 Manual Excerpts
The Anttron Story
Anttron 305 Revisited
New Antrron Products
Aries A-SWR 460 Digital Meter
Barjan Buys Wilson Antenna
Wilson Electronic In Cell Phone Market
First Web Issue
Help Get The Word Out
Sneak Preview: The New Maverick A24
Maverick A24 Front Panel Controls
Inside The Magnum Maverick A24
Barjan Buys Francis Antenna
Wilson Antenna, 1 Year After Barjan Buyout
Solarcon I-Max 2000
False Performance Claims
CAUTION: Don't Burn Out That Radio
Magnum's Filtered Power Cord
Dragon Super Heavy Duty SO-239 Stud
CBWI...Give Us Your Opinion
Reveiw Of The RCI 2950DX
RCI 2950DX Image Rejection Modification
RCI 2950DX Coversion & Clarifier Mods
RCI 2950DX Photos
RCI 2950DX Board Component Layout
RCI 2950DX Adjustment Layout
RM-9807: Petition To Remove 155 Mile Limit
Slip-Sear Radio Box
RF Limited UTB-1 Adjustable Talkback Board
A Message From The Editor
Review Of The General Lee
General Lee Conversion
General Lee Tune-Up
Genral Lee Pot. Adjustments
Proposal For Citizens Radio Network
RF Limited EC-2018 Turbo Echo Mic
RF Limited EC-2018XTR Xtreme Echo Mic
RF Limited TRB-1 Turbo Echo Board
Santas CB Gift List
Review: RCI 6900F TB 10 Meter Transceiver
RCI 6900F TB Frequency & Clarifier Mods
RCI6900F TB Tune-Up & PCB Adj. Locations
RCI 6900F TB Photo Gallery
UPDATE: Maverick A24 Transceiver
Texas Ranger SRA-158 Stock Mic
Astatic's Final Edition D-104 Silver Eagle
Firestik's All American Limited Edition Antennas
Uniden PC 68XL Turbo 121 Combo From Mexico
The Cobra 29 Night Watch Classic
Cobra XL 450 Amplifier From Mexico
Review: The Magnum 357DX
Magnum 357DX Photo Shoot
Magnum 357DX Conversions
Magnum 357DX Adjustments
MD-4 External S/RF Meter
Adding MD-4 To The Magnum 357DX
Adding MD-4 To Other Radios
PanaVise Pedesal Mount
A Little History On The CCC People
AR-3500 Operating Manual
AR-3300 & AR-3500 Photos
Maverick A24 Technical Bulletin
Super Star Silver Salute Specifications
Magnum Delta Force 2002 Conversion
Top Gun 56 Mic
Top Gun 56 Story
Top Gun 56 Audio Test
QUAD-5 10 Meter Transceiver
QUAD-5 Magnified Photo
QUAD-5 Operator's Manual
QUAD-5 Photo Gallery
Top Gun SP-1a Speech Processor
DAS Acquires Turner
RCI Move Manufacturing