For most installations the RF environments
are fairly benign and the SL 9003Q factory defaults reflect this
by optimizing sensitivity, spectral efficiency, and delay. However,
cases often arise where the RF environment can be downright nasty
such as with nearby paging transmitters, strong co-channel and
adjacent channel interference, lightening, and the nearby unlicensed
ISM band. Moseley prides itself on a reputation for building
robust radio products, and the SL9003Q is no exception. The SL9003Q
can be easily configured for optimal performance from the benign
to the most brutal environments from the front panel. The following
discussion will show the user how to configure the frequency,
front-end attenuator, QAM mode, interleaver, and pre-selector
for best results and tradeoffs that result.
The easiest place to start is with
the front-end attenuator. The receiver has a 20 dB variable pin-diode
attenuator in front of the pre-amp to protect the receiver from
overload when faced with strong in-band and out-of-band undesired
signals that find their way past the pre-selector filter (Figure 1.0).
This attenuator is controlled from the front panel under QAM
RADIO RX CONTROL to one of three modes, ON/ OFF/ AUTO.
AUTO: (Factory default) In this
mode the front-end attenuation is controlled by a leveling loop
that begins to insert attenuation in front of the pre-amp when
the input signal exceeds 28 dBm. It continues to increase
attenuation with increasing input signal up to 8 dBm. In
general this mode insures that your receiver will operate with
greatest sensitivity and yet provide protection against occasional
OFF: This mode disables the attenuator
completely. Use this mode if strong bursty interfering signals
are sporadically triggering the attenuator leveling control and
causing errors (this is a fairly low likelihood).
ON: This mode forces the attenuator
on essentially placing a 20 dB pad in front of the pre-amp. This
mode provides the greatest continuous protection against interference
but also eats up 20 dB of threshold and fade margin. Use
this mode if your received signal exceeds 43 dBm
or when strong continuous interferer(s) existing in-band cause
1.1 depicts the
typical shape of the BER curve with and without high-level signals
in-band. It should be emphasized that it is not necessarily only
high-level adjacent channels that cause interference. There are
many combinations of signals that can give rise to intermodulation
distortion, which cause the resultant product to fall within
the desired passband.
This method is very useful to assess
interference at your STL receiver (especially if you do not have
a spectrum analyzer available).
Turn OFF the STL transmitter at the studio. At the receiver from
the front panel navigate to QAM RADIO MODEM - STATUS.
The first line entry "QAM Modem" will indicate the
RSL (Received Signal Level) in dBm. With no interference present
the RSL will be below 120 dBm, typically. If this is not
the case and RSL is above this level then you are receiving undesired
interference within your STL passband.
For the QAM data to be properly demodulated at the STL receiver
the RSL must be greater than the interference noise floor by
the following amounts:
21 dB for 16 QAM
24 dB for 32 QAM
27 dB for 64 QAM
(To determine your QAM mode navigate down 5 more menus under
MODEM STATUS until you read "MODE".) For instance,
if your STL is operating in 32 QAM mode (i.e., 32Q) and your
RSL interference is 90 dBm, then the minimum signal that
your STL receiver can acquire must be greater than 66 dBm.
Add 10 dB more for fade margin then you will want to see an RSL
of at least -56 dBm.
Lets say youve discovered
there is interference within your passband and there is nothing
you can do about. You cant change frequency, you cant
install larger antennae, youre just plain stuck with it.
From the previous example it is
seen that 16 QAM provides a 6 dB improvement in threshold. Basically
16 QAM is more robust than 64 QAM because it spreads its information
out more and is easier to decode (there are only 16 states to
detect compared to 64). 16 QAM will work down to a 18 dB CNR
versus 24 for 64 QAM. Along with this 6 dB sensitivity improvement
16 QAM is more robust against interference, microphonics, and
impulse noise such as lightning.
The trade-off is bandwidth. The
approximate RF bandwidth for 64 QAM is the DATA RATE divided
by 6 versus 16 QAM for which the RF bandwidth is DATA RATE divided
by 4. Stated another way, 16 QAM utilizes 33% more bandwidth
than 64 QAM, which is a fair price to pay. Though this is a very
effective way to increase the radios robustness, keep in
mind that the IF bandwidth of the receiver is fixed at 300 kHz.
The receiver can effectively decode modulation that exceeds this
bandwidth but performance will begin to degrade as the transmission
bandwidth increases. Changing the transmission bandwidth is left
to the users discretion; exercise caution not to exceed Part
74 bandwidth allocation.
To change QAM rate navigate to QAM
RADIO CONFIGURE MODEM Mode/Effic. Switch from 64Q/6
to 16Q/4. It is imperative to match the QAM mode on both
transmitter and receiver or the system will not operate. Dont
forget to change both.
Bit errors may also result from
sources other than traditional RF interference. Some of these
noise sources include microphonics, lightening bursts, ignition
noise, and basically anything bursty in its nature. The problem
with these sorts of burst errors is when too many errors appear
together the Reed-Solomon error correction algorithm will be
unable to correct for them. To combat this phenomenon an interleaver
is used to spread out the error bursts (Figure
1.2) so the error correction algorithm doesnt have
to deal with all the errors at once.
The trade off here for using interleaving
is added delay. Table 1 shows the correlation
between the interleave setting and delay.
1. Interleave Setting vs. Delay
* delay is for 1408 kbps
To change interleave length navigate
to QAM RADIO CONFIGURE MODEM Intrlv. The factory
setting is 3 (5 ms). Just like with the QAM mode setting the
user must change the interleave setting to match on both
transmitter and receiver or the system will not operate.
PRE- & POST-
BIT ERROR RATE MENU
The receiver BER status screen is
the most important indicator to the health of the link. From
the front panel navigate to QAM RADIO MODEM STATUS. The
first screen that is shown is the "BER POST" and RSL
status. "Post" refers to post-error correction count,
or the bit-error-rate after Reed-Soloman error correction. This
is the actual error rate. It is a long-term error count which
means it will reflect every error that has been accumulated since
the last time it was reset by pressing enter on the front-panel.
The system should be error free (displayed as 0.00E+0) under
normal operating conditions but it is quite reasonable to expect
occasional errors due to external or environmental conditions.
For a healthy link the error rate should not exceed 1.0E-10 (about
1 error in 3 hours at 1 Mbps).
Navigate down one more screen to
find "BER Pre". This is the pre-corrected error rate,
or the error count before error correction has been applied.
There will usually be some none-zero error rate before error
correction due to errors caused by nonlinearities within the
radio link itself. This is especially true for 64 QAM modulation,
which is quite sensitive to amplifier linearity and amplitude
and group delay variations. The 16 QAM modulation isnt
nearly so sensitive. Pre-BER is a good indicator of proper circuit
operation such as whether the power amplifier is being driven
too hard. An increase of only 1 dB above the factory-calibrated
level can be enough to cause a substantial pre-corrected error
increase. For this reason the power amplifier output level is
accurately controlled and compensated over temperature.
For some types of interference,
such as strong co-channel and adjacent channel signals, the only
remedy may be to move the carrier frequency away from the interference.
This is also a good test to see where the interference lays.
The frequency is changed from the
front panel. Navigate to QAM RADIO TX/RX CONFIGURATION
and set the frequency as needed.
Frequency may be programmed from
the front panel to +/- 1 MHz of center (factory calibrated frequency)
without adjustment. Frequency change beyond this range requires
re-tuning the VCO on the up/down converter to the center of its
range as follows:
- Disconnect AC power cord.
- Unscrew the two retaining thumbscrews
from the Up/Down Converter Module, disconnect the RF semi-rigid
cables from the module and pull the module from its slot.
- Plug the VME extension card into
the Up/Down Converter slot and plug the Up/Down Converter into
extension card. (There is no need to reconnect the RF cables
at this point).
- Re-apply AC power. You should observe
positive lock indications for TX LOCK and RX LOCK (green LEDs)
on converter back panel.
- On the front panel navigate to
QAM RADIO TX/RX STATUS. Scroll down to the menu that shows
AFC. It should be between 4.5 and 5.0 VDC.
- Navigate to QAM RADIO TX/RX
CONFIGURATION and set to desired frequency.
- Navigate back to QAM RADIO
TX/RX STATUS and scroll down to AFC.
- Locate respective tuning access
hole on the module, RX AFC on receiver down converter and TX
AFC on transmitter up converter and adjust with small tuning
screwdriver. A metal jewelers screwdriver works fine for
this. If the new frequency is higher than the old, rotate adjustment
counter clockwise until the synthesizer locks again. If the new
frequency is lower, rotate clockwise until lock is achieved.
Once the synthesizer is locked, adjust AFC for 4.7 V.
- Unplug the radio, remove the VME
extender card, and replace Up/Down Converter back into its slot.
Reconnect semi-rigid RF cables.
- Plug in radio and you should be
back in business. This procedure must be repeated at both ends
of the link.
The pre-selector filter that is
shipped with the SL9003Q is a 5-pole inter-digital waveguide
bandpass filter. It has been optimized for lowest loss, high
ultimate selectivity, and reasonable cost. The bandpass is 20
MHz, which was designed to keep the loss consistent between the
inside and outside channel allocations. For most applications
this pre-selector should provide the best overall performance,
but for extremely powerful near band interference such as pagers
this pre-selector may not provide adequate protection.
Celwave, EMR and Microwave Filter
Company are just three of the many companies that manufacture
bandpass filters for additional protection in antenna farm environs.
Moseley recently introduced the
Starlink Bandpass Cavity for the 944-952 MHz STL band. As you
can see by clicking here
(Thanks to Dennis Martin at KZLA Los Angeles) the cavity offers
over 65 dB protection from potential interference.