NU9N SSB Audio News Editorial - August, 2006
Good RF Modulation Checklist
| Good RF Modulation Checklist
How Does Your RF Modulation Measure Up ?
NU9N News - August, 2006
The details of
how you modulate your audio with your particular mode of operation, be it AM, SSB or FM will determine the final quality of your signal, regardless of the bandwidth you select.
I would like to present a simple check list of items to consider when modulating your preferred mode of RF with speech audio.
First, we should consider the transmitter's state of affairs and it's proper alignment. If the transmitter's internal circuitry is not adjusted properly, then modulating it with audio will just compound the issues already present. So, let's
examine the checklist below from beginning to end and make sure that everything is as it should be for a final product that is linear, clean, pleasing and properly modulated.
- TRANSMITTER ALIGNMENT CHECKLIST
- Transmitter Final PA BIAS Adjustments
The final power amp section of your transmitter / exciter should be linear because if it's not, then applying power to the signal will just amplify the problem! (No pun intended)
There is a BIAS adjustment procedure with every transmitter / exciter that when properly adjusted, will ensure that output signal will be as clean and as linear as possible within the rated specifications of the manufacturer. With
modern solid state SSB rigs, this will result in Class AB2. See your exciter's technical manual for more details.
- Carrier Balance / Carrier Set Point / Sideband Suppression Adjustments
SSB carrier balance null is a compromise between USB and LSB. Often, in some rigs, there is *both* a null pot and a null capacitor to balance out the carrier leakage. A carrier null of -40 db or better is typical. See your exciter's
technical manual for more details. (Not usually an issue with DSP based rigs)
In SSB analog mode, the carrier set point must be aligned properly so that the audio applied ends up on one sideband or the other as much as possible without audio being applied to the unwanted opposite sideband. In DSP based transmitters,
this is not an issue due to the way the sidebands are generated. See your exciter's technical manual for more details. Carrier Suppression is achieved with another alignment procedure to make sure that the carrier is suppressed as
much as possible equally between both sidebands. See your exciter's technical manual for more details.
- LINEAR AMPLIFIER LINEARITY
- Is Your Linear Linear?
To monitor linear amplifier linearity, you will need an oscilloscope, RF demodulator and RF Sampler of some kind. (See my "Scope Your Audio" page
One of the most common mistakes made when tuning a linear amplifier is under loading of the "Load" control. A simple procedure that will get you close to proper loading of your amplifier is to simply tune for maximum rated
power with the amplifier and then increase the loading to the point where you just start to see a power decrease. This will get you in the ballpark.
- AUDIO PROCESSING CHECKLIST
This procedure is the most important in determining the audio "Persona" of your station. One must be careful here to not over-emphasize certain frequencies, while at the same time emphasizing what's important in the audio
spectrum without adding distortion or unwanted artifacts through the I. F. filtering section of your transmitter.
The basic concept here is to custom tailor your audio to achieve the type of audio you want to present to your listeners, whether it be for a pleasing smooth full sound with fidelity, or a narrow crispy DX punch. Either way, your
EQ settings will be critical.
For a full, smooth and natural sound, you will want to open your transmit filters to 3kHz or above and then boost low frequencies (from 50 Hz ~ 90 Hz), cut midrange frequencies (from 200 Hz ~ 800 Hz) and boost upper midrange frequencies
(from 2kHz ~ and above).
For a narrow crispy DX punch, you will want to narrow up your transmit filters below 3kHz and then cut all low frequencies (below 200Hz), and boost upper midrange frequencies (from 1kHz ~ 2.9kHz) with the most energy being applied
at about 2.5 kHz.
See my "DSP1100 / DSP1124" page for general EQing guidelines.
Note: Boosting and cutting of frequencies should NEVER exceed more than 20dB in either direction. If you boost your lows and highs by +20dB, and you cut your midrange frequencies by -20dB, be conscious of the fact
that this will result in a 40dB difference in comparison when referencing the mids to the lows and highs. Your transmitter's ability to handle these extremes may be compromised resulting in unwanted transmitter products. Use some
care and common sense when EQing. Know your transmitters limits and don't try to stuff a gallon of whisky into a shot glass !!!
See my "News_November_2003" page for more on knowing your transmitter's limits.
- Dynamic Processing
Compression and Peak Limiting can be a great asset to the final quality of your sound. These devices are meant to take control over the wide dynamics present in human speech, keeping levels in a managable range to prevent over modulation
that results in distortion.
The basic compression settings to keep in mind is a fast attack time, fast release time, a compression ratio of about 5:1 (for every 5dB of gain at the compressor input, only 1db of gain results at the output).
Peak Limiting is a more aggressive form of compression with fast attack and release times and usually a ratio that exceeds 20:1 (for every 20dB of gain at the input, only 1db of gain results at the output). A combination of compression
and peak limiting can assure that no matter how aggressive you get at the microphone, the output will remain fairly constant and under control.
Careful control of your audio rack input / output levels are important in order to achieve the best signal to noise ratio (S/N) while not overdriving any input / output audio stage along the way. Keep audio levels entering audio processor
inputs at about 0dB line level reference and keep audio levels leaving audio processors at about -6dB line level reference. This is especially critical with DSP based digital processors where the output should never exceed -6dBU.
Digital clipping will occur above these levels and will sound really nasty!
Also, proper adjustment of your exciter's mic level (if you are feeding your rack into the transmitter's mic input) is obviously very important. Some operators have a habit of cranking up their mic gain so that they constantly see
Mid or Full-Scale ALC when talking. This is too much! When incoming signals to the transmitter cause the ALC meter to deflect at all on voice peaks, this means that the transmitter's MAXIMUM allowed input gain has been reached at
those peaks. Any peak level above that is eliminated via the ALC circuit anyway, assuming that the ALC is properly responding to the instantaneous peaks.
Even worse, the ALC circuit may not be properly suited for an enhanced processed input signal on any given instantaneous peak. These peaks can occurs so quickly, that by the time the ALC circuit responds, it is already too late and
a nasty peak (usually in the high frequencies) has slipped through causing severe distortion of the transmitted audio.
Let the audio compressor and peak limiter in the audio rack control your voice dynamics, NOT the ALC circuit in your transceiver. When the external processing is properly set up, the ALC should only occasionally move (1 or 2 bars
or marks at the most) on voice peaks!
See my "TX Setup" page for detailed processor guidelines.
- RACK to RIG INTERFACING -- GROUND LOOPS and RFI
- Getting Your Rack Audio To Your Transmitter Cleanly
Interfacing your audio rack to your transmitter may present many problems including the development of ground loops resulting in 60 HZ hum, or worse yet the dreaded "Radio Frequency Interference" (RFI).
This is why I strongly recommend the use of "Audio Isolation Transformers" and high quality double shielded "Star quad" interface cabling through the rack and to the transmitter input. If careful attention is applied
when interfacing the systems, the resulting output signal will be clean, quiet and well formed.
See my "TX Setup" page for solving tough RFI / Hum problems.
With a little patience, proper planning and good engineering, your final RF output signal will be clean, pure, linear, free from noise, RFI and hum and sound exactly how you want it to sound, given the inherent limitations of your transceiver's
May you find that sweet sound you are looking for and be proud knowing that you put your heart and soul in the production of your personalized and fabulous on-air signal. It's worth the time and effort!