Frequency
Coordination Versatility Factor
Andrew
S. McHaddad
10/5/2013
Frequency
Coordination Versatility Factor is
a method of quantifying equipment for the purpose of comparing the ease or difficulty
of including that equipment in a frequency band-plan. As the available radio
frequency spectrum decreases, the need to adequately coordinate the occupation
of the spectrum ever increases. Several
frequency coordination programs are on the market which have
all proven to be invaluable in the determination of potential interference as
well as serving as an overall management tool.
These applications take into account the frequency-step, available
band-width (ABW) and to a degree, the quality of the design of the transmitting
and receiving equipment. These different pieces of equipment each exhibit or
have specific natures which can make them easier of more difficult to
coordinate. The FCV factor provides a metric
that can be assigned to any relevant product as a marketable feature and as a
way of helping customers make intelligent purchasing choices.
1 Frequency coordination versatility is
an attempt to quantify the ease of introduction or inclusion of a piece of
equipment into an existing or new frequency band plan. The FCV value is determined by quantitative
analysis of stated and interpolated performance specifications processed
against the optimum set of features determined to be necessary for band-plan
designing.
2 The FCV value is determined by the
simple equation below. This equation
applies to basic equipment. More
sophisticated equipment requires additional considerations.
( (FHigh - FLow)
/ Number of channels) / Frequency step
This
arrived at by:
High
band limit - low band limit = Accessible Bandwidth (ABW)
ABW/number
of channels in the equipment = Channel Bandwidth (CBW)
CBW/frequency
step = Simple Density Bandwidth (SDBW)
SDBW/SNR
Factor = FCV
3 The range of values is quite large
with some as low as 120 and others as high as 33000. The higher the number, the greater the
versatility; The lower the number, the less
versatility.
When designing a
band-plan, the FCV determines which equipment should be coordinated first. Equipment with low versatility should be
coordinated first. The most versatile
will be coordinated last as it is best able to be placed in whatever spaces
remain.
4 Equipment with one channel, wide
tuning bandwidth and small increments will be the most versatile. Multi-channel, narrow bandwidth and large
steps will be the least versatile.
5 The importance of the quality of the
equipment being coordinated should not be overlooked. Lower cost and less sophisticated RF circuits
can generate more and be more sensitive to spectrum density, de-sensing, signal
propagation and proximity inter-modulation.
One of the most important quality factors is inter-modulation. The assessment of this quality is detailed in
the following section.
6 Transmitters and receivers
have polor opposite natures. The issues that affect receivers, such as
diversity switching, de-sensing are not relevant to transmitters, such as power
output, inter-modulation and deviation levels.
These differences mandate that the criteria for FCV be independently
addressed.
7 System design considerations can also
impact the selection of equipment. If a
single transmitter needs to be received by one channel of a dual receiver and
picked up by an ENG style receiver, the dual channel rack-mount receiver is the
defining limitation as the ABW for each receiver may be different. Beyond the actual transmitters and receivers,
other equipment such as multiple antenna locations, external preamplifiers and
active antennas all contribute to overall performance. An over driven active antenna can severely
reduce the quality of performance do to the generation of RF-products.
Inter-modulation
Products Assessment
Transmitters
demonstrate proximity-based inter-modulation (IM) products. This can significantly affect the operational
density; The lower the amplitude of the products, the
greater possible density. This is
measured on a spectrum analyzer with two frequencies and a 2x1 50-ohm signal
combiner. Bear in mind that this test is only appropriate for transmitters with
detachable antennas. Hand-held
microphones and other forms of integrated antennas can’t connect to the
combiner so less consistent methods of proximity must be used such as
mechanical jigs or in some cases, test cables connected to circuit boards. Two
test conditions are used (1) Highest/lowest and (2) Center/offset.
5.1 Test Condition #1 (Highest/Lowest)
F1 = lowest possible
tunable frequency
F2 = Highest possible
tunable frequency
5.2 Test Condition #2 (Center/Off-set)
Use the manufacturer’s stated
maximum number of operational units within a stated bandwidth. This is the
Manufacturers Stated Operational Density (MSOD). This number is divided into the total
bandwidth to determine the minimum spacing to achieve MSOD. One-half the minimum spacing value is added
to the center frequency and one-half is subtracted from the center frequency to
generate F1 and F2.
For example:
First, determine
minimum spacing for maximum density: BW is 470-495MHz, MSOD is 16 units.
(494-470)/16
= 1.5625MHz
Then figure the
center frequency
(494-470)/2
= 482.500MHz
Next figure
one-half the minimum spacing
1.5625/2
= .78125MHz
Finally add and
subtract one-half the spacing from the center frequency
482.500
+ .78125 = 483.28125 and 482.5 - .78125 = 481.71875
These values need
to round to the nearest tunable frequency.
For a typical .025MHz step, those values would be 481.725 and 483.275
In each test:
Measure the lower primary carrier
(F1)
Measure the higher
primary carrier (F2)
Measure the lower frequency
product (FPL)
Measure the higher
frequency product (FPH)
Average the F1/F2
measurement by:
(F1
+ F2)/2
Average the FPL/FPH
measurement by:
(FPL
+ FPH)/2
The F1/F2
Average is the desirable energy which should be as close to the manufacturers specification as possible allowing for ~ -4dBm
for the losses of the combiner and connecting cables. The higher the value the
stronger the transmitter.
The FPL/FPH
Average is the undesirable energy; the lower the value or even non-existent,
the better.
It is the ratio
between these numbers that is most significant.
The higher the primary carriers and the lower the products, the better
the equipment and the more density can be attained. The greater the absolute distance between the
values the better. This is commonly
referred to as a signal-to-noise (SNR).
Test #1: ((F1 + F2)/2)
+ ((FPL + FPH)/2) = Test1 Average
Test #2: ((F1 + F2)/2)
+ ((FPL + FPH)/2) = Test2 Average
Then, average the Test1
Average and Test2 Average to determine the over-all average IM-SNR.
Then the F1 and F2
average are compared to this value. In a
high quality system with nearly immeasurable products, the FPL/FPH
average will be a high absolute value number, something on the order of
-90dBm. This value when compared with
the F1/F2 average produces the largest possible range of
value.
Example
Equipment #1 (Fixed Base Transmitter)
Test #1 Example
Data:
F1 =
470.000MHz @ +23dBm
F2 =
506.000MHz @ +22.5.5dBm
FPL =
452MHz @ -30dBm
FPH =
524MHz @ -40dBm
(23
+22.5)/2 = 22.75
(-30
+ -40)/2 =-17.5
Next:
(22.75
- -17.5)/2 = -40.25 SNR
●●●
Test #2 Example
Data:
F1 =
481.725 MHz @ +22.5dBm
F2 =
483.275MHz @ +23.5dBm
FPL =
480.175 MHz @ -35dBm
FPH = 484.825
MHz @ -50dBm
(22.5
+23.5)/2 = 23
(-35
+ -50)/2 =-42.5
Next:
(23
- -42.5)/2 = -32.75 SNR
●●●
These
two SNR values are then averaged and rounded to a whole number:
(40.25 + 32.75)/2 = -36dBm average SNR spread.
Example
Equipment #2 (High Quality Analog Microphone)
Test #1 Example
Data:
F1 =
470.000MHz @ +14dBm
F2 =
506.000MHz @ +13.5dBm
FPL =
452MHz @ -70dBm
FPH =
524MHz @ -80dBm
(14
+13.5)/2 = 13.75
(-70
+ -80)/2 =-75
Next:
(13.75
- -75)/2 = -88.75 SNR
●●●
Test #2 Example
Data:
F1 =
481.725 MHz @ +14.5dBm
F2 =
483.275MHz @ +14dBm
FPL =
480.175 MHz @ -60dBm
FPH =
484.825 MHz @ -65dBm
(14.5
+14)/2 = 14.25
(-60
+ -65)/2 =-62.5
Next:
(14.25
- -62.5)/2 = -76.75 SNR
●●●
These
two SNR values are then averaged and rounded to a whole number:
(88.75 + 62.5)/2 = -75dBm average SNR spread.
SNR
Factor Relevance
In determining the
FCV-Factor, if the average SNR is below -75dBm, the equipment is assigned a
value of one (1). If
it is greater than -75 and below -50, it is assigned a value of two (2).
If it is greater than -50 it is assigned a value of three (3). This figure is divided into the FCV factor to
diminish its versatility due to its poor SNR value. It is only applicable to transmitters as only
transmitters can inter-modulate on the carrier frequency. Note that when
dividing by one (1) the net effect is no change since the products are not
sufficiently strong to impact the analysis.
This SNR factor is
the most challenging to quantify in the scope of determining FCV for several
reasons. (1) The passive 2x1 combiner is
the least forgiving method of combining.
(2) It is only a small percentage of the time that transmitters are in
close enough proximity to affect each other.
(3) Manufactured, active transmitter combiners for
fixed-base-transmitters provide high isolation through amplification and
isolator circuits to all but eliminate the generation of these products.