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RF Filtering Fundamentals
Introduction
Today’s crowded communication bands and closely located receivers and transmitters operated simultaneously (SIMOP) require RF System designers to pay increasing attention to their equipment’s generation and rejection of signals and noise. The receiver must operate in the presence of large interfering signals on adjacent channels while the transmitter noise and spurious signals can artificially raise the system noise floor for collocated (cosite) receivers with the end result of system desensitization and diminished communications range.
Pole/Zero offers a full suite of products for these highinterference environments. For the equipment designer, Pole/Zero offers multiple families of rapidly tunable notch filters, bandpass filters and pre/postselection filters for incorporation into our customer’s equipment. For the system engineer with the challenge of enhancing a modern transceiver’s performance in a cosite environment, Pole/Zero offers our Integrated Cosite Equipment (ICE) which directly interfaces with the transceiver to provide the required cosite interference mitigation – even under fast frequency hopping applications.
Pole/Zero offers a Cosite Analysis and Integration service to our customers to determine the level of cosite interference mitigation required for a specific communications application. The goal of this analysis is to determine the level of mitigation required to meet the system’s concept of operations and ensure full communications range – at minimum cost.
Tuning Components
Changing the resonant frequency of a tuned circuit can be done by varying either its inductance or capacitance. Because of their smaller size and higher Q, capacitors are generally chosen as the tuning element. One way to accomplish this is by allowing the capacitance to be a bank of switched discrete capacitors. Far from ideal, the PIN diode remains the choice RF switching component when medium to high inband RF power handling, 1 to 100 watts or greater, is required. Once a high cost component, diode manufacturers now offer high performance parts in low cost SMD packages. Some JFETs are available that exhibit usable RF switch characteristics, but RF power handling can only approach the 1 watt level. Varactor diodes remain the choice tuning element for RF power handling to 20mW.
Selecting the Right Tunable Filter
Selecting the right filter for the job requires the designer to consider a number of aspects. These relate to technical performance, size/weight, and cost. On the technical side, the filter performance characteristics can be summarized:
Insertion Loss
Bandwidth/Selectivity
RF Power Handling
Intercept Point (Third Order Intercept)
Tuning Range
Tuning Speed
Power Consumption
Additionally, the size and weight of the filter must be considered, especially for portable and airborne equipment. And as always, cost is a major driver in engineering decisions.
Insertion Loss (IL) and Bandwidth/Selectivity
An unfortunate rule of nature dictates that a filter’s insertion loss and bandwidth are inversely related; the narrower the bandwidth of a filter of a given technology, the higher its loss. The bandwidthloss relationships are measured by a filter designer using the property called “unloaded Q”. This property measures the Q of an unloaded resonant circuit. Q is mathematically defined for a resonant circuit by the equation:
Q = wo*R*C or Q = R/(wo*L)
It is readily obvious that the higher the value of R, which for an unloaded resonator represents the lossy component, the higher the value for Q. The insertion loss of a 2pole Butterworth filter is given by the equation:
IL = 20*log[Q/(Q – 1.414/BW3dB)]
Again, the only way to improve a filter’s IL, for a given technology and bandwidth, is to increase the Q of its resonant circuits. Generally, this means larger size and/or increased DC power consumption, and higher cost, due to higher quality components.
RF Power Handling
This parameter can be the most important one in selecting a tunable filter. As opposed to fixed tuned filters that consist of passive components, tunable filters contain active components, which have limited linearity. The 1dB compression point of a filter is the RF signal level where IL increases by 1dB. For a tunable filter, this occurs when the RF signal’s peak voltage imposed across an active tuning component, whether PIN diode or varactor, approaches the DC bias voltage applied. For PIN diodes, power handling can be improved with increased reverse bias; however, care must be taken to ensure the sum of the bias voltage and the peak RF voltage do not exceed the breakdown voltage of the parts. Insufficient forward bias current can also limit power handling but is usually of secondary importance.
Intercept Point (IP3)
The Third order intercept point is a figure of merit for linearity and is closely related to the 1dB compression of the filter. When two large “interfering” signals (F1 and F2) are applied to a filter (input or output), two new signals are generated which appear one on either side of the interferers and spaced from them by F1 – F2. If these interfering signals occur within the filter’s passband, the distortion products can be large and easily fall right on top of a desired signal. In a tunable filter, this distortion is caused by the nonlinearity of the active components when large RF voltages are imposed on them. Inband Third Order intercept is generally 10 to 15dB higher than the 1dB compression level of a filter. The amplitude of the distortion products decreases as the interfering signals are moved out of the passband and on to the filter skirts. Note that even though the filter being specified may not have to handle high RF levels, the requirement for IP3 may drive its size, weight, and cost due to the relationship between RF power handling and IP3.
Tuning Range
Pole/Zero filter products offer frequency coverage up to a full octave. The narrower the tuning range required of the filter, the higher the performance. If your tuning range can be reduced, or two halfband filters can be utilized, usually at least one other technical parameter can be significantly improved.
Power Consumption
PIN diodes require DC power when forward biased. Generally, by increasing the forward bias of a diode, unloaded Q is increased and thus Insertion Loss improved.
Standard Filter Designs
Pole/Zero’s standard MINIPOLE®, MAXIPOLE®, and POWERPOLE® bandpass filters are 2pole, constant Q designs and are aligned to provide a close approximation to a Butterworth response. Filter tuning ranges are available based on popular communication bands covering 1.5MHz to 1GHz. Units are available with standard 3dB bandwidths from 1.8 to 20%. Figure 1 shows a simplified Block Diagram of a filter module.
Tuning is accomplished via a PIN diode switched binary capacitor array placed in parallel with a high Q inductor or resonator. A single +5vdc input provides the current for diode forward biasing and an additional input voltage between +30 and +100vdc is required for diode reverse biasing. An internal DCDC converter running off the +5vdc supply for generating the high bias is an option on the MAXIPOLE®, and is standard on the POWERPOLE®.
The tuning arrays are driven by a decoder/driver that contains all of the necessary circuitry to receive digital tuning commands, translate them to the internal filter tuning codes, and drive the PIN diodes with the proper bias. The entire tuning process is accomplished in under 10 microseconds for most bands. The standard input format is a parallel 8 bit binary word allowing each filter 251 tune positions (the last 5 tune words are reserved for special functions), linearly spaced over it’s RF tuning range.
For lower RF power handling requirements, small size, or battery operated equipment, Pole/Zero’s “MICROPOLE®” offers designers an offtheshelf DIGITALLY TUNED filter solution. With insertion loss and selectivity performance similar to the “MINI” series, the “MICROPOLE®” requires a single +3 to +5vdc input at less than 1 mA current. Tuning is controlled via an 8 bit binary sequence similar to that described above. This series is packaged as a SMD module with dimensions of 0.5″ x 1.0″ x 1.5″. This latest addition to the P/Z family of tunable filters offers communication engineers the final building block for low power frontend receiver design. As easy to specify as 50mW MMIC amplifiers or mixers, the “MICROPOLE®” provides a small, lowcost, lowpower solution for RF receiver front ends.
Customized units are always available as well as Pole Zero’s series of tunable Notch Filters and Filter/Amp cascades.
Cosite Analysis
Designing and Upgrading Command and Control Communications Systems for FirstTime Success
Pole/Zero®’s Cosite Interference Analysis models the following cosite interference mechanisms:
Transmit Mode
– Broadband noise and spurious output
– Reverse intermodulation distortion
Receive Mode
– Receiver desensitization
– Reciprocal mixing
– Cross modulation
– Receive intermodulation
With antenna to antenna coupling data derived either from a Delcross Technologies simulation or via empirical techniques, the Pole/Zero cosite analysis proceeds as follows:
Military Command & Control (C2) platforms employ several RF communications channels to ensure force coordination over long distances. Unbeknownst to the warfighter, often these critical communication distances are not realized due to selfgenerated or cosite interference. This degradation has, at its source, insufficient antenna isolation on the C2 platform, lack of transmitter spectral purity and receiver vulnerability to desensitization from local interference. Therefore, antenna placement, transceiver selection and cosite interference mitigation techniques are central to ensuring full communications range is realized.
To meet these challenges, Pole/Zero Corporation and Delcross Technologies have partnered to offer our system integration customers a “turnkey” source for solutions to the challenges of antenna placement optimization and RF cosite interference mitigation in shipboard, airborne and vehicular applications.
Filter Selection Guide
The Pole/Zero filter selection guides provide the basic specifications to help select the required filter series for your application. This is only to be used as an initial selection as other factors may affect your application. Customized units are always available as well. If you have any questions please send them to support@polezero.com or call 5138709060.
Tunable Bandpass Filters
Extended Range Tunable Bandpass Filters
Tunable Notch Filters
Pre/Post Selectors
Integrated Cosite Equipment (ICE)
Tunable Bandpass Filters
Filter Series 





MEGAPOLE® 
Insertion Loss (4%BW_{3dB}): 
6.0 dB 
5.0 dB 
2.5 dB 
2.2 dB 
4.5 dB 
1.0 dB 
IL X BW Product: 
24 (Typ) 
20 (Typ) 
10 (Typ) 
8.5 (Typ) 
18 (Typ) 
4.5 (Typ) 
Shape Factor (30db / 3dB): 
6.9 (Typ) 
6 (Typ) 
6 (Typ) 
6 (Typ) 
3.3 to 3.75 (Typ) 
6 to 8 (typ) 
1dB Input Compression (4% BW_{3dB}): 
+6 dBm 
0 dBm 
+32 dBm 
+32 dBm 
+50 dBm 

Input IP3: 
+15 dBm 
+45 dBm 
+45 dBm 
+53 dBm 
+45 dBm 
+63 dBm 
+5VDC Current: 
< 2.6 mA 
< 250 mA 
< 500 mA 
< 1.5 Amps 
< 1.0 A 
Option 
Tuning Speed (Typ)*: 
15 µS 
10 µS 
15 µS 
15 µS 
50 µS 
<50 µS 
Size (inch) (HxWxL): 
0.25x.78x.78 
0.6×1.4×2.3 
1.5×2.5×3.3 
2.6×3.0x4.0 
3.1×3.5×7.0 
6x7x3.6 
Size (mm) (HxWxL): 
6.4×19.8×19.8 
15.2×35.6×58.4 
38×63.5×82.6 
66.7×76.2×102 
79.4×91.4×178 

Weight (ounce) (Typ)**: 
0.18 
3.2 
9.2 
18.7 
43.2 
64 
Weight (grams) (Typ)**: 
5.1 
90.7 
260.8 
530.1 
1255 
1815 
Temperature range (°C): 
40 to +85 
40 to +85 
40 to +85 
40 to +65 
40 to +85 
40 to +55 
*Tuning speed shown is a typical value for each filter series. The actual tune time can vary significantly based on frequency range and configuration. Please refer to the Switching Characteristics section of each filter series for the maximum tune time.
** Weight varies by configuration, power supply, options, etc.
Extended Range Tunable Bandpass Filters
Filter Series  NANOERF®  MINIERF®  ERF5W™ 
Insertion Loss _{(Typ)}:  5 dB  5.2 dB  2.5 dB 
Percent Bandwidth (Typ):  6.5 %  4.7%  10% 
Shape Factor (30dB / 3dB):  6±0.5 (Typ), 8 (Max)  6±0.5 (Typ), 7 (Max)  
Inband RF Power Handling:  4 milliwatt (input)  1 watt (input avg)  5 watt (input avg) 
Out of Band RF Power Handling:  13 milliwatt (input)  2 watt (input)  20 watt (input) 
Inband Input/Output VSWR:  1.5:1 (Typ), 2.2:1 (Max)  1.5:1 (Typ), 2.2:1 (Max)  1.65:1 (Max) 
Input IP3:  +10 dBm  +40 dBm  
DC Power: 
+3.3 VDC @ 15 mA 25

+3.3 VDC @ 200 mA (Max) +100 VDC @ 2.5 mA (Max) 
+5 VDC @ 1.5 A (Max) +1.65 to +5.5 VDC 18u (Max) 
Tuning Speed (Typ)*:  25 µsec (Typ), 35 µsec(Max)  15 µsec (Typ) @0 dBm  
Size (inch) (HxWxL):  0.216 x 1.1 x 1.1  .385 x 1.75 x 2.4  
Size (mm) (HxWxL):  5.5 x 28 x 28  9.8 x 44.5 x 61.0  
Weight (ounce) (Typ):  
Weight (grams) (Typ):  
Temperature Range (°C)  40 to 85°  40 to 85°  40 to 85° 
*Tuning speed shown is a typical value for each filter series. The actual tune time can vary significantly based on frequency range and configuration. Please refer to the Switching Characteristics section of each filter series for the maximum tune time.
Filter Series  MINIPOLE®  MAXIPOLE®  POWERPOLE®  MAXI 4/R 
Passband Insertion Loss:  1 dB max  1 dB max  1 dB max  1 dB max 
Notch Depth:  20 dB  20 dB  20 dB  35 dB min 
3dB Bandwidth (Typ):  7.5% typ  5% typ  4% typ  10 MHz typ 
Passband VSWR (max):  2:1 max  2:1 max  2:1 max  2:1 max 
Passband IP3 dBm:  +50 dBm  +50 dBm  +50 dBm  +50 dBm 
DC Power: 
+5 VDC @10 to 250mA +100 VDC @ 1mA 
+5 VDC @10 to 500mA +100 VDC @ 1mA 
+5 VDC @400mA to 1.5A 
+5 VDC@ 1.5A max +100 VDC @ 2mA 
Tuning Speed (Typ)*:  10 µS  10 µS  20 µS  40 µS 
Size (inch) (HxWxL):  0.6 x 1.4 x 2.3  1.5 x 2.5 x 3.3  2.6 x 3.0 x 4.0  3.1 x 3.5 x 7.0 
Size (mm) (HxWxL):  15.2 x 35.6 x 58.4  38.1 x 63.5 x 84  66.7 x 76.2 x 101.6  79.4 x 88.9 x 177.8 
Weight (ounce) (Typ):  3.2  9.2  18.7  56 
Weight (grams) (Typ):  90.7  260.8  530.1 / .5 (kg)  1588 / 1.6 (kg) 
Temperature range (°C):  40 to +85  40 to +85  40 to +65  40 tp +85 
*Tuning speed shown is a typical value for each filter series. The actual tune time can vary significantly based on frequency range and configuration. Please refer to the Switching Characteristics section of each filter series for the maximum tune time.
Pre/Post Selectors
Filter Series  PSEL1002  PSEL1003 
Operation  Pre/Post selector; filter/amplifier cascade  Broadband Preselector 
Frequency Coverage:  22000 MHz (2 to 30MHz bypass)  203000 MHz 
Inband IP3:  +13 dBm, (+2 dBm @ >1 GHz)  +3 dBm (average) 
Noise Figure: 
10 dB up to 1000 MHz 12 dB up to 2000 MHz 
8 dB (average) 
Gain: 
Receive: 7dB Transmit: 22dB with tunable 1dB steps of attenuation up to 19 dB 
20 dB ± 3 dB 
Selectivity: 
27 dB @ ± 10% 50 dB @ ± 20% 65 dB @ ± 30% 
35 dB @ ± 10% 58 dB @ ± 20% 65 dB @ ± 30% 
Power: 
+24 V @ 500 mA +12 V @ 350 mA +6.25 V @ 1A 12 V @ 200 mA +95 V @ 5 mA 
+24 V ± 5% +12 V ± 5% 2 Watts / path 
Tuning Speed (Typ)*:  25 µS  < 1 mS 
Size (inch) (HxWxL):  1.3 x 6.4 x 10 
1.0 x 6.5 x 5.0 (single) 1.0 x 8.0 x 9.5 (dual) 
Size (mm) (HxWxL):  33.0 x 162.6 x 254.0 
25.4 x 165.1 x 127.0 25.4 x 203.2 x 241.3 
Weight (lbs.) (Typ):  3 lbs. 
1.4 lbs. (single) 2.8 lbs. (dual) 
Weight (kgrams) (Typ):  1.4 kg 
.64 kg (single) 1.27 kg (dual) 
Temperature Range (°C)  40 to +50  40 to +50 
*Tuning speed shown is a typical value for each filter series. The actual tune time can vary significantly based on frequency range and configuration. Please refer to the Switching Characteristics section of each filter series for the maximum tune time.
Integrated Cosite Equipment (ICE) 

Part Number  Frequency Range  SATCOM Compatible 
Transmit (T) / Receive (R) Applications 
Selectivity (1)  Output Power 
Relative Price (2) 
ICE2003TR30512  30 MHz – 512 MHz & Lband 
T & R  +47 dBm  $$  
ICE2004R30512  30 MHz – 512 MHz  R  Cancelation  N/A  $$$$  
ICE3001TR243318  243 MHz – 318 MHz  YES  T or R  +18 dBm  $$  
ICE3002R243270  243 MHz – 270 MHz  YES  R  N/A  $$$  
ICE3003T292318  292 MHz – 318 MHz  YES  T  +53 dBm  $$$  
ICE3005TR225400  225 MHz – 400 MHz  T & R  +45 dBm  $$$$  
ICE3006TR225400  225 MHz – 400 MHz  YES  T & R  +51 dBm  $$$$  
ICE3007T292318  292 MHz – 318 MHz  YES  T  +43 dBm  $$$  
ICE3008TR30174  30 MHz – 174 MHz  T & R  +47 dBm  $$$$  
ICE3009TR108400  108 MHz – 400MHz  T & R  +47 dBm  $$$  
ICE3011TR30400  30 MHz – 400 MHz  T & R  +45 dBm  $$$  
ICE4001T225400  225 MHz – 400 MHz  T  +41 dBm  $$  
ICE4002T225400  225 MHz – 400 MHz  YES  T  +52 dBm  $$  
ICE40032TR225400  225 MHz – 400 MHz  2T & 2R  +44 dBm  $$$$$  
ICE4004TR118400  108 MHz – 400 MHz  T & R  43 dBm  $$$  
ICE4005T290320  290 MHz – 320 MHz  YES  T  +51 dBm  $$$  
ICE5001TR225400  225 MHz – 400 MHz  YES  T & R  +52 dBm  $$$$$  
ICE5002TR30400  30 MHz – 400 MHz  T & R  +51dBm  $$$$$  
ICE5002TR30406  30 MHz – 406 MHz  T & R  +51dBm  $$$$$ 
 Selectivity is relative to all ICE products with representing the least selective and representing the most selective.
 Price is a relative factor of price per channel with “$” being the lowest price and “$$$$$” being the highest price.