Arrl - Qst Magazine - Receiver Band-Pass Filters (1999) Ww.pdf - Krótkofalarstwo - ei6kd

Receiver Band-Pass

Maximum Attenuation

in Adjacent Bands

Third-order Cauer filters can boost performance of

multi-transmitter, multi-operator contest stations to

the “next level.” The filters are practical and you

don’t need expensive test equipment to align them.

By Ed Wetherhold, W3NQN

ARRL Technical Advisor

plained how to design, assemble

and test six three-resonator band-

pass filters (BPFs) for attenuating the

phase noise and harmonics of the typi-

cal 150-W transceiver, both on trans-

mitting and receiving. In this article,

I will explain how to design, assemble

and tune smaller-sized four-resonator

BPFs having maximum attenuation in

the two ham bands adjacent to the

band being received. The BPFs are in-

tended for connection to the 50-

contester, where six receivers and six

high-power transmitters are in simul-

taneous operation, and the receivers

need preselection filtering to prevent

front-end overload.

Several receiving band-pass filter

designs are in current use by the multi-

multi contest fraternity, but they are

either difficult to assemble, have in-

sufficient attenuation, or lack design

information so the interested reader

can confirm the correctness of the de-

sign or try a different design. For ex-

ample, an article in CQ CONTEST

Magazine 2 described a group of band-

pass filters for the multi-multi opera-

tor station. Although the BPFs had

exceptional stop-band attenuation (on

the order of 80 dB in adjacent bands),

the number of components (seven in-

ductors and seven capacitors) was

more than really needed, and the con-

struction was difficult. The author

made a passing reference to Zverev’s

Handbook of Filter Synthesis 3 as a

source of the designs, but no explana-

tion of the design procedure was in-

cluded; consequently, none of the BPF

designs could be confirmed. Other re-

ceiver BPF designs used over the past

15 years by the better-known multi-

multi operators used a capacitively

coupled three-resonator design with

capacitor input and output. The at-

tenuation of low-frequency signals was

very good because of the capacitive

coupling, but the high-frequency per-

formance was poor. In addition, the

tuning procedure was difficult unless

you used a network analyzer.

In comparison, the new four-resona-

tor receiver BPFs described below

input terminals of a receiver. They are

especially useful for the multi-multi

Ω

1 Notes appear on page 33 .

1426 Catlyn Pl

Annapolis, MD 21401

tel 410-268-0916

July/Aug 1999 27

Filters Having

I n a recent QST article, 1 I ex-

need only four inductors and four ca-

pacitors for each BPF. Preliminary tun-

ing of the four resonators requires a

signal generator and detector, with fi-

nal tuning using the return-loss test

described in a previous article. 4 Stop-

band attenuation of 60 to 80 dB is

obtained in the center of the bands ad-

jacent to the passband. The four induc-

tors and capacitors of each BPF can be

mounted on a piece of 1×1 1 / 2 -inch perf-

board and installed in a 2 1 / 8 ×1 5 / 8 ×3 1 / 2 -

inch aluminum Minibox. The design

procedure is fully explained. Anyone

having a computer can duplicate the

designs and confirm the correctness of

each design by means of free software

that is available.

Whether you want to update your

receiver BPFs for better selectivity, or

design different BPFs, this article will

show you how to do it.

third-order filter, the software costs

nothing! This unusual offer to the

Amateur Radio fraternity is made by

Jim Tonne, president of Trinity Soft-

ware. 8 Jim’s intent is that those seri-

ously interested in filter design and

analysis—either for amateur or profes-

sional purposes—should have the op-

portunity to become familiar with his

ELSIE (LC) filter-design and analysis

software. He is therefore offering a

demo disk of his DOS-based ELSIE

software to anyone who asks. Although

the program on the demo disk is lim-

ited to filters of the third order only, all

options of ELSIE are available for use.

These include plots and tables of all

parameters, ELSIE can design filters,

and tune the designs.

Those interested in either duplicat-

ing these third-order Cauer BPF de-

signs or designing other third-order

BPFs for different bands may obtain

ELSIE software on a 3 1 / 2 -inch floppy

disk by writing to Jim. In your letter,

please include a description of your in-

tended application and your filter-de-

sign background.

for the 160, 80, 40, 20 and 15-meter

BPFs are shown in Table 1 . With these

data, you can assemble and tune a set

of BPFs with confidence.

Fig 1 shows the schematic diagram

and the L, C and frequency values of the

40-meter BPF. As specified in rows 4, 5

and 6 of the 40-meter data in Table 1 ,

inductors L1 and L4 each have five

quintifilar turns of #18 and #20 magnet

wire wound on T94-6 powdered-iron

cores. A tap at the fifth turn above

ground serves as the input and output

connection to a 50-Ω source and load.

The other BPFs are wired in a similar

manner, except the 160 and 80-meter

BPFs use quadrifilar windings for L1

and L4 instead of quintifilar windings.

Using quadrifilar or quintifilar

windings on L1 and L4 results in an

interleaving of all turns, with a corre-

spondingly greater coupling between

turns than that obtained with the

more-customary single continuous

winding. The inter-winding coupling

reduces leakage inductance while op-

timizing the filter performance. This

same winding technique was used in

the wiring of the input and output reso-

nators in the transmit BPFs discussed

in an earlier article (see Note 1 ).

It was necessary to connect resona-

tors 2 and 3 of the BPFs to taps on L1

and L4 at 1 / 4 or 1 / 5 of the total turns so

that the component values of resona-

tors 2 and 3 would be practical. For

example, the inductive reactances of

L2 and L3 in the 40-meter BPF design

are 413 Ω and 391 Ω at 14.287 and

3.734 MHz, respectively. These rea-

sonable reactances can be achieved

Background

Some previously published designs

used two or three top-coupled resona-

tors, such as the N1AL designs, 5 or the

W3LPL designs. 6 K4VX used three-

resonator Butterworth designs for his

BPFs, 7 and the most-selective BPFs

by N6AW used seven resonators in a

series-parallel configuration (See

Note 2 ) . I elected to base my BPF de-

signs on the four-resonator, third-or-

der Cauer. The input and output shunt

resonators are tuned to the center fre-

quency of the passband and the two

series-connected resonators are tuned

to the center frequencies of the adja-

cent bands. The intent is to have one

more resonator than used in the sim-

pler designs while getting maximum

attenuation in the adjacent bands by

having two of the resonators tuned to

the frequencies where maximum at-

tenuation is needed.

Although the stop-band attenuation

of the third-order Cauer may be less

than that of the N6AW seven-resona-

tor design, the less-complex Cauer has

less passband insertion loss, and is

easier to assemble and tune. The de-

sign procedure to be explained shows

how to confirm each BPF design and

how to calculate other designs having

different center frequencies or band-

widths. The computer used for design-

ing needs only a DOS operating sys-

tem. The computer I used has a 386SX

microprocessor operating at 20 MHz

with MS-DOS Ver. 4.01.

In addition to a computer, you need

filter-design and analysis software.

Normally, such software would cost

more than $100, but for you to design,

analyze and plot the responses of any

BPF Design and

Confirmation Procedure

The design of these third-order Cauer

BPFs involves discovering the optimum

values of many parameters, such as

passband and stop-band widths, cen-

ter frequency, stop-band attenuation,

passband return loss, and impedances

of the input and output resonators.

Finding the optimum values of all these

would have been impossible without

the help of ELSIE . My ELSIE designs

C1, 4 = 100 pF

L1, L4 = 4.926

H F1, F4 = 7.17 MHz

C2 = 27 pF

L2 = 4.60 µ H

F2 = 14.29 MHz

C3 = 110 pF

L3 = 16.5

F3 = 3.734 MHz

Fig 1—Schematic diagram and component values of the 40-meter receiver band-

pass filter. The diagram is representative of all receiver BPFs, except for the 160

and 80-meter BPFs, which have quadrifilar windings for L1 and L4. See Table 1 for

the component values and coil-winding details.

28 QEX

g 9 9

with toroidal powdered-iron cores. If

these resonators had been connected

to the tops of resonators 1 and 4, the

reactances would have been impracti-

cal at 25 times greater; that is, at

10.3 kΩ and 9.7 kΩ.

If resonators 1 and 4 had been de-

signed for an impedance of 50 Ω at the

start, this would have eliminated the

need for taps, but then the inductance

and reactance of L1 and L4 would have

been much too low at 0.197 µH and

8.88 Ω to realize these inductance val-

ues with acceptable Q. The procedure

to obtain optimum component values

for all resonators is to design resona-

tors 1 and 4 for an impedance equal to

the square of 2, 3, 4 or 5 times 50 Ω,

and then to connect the center resona-

tors between 50-Ω taps on L1 and L4.

Whether a quadrifilar or quintifilar

winding is used for L1 and L4 depends

on the BPF percentage bandwidth. For

example, the percentage bandwidth of

the 80-meter BPF is (100 × BW )/ F c =

82.385 / 3.74 = 22%. This is a relatively

broad bandwidth, and a quadrifilar

winding is satisfactory. In comparison,

the percentage bandwidths of the 40,

20 and 15-meter BPFs are 11.3%, 5.0%

and 6.8%, and quintifilar windings are

more appropriate. The 160-meter BPF

has a relative percentage bandwidth of

11.7%, and either quadrifilar or quin-

tifilar windings could be used.

The BPF designs listed in Table 1

may be confirmed in two ways. The

simplest way is to use the analysis

option of ELSIE wherein the listed

component values are entered at the

ELSIE prompts, and the insertion loss

and return-loss response plots are

viewed to confirm that the design is

satisfactory. However, a minor correc-

tion to the tabular data for resonators

2 and 3 must be made before entering

their component values at the ELSIE

prompts because ELSIE is not capable

of evaluating tapped inductors. Con-

sequently, the two series-connected

resonators 2 and 3 must be moved to

the tops of resonators 1 and 4, and the

component values of resonators 2 and

3 corrected to account for the change

in impedance level. This is accom-

plished by multiplying and dividing

the tabular inductance and capaci-

tance values, respectively, of resona-

tors 2 and 3 by a factor equal to the

impedance of resonators 1 and 4 di-

vided by 50, or 1250 / 50 = 25. Fig 2

shows the schematic diagram and

component values of the 40-meter BPF

in a corrected form suitable for ELSIE

to analyze the design and plot the at-

tenuation and return-loss responses.

The attenuation peaks should fall in

the center of the 80 and 20-meter

bands, and the minimum passband

return loss should be 30 dB.

The BPF designs may also be con-

firmed by letting ELSIE assist you in

designing a BPF. At the ELSIE

prompts, enter the width of the pass-

band, the center frequency, the stop-

band width, the depth of the stop-band

attenuation and the impedance.

ELSIE will then design a BPF to meet

these requirements. The design val-

ues to use are listed in the first two

rows of each column in Table 1 , except

for the passband return loss, which is

not needed by ELSIE , and is included

only for reference.

After reviewing the attenuation and

return-loss response plots, you then

manually tune the design so the at-

tenuation peaks fall at the center of

the bands adjacent to the passband.

This is done by varying the C and L

values of resonators 2 and 3 while

maintaining a passband return loss

greater than 20 dB. Additional minor

adjustments can be made to the center

frequency so that the values of C1 and

C4 are convenient. For example, the

center frequency of the 40-meter BPF

was increased slightly from 7.15 MHz

to 7.17 MHz so C1 and C4 would be-

come exactly 100 pF instead of the

original nonstandard value.

When you are satisfied with the

tuned design, the impractical compo-

nent values of resonators 2 and 3 are

scaled from the design impedance to

50 Ω. The 50-Ω taps on resonators 1

and 4 serve as the BPF input and out-

put connections. Fig 1 shows the sche-

matic diagram of the completed design

of the 40-meter BPF.

The third-order BPFs designed by

ELSIE originated as classic Cauer

designs, where the minimum stop-

band attenuation both below and

above the passband are identical.

However, after the modifications, the

lower and upper-frequency minimum-

attenuation levels are no longer iden-

tical, thus showing that the design is

no longer a legitimate Cauer. For this

reason, these modified Cauer designs

cannot be duplicated using the pub-

lished Zverev tables. For our pur-

poses, this is of no concern as long as

the attenuation peaks are in the cen-

ter of the adjacent ham bands, and the

computer-calculated minimum pass-

band return loss is greater than 20 dB.

By using ELSIE to design these third-

order Cauers, what before was impos-

sible now becomes simple!

BPF Assembly and Tuning

Fig 3 shows the 40-meter BPF as-

sembled on a piece of perfboard in-

stalled in an LMB 873 aluminum

Minibox. The toroidal inductors are

secured to the perfboard by passing

their leads through the holes in the

perfboard, then sharply bending the

leads sideways. All capacitors are con-

nected to the inductor leads under the

perfboard. A cardboard strip insulates

the capacitor and inductor leads (un-

der the perfboard) from the bottom of

the aluminum box. The #18 wire leads

of L1 and L4 connect at each end of the

assembly to the center pins and

ground lugs of the phono connectors.

These four #18 leads provide sufficient

support to hold the assembly in place.

The other BPFs are assembled in a

similar manner.

The assembly of the BPF components

is greatly simplified by the omission of

shielding partitions between stages.

The lack of any shielding apparently

had no effect on the BPF stop-band

performance, since attenuation levels

greater than 80 dB were noted in the

upper frequencies in all the BPF tests.

Because resonators 1 and 4 must be

C1, 4 = 100 pF

L1, 4 = 4.926

H F1,4 = 7.17 MHz Z = 1250

Ω

C2 = 1.08 pF

L2 = 114.90 µ H F2 = 14.29 MHz

C3 = 4.40 pF

L3 = 412.80

H F3 = 3.734 MHz

taps on L1 and L4. Use these component

values if you want ELSIE to analyze the BPF performance.

Ω

30 QEX

Fig 2—Schematic diagram of the prototype third-order Cauer 40-meter BPF before

resonators 2 and 3 are moved to the 50-

tuned to the same center frequency,

successful tuning depends on using

precisely matched capacitors, prefer-

ably both having the same value, and

within one percent of the design value.

For the 40-meter BPF, this frequency

was 7.17 MHz, so C1 and C4 could be

standard values of 100 pF. Capacitors

2 and 3 can be within two percent of

the design values. Sufficient room

should be left on the T94 cores so the

windings can be squeezed or spread to

fine tune each resonator. This is im-

portant so that all resonators can be

tuned either to the center of the BPF

passband, or to the center frequency

of the adjacent amateur bands.

Initially, tune each resonator before

installation on the perfboard. First,

pass a single-turn wire loop from a

signal generator through the center of

the inductor. Then put a second loop

through the inductor, and connect it to

a sensitive wide-band detector. Vary

the signal-generator frequency until

you see a voltage peak on the detector

output meter; that indicates circuit

resonance. Measure the generator fre-

quency with a frequency counter, then

squeeze or spread the inductor turns

until a resonance peak is obtained at

the design frequency. After this, in-

stall the resonator on the perfboard

without disturbing the turns on the

core. A final check on the BPF tuning

is made with the return-loss response

test as explained in the referent of

Note 4 . After the final check, the in-

ductor turns may be secured with a

coating of polystyrene Q-dope. 9

BPF could be duplicated with an

ELSIE analysis only when C3 was

made equal to 490 pF instead of the

470-pF value, and F3 was 0.984 MHz.

The actual value of C3 is 20 pF

greater than the 470-pF capacitor in-

stalled on the perfboard because of

the inter-winding capacity of L3. Con-

2.6 inches) with the prepunched holes on a 0.1-inch grid. All

capacitors are mounted under the perfboard. A strip of cardboard glued to the

inside of the box bottom provides insulation between the BPF leads and the

aluminum box. The BPF input and output leads that are connected at each end to

the phono-connector center pins and ground lugs are stiff enough to hold the

assembly in place.

Special Tuning Considerations

To find the optimum parameters for

the 160-meter BPF, I used ELSIE’s

“tune” mode to find convenient capaci-

tance values while keeping the upper-

frequency attenuation peak in the cen-

ter of the 80-meter band and while

keeping the minimum passband re-

turn loss greater than 20 dB. The

placement of the lower-frequency at-

tenuation peak, established by reso-

nator #3, was not critical, and the C3

and L3 values were varied until a con-

venient C3 value of 470 pF was ob-

tained with a computer-calculated

minimum return loss of more than

20 dB. However, when the design was

assembled and final tuning adjusted

by observing the measured return-loss

response as seen on an oscilloscope, I

discovered that the optimum return-

loss response occurred when resonator

#3 was tuned to 0.984 MHz, not to the

original frequency. The measured re-

turn-loss response of the assembled

Fig 4—The plot shows the insertion-loss response of the 40-meter BPF as

measured with a network analyzer. The attenuation of signals in the adjacent 80

and 20-meter bands is greater than 58 and 85 dB, respectively. The passband loss

is about 0.5 dB and the passband return loss (not shown) is greater than 20 dB.

July/Aug 1999 31

× 1 5 / 8 inches (LMB 873). The T94 cores are installed on the top of a piece of

perfboard (1

× 2 1 / 8

Fig 3—The photo shows the 40-meter BPF assembled in an aluminum Minibox

3 1 / 2

Arrl - Qst Magazine - Receiver Band-Pass Filters (1999) Ww.pdf

Plik z chomika:

Inne pliki z tego folderu:

Inne foldery tego chomika: