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) Agenda 1. 2. 3. 4. 5. What is Reverse IMD? Observation test system Amplifier measurements and data interpretation Mitigating the phenomenon Conclusions 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 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 IMD Fin F1, F2, ... Test System SPECTRUM OF REV IMD TEST SIGNALS INTERFERER IMD 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) 200 W GaN SSPA REV IMD PERFORMANCE NO SIMPLE RELATIONSHIP 10 W GaN SSPA REV IMD PERFORMANCE ~2 dB/dB DECREASE 20 W BIPOLAR SSPA REV IMD PERFORMANCE ~2 dB/dB TWTA REVERSE IMD PERFORMANCE ~2 dB/dB: PROVIDED BEST PERFORMANCE REVERSE IMD OF DIFFERENT HPAs QUADRATURE COMBINING AMPS SUPPRESSES REVERSE IMD EFFECT INTERFERER ON IMD 1 dB/dB 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 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 Observed Isolator Mitigation Phasing Principle – Sample the interfering transmitters – Couple the resulting signals into the outputs of the corresponding sending transmitters 180° out of phase Phasing Configuration Used 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 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. 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.