PROMISSORY NOTE

$1,000,000                                                           May 5, 2000


FOR VALUE RECEIVED,  the  undersigned,  Hamid Akhavan (the  "Borrower"),  hereby
promises to pay to Teligent Services,  Inc., a Delaware company (the "Company"),
the  principal  sum (the  "Principal  Sum") of One Million Six Hundred  Thousand
Dollars  ($1,600,000)  in lawful  money of the  United  States of  America.  The
Borrower also agrees to pay interest  (computed on the basis of a 365 or 366 day
year,  as the case may be) on any unpaid  amount of the  Principal  Sum from and
after the date of this Promissory  Note set forth above (the  "Effective  Date")
until the entire  Principal  Sum has been paid in full, at a rate equal to 7.45%
per  annum;  provided  that in no event  shall such  interest  be charged to the
extent it would violate any applicable  usury law.  Payment of the Principal Sum
and accrued interest thereon shall not be secured.  Borrower shall be personally
liable for the Principal  Sum and the accrued  interest  thereon,  calculated in
accordance with this Promissory Note.

This Promissory Note is subject to the following further terms and conditions:

1.       PAYMENT  UPON  MATURITY.  The  Principal  Sum and all accrued  interest
         thereon  will become due and payable on the second  anniversary  of the
         Effective Date (the "Maturity Date").

2.       PAYMENT AND  PREPAYMENT.  All payments and prepayments of the Principal
         Sum of, and the accrued interest thereon,  shall be made to the Company
         or its order,  in lawful  money of the United  States of America at the
         principal offices of the Company (or at such other place as the Company
         shall notify the Borrower in writing). The Borrower may, at his option,
         prepay this  Promissory  Note in whole or in part at any time from time
         to time without  penalty or premium.  Any prepayments of any portion of
         the  Principal  Sum of this  Promissory  Note shall be  accompanied  by
         payment of all  interest  accrued but unpaid  hereunder.  Upon full and
         final payment,  or  forgiveness,  of the Principal Sum of, and interest
         accrued on, this  Promissory  Note, it shall be canceled by the Company
         and  surrendered  by the  Borrower.  All  payments  hereunder  shall be
         applied  first  to the  payment  of any  costs of  collection  then due
         hereunder,  second to the payment of accrued and unpaid  interest  then
         due  hereunder,  and the  remainder,  if any,  shall be  applied to the
         unpaid  Principal Sum. The Borrower agrees that, if any payment made by
         the  Borrower  is  applied to this  Promissory  Note and is at any time
         annulled, set aside, rescinded,  invalidated, declared to be fraudulent
         or preferential or otherwise  required to be refunded or repaid,  then,
         to the extent of such payment, the Borrower's liability hereunder shall
         be and remain in full force and effect as fully as if such  payment had
         never been made.
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3.       LOAN FORGIVENESS. Notwithstanding the foregoing, the Principal Sum, and
         accrued and unpaid interest  thereon,  shall be automatically  forgiven
         upon the  happening of either of the following  events:  (a) the second
         anniversary of the Effective  Date, if, and only if, the Borrower shall
         be employed by the  Company on such date,  (b) if,  prior to the second
         anniversary  of the  Effective  Date,  Borrower's  employment  with the
         Company is  terminated  by the  Company  for any reason  other than for
         Cause (as defined  below) or (c) the  occurrence of a Change of Control
         (as defined  below) prior to the second  anniversary  of the  Effective
         Date. For purposes  hereof,  a "Change of Control" means (i) any person
         or entity, or group of affiliated  persons or entities,  other than the
         Liberty  Media  Corporation,  a Delaware  corporation,  and  Telcom-DTS
         Investors,  L.L.C., a Delaware limited liability company (collectively,
         the "Shareholders")  and/or their respective  affiliates acquires stock
         of Teligent  representing more than 50% of the voting power of all such
         outstanding  stock;  or (ii) the  majority of the Board of Directors of
         Teligent  consists of persons who are designees of any person or entity
         or  group  of  affiliated  persons  or  entities  which  hold  stock in
         Teligent,   other  than  the   Shareholders   and/or  their  respective
         affiliates;  or (iii) Teligent  adopts a plan of liquidation  providing
         for the distribution of all or substantially all of its assets; or (iv)
         all or  substantially  all of the  business  enterprise  of Teligent is
         disposed  of  pursuant  to a sale of  assets  transaction  or a merger,
         consolidation  or  similar  transaction  in which  Teligent  is not the
         surviving  entity  (unless  (A)  no  person  or  entity,  or  group  of
         affiliated  persons or  entities,  other than the  Shareholders  and/or
         their respective  affiliates,  owns immediately  after such transaction
         stock or other equity  interests  of the entity  which  succeeds to the
         business of Teligent as a result of such transaction  representing more
         than 50% of the voting power of all such  outstanding  stock,  or (B) a
         majority of the board of directors (or  comparable  governing  body) of
         the entity  which  succeeds to the  business of Teligent as a result of
         such  transaction  consists of persons (or persons  designated  by such
         persons)  who  constituted  a  majority  of the Board of  Directors  of
         Teligent  immediately prior to such  transaction.  For purposes of this
         definition,  "affiliate"  (or  derivations  thereof)  of any  person or
         entity  means  any  other  person  or  entity  directly  or  indirectly
         controlling or controlled by or under direct or indirect common control
         with such  person  or  entity;  and for  purposes  of this  definition,
         "control"  when used with  respect  to any  person or entity  means the
         power to direct the  management  and policies of such person or entity,
         directly  or  indirectly,  whether  through  the  ownership  of  voting
         securities or other equity interests, by contract or otherwise, and the
         terms  "controlling" and "controlled" have meanings  correlative to the
         foregoing.

4.       LOAN  PRORATION;  LOAN  ACCELERATION.  In the  event of the  Borrower's
         voluntary  termination of his employment  with the Company prior to the
         second  anniversary  of the  Effective  Date,  the  Borrower  agrees to
         reimburse the Company for the entire outstanding  Principal Sum and all
         accrued interest thereon immediately, prorated (i.e. multiplied) by the
         following  ratio:  the number of days between the termination  date and
         the second  anniversary  of the Effective Date divided by 730. Upon (a)
         any termination of the Borrower's employment with the Company for Cause
         (as defined below),  (b) the Borrower's death, (c) the inability of the
         Borrower to continue  employment  with the Company due to a disability,
         or  (d) an  Event  of  Default  (as  defined)  the  entire  outstanding
         Principal  Sum and all accrued  interest  thereon  shall become due and
         payable  by  the   Borrower   to  the  Company   immediately,   without
         presentment,  demand, protest, notice of dishonor and all other demands
         and notices of any kind, all of which are hereby expressly waived,  For
         purposes  hereof,  "Cause"  means (i) the  engaging by the  Borrower in
         willful  misconduct or negligence  that is materially  injurious to the
         Company; (ii) the embezzlement or misappropriation of funds or property
         of the Company by the Borrower or the  conviction  of the Borrower of a
         felony or the  entrance  of a plea of guilty or nolo  contendre  by the
         Borrower to a felony;  or (iii) the  willful  failure or refusal by the
         Borrower to perform his duties or responsibilities that continues after
         being brought to the attention of the Borrower.  Determination of Cause
         shall be made by Company in its discretion,  and any such determination
         shall be final and binding on the  Borrower.  The Borrower  promises to
         pay to the Company on demand by the Company  all  reasonable  costs and
         expenses  incurred  by the  Company  after  an  event  giving  rise  to
         acceleration  in connection with the collection and enforcement of this
         Promissory Note, including,  without limitation,  reasonable attorneys'
         fees and expenses and all court costs.

5.       EVENTS OF DEFAULT; DEFAULT INTEREST; DEFAULT ACCELERATION AND EXPENSES.
         The  occurrence  of any  one or  more  of the  following  events  shall
         constitute an event of default (individually, an "Event of Default" and
         collectively,  the  "Events  of  Default")  under  the  terms  of  this
         Promissory  Note: (a) the failure of the Borrower to pay to the Company
         within  fourteen  (14) days of when due any and all amounts  payable by
         the Borrower to the Company  under the terms of this  Promissory  Note;
         (b) if a petition in  bankruptcy  is filed by  Borrower  under the U.S.
         Bankruptcy Code, as amended, or under any other insolvency laws or laws
         providing  for the  relief  of  debtors;  (c) if the  Borrower  becomes
         insolvent or executes a general assignment for the benefit of creditors
         or if any appointment is made or a receiver or trustee for the property
         of the  Borrower;  or (d) if a petition is filed  against the  Borrower
         under the U.S. Bankruptcy Code, as amended, and is not dismissed within
         sixty (60) days of filing.

6.       NOTICE. For the purposes of this Promissory Note, notices,  demands and
         all other  communications  provided  for herein shall be in writing and
         shall be deemed to have been duly  given  when  delivered  in person or
         five  business  days after being mailed by United  States  certified or
         registered mail, return receipt requested,  postage prepaid,  addressed
         as follows:

If to the Borrower:                 If to the Company:

Hamid Akhavan                       Teligent Services, Inc.
11620 Piney Spring Lane             8065 Leesburg Pike, Suite 400
Potomac, MD 20854                   Vienna, VA 22182
                                    Attn: Laurence E. Harris, General Counsel

         Or such  other  address  as any party may have  furnished  to others in
writing in accordance herewith.

7.       MISCELLANEOUS.

A.       No  delay  or  failure  by the  Company  or the  legal  holder  of this
         Promissory Note in the exercise of any right or remedy shall constitute
         a waiver thereof, and no single or partial exercise by the legal holder
         hereof of any right or remedy  shall  preclude  other  future  exercise
         thereof,  or the exercise of any other right or remedy. This Promissory
         Note may be assigned by the Company  without the prior written  consent
         of the Borrower.  This Promissory Note may be amended or modified,  and
         the terms and conditions hereof may be waived,  only by a majority vote
         of the  Board of  Directors  of  Teligent;  provided,  however,  if any
         amendment  or  modification  to this  Promissory  Note  materially  and
         adversely   affects  the  rights  of  the  Borrower   hereunder,   such
         modification  or  amendment  shall  require the written  consent of the
         Borrower.

B.       The  headings  contained  in this  Promissory  Note  are for  reference
         purposes only.

C.       No  provisions  hereof  shall  confer  upon the  Borrower  the right to
         continue in the employment of the Company,  any of its  subsidiaries or
         any of their  perspective  successors  or affect any  rights  which the
         Company or any of such  subsidiaries or successor may have to terminate
         the employment of the Borrower.

D.       The Provisions of this  Promissory Note shall be governed and construed
         in accordance  with the laws of the State of Delaware,  without  giving
         effect to the choice of law principals  thereof.  The Company,  but not
         the Borrower, shall have the right to assign this Promissory Note.

         IN WITNESS  WHEREOF,  this  Promissory  Note has been duly executed and
delivered to the Company by the Borrower on the date first above written.

                                                   /S/ HAMID AKHAVAN
                                                   -----------------------------
                                                       Hamid Akhavan