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                                                                   Exhibit 10.57
                                                                    ------------

                SECOND AMENDMENT TO CREDIT AND SECURITY AGREEMENT

     This Amendment, dated as of August 23, 2001, is made by and between RF
MONOLITHICS, INC., a Delaware corporation (the "Borrower"), and WELLS FARGO
BUSINESS CREDIT, INC., a Minnesota corporation (the "Lender").

                                    Recitals

     The Borrower and the Lender are parties to a Credit and Security Agreement
dated as of December 8, 2000, as amended by the First Amendment to Credit and
Security Agreement and Waiver of Defaults dated as of March 30, 2001 (as so
amended, the "Credit Agreement"). Capitalized terms used in these recitals have
the meanings given to them in the Credit Agreement unless otherwise specified.

     The Borrower has requested that certain amendments be made to the Credit
Agreement, which the Lender is willing to make pursuant to the terms and
conditions set forth herein.

     NOW, THEREFORE, in consideration of the premises and of the mutual
covenants and agreements herein contained, it is agreed as follows:

     1. Defined Terms. Capitalized terms used in this Amendment which are
        --------------
defined in the Credit Agreement shall have the same meanings as defined therein,
unless otherwise defined herein. In addition, Section 1.1 of the Credit
Agreement is amended by adding or amending, as the case may be, the following
definitions:

          "Accounts" means all of the Borrower's accounts, as such term is
     defined in the UCC, including each and every right of the Borrower to the
     payment of money, whether such right to payment now exists or hereafter
     arises, whether such right to payment arises out of a sale, lease or other
     disposition of goods or other property, out of a rendering of services, out
     of a loan, out of the overpayment of taxes or other liabilities, or
     otherwise arises under any contract or agreement, whether such right to
     payment is created, generated or earned by the Borrower or by some other
     person who subsequently transfers such person's interest to the Borrower,
     whether such right to payment is or is not already earned by performance,
     and howsoever such right to payment may be evidenced, together with all
     other rights and interests (including all Liens) which the Borrower may at
     any time have by law or agreement against any account debtor or other
     obligor obligated to make any such payment or against any property of such
     account debtor or other obligor; all including but not limited to all
     present and future accounts, contract rights, loans and obligations
     receivable, chattel papers, bonds, notes and other debt instruments, tax
     refunds and rights to payment in the nature of general intangibles.

          "Collateral" means all of the Borrower's assets, including Accounts,
     Chattel Paper, deposit accounts, documents, Equipment, General Intangibles,
     goods, Instruments, Intellectual Property, Inventory, Investment Property,
     letter-of-credit rights, letters of

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     credit, Receivables, all sums on deposit in any Collateral Account, and any
     items in any Lockbox; together with (i) all substitutions and replacements
     for and products of any of the foregoing; (ii) in the case of all goods,
     all accessions; (iii) all accessories, attachments, parts, equipment and
     repairs now or hereafter attached or affixed to or used in connection with
     any goods; (iv) all warehouse receipts, bills of lading and other documents
     of title now or hereafter covering such goods; (v) all collateral subject
     to the Lien of any Security Document; (vi) any money, or other assets of
     the Borrower that now or hereafter come into the possession, custody, or
     control of the Lender; (vii) all sums on deposit in the Special Account;
     and (viii) proceeds of any and all of the foregoing.

          "Lien" means any security interest, mortgage, deed of trust, pledge,
     lien, charge, encumbrance, title retention agreement or analogous
     instrument or device, including the interest of each lessor under any
     capitalized lease and the interest of any bondsman under any payment or
     performance bond, in, of or on any assets or properties of a Person,
     whether now owned or hereafter acquired and whether arising by agreement or
     operation of law.

               "Special Charges" means certain restructuring changes of the
          Borrower to be taken during fiscal year ending August 31, 2001
          including approximately (a) $1,000,000 for a fixed asset reserve, (b)
          $1,000,000 for an inventory reserve and (c) $200,000 of severance
          package accruals related to the plan for a reduction in work force.

     2. Rules of Interpretation. Section 1.2 of the Credit Agreement is amended
        -----------------------
to read as follows:

          Section 1.2 Other Definitional Terms; Rules of Interpretation. The
                      -------------------------------------------------
     words "hereof", "herein" and "hereunder" and words of similar import when
     used in this Agreement shall refer to this Agreement as a whole and not to
     any particular provision of this Agreement. All accounting terms not
     otherwise defined herein have the meanings assigned to them in accordance
     with GAAP. All terms defined in the UCC and not otherwise defined herein
     have the meanings assigned to them in the UCC. References to Articles,
     Sections, subsections, Exhibits, Schedules and the like, are to Articles,
     Sections and subsections of, or Exhibits or Schedules attached to, this
     Agreement unless otherwise expressly provided. The words "include",
     "includes" and "including" shall be deemed to be followed by the phrase
     "without limitation". Unless the context in which used herein otherwise
     clearly requires, "or" has the inclusive meaning represented by the phrase
     "and/or". Defined terms include in the singular number the plural and in
     the plural number the singular. Reference to any agreement (including the
     Loan Document), document or instrument means such agreement, document or
     instrument as amended or modified and in effect from time to time in
     accordance with the terms thereof (and, if applicable, in accordance with
     the terms hereof and the other Loan Documents), except where otherwise
     explicitly provided, and reference to any promissory note includes any
     promissory note which is an extension or renewal thereof or a substitute or
     replacement therefor. Reference to any law, rule, regulation, order,
     decree, requirement, policy, guideline, directive or interpretation means
     as amended, modified, codified, replaced or reenacted, in whole or in part,
     and in effect on the determination date, including rules and regulations
     promulgated thereunder.


                                       -2-

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     3.   Financing Statements. Section 3.6 of the Credit Agreement is
          --------------------
amended by adding the following new sentence before the first sentence of that
Section:

          The Borrower authorizes the Lender to file from time to time where
     permitted by law, such financing statements against collateral described as
     "all personal property" as the Lender deems necessary or useful to perfect
     the Security Interest.

     4.   Amendment to Section 6.16. Section 6.16 of the Credit Agreement is
          -------------------------
amended in its entirety to read as follows:

          Minimum Book Net Worth. The Borrower will maintain, during each period
          ----------------------
     described below, its Book Net Worth in an amount not less than the amount
     set forth below:

          (a) From June 1, 2001 through August 31, 2001, a minimum Book Net
     Worth of not less than $26,750,000; and

          (b) From September 1, 2001 through November 30, 2001, a minimum Book
     Net Worth of not less than the Book Net Worth as of FYE
     August 31, 2001, minus $250,000.

Notwithstanding the foregoing, if the Special Charges are not taken during the
fiscal year ending August 31, 2001, the covenants for the above-listed periods
will be adjusted to new levels to be determined by the Lender in its sole
discretion.

     5.   Amendment to Section 6.17. Section 6.17 of the Credit Agreement is
          -------------------------
amended in its entirety to read as follows:

          Quarterly Minimum Net Income. The Borrower will achieve (a) during the
          ----------------------------
     fiscal year ending on August 31, 2001, a minimum Net Income of greater than
     ($6,240,000); and (b) during the three month fiscal period from August 31,
     2001, to November 30, 2001, a minimum Net Income of greater than
     ($250,000). Notwithstanding the foregoing, if the Special Charges are not
     taken during the fiscal year ending August 31, 2001, the covenants for the
     above-listed periods will be adjusted to new levels to be determined by the
     Lender in its sole discretion.

     6.   Amendment to Section 6.18. Section 6.18 of the Credit Agreement is
          -------------------------
amended in its entirety to read as follows:


          Monthly Minimum Net Income. The Borrower will achieve, as of the end
          --------------------------
     of each month (a) during the fiscal year ended August 31, 2001, before the
     effectiveness of the Special Charges a minimum Net Income of greater than
     ($500,000); and (b) from August 31, 2001 through November 30, 2001, a
     minimum Net Income of greater than ($500,000).

     7.   Amendment to Section 6.21. The first sentence of Section 6.21 of the
          -------------------------
Credit Agreement is amended in its entirety to read as follows: "The Lender may
arrange for one or

                                       -3-

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more appraisals of the Borrower's machinery and Equipment by a qualified
appraiser selected by the Lender at such times as Lender may determine."

     8.   Amendment to Exhibit C. Exhibit C to the Credit Agreement is hereby
          ----------------------
amended to be in the form of Exhibit C to this Amendment.

     9.   No Other Changes. Except as explicitly amended by this Amendment, all
          ----------------
of the terms and conditions of the Credit Agreement shall remain in full force
and effect and shall apply to any advance or letter of credit thereunder.

     10.  Waiver of Defaults. The Borrower is in default of Sections 6.16, 6.17,
          ------------------
and 8.1(p) of the Credit Agreement and Section 5(a) of the Patent Security
Agreement (collectively, the "Existing Defaults"). The defaults under Section
6.16 and 6.17 of the Credit Agreement may be summarized as follows:

--------------------------------------------------------------------------------
Section/Covenant              Required Performance          Actual Performance
-------------------------------------------------------------------------------
Section 6.16/Minimum      Through August 31, 2001,          As of June 30, 2001,
Book Net Worth            a minimum Book Net Worth not      Book Worth was
                          less than $29,500,000             $29,334,000
--------------------------------------------------------------------------------
Section 6.17/Quarterly    During the fiscal year ended on   As of June 30, 2001,
Minimum Net Income        August 31, 2001, a                Net Income was
                          minimum NetIncome of              ($3,619,000)
                          ($3,200,000)
--------------------------------------------------------------------------------

Upon the terms and subject to the conditions set forth in this Amendment, the
Lender hereby waives the Existing Defaults. This waiver shall be effective only
in this specific instance and for the specific purpose for which it is given,
and this waiver shall not entitle the Borrower to any other or further waiver in
any similar or other circumstances.

     11.  Amendment Fee. The Borrower shall pay the Lender as of the date hereof
          -------------
a fully earned, non-refundable fee in the amount of $125,000 in consideration of
the Lender's execution and delivery of this Amendment. The Amendment Fee shall
be payable as follows: (a) $50,000 upon the execution of the Amendment; (b)
$25,000 on September 15, 2001; (c) $25,000 on October 15, 2001; and (d) $25,000
on November 15, 2001. Notwithstanding the foregoing, if a Default or an Event of
Default occurs under the Credit Agreement or a breach, default or event of
default occurs under any Security Document or other document held by Lender
occurs at any time prior to November 15, 2001, the entire remaining amount of
the Amendment Fee shall be immediately due and payable in full.

     12.  Conditions Precedent. This Amendment, and the waiver set forth in
          --------------------
Paragraph 10 hereof, shall be effective when the Lender shall have received an
executed original hereof, together with each of the following, each in substance
and form acceptable to the Lender in its sole discretion:

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               (a)  The Acknowledgment and Agreement of Guarantor set forth at
          the end of this Amendment, duly executed by the Guarantor.

               (b)  A Certificate of the Secretary of the Borrower certifying as
          to (i) the resolutions of the board of directors of the Borrower
          approving the execution and delivery of this Amendment, (ii) the fact
          that the articles of incorporation and bylaws of the Borrower, which
          were certified and delivered to the Lender pursuant to the Certificate
          of Authority of the Borrower's secretary or assistant secretary dated
          as of December 8, 2000, continue in full force and effect and have not
          been amended or otherwise modified except as set forth in the
          Certificate to be delivered, and (iii) certifying that the officers
          and agents of the Borrower who have been certified to the Lender,
          pursuant to the Certificate of Authority of the Borrower's secretary
          or assistant secretary dated as of December 8, 2000, as being
          authorized to sign and to act on behalf of the Borrower continue to be
          so authorized or setting forth the sample signatures of each of the
          officers and agents of the Borrower authorized to execute and deliver
          this Amendment and all other documents, agreements and certificates on
          behalf of the Borrower.

               (c)  Payment of the fee described in Paragraph 11.

               (d)  Such other matters as the Lender may require.

     13.  Representations and Warranties. The Borrower hereby represents and
          ------------------------------
warrants to the Lender as follows:


               (a)  The Borrower has all requisite power and authority to
          execute this Amendment and to perform all of its obligations
          hereunder, and this Amendment has been duly executed and delivered by
          the Borrower and constitutes the legal, valid and binding obligation
          of the Borrower, enforceable in accordance with its terms.

               (b)  The execution, delivery and performance by the Borrower of
          this Amendment have been duly authorized by all necessary corporate
          action and do not (i) require any authorization, consent or approval
          by any governmental department, commission, board, bureau, agency or
          instrumentality, domestic or foreign, (ii) violate any provision of
          any law, rule or regulation or of any order, writ, injunction or
          decree presently in effect, having applicability to the Borrower, or
          the articles of incorporation or by-laws of the Borrower, or (iii)
          result in a breach of or constitute a default under any indenture or
          loan or credit agreement or any other agreement, lease or instrument
          to which the Borrower is a party or by which it or its properties may
          be bound or affected.

               (c)  All of the representations and warranties contained in
          Article V of the Credit Agreement are correct on and as of the date
          hereof as though made on and as of such date, except to the extent
          that such representations and warranties relate solely to an earlier
          date.

     14.  References. All references in the Credit Agreement to "this Agreement"
          ----------
shall be deemed to refer to the Credit Agreement as amended hereby; and any and
all references in the Security Documents to the Credit Agreement shall be deemed
to refer to the Credit Agreement as amended hereby.

                                       -5-

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     15.  No Other Waiver. Except as set forth in Paragraph 10 hereof, the
          ---------------
execution of this Amendment and acceptance of any documents related hereto shall
not be deemed to be a waiver of any Default or Event of Default, breach or
default under the Credit Agreement or breach, default or event of default under
any Security Document or other document held by the Lender, whether or not known
to the Lender and whether or not existing on the date of this Amendment.

     16.  Release. The Borrower and the Guarantor by signing the Acknowledgment
          -------
and Agreement of Guarantor set forth below, each hereby absolutely and
unconditionally releases and forever discharges the Lender, and any and all
participants, parent corporations, subsidiary corporations, affiliated
corporations, insurers, indemnitors, successors and assigns thereof, together
with all of the present and former directors, officers, agents and employees of
any of the foregoing, from any and all claims, demands or causes of action of
any kind, nature or description, whether arising in law or equity or upon
contract or tort or under any state or federal law or otherwise, which the
Borrower or such Guarantor has had, now has or has made claim to have against
any such person for or by reason of any act, omission, matter, cause or thing
whatsoever arising from the beginning of time to and including the date of this
Amendment, whether such claims, demands and causes of action are matured or
unmatured or known or unknown.

     17.  Costs and Expenses. The Borrower hereby reaffirms its agreement under
          ------------------
the Credit Agreement to pay or reimburse the Lender on demand for all costs and
expenses incurred by the Lender in connection with the Loan Documents, including
without limitation all reasonable fees and disbursements of legal counsel.
Without limiting the generality of the foregoing, the Borrower specifically
agrees to pay all fees and disbursements of counsel to the Lender for the
services performed by such counsel in connection with the preparation of this
Amendment and the documents and instruments incidental hereto. The Borrower
hereby agrees that the Lender may, at any time or from time to time in its sole
discretion and without further authorization by the Borrower, make a loan to the
Borrower under the Credit Agreement, or apply the proceeds of any loan, for the
purpose of paying any such fees, disbursements, costs and expenses and the fee
required under paragraph 11 hereof.

     18.  Security Agreement. The Borrower shall cause the Guarantor to deliver
          ------------------
a security agreement, in favor of Lender and in form and substance satisfactory
to the Lender in its sole discretion, to be properly executed by the Guarantor
and delivered to the Lender within 30 days from the date hereof.

     19.  Appraisal of Borrower's Machinery and Equipment. The Borrower hereby
          -----------------------------------------------
acknowledges and agrees to an appraisal of the Borrower's machinery and
Equipment to be conducted in accordance with Section 6.21 of the Credit
Agreement as amended.

     20.  Miscellaneous. This Amendment and the Acknowledgment and Agreement of
          -------------
Guarantor may be executed in any number of counterparts, each of which when so
executed and delivered shall be deemed an original and all of which
counterparts, taken together, shall constitute one and the same instrument.

                                       -6-

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     21.  Governing Law; Jurisdiction, Venue; Waiver of Jury Trial. This
          --------------------------------------------------------
Amendment, the Acknowledgment and Agreement of Guarantor and the Loan Documents
shall be governed by and construed in accordance with the substantive laws
(other than conflict laws) of the State of Texas. Each party hereto and the
Guarantor hereby (i) consents to the personal jurisdiction of the state and
federal courts located in the State of Texas in connection with any controversy
related to this Amendment; (ii) waives any argument that venue in any such forum
is not convenient, (iii) agrees that any litigation initiated by the Lender or
the Borrower in connection with this Amendment or the other Loan Documents shall
be venued in either the District Court of Collin County, Texas, or the United
States District Court for the Northern District of Texas; and (iv) agrees that a
final judgment in any such suit, action or proceeding shall be conclusive and
may be enforced in other jurisdictions by suit on the judgment or in any other
manner provided by law. THE PARTIES WAIVE ANY RIGHT TO TRIAL BY JURY IN ANY
ACTION OR PROCEEDING BASED ON OR PERTAINING TO THIS AMENDMENT.

     22.  ENTIRE AGREEMENT. THIS AMENDMENT AND ALL OTHER INSTRUMENTS, DOCUMENTS
          ----------------
AND AGREEMENTS EXECUTED AND DELIVERED IN CONNECTION WITH THIS AMENDMENT EMBODY
THE FINAL, ENTIRE AGREEMENT AMONG THE PARTIES HERETO AND SUPERSEDE ANY AND ALL
PRIOR COMMITMENTS, AGREEMENTS, REPRESENTATIONS AND UNDERSTANDINGS, WHETHER
WRITTEN OR ORAL, RELATING TO THIS AMENDMENT, AND MAY NOT BE CONTRADICTED OR
VARIED BY EVIDENCE OF PRIOR, CONTEMPORANEOUS OR SUBSEQUENT ORAL AGREEMENTS OR
DISCUSSIONS OF THE PARTIES HERETO. THERE ARE NO ORAL AGREEMENTS AMONG THE
PARTIES HERETO.

                            (Signature Page Follows)

                                       -7-

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     IN WITNESS WHEREOF, the parties hereto have caused this Amendment to be
duly executed as of the date first written above. WELLS FARGO BUSINESS CREDIT,
INC. RF MONOLITHICS, INC.


By /s/Terrance O. McKinney                     By /s/David M. Kirk
   -----------------------------                  ------------------------------
      Terrance O. McKinney                        David M. Kirk
      Assistant Vice President                    President