Evanescent Mode Circular Waveguide Filtersk5tra.net/tech library/filters - general/WG filters...3...

K5TRA

EVANESCENT MODE CIRCULAR WG FILTERS

2K5TRA

MOTIVATION: NEED IMAGE FILTERS for X-BAND TEST-SET

Splitter

Image

Filter

Image

Filter

11.250 GHz

DRO

IF

Port -1

IF

Port -2

RF

Port -1

RF

Port -2

3K5TRA

X-BAND TEST-SET CONFIGURATIONS

S21 MEASUREMENT

S11 MEASUREMENT

Splitter

Image

Filter

Image

Filter

11.250 GHz

DRO

IF

Port -1

IF

Port -2

RF

Port -1

RF

Port -2

DUT

Splitter

Image

Filter

Image

Filter

11.250 GHz

DRO

IF

Port -1

IF

Port -2

RF

Port -1

RF

Port -2

Directional

Coupler

Measurement

Port

4K5TRA

X-BAND TEST-SET

5K5TRA

LADDER FILTER BASIC BUILDING BLOCK

• Passband: Z1 short and Z2 open

• Stopband: Z1 open and Z2 short

6K5TRA

J and K INVERTERS PROVIDE REUSE OF RESONATOR TYPE

• Impedance inverter (K) with a series resonator behaves like a

parallel resonator

• Admittance inverter (J) with a parallel resonator behaves like a

series resonator

• Impedance/admittance inverter interface between similar

resonators provides maximum stopband attenuation

• Most common impedance inverter is transmission line that is an

odd multiples of l/4

Z INVERTER Y INVERTER

K J

7K5TRA

IMPEDANCE/ADMITTANCE INVERTERS

• Impedance (or admitance) inverters

can be used to convert parallel

resonance to a series resonance

characteristic.

• The canonic impedance inverter is the

l/4 line.

• LC forms provide moderate bandwidth

Z inversion.

• Capacitive T and p sections are for

narrow band applications. Negative C

or L is absorbed into resonator

(cancels some positive C or L).

4,0

l Z

Transmission Line

J (or K) Inverter

CZ

0

0

1

- C

C

- C C

- C - C

LZ 00

L

- L - L

- L

L

- L

LCC

LZ

1, 00

L

C C

L L

C

8K5TRA

BANDPASS FILTER STRUCTURE WITH J INVERTERS

• Filters are formed as cascade of parallel resonators

and inverters

• How do we realize a structure like this in WG?

J J J J

9K5TRA

BANDPASS FILTER WITH INDUCTIVE J INVERTERS

• Negative inductors of inverter cancel some of the resonator

inductance

• Inverter admittance/impedance sets coupling between

resonators. In this case, coupling is set by ( L)-1

Y INVERTER

J

L

L" L"

L

- L - L

10K5TRA

EVANESCENT WG MODEL

• Operation BELOW TE11 cutoff

frequency

• Propagation falls off quickly

• Behavior is reactive (inductive)

Ls

Lp Lpa

l

COPPER PIPE

CIRCULAR WG

𝛾 =2𝜋

𝜆0

𝐹𝑐𝐹0

2

− 1

𝑋0 ~ 599 𝛺

11K5TRA

EVANESCENT WG SECTIONS PROVIDE J INVERTER and RESONATOR L

• RESONATOR C CAN BE ADDED WITH TUNING SCREWS

• SCREW SPACING SETS Ls (SETS COUPLING)

Ls

Lp Lpa

l

COPPER PIPE

CIRCULAR WG

12K5TRA

EVANESCENT WG FILTER CONSTRUCTION

• 4-40 tuning screws

• Tapped holes and soldered

brass nuts

• Stainless locking nuts

• Coupling loop with 2-56 tuning

• 0.25” 4-40 stand-off mounting

• Operation BELOW

cutoff frequency

• 0.5” Cu pipe for X-band

(actual ID = 0.565”)

13K5TRA

EVANESCENT WG FILTER ANALYSIS 6th ORDER EXAMPLE

INDUCTOR Qo FOUND EMPERICALLY

TO BE APPROXIMATELY 200

14K5TRA

DESIGN TABLES N= 2, 3, & 4

15K5TRA

DESIGN TABLES N= 5, & 6

16K5TRA

EVANESCENT WG FILTER INSERTION LOSS, 2 dB/DIV

17K5TRA

EVANESCENT WG FILTER INSERTION LOSS, 5 dB/DIV

18K5TRA

EVANESCENT WG FILTER -40 dB BW = 705 MHz

19K5TRA

EVANESCENT WG FILTER USED IN K5TRA X_BAND TEST SET