Adjusting Modulation Balance Overview 2-Level verses 4

Adjusting Modulation Balance Overview 2-Level verses 4
Technical Brief
AN142 Rev A0
Adjusting Modulation Balance
By John Sonnenberg
Raveon Technologies Corp
Overview
The M7 series of radio’s from Raveon use a transmit modulation technique called
“dual-port modulation”. The transmitted signal is applied to both the VCO and to the
VCTCXO in the radio’s RF synthesizer. The modulation of these two circuits must be
exactly the same. When Transmitting two-level data, the balance between VCO
modulation and TCXO modulation is much less critical.
When transmitting 4-level modulation, the balance between VCO and TCXO
modulation must be precise. When sending 4-level data, it is highly recommended
that the radio’s modulation balance be adjusted on the desired operating frequency,
before the radio is installed.
Modulation balance tends to change across the radio’s frequency band, so it is best to
adjust it on the actual operating frequency. Raveon’s design has built-in
compensation for the balance of the modulation, to keep it relatively consistent across
the band.
2-Level verses 4-Level data
“2-level” data, refers to a signal that digital 1s and 0s are represented by one of two
different levels. On an FM radio, such as the Raveon’s M7 series, 2-level data is sent
as either a positive deviation from center, or a negative deviation.
2-Level Modulation
For high-speed data, the digital data is encoded into 4 levels, and sent over-the-air as
one of 4 frequency deviations. The 4 different levels correspond to the 4 binary
combinations of two bits (00, 01, 11, and 10).
4-Level Modulation
It is obvious looking at the two pictures, that the difference between signals on a 2level signal is much greater that the distance between levels with a 4-level signal. This
is why 2-leve radio system have much better communication range than 4-level
systems, although their data rate is ½.
Modulation Balance
Because of the dual-port modulation method used in the M7, there is an adjustment in
the M7 that effects the fidelity of the modulation. This is called the Mod Balance
setting, and it is stored in register R9 of the radio.
If Mod Balance is too low, the rise-time of the leading edge of the signals is too slow.
If Mod Balance is too high, then the waveform will overshoot. Both phenomena will
degrade the performance of the radio.
Modulation balance is set at the factory, and normally should not be adjusted in the
field.
Only in the case where the radio is changed from two-level to 4-level modulation, or if
it is a 4-level radio is changed in operating frequency, should the balance be adjusted.
Mod Balance must only be adjusted by trained technicians with appropriate RF
communication test equipment.
Proper Balance
The following screen captures illustrate properly balance 2 and 4-level waveforms.
Well-balance 2-level signal
Well-balance 4-level signal
These waveforms are created by causing the M7 to transmit a test pattern into an RF
service monitor, and then viewing the recovered FM signal on an oscilloscope.
To properly view this, it is vital that the FM service monitor, FM demodulator, and
oscilloscope be DC coupled.
Improper Balance
The following screen captures illustrate what an improperly balance M7 will look like
when viewing the recovered FM signal.
Over-balance 4-level signal
Under-balance 4-level signal
Over-balance 2-level signal
Under-balance 2-level signal
Test Equipment
An Agilent HP8910 or HP8921 is the ideal service monitor for testing FM deviation,
and looking at the Modulation Balance. The M7 modulation is FM, so a high-quality
FM modulation analyzer, FM deviation meter, or FM service monitor is required.
Whatever FM deviation monitor is used, it must meet as a minimum these
specifications:
RF Frequency:
500MHz
Audio Frequency:
DC to 10KHz
De-emphasis:
None
Residual FM/noise:
<50Hz
An instrument with AC coupled audio cannot be used to adjust Modulation Balance.
Only DC coupled instruments. It is easy to test if it is DC coupled. Simply change the
frequency of the M7 by 1000Hz, and the audio output of the FM modulation analyzer
should shift in DC voltage.
Connect the audio-output of the FM modulation analyzer to an oscilloscope, preferably
an old phosphor type, or a DSO will persistence. The oscilloscope must he DC
coupled also.
Setting the Balance
This procedure is only to be used when adjusting the modulation for optimal
performance on one specific frequency. After doing this procedure, the factory
Modulation Balance settings are modified, and the radio should only be used on the
one frequency it was compensated for.
Setting the balance is done in the “Configuration” mode. See the user manual on how
to enter the configuration mode. (+++ from a terminal)
The balance setting is stored in register R9. Always record its setting before you
begin balancing, so that you can revert back to the original setting if needed.
Raveon calibrates the M7’s balance in the middle of the frequency band, using the R9
register. For the RV-M7-UC, the middle of the band is 460MHz.
There are three test-patterns available to the technician to generate signals that are
easy to see on an oscilloscope. The three test signals are generated with the
following commands:
ATTD 1
(send random data)
ATTD 3
(send 10101.. pattern at ¼ the baud rate)
ATTD 8
(send 10101... pattern at the baud rate)
You may want to turn the RF power down before adjusting the Modulation Balance.
RF power may be set to 5 (ATPO 5), to reduce heat during this procedure.
Remember after adjusting modulation, set the ATPO back to the setting as it was
before turning it down.
Begin by transmitting the ATTD 3 pattern, and adjusting ATR9 for best balance. Once
a properly balanced 2-level signal is achieved, verify it by sending random data with
the ATDT 1 command.
For M7 radios that use 4-level modulation, verify that the “eye opening” is wide open.
The ATR9 setting will adjust this, and while sending random data, adjust ATR9 for
best eye opening.
The oscilloscope must be adjusted to trigger at the 50% point in the wavefom to get a
good view of the eye opening.
Raveon Technologies Corporation
2780 La Mirada Drive, C
Vista, CA 92081
sales@raveontech.com
760-727-8004
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