Frequency Coordination Versatility Factor
Andrew S. McHaddad
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.
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
(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
(23 - -42.5)/2 = -32.75 SNR
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
(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
(14.25 - -62.5)/2 = -76.75 SNR
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.