Reverse IMD Performance Presented by: Chris Brinton Advisors: Dr. Allen Katz Dave McGee

1. Reverse IMD Performance Presented by: Chris Brinton Advisors: Dr. Allen Katz Dave McGee The College of New Jersey (Ewing, NJ) Linearizer Technology, Inc. (Hamilton, NJ)

2. Agenda • What is Reverse IMD? • Observation test system • Amplifier measurements and data interpretation • Mitigating the phenomenon • Conclusions

3. Effect of Reverse IMD • Can be a major problem where multiple high power transmitters operate in close proximity • Cell telephone sites • Broadcast facilities • Satellites • Causes interference at receiver • Source of error in non-linearity measurements

4. What is Reverse IMD? • Reverse Intermodulation Distortion (IMD) • Little studied on the phenomenon • Unwanted signals enter amplifier output port • Beating with main carrier produces distortion products

5. Test System

6. SPECTRUM OF REV IMD TEST SIGNALS INTERFERER IMD

7. Testing Details • L-band frequency range (1,000 to 2,000 MHz) • Center 1.3 GHz, C1 to C2 spacing of 1 MHz • C1/C2 levels between 20 and 50 dB • Amplifiers tested • 200W wideband GaN solid state SSPA • 200W LDMOS SSPA • Several lower power GaAs SSPAs • Bipolar SSPA • Completeness: Traveling wave tube amplifier (TWTA)

8. 200 W GaN SSPA REV IMD PERFORMANCE NO SIMPLE RELATIONSHIP

9. 10 W GaN SSPA REV IMD PERFORMANCE ~2 dB/dB DECREASE

10. 20 W BIPOLAR SSPA REV IMD PERFORMANCE ~2 dB/dB

11. TWTA REVERSE IMD PERFORMANCE ~2 dB/dB: PROVIDED BEST PERFORMANCE

12. REVERSE IMD OF DIFFERENT HPAs QUADRATURE COMBINING AMPS SUPPRESSES REVERSE IMD

13. EFFECT INTERFERER ON IMD1 dB/dB

14. Reverse IM Distortion Mitigation • For narrow band transmission systems, suppression can be achieved using filtering • For wideband applications, and where the frequency of the interfering signal changes, other techniques are need • Techniques • Isolation • Phasing

15. Isolators • Introduce some loss • Expensive • Several broadband (1 – 2 GHz) 200 W isolators/circulators were purchased • Specified isolation of better than 20 dB • Insertion loss of 0.5 dB or less

16. Observed Isolator Mitigation

17. Phasing Principle • Sample the interfering transmitters • Couple the resulting signals into the outputs of the corresponding sending transmitters 180° out of phase

18. Phasing Configuration Used

19. Phasing Results • More than 35 dB of additional rejection achieved • Maximum rejection measured limited by noise floor • In reality, need adaptive adjustment of attenuation and phase settings: Changes with frequency and environment

20. Summary • The effect of Reverse IMD varies at 1 dB/dB in relation to the interfering signal • At high OPBO, the Reverse IMD displayed a variety of different characteristics that could not be associated with a particular type of amplifier. • Quadrature combining amplifiers provide the best improvement overall (Aethercomm and TWTA). • The Aethercomm GaN SSPA provided the best Reverse IMD performance of all the SSPA tested.

21. Summary • A TWTA provided best performance. • Isolators are shown to increase performance by a dB factor of their coupling. • An adaptive IMD cancelation system is practical and can reduce Reverse IMD levels over 35 dB.