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Lumped electronic RF-filters content:

• Rf-filter design
• Parametric filter dedects corona in molted transformers
• 2 Pole parametric IF bandpass matches OP-AMP
• A simple parametric IF-Filter
• A two bandwidth filter
• DVB-covering filter
• 1 GHz mixer- filter
• High selectivity universal filter
• 140 MHz broadband If-filter for noise measurement
• QPSK-Data Filter for Digital Radio
• Minimum Coil parametric 70 MHz If Filter
• IF Parametric-Filter has cos roll off
• Simplified IF Parametric-Filter has cos roll off
• Classical Butterworth Bandpass is realized with SMD Coils
• Parametric filter matches receiver input
• Parametric Data IF Bandpass has 2 finite poles

Parametric filter dedects corona in molted transformers

The insulation of high voltage transformers usually is measured by watching the DC-leakage current. But in molted transformers the problem of unknown tiny air bubbles exist. This bubbles my create ionization in the captive air.The corona effect may exist in a transformer for years after the high voltage breakdown will happen.The ionization bubbles can be found, watching the MHz leakage current with a scope. But this will lead to lot of mismeasuremends due of added radio frequencys.To filter this radiowaves , a proper filter must used. The requirement of the filter is : Fo = 1 MHz, Zin = 1 kOhm, Zout = 50 Ohm, B =100 khz, Fig. 1 shows the measurement circuit, using a sensitive oscilloscope to watch the corona.

             Fig 1 Corona measurement equipment

The Filter is a transforming parametric circuit using lumped components rare then an OP-active filter which could be damaged from high voltage sparcs. Fig.2. Fig.3 shows S21 .

Fig.2 Corona measurement filter                                                                                                                                                                                                                                                                 Fig.3 corona measurement filter S12

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2Pole parametric IF bandpass matches OP-AMP

The above simple 2 pole parametric bandpass can be modified to be a 10Mhz IF-Filter matched to the OP-AMP in and outputs.The input of the filter has 1 Ohm to fit to the OP-ouput, whereas the output is resistive matched with a 100 Ohm resistor to fit the Megaohm input of OP-AMPs.The series connection of two or three filters between OPs give an high selectivity If-Filter.Fig 1 shows the circuit and Fig.2 the selectivity of one single filter.

Fig.1 OP-AMP bandpass

           Fig.2 S21 of op bandpass

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A simple parametric IF-Filter

We get a simple narrowband IF filter using:

The circuit is gained using Norton’s transformation and matches the output of RF-Amp to a transistor. Fig.1

  Fig. 1 IF-Filter 10 MHz

• C1 = 200 pF; C2 = 100 pF; C3 = 400 pF; C4 =1 2 pF; C5 = 560 pF; C6 = 200 pF; C7 = 200 pF; C8 = 2 pF; L1 = 450 nH; L2 = 430 nH; L3 =4 000 nH; L4 = 1014 nH;
• C1, C8 and L3 are problem less components and may be fixed SMD-components.
• The parasitics at this frequencies are uncritical

Fig. 2 Simple IF-Filter

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Bandwidth switchable If-filter

To enhance the selectivity in a receiver, it is often help full to decrease the bandwidth with a switch. Fig.1 shows a IF-filter having two bandwidthes.The switches can be a small coaxial relais.Their little parasitic capacitors will not affect the function of the filter to much, because on their nodes are big capacitors of 400 pF:

Fig.1 Double bandwidth if-filter

The component values are:

• C1 = 200 pF; C2 = 100 pF; C3 = 400 pF(+10 pF); C4 =15 pF (-7 pF); C5 = 555 pF (+5 pF); C6 = 300 pF; C7 = 2000 pF; C8 = 2 pF; L1 =440 nH; L2 = 430 nH; L3 = 4000 nH; L4 =1014 nH;

             Fig.2 S12 of the double bandwidth filter

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DVB-Covering Filter

Outside band signals my disturb the digital television DVB especially at the upper frequencies of 800 Mhz. To reject this unwanted signals, a DVB-Covering filter was developed.We get a practical circuit using a the following parametric filter:

  The circuit shows Fig 1 and is developed using a parametric filter software and Norton’s transformation.

 Bild 3 DVB-Covering Filter   !!! Z=50Ohm

• C1 = 0,5 pF; C2 =10 pF; C3 = 2,2 pF; C4 = 1,5 pF; C5 = 2,4 pF; C6 = 0,5 pF;
• C7 = 0,5 pF; C8 = 1 pF; Cpar.= 0,5 pF
• L1 =15 nH; L2 = 20 nH; L3 = 160 nH; L4 = 18 nH;  L5 = 11,6 nH;
• C -Values are CAD-values, in a practical circuit, the measured parasitic values must taken into account

                                      Fig. 2 S21of Pf341

• Frequency band = 400 MHz to 800 Mhz

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1 GHz Mixer-output filter

A Mixer must work into some resonating device to gather the wanted mixing products. A small filter can do this job, but it must be properly matched. We use: and find the circuit of Fig.1 where the parasitics do not affect the function as Fig.2 shows.The low output coil in a practice circuit is only a small path on the PCB and is calculated with a Q of 20. The black component values are computed values.The PCB -capacitors must be subtracted.

Fig.1 Mixeroutputfilter-circuit

                                                               Fig.2 S21 of Mixer filter.   Matching values are :S11 and S22 = 12dB.

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A receiver-unversal-polefilter

This filter is an example of filters having certain frequency poles.This example shows the high increase on rejection outside the band. We use a wave parameter filter from [3] having two resonators at Fo and two poles at frequencies near the bandend. As we want high low insertion loss in the band, we add a third resonator at Fo. This filter is an universal filter to be changed for many purposes. For instance -narrow band -channel filter or high selectivity if filter. Impedance or frequency changing is easy. To change the bandwidth , the poles at band end must equally changed in percentage to broaden the band. This filter is easy to handle in a CAD-program, the three resonators are always at Fo and the poles can than be adjusted.This allows to build a bandwidth switchable filter. Fig.1 shows the circuit for Fo =100 Mhz and Fig.2 the transmission S21setting Q=100 . Lower impedance on the input or output can be realized as it is well known , using taping of the first coil.

                                                   Fig.1 Wpo2 -high selectivity universal filter

                                              Fig.2 wpo2 universal-high selectivity-filter

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Noise measurement using a parametric if filter

To measure the noise produced in a communication system, the bandwidth of the noise measurment device has to be limited over the bandwidth of the system. The noise power must be measured after the filter and the generated noise in the source can be computed from the ENR value:

     Fig:1 Noise measurement in a communication channel

                          Fig.2 Formulas to compute the generated noise

The necessary data’s are: fo=140 Mhz; Bandwidth 1dB =+-6 Mhz, Bandwidth 3dB =+- 10 MHz ,  Bandwidth 20dB = +-15 Mhz. The used filter is a parametric filter having the grade of 11. The poles are :   4,1,3,  Grad =11 . A  PCB circuit using fixed air coils is used. To tune the filter, SMD-trimmers have been used .To manufacture a series product a lot of tolerance considerations still has to be made.The circuit up to now is shown in Fig. and the transmission S12 in Fig.4.

                                                     Fig.3 140Mhz noise measurementfilter

                                      Fig.4 S21; 413-Filter

                                          Fig.5 S12; 413-Filter

                                            Fig.6 S22; 413-Filter

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QPSK-Data Filters for digital radio

Today , digital modulated communication channels are everywhere. Ignorant people may think its digital communication, using data jumps from 1 to zero, the same as digital logic . But this is not the case.We look how the communication bit’s look like and show how to design lumped RF-data filters for high bit rates. Look at two different digital bit’s, the hard jump and the soft cosines shape jump and its amplitude density. We get this density from a Fourier transformation . Fig.1.

                                                 Fig.1 Amplitude density of data bits

 Fig. 2 shows the graphical results of this power density .The hard data bit produces power  at multiples of the bitrate. To avoid data confusion due of this unwanted power, soft square cosines bit’s are used in digital data communication.

Fig.2 Power density of different bit’s..                                     Fig. 3 Filter specification of digital radio mod-demod filter

Now we look to a classical specification of a filter to change the digital output bit’s of a QPSK-modulator into a cosines square shape. And look too, to a QPSK- demodulator specification to transform the data bit’s content , back to analog values. Fig.3. We see the shape of a very broadband low pass filter having a little sinx /x over swing before crossing the Nyquist frequency fbit. At this frequency the roll off function is working, which is a cosines square function having a variable steepness n. The demodulator filter is indeed a sinus function, written in cosines form without sin x/x increase. At Fig 4, the equations can be found:

Fig.4 Digital radio mod-demod -filter specification. formulas

 The Filters

Mod and demod filters are both low pass filters, I use the same simple basic filter , it’s a n-pole low pass from a catalogue[3] and select it to match approximately the specification requirements. Now how can we get the cos roll off and the sinx / x increase? We get the roll off, by shaping the poles resonator with a resistor only damping at a certain series resonator frequency .The sinx / x  increase is very easy to get. A little differentiating resistor into the infinite pole capacitor enhances the corner frequency over swing.This filter is much more easier to adjust than an full computed overswing filter without resistors, where all components my change the roll off too. Fig 5 shows one pole of the filter.  Fig.6 Shows the transmission of such a modulator filter, using 5 poles in series.At the 2 upper frequencys, the poles work without resistors..

Fig.5 Universal data filter pole

Fig.6 Example of communication modulator filter transmission

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Minimum Coil Parametric 70 MHz If Filter

Parametric filters need less coils than every other filtertype. Here is a minimun coil broadband 70 Mhz Parametric Filter The circuit of Fig.1 is the result of  computerized solving of the H(s) and K(s) equations, of a parametric filter.The components are found in a computer  iteration process to nullify the equation H(s).The Circuit shows the typical C-Input of Parametric filters and has only 5 coils to get a grade of 11. Fig.2 presents the transmission .

Infinit poles = 3. Zero poles =4. Finite Poles = 2. Grad = 2*2+4+3=11.

                                       Fig.1 Circuit and values of Fil342

                        Fig.2 Fil.342 Transmission

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 IF Parametric-Filter has cos roll off

RF Receivers for digtital data have three main sections, the RF section, including the input front end, the If section and a digital demodulator. As it can bee seen at PQSK-Data Filters for digital radio , the digital demodulator needs a low pass having a very tight specified cos roll off. This low pass, can be simplified, if the already existing IF filter has a cos roll off. . But realizing an IF Filter having this roll off; is not very easy . If we look to the filter catalogues; only very high graded Tschebischeff or Cauer filter can met this specification, but both filter circuits will not work prober at RF frequencies. Beside the specification, the filter to be designed must have a minimum number of coils  having a value greater than 30nH. The here shown filter is a computer designed parametric filter having a grade of 10, see Fig.1. The values of the coils, are big enough to use fixed standard SMD Coils with a Q of >70 at 70 MHz. ( For instance Stettner, Componex or Coilcraft types) To adjust the very tight roll off specification an adjustment is used for the roll off. (R1 and R2). The poles must be adjusted, using SMD trimmers. The adjustment is easy , only the poles must be set to its frequencys. Fig.2 shows the computed filter transmission and its specification .Matching is S11= S22 =15dB; .          Fig.1 If Filter with cos roll off

The values are:

• C1=4.2pF; C2=53pF; C3=0.5pF; C4= 500pF; C5=42pF; C6=0.5pF; C7=29,6pF; C8=53pF; C9=25pF; C10=4.2pF;C11=16pF; C12=132pF; C13=91pF; C14=40pF; C15=27pF; C16=4.2pF;
• L1=42nH; L2=378nH; L3=56nH; L4=127nH; L5=200nH; L7=500nH;
• R1,2=100-300Ohm ;
• POLES:                   38MHz    48MHz                                  92MHz  106MHz                 

                      Fig.2 Transmission of the cos roll off filter

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 IF Parametric-Filter has cos roll off for wider specification

RF Receivers for Digital data have three main sections, the RF section, including the input front end, the If section and a digital demodulator. As it can bee seen at PQSK-Data Filters for digital radio , the digital demodulator needs a low pass having a very tight specified cos roll off. This low pass, can be simplified, if the already existing IF filter has a cos roll off. Above, a very complicated filter has been shown, fulfilling a very tight specification. Here a 70 MHz bandpass is shown, having a cosines roll off, to fit in a somewhat wider specification: The electrical values fits SMD components. Adjustment is easy if the two poles are tuned to its frequencis.Fig. 1 shows the the circuit and Fig.2 the transmission.

                   Fig.1 Simplified Parametric bandpass having cosines roll off.

The values are:

C1=30pF; C2=28pF; C3=20.4pF; C4=1pF parasitic; C5=13.8pF; C6=5pF; C7=63pF; C8=120pF; C9=83pF; L1=185nH; L2=137nH; L3=185nH; L4=200nH;L5=118nH;

                  Fig.1 Simplified parametric cos roll off band filter

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Classical Butterworth Bandpass is realized with SMD Coils

Classical Radio Bandpasses have been realized by means of  magnetic coupled trimmed coils in a metallic housing. A better modern design, is a capacitor coupled filter having small SMD coils used as resonators. Here is an example of a 4 resonator 70 MHz broadband bandpass to be used in a telecommunication data receiver. Fig.1 shows the circuit. As the SMD coils are not trimmed at all, three adjustment SMD capacitors are used to compensate the 5 % delivery tolerances of the coils. Fig 2 shows the S12 transmission at 200 Ohm matching, S11/ 22 is 20 dB. On a PCB, the coils must be placed between small flat metal shields, soldered on the PCB, to prevent magnetic coupling.

 The circuit values are :

• C1=30pF; C2=10pF ; C3=1,5 -2,5pF, C4=0,8-1,5pF ; C5=1,5 -2,5pF; C6=10pF; C7=47pF;
• L1 =415nH; L2 =520nH; L3 =470nH; L4=330nH;
• Z = 200Ohm; Q = 100;
• Tolerances of capacitors 1%
• Tolerances of coils 5%

As the components are very tight selected, adjustment time of the filter is only a few minutes.

                                     Fig.1        Classic Butterworth bandpass data filter

                                                                 Fig.2 Tuned bandpass

The numbered shapes  are : (S12 is matched gain) :

• #1 is  S12 IF all coil values at +5% .
• #2 is  S12 IF all coil values at -5% .
• #3 is S12 of #1 tuned to fo using C3, C4, C5.
• #4 is S12 of #2 tuned to fo using C3, C4, C5.
• S22/S11 = 22dB

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Parametric filter matches receiver input

Show IF-filter The IF- Filter shown above in this site can be transformed using Norton’s transformation, to match input antennas to a RF inputamplifier. To tune this grade 7 filter, only two variable capacitors are necessary.  The matchings are:

• 50 Ohm antenna to 25 Ohm common base transistor input. The circuit is at Fig. 1 and the transmission S21 at Fig.2
• High Q Coil on a ferrite antenna having a impedance of 2kOhm to a 100 Ohm Amplifier input. It is the same circuit, but input and output have been changed.

                                     Fig.1, 50 Matching of a 50 antenna to a common base  transistor

The values are: C1=8pF; C2=15pF, C3=27pF; C4=1,5pF; C5=40pF; C6=5pF; C7=40pF;C8=1pF; L1=33nH;L2=33nH;L3=300nH; L4=100nH; (High Q SMD Coilcraft Aircoils)

                                           Fig.2  S21 at frequency tuning . C3 from 20 to 28 pF and synchrony C5 from 30 to 40 pF

                                                   Fig.3 Matching of a ferrite antenna

The values at  fo = 80 Mhz are: C1=15pF; C2=30pF, C3=1pF; C4=2pF; C5=25pF; C6=25pF; L1=200nH;  L2=160nH; L3=4uH; L4=180nH; The resonance frequency of L3 must be >3fo, therefore this coil must be realized using a high permeability RF-Ferrite, to need only 3 to 7 turns. Component Values can be scaled by fo1/fo2 to change fo.

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Parametric Data IF Bandpass has 2 finite poles

Parametric Bandpasses with finite poles  are a good choice for broadband data filtering .As an example, a 2 finite pole bandpass has been computed using a FORTRAN Program . The circuit was  found by means of nullifying the roots with circuit elements in an interactive process. The program finds many solution circuits which all solves the roots.The designer has to decide which circuit has to be used. Fig 1 shows the chosen circuit.The output impedance is set to the 90 Ohm to optimize PCB wiring. Fig.2 shows the computed S21 transmission values for broad and narrowband. If the infinite pole frequencies are changed using C2 and C6 synchronously, the bandwidth may be changed up to 30 % .

                                Fig.1 Broadband parametric-bandpass realized with 2 infinite Poles

The Component values are:

• C1=300pF;C2=57pF; C3=35pF; C4=15PF;C5=130pF;C6=90pF; C7=18pF;
• L1=230nH; L2=350nH; L3=60nH; L4=30nH;
• Q1,2,3,4 = 120;

               Fig.2  Transmission of Broadband parametric bandpass

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