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                                                                   EXHIBIT 10.40
                                 PROMISSORY NOTE

$19,500.00                                                          May 12, 1999
                                                                   Dallas, Texas

     FOR VALUE RECEIVED, David M. Kirk, ("Borrower"), hereby unconditionally
promises to pay to the order of RF Monolithics, Inc., a Delaware corporation
("Lender"), in lawful money of the United States of America and in immediately
available funds, the principal sum of Nineteen Thousand, Five Hundred Dollars
and No Cents ($19,500.00) (the "Loan") together with accrued and unpaid interest
thereon, payable on the dates and in the manner set forth below.

      1.  Principal Repayment. The outstanding principal amount of the Loan
shall be due and payable on May 12, 2002.

     2.   Interest Rate. Borrower further promises to pay interest on the
outstanding principal amount hereof from the date hereof until payment in full,
which interest shall be payable at the rate of Four and Ninety/One Hundredths
percent (4.90%) per annum or the maximum rate permissible by law (which under
the laws of the State of Texas shall be deemed to be the laws relating to
permissible rates of interest on commercial loans), whichever is less. Interest
shall be payable quarterly in arrears not later than the first day of each
calendar quarter for the preceding quarter and shall be calculated on the basis
of a 360-day year for the actual number of days elapsed.

     3.   Place of Payment. All amounts payable hereunder shall be payable at
the office of Lender, unless another place of payment shall be specified in
writing by Lender.

     4.   Application of Payments. Payment on this Note shall be applied first
to accrued interest, and thereafter to the outstanding principal balance
thereof.

     5.   Secured Note. The full amount of this Note is secured by the
Collateral identified and described below. As collateral security for prompt and
complete payment and performance of all obligations of Borrower under this Note
and to induce lender to extend credit, Borrower hereby assigns, conveys, grants,
pledges, and transfers to and creates in favor of Lender a security interest in
all goods and personal property of Borrower whether tangible or intangible and
whether now or hereafter owned by Borrower ("Collateral").

     6.   Default. Each of the following events shall be an "Event of Default"
hereunder:

          (a)  Borrower fails to pay timely any of the principal amount due
under this Note on the date the same becomes due and payable or any accrued
interest or other
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amounts due under this Note on the date the same becomes due and payable or
within five (5) calendar days thereafter;

          (b)  Borrower files any petition or action for relief under any
bankruptcy, reorganization, insolvency, or moratorium law, or any other law for
the relief of, or relating to, debtors, now or hereafter in effect, or makes any
assignment for the benefit of creditors, or takes any corporate action in
furtherance of any of the foregoing; or

          (c)  An involuntary petition is filed against Borrower (unless such
petition is dismissed or discharged within sixty (60) days), under any
bankruptcy statute now or hereafter in effect, or a custodian, receiver,
trustee, assignee for the benefit of creditors ( or other similar official) is
appointed to take possession, custody, or control of any property of Borrower.

Upon the occurrence of an Event of Default hereunder, all unpaid principal,
accrued interest and other amounts owing hereunder shall, at the option of
Lender, be immediately collectible by Lender pursuant to applicable law.

     7.   Waiver. Borrower waives presentment and demand for payment, notice of
dishonor, protest and notice of protest of this Note, and shall pay all costs of
collection when incurred, including, without limitation, reasonable attorneys'
fees, costs, and other expenses.

     The right to plead any and all statutes of limitations as a defense to any
demands hereunder is hereby waived to the full extent permitted by law.

     8.   Governing Law. This Note shall be governed by, and construed and
enforced in accordance with, the laws of the State of Texas, excluding conflict
of laws principles that would cause the application of laws of any other
jurisdiction.

     9.   Successors and Assigns. The provisions of this Note shall inure to the
benefit of and be binding on any successor to Borrower and shall extend to any
holder hereof.

Borrower                                RF Monolithics, Inc.

/s/ David M. Kirk                       /s/ Sam L. Densmore
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David M. Kirk                           Sam L. Densmore
                                        President and CEO