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Agilent
Network Analysis Solutions
Advanced Filter Tuning Using
Time Domain Transforms
Application Note 1287-10
    Introduction
    The level of experience and            The basic technique has been        This application note reviews
    expertise required to accurately       comprehensively covered in          these time domain tuning
    tune coupled-resonator cavity          Agilent application note 1287-8,    techniques, and extends the
    filters, cross-coupled filters, and    which also describes how            technique for use in tuning
    duplexers effectively precludes        coupled-resonator band-pass         filters with cross-coupled
    these devices from mass produc-        filters can be easily and deter-    resonators that produce
    tion at high speed. Ironically,        ministically tuned. To achieve      transmission zeros near the
    these same filters are increasing-     the proper passband response,       filter passband, as well as
    ly needed in large quantities, as      and to achieve good return loss     duplexer filters that have a
    a result of the spectral density       and passband ripple, the center     common (antenna) port, an
    resulting from the runaway suc-        frequency of each resonator is      upper passband (transmit) port,
    cess of wireless communications        precisely tuned, and each           and a lower passband (receive)
    services. The time required to         coupling between resonators         port.
    tune these filters accurately lim-     precisely set. The method is
    its manufacturers from increas-        based on the time-domain            Together with application note
    ing their production volumes           response of a filter's return       1287-8, this application note
    and reducing manufacturing             loss, in which the time-domain      provides a comprehensive
    cost. Fortunately, it is possible to   response is obtained by a special   compilation to filter tuning in the
    dramatically reduce both the           type of discrete inverse Fourier    time domain, including theory,
    time required to tune these            transform of the frequency          application, set-up, and tuning
    types of filters, as well as the       response. Readers are encouraged    procedures.
    experience and expertise               to review the material contained
    required. The method removes           in application note 1287-8 for
    filter tuning from the realm of        information about the basic
    art, and makes the process pre-        technique and how it is applied
    dictable and repeatable. Even          to tuning coupled-resonator
    relatively inexperienced filter        cavity filters.
    tuners can tune multiple-pole
    filters with great success with a
    minimal amount of training.




2
The technique defined
A five-pole coupled resonator fil-
ter with four coupling structures
will be used to illustrate the
basic tuning technique. A
schematic of the filter is shown
in figure 1, with the distributed
loss of the filter represented as
shunt resistance. To apply the
                                      Figure 1.
tuning method, the network
analyzer's frequency sweep is
centered at the desired center
frequency of the bandpass filter.
The frequency span is set to two
to five times the expected filter
bandwidth. The bandpass mode
of time domain transform is
applied to the return loss trace.
Figure 2 shows the frequency
response and the bandpass
mode time response of the filter,
a fifth-order Chebyshev with
0.25 dB of passband ripple.

Each plot shows two traces. The
lighter (red) one is the filter
return loss response with ideal
values for all the components,
and the darker (blue) trace
shows the effect of mistuning
one of the resonator elements
(in this case, the second res-
onator). The upper plot is the
frequency response and the
lower plot is the time domain
response. Notice the
                                      Figure 2.
distinctive dips in the time
response S11 of the filter            The essence of the tuning tech-      domain response with only the
(indicated by the triangles           nique is that the dips in the time   second resonator mistuned from
labeled 1-5). These are charac-       domain response correspond           its ideal (derived) value. In this
teristic nulls that occur if the      exactly to each resonator in the     case the capacitor CII was tuned
resonators are exactly tuned. If      filter. When the resonator is        to a few percent above its ideal
the center frequency of the           tuned properly, the null is deep.    value. It is clear that the dip has
measurement is changed even           If the resonator is not tuned, the   nearly disappeared. The dip will
slightly, the nulls start to disap-   null starts to disappear. Though     only be maximized when the
pear, indicating that the filter is   it may seem remarkable that this     capacitor is returned to its
no longer tuned. The peaks            exact relationship exists, exten-    ideal value. Note that mistuning
between the nulls relate to the       sive testing with many different     one resonator can affect the
coupling factors of the filter.       kinds of filters, as well as simu-   response from the other
This type of response holds true      lations and direct mathematical      "downstream" resonators.
for any all-pole filter, regardless   derivation, confirm this relation-
of filter type.                       ship. Figure 2 shows the time




                                                                                                                 3
    Basic tuning method
    The basic time domain tuning           This first adjustment will exactly   3. Adjust the next coupling from
    method for simple all-pole             center the filter and provide           the input and output to match
    filters, is to measure the time        optimum tuning for the given            the associated peak in the
    domain response of S11 and S22         coupling factors. Many filters          template response. Readjust
    of the filter. The filter resonators   have adjustable coupling factors        the resonators adjacent to this
    are adjusted with the following        that must be tuned to generate          coupling to restore the nulls
    steps:                                 the desired filter response, par-       to be as deep as possible.
                                           ticularly bandwidth and return
    1. Starting with the first and last    loss. The coupling adjustment        4. Continue in this manner until
       resonators, tune to create a        can be accomplished with the            all couplings have been
       deep null in the time responses     following steps:                        adjusted to match the peaks
       of the S11 and S22 measure-                                                 of the filter template, and all
       ments respectively (the nulls       1. Create a filter template by          resonators have their
       will be at approximately t=0).         measuring an existing tuned          associated nulls as deep as
                                              filter or from a filter simula-      possible.
    2. The next resonator from the            tion, and load it into the net
       input and output are then              work analyzer's memory            Note that adjusting one coupling
       tuned for deep nulls (which            traces for S11 and S22.           will affect all couplings that
       will appear approximately at                                             follow, so it is important to start
       t=1/BW where BW is the filter       2. After the initial resonator       with the couplings at the input
       bandwidth). Tuning the                 tuning described above, adjust    and output and work toward the
       second resonator will slightly         the input and output coupling     center.
       pull the first, since they are         to match the amplitude of the
       coupled.                               first peak of the target S11
                                              and S22 filter response.
    3. The previous resonators (first         Readjust the first and last
       and last, in this case) are            resonator to restore the first
       readjusted to restore the null         S11 and S22 nulls to make
       in the time domain trace to            them as deep as possible.
       make it as deep as possible.

    4. Continue in this manner,
       working in toward the center,
       until all the resonators have
       been adjusted for a deep null.




4
Tuning filters with
cross-coupled
resonators
For many communication appli-
cations, it is necessary to make a
filter skirt response steeper than
normally obtained by all-pole
type filters. Discrete transmis-
sion zeros (where the S21 goes         Figure 3.
to zero) can be obtained in the
filter stopband by adding cross-
coupling (coupling between res-
onators other than nearest
neighbors). The number of res-
onators that the coupling "skips
over" will determine the charac-
teristics of the transmission
zeros. Skipping over an odd
number of resonators, as seen in
figure 3, results in an asymmet-
ric frequency response, with a
zero on only one side of the
passband. Skipping over two res-
onators results in transmission
zeros on both sides of the pass-
band. The time domain response
of these filters differs from the
all-pole filters, in that tuning the
characteristic nulls to be as deep
as possible does not result in the
                                       Figure 4.
filter being properly tuned.
                                       Figure 4 shows the frequency          The filter was optimized for
                                       and time response of the four-        return loss in the passband and
                                       pole filter with asymmetric cross     rejection in the upper stop band.
                                       coupling from figure 3. The filter,   Notice from the time response
                                       in this case, had coupling adjust-    that the nulls are not deep for
                                       ments for only the input, output,     many of the resonators. The
                                       and cross-coupling. The coupling      design methods for simple,
                                       between resonators was fixed.         all-pole filters help illustrate
                                                                             why this is so, and how to tune
                                                                             these filters.




                                                                                                                 5
    All-pole filters
    All-pole filters are designed by
    starting with a low-pass proto-
    type filter, then applying a trans-
    form to shift it up in frequency
    from "DC-centered" to the
    desired center frequency. The
    essence of the design process is
    that the coupling values are
    derived only from the low-pass
    prototype component values.
    The resonator values are derived
    by making the resonant frequen-
    cy of the node (which includes         Figure. 5. The time domain response separates
    the input and output coupling)         the response from each node.
    equal to the center frequency of
    the filter. For example, in the fil-   To illustrate this point, consider              time zero, the reflection from
    ter in figure 1, the resonant fre-     the response of a filter to an                  node 2 will look as though both
    quency of the second node is           impulse, as shown in figure 5.                  C12 and C23 are grounded. The
    defined by the elements L2 in          As the impulse proceeds though                  delay between these pulses will
    parallel with CII plus C12 and         each node of the filter, part is                be due to the coupling, so less
    C23 (the coupling elements), and       reflected and most is transmit-                 coupling (which results in a
    it exactly equals the filter center    ted. If the filter is uncharged                 narrower filter) will have more
    frequency. This is true for all the    before the pulse arrives, the                   delay. This is the same relation-
    nodes, including the first and         reflection from the first node                  ship used to design the all-pole
    last, which have only one cou-         will look as though the coupling                filter. It then becomes clear why
    pling added.                           capacitance, C12, is grounded on                tuning for deep nulls with the
                                           the far side. That is, the time                 network analyzer tuned to the
    The time domain response of a          domain reflection will be the                   filter center frequency succeeds:
    filter node has a deep null when-      same as a circuit that is tuned to              The response from each individ-
    ever the frequency sweep of the        the "node frequency" consisting                 ual node is centered on the same
    network analyzer is exactly cen-       of C12 + C1 in parallel with L1.                frequency.
    tered on the resonant frequency        Since the pulse goes to zero after
    for that node. Further, the time
    domain response shows the
    response of the filter nodes
    separated in time. This separation
    is caused by the delay through
    each filter section, which Fano
    showed to be inversely propor-
    tional to the filter bandwidth.
    The time domain response will
    have sufficient resolution if the
    frequency sweep is at least twice
    as wide as the filter bandwidth.




6
Effects of cross coupling
With cross-coupling added to the
filter, the time domain response
no longer has the simple
relationship to filter tuning.
Further, especially in filters with
asymmetric transmission zeros,                                                 VNA center frequency tuned
                                                                               for deepest null on resonator 2
tuning of the filter is not optimum
when each node frequency is                     VNA center frequency =
tuned to the filter center                      filter center frequency
frequency. Recall that the node                                            Time ( ns)
frequency is defined to be the
resonant frequency of the node        Figure 6. Change in time response when the
with all connected couplings,         VNA center frequency is tuned.
including cross coupling,
grounded. The resonators are          The argument still holds for the                  Figure 6 illustrates the time
often "pulled" to compensate for      time response of any particular                   response of the filter tuned at
the effect on the pass-band of        node of a filter having a deep                    the filter center frequency, and
the transmission zeros in the         null when the node frequency is                   then tuned to a frequency that
stopband, thus achieving the          exactly centered on the network                   maximizes the null associated
desired passband return loss          analyzer frequency. The difficul-                 with resonator 2 (one of the res-
specification. This results in an     ty with these complex filters is                  onators with cross coupling).
asymmetric shape to the return        that the node frequencies are no
loss, as demonstrated in figure 4.    longer easy to determine. But                     This process is repeated for each
Tuning for deep nulls results in      the network analyzer itself can                   of the filter's resonators, adjust-
a filter that does not meet the       be used, on a properly tuned or                   ing the VNA center frequency
return loss specifications.           "golden" filter, or on a simulated                until each null is maximized. For
However, the discussion about         filter, to discover the individual                best sensitivity, the frequency
figure 5 points to a method that      node frequencies. This is done                    span is reduced to just two times
will allow tuning filters with        by setting up the vector network                  the bandwidth. Table 1 gives the
cross coupling in the time            analyzer (VNA) in dual-channel                    node frequencies determined for
domain.                               mode, with one channel on fre-                    each resonator for the filter from
                                      quency domain and one on time                     figure 4. Armed with this infor-
                                      domain. The center frequency of                   mation, and using the measure-
                                      the VNA is adjusted while look-                   ment from figure 4 as the tuning
                                      ing at the null associated with a                 template, a filter tuning process
                                      particular resonator. When the                    for complex filters can be
                                      null is maximized, that frequen-                  defined.
                                      cy is recorded as the node
                                      frequency for that resonator.

                                      Table 1. Node frequency for each resonator
                                      Resonator no.      Node frequency
                                            1              836.25 MHz
                                            2              833.85 MHz
                                            3              834.55 MHz
                                            4              836.45 MHz




                                                                                                                              7
    Tuning of complex filters
    The filter from figure 4, with all                    2. Adjust the coupling to align            4. Finally, to get the resonators
    four resonators, the input and                           the time domain response                   tuned to their correct final
    output coupling, and the cross                           peaks with those of the target             values, set the VNA center
    coupling detuned, is used to                             filter, remembering to readjust            frequency to that listed in
    demonstrate this process.                                the resonators to get deep                 table 1 for each resonator,
                                                             nulls. Figure 8 shows the                  and tune that resonator for
    1. Assuming that the input and                           result of coupling adjustment.             maximum null. After a first
       output coupling is sufficient                                                                    pass, go back again and retune
       to produce an approximate                          3. Adjust the cross coupling to               each resonator to account for
       filter shape, start by tuning                         set the zero frequency to                  the pulling effect of tuning the
       the filter as though it were an                       match the S21 frequency                    other resonators. Figure 10
       all-pole filter. Figure 7 shows                       response target, as shown in               shows the final result of
       the frequency response before                         figure 9.                                  tuning this filter. It is clear
       any tuning, and after the res-                                                                   that the final response is
       onators (but not coupling)                                                                       nearly identical. Remember
       have been adjusted for                                                                           that the return loss tuning
       maximum nulls.                                                                                   was done entirely in the time
                                                                                                        domain.




                                                                 S21 &
                          S21 and S11                            S11
                         after first tuning                      Untuned
                                                                                                                            S11 Target
                Freq       .
                         MHz                                                                Freq . MHz

                                                   S11 before tuning




                   S11 after first tuning
                                                                  S11 Target                                                             S11 Target


                                     Time ( ns )                                                             Time ( ns )
    Figure 7.                                                                  Figure 8.




                                                                S21 Target




                                                                                                Freq . MHz
                       Fr eq . MHz


    Figure 9.




                                                                                                              Time ( ns )

8                                                                              Figure 10.
Duplex filter tuning
Duplex filters (sometimes called
duplexers), as seen from the
antenna port, have two paths
that contribute to the return loss
response, each with its own
delays and responses. The task
for the filter tuner, and the focus
of this section, is to separate
these responses so that each
side of the filter can be deter-
ministically tuned.
                                      Figure 11.
Duplex filters are used primarily
to separate the transmission
channel (Tx) from the receive
channel (Rx) in a wireless com-
munications base station.
Because the Tx and Rx are near-
ly adjacent, the filters tend to be
very asymmetric to create sharp
cutoffs for each band. Figure 11
shows the schematic of such a
duplexer. Note that a single
cross-coupling is used in each
side, but that the cross-coupling
is capacitive in one side and
inductive in the other. This gives
a lower transmission zero for the
Rx band (Rx is upper in this
case) and an upper transmission
zero in the Tx band as shown in
figure 12.                            Figure 12.


                                      Duplexers that have more than      resonator responses at the
                                      a bandwidth of separation          common port can come from
                                      between the Tx and Rx bands        either the Tx side or the Rx side.
                                      are easily tuned with the method
                                      noted above for tuning filters     In figure 11, the duplexer uses
                                      with cross-coupling. That is       quarter-wave transformers to
                                      because the network analyzer       isolate each side of the duplexer
                                      can be centered on the Tx band,    (the input impedance of the Tx
                                      with the span at greater than      side is a short circuit at the Rx
                                      two bandwidths, and still not      frequency). Other topologies
                                      have the Rx band interfere with    couple the common port to a
                                      the input or output reflection     broader-band common resonator,
                                      response. However, most duplex-    which is in turn coupled to the
                                      ers have substantially less than   first resonator on both the Tx
                                      one bandwidth between the          and Rx sides. With this configu-
                                      edges of the Tx and Rx bands       ration, the common resonator
                                      (a typical filter might have an    clearly cannot be centered on
                                      80 MHz bandwidth with 20 MHz       either the Tx or Rx passbands,
                                      of separation). These types of     instead it is centered somewhere
                                      duplexers make time-domain         in between.
                                      tuning difficult, because


                                                                                                              9
     Time domain response of
     duplexers
     The time domain response of
     duplexers is complicated by the
     fact that at the common port,
     reflections from both the Tx side
     and Rx side will cause some
     nulls in the time domain. Figure
     13 shows the time domain and
     frequency response of a real
     duplexer. To view the time-
     domain response in a way that
     makes sense, it is necessary to
     set the network analyzer center
     frequency to the frequency
     between the Rx and Tx pass-
     bands. The span of the analyzer
     must be set to at least two times
     the overall bandwidth of the Tx      Figure 13. Top half of display:   Upper trace = antenna common
     and Rx bands. The following                                            Lower trace = Rx
     example of tuning a real duplex                 Lower half of display: Upper trace = antenna common
                                                                            Lower trace = Rx
     filter uses a duplexer which has
     the common port coupled to a
     common resonator, which in
     turn is coupled to both the
     last (5th) Tx resonator and the
     last (6th) Rx resonator.

     Setting up the tuning process
     Just as with the complex filter of
     figure 4, the tuning process for a
     duplexer requires a properly
     tuned prototype filter to allow
     the node frequencies and target
     couplings to be determined.
     However, the nodes will be more
     difficult to associate with indi-
     vidual resonators, especially
     from the common port.




10
Identifying the resonator
The upper half of figure 13
shows that there are more nulls
in the time domain response of
the reflection from the common
port than there are from the Tx
port. The first null is associated
with the common resonator
(figure 14). The second null asso-
ciation is found by changing the
tuning slightly on the last Tx res-
onator, and in the same manner
the last Rx resonator can be
associated with the third null
from the common port (figure 15).
Depending upon the filter, it may
also be possible to identify other                                  ANT
resonators in the Tx or Rx filter,
but soon the nulls become con-
                                      Figure 14. Tuning the common ANT resonator shows a response change primarily in
fusing, with the tuning of one        the first null. In this way the first node resonator is determined and the first node
resonator affecting two nulls.        frequency can be found by changing the VNA frequency to find the deepest null




                                                                            Rx 6

                                      Figure 15. Tuning the Rx 6 resonator shows the primary effect at the second null. By
                                      looking for the frequency of the VNA, which makes the null deepest, we know this
                                      node frequency. Note: the next null also shows some effect from tuning




                                                                                                                              11
     Finding node frequencies             Table 2. Node frequency for tuned duplexer
     Once the association of nulls        Common port         Tx port             Rx port
     with resonators has been done        Node Freq.          Node Freq.          Node Freq.
     from the common port for the         Com      1800           TX1     1747           RX1        1848
                                          RX6      1800           TX2     1749           RX2        1848
     last Tx and Rx resonators, the
                                          TX5      1796           TX3     1750           RX3        1851
     individual node frequency for
                                          RX5      1805           TX4     1760           RX4        1841
     each resonator is found by tun-      TX4      1788
     ing the analyzer's center fre-       RX4      1810
     quency until the associated null
     is deepest. This frequency is also
     recorded for each null while
     measuring reflection from the Tx
     and Rx ports, and for the first
     several nulls from the common
     port. These frequencies (in MHz)
     are shown in table 2.


     Separating Tx and Rx
     Responses
     These node frequencies will be
     used for the final tuning of the
     duplexer, but experimental
     research shows that it is not
     practical to try to tune the
     duplexer directly to these fre-
     quencies. This is because there                                                         Rx 6
     is so much interaction from the
                                                                                     Rx 1
     Rx side on the Tx response,
     especially at the common port,
     that the resonators cannot be        Figure 16. The upper trace shows the Rx path frequency response with a VNA center
     sufficiently isolated unless they    frequency selected to obtain the deepest null for the respective resonators. These
                                          frequencies are recorded for the Rx filter tuned and the Tx first resonator tuned low.
     are already very close to their
     correct values. The solution for
     initial tuning is to mistune one     Table 3. Node frequency for duplexer with sides isolated
     side (say the Tx side) and then      Common Port           Tx Port *              Rx Port **
     recharacterize the filter for the    Node   Freq.          Node      Freq.        Node       Freq.
     Rx side node frequencies. Figure     Com       1803**     TX1          1746       RX1          1848
     16 shows the response of the                   1793*
     duplexer with Tx5 (the one           RX6**     1829       TX2          1749       RX2          1848
     closest to the common port           TX5*      1762       TX3          1749       RX3          1850
     resonator) mistuned.                 RX5**     1848       TX4          1787       RX4          1850
                                          TX4*      1738
     In figure 16, the VNA center fre-    RX4**     1860
     quency is changed such that the      *Rx untuned; **Tx untuned
     null associated with the Rx 6
     resonator measured at the com-
     mon port is deep (time domain,       After the Rx frequencies are                         Note that from the Tx and Rx
     upper trace). This frequency is      determined, the Rx 6 resonator                       ports, the node frequencies are
     recorded in table 3 as Rx 6 fre-     is set high, and the Tx resonator                    nearly unchanged, indicating
     quency. But with the same filter     frequencies are determined in a                      that these are very nearly isolat-
     measured at the Rx port, with an     similar way. The precise node                        ed from their respective other
     analyzer center frequency of         frequency for each node was                          sides even in a tuned duplexer.
     1850 MHz (time domain, lower         recorded in table 3.
     trace), each Rx node is nearly a
     null.


12
Tuning a filter
A duplexer tuning process
proceeds as follows:

1. Start with resonator RX6
   tuned high in frequency. Tune
   the Tx side of the filter, and
   common port according to the
   starred (*) frequencies in
   table 3. Tune coupling and
   cross-coupling as described in
   application note 1287-8.

2. Tune resonator TX5 as low
   as possible. Tune the Rx side
   of the filter using the double
   starred (**) frequencies in
                                    Figure 17. The Rx side of the filter is being tuned here. The upper plot shows
   table 3. Figure 17 shows         the S11 and S22 of the filter; each set to a different center frequency appro-
   tuning starting with Rx1 and     priate for the first and last resonator. The lowest plot shows the null from
   common. The result of tuning     each resonator.
   all Rx resonators is shown in
   figure 18. The VNA is set to
   the common resonator
   frequency (about 1800 MHz)
   so the Rx nulls don't appear
   deep. Here, Tx 5 resonator is
   not yet tuned.

3. Final tune TX5 and TX6 to
   the frequency in table 2.
   Final tune all resonators to
   table 2 values. Results shown
   in figure 19.




                                    Figure 18.




                                    Figure 19.


                                                                                                                     13
     More complex
     filter tuning
     Dealing with multiple or
     strong cross couplings
     In the example filter shown in       This is similar to the method        pass band response. To do
     figure 3, the coupling value for     used to isolate the Tx side of the   this, one may short out the res-
     the cross coupling was much less     duplexer from the Rx side. The       onators beyond the cross cou-
     than the main coupling. For          resulting filter has only one path   pling, essentially making a new
     such cases, the cross coupling       for coupling, and can be charac-     filter with the cross coupling
     does not have a strong effect on     terized as an all pole filter. A     being the main path through the
     the time domain response.            "golden" trace of this filter can    filter. A "golden" trace may be
     However, some filters have very      be captured without the cross        taken with a filter thus modified,
     strong cross coupling (coupling      coupling.                            and the value of the cross cou-
     of the same order as the main                                             pling in the time domain may be
     coupling), or multiple cross cou-    When tuning an untuned filter, it    recorded. When tuning an
     plings. In these cases, it may be    can be set to have the same          untuned filter, the process is
     necessary to take a different        response as the "golden" filter,     reversed. The resonators beyond
     approach to tuning the filters.      with the cross coupling removed.     the cross coupling are shorted
                                          All that remains is to set the       and the cross coupling is set in
     One approach that has been           cross coupling back to verify the    the time domain. The shorting of
     effective is to remove the cross     final filter response.               the resonators is removed, and
     couplings (either by tuning them                                          the filter is tuned as described
     to a very low value, or shorting     Another option for filters with      above. This may be effective in
     out cross couplings if they are      adjustable cross couplings is to     dealing with cross coupling that
     not adjustable). This will result    set the cross coupling first,        is used for linearizing group
     in a filter that does not have the   before tuning the rest of the fil-   delay in filters.
     desired shape, but does have         ter. This method may be effective
     the correct settings for the res-    for filters where the cross cou-
     onators and main coupling.           pling has a strong effect on the




14
Conclusions                          Other resources
In this application note, we have    Tuning coupled resonator
shown ways to extend the time
domain tuning techniques to
                                     cavity filters
                                     1. Joel Dunsmore, "Simplify
more complex filters. These fil-
                                        Filter Tuning Using Time
ters may contain complex trans-
                                        Domain Transforms",
mission responses, with cross
                                        Microwaves & RF, March 1999.
couplings. These filters may also
                                     2. Joel Dunsmore, "Tuning Band
contain multiple paths, such as
                                        Pass Filters in the Time
in duplexers, or even multiplex-
                                        Domain, Digest of 1999 IEEE
ers. While good progress has
                                        MTTS Int. Microwave Sym.,
been made on extending these
                                        pp. 1351-1354.
techniques, there remain many
                                     3. "Simplified Filter Tuning
opportunities for enhancements
                                        Using Time Domain,"
to these methods, and many fil-
                                        Application note 1287-8,
ter types that require further
                                        literature number 5968-5328E
investigation. Agilent
Technologies is continuing
research into the area of filter     Tuning cross-coupled
tuning, and will continue to pro-    resonator filters
vide state-of-the-art tuning tech-   4. Joel Dunsmore, "Advanced
niques and applications to              Filter Tuning in the Time
support innovation in the area of       Domain,"Conference
coupled resonator filter design.        Proceedings of the 29th
                                        European Microwave
                                        Conference, Vol. 2, pp. 72-75.

                                     Tuning duplexer filters
                                     5. Joel Dunsmore, "Duplex
                                        Filter Tuning Using Time
                                        Domain Transformers,"
                                        Conference Proceedings of
                                        the 30th European Microwave
                                        Conference, Vol. 2, pp. 158-161.

                                     Filter design
                                     6. Zverev, "Handbook of Filter
                                        Synthesis," John Wiley and
                                        Sons, 1967.
                                     7. Williams and Taylor,
                                        "Electronic Filter Design
                                        Handbook, 2nd Edition,"
                                        McGraw Hill Publishers,
                                        Chapter 5, 1988.




                                                                           15
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