PHASE LOCK LOOP AND ASSOCIATED COMPONENTS FOR
THE RADIO FREQUENCY SYSTEM OF THE PF STORAGE RING PARTIII

A.K. Mitra and H. Kobayakawa
National Laboratory for High Eenrgy Physics

Filters are often necessary in electronic systems where undesired frequency
components pollute the signal. The general design and constructio n of a band
pass comb line filter which will be used in the rf system of the PF storage
ring 1 is described here. Comb filters are easy to construct and give goodelectrical performance . These filters being small (A/8 or less) also have theadvantage of very broad stop bands above their primary pass bands. This typeof filter consists of an array of resonating lines coupled together. Theresonators are shorted at one end and the other end being loaded by a lumped
capacitor. The theory and design of comb filters is well known: However, adesign modificatio n which makes use of only n resonators (round rods) for a npole filter will be used~
It is convenient to work out the design of resonator lines in terms of
their capacitance s to ground per unit length and the mutual capacitance s perunit length between neighbourin g lines (Figure 1). For s + ~. the characteristic impedance of a single line
Zo(60/IE ) log (4b/nd)=rlog coth(ns/2b)

and one obtains=(Cab/s)(60/n o){log(4b/n d)}

The filter canthbene built with identical rods and choosing Z0
b/d = 2.52,resonator Q,
we get Cab/E=270 ~ for optimum

4.629 log coth (ns/2b)This gives the distance between rods (s/b) correspondi ng to the Cab's required.
This is valid for relatively small fractional bandwidth (< 10%) filters.
Instead of using additional bars for coupling to the 50 ~ input and output
ports of the filter, tapping onto the first and the last rod is a practicalway of producing the required loaded Q (loaded by resistor R) for the tworesonators. The tapping point 8 1 from the short circuit end (Figure 2) can beobtained from the following equations
Q1
R-------- ce +sin8 cose)ooo
2Z sin2 81

fandQ0qn (bW)3dB

80 is the electrical length of the resonators and q can be obtained fromfilter table. The tuning capacitor for center freqUency w (radious per0second) is
C = Y /(w tane 0 )00

A band pass comb filter is to be designed with the specificatio ns inTable 1. Choosin~ electrical length e as n/5 radians and from curves andThe filter can be Hesigned fully and the values are given
design formulae:•in Table 2. The photograph of the actual filter is shown in Figure 3 and the-223-

measured electrical performance is listed in Table 1. Figure 4 shows theresponse of the filter over a sweep bandwidth of 4 to 1300 MHz. The deviationof 0.1 dB bandwidth from the designed value is due to tolerances on thephysical dimensions of the filter. The performance of the filter is adequate
for suppressing frequency components outside its 490±20 MHz bandwidth,
References
1)
2)

Photon Factory Design Manual (1979).
G.L. Matthae, L. Young and E.M.T. Jones, Microwave Filters,
Impedance-Matc hing Networks and Coupling Structures, ppl74-197, 497-519
D. Boussard, How To Design A Com Line Band-Pass Filter,
CERN: LABII/RF/Int. Note/74
Reference Data For Radio Engineers, 177, Sixth Edition, Chapter 83)4)

Table 1

Elec.trical Performance of the Filter

Filter Perfcn~~anc.e~~~t=~ !~::~:~!n~8t::~ ~z3 dB bandwidth MHzRejection at ±20 MHz bandwidth dBInsertion loss d»Group delay nsecond
Input/Output VS~'R.

Table 2

Designed

Measured

490
4.95
6.0
> 70

490
2.5
6.0
> 70
7.1//J/fl'f/'lfi"/%/fl"A'i'~:.qLine a~GC~b·Y:'~"-·

156
1.5:1

Designed Physical Parameters of the Filter

Number of Resonators nDistance Between Plates b
Diameter of Rods dLength of Resonating Rods €1 0Tapping Point From Short-End 9 1
Spacing Between Rods s 12 "' S3 44105 23. s
Value of Lumped Capacitor C PF

17.22
6.38

_A z ~/7///~;//7/.%////boJs4
61.22
5.85
16.46

Figure 1

A Pair of Parallel-Coupled Lines

Figure 3

PhotoO'ranh of the Actual Filter

All Dimensions are in 1111!1e1 ' --.1

1nIc"
3

Figure 2~c"nJ

A Comb-Line n-Pole Band-Pass Filter

Figure 4

Amplitude Response for a Sweep Bandwidthof 4 to 1300 MHz. Vertical Scale 10 dB/ div.

-224-