Reverse IMD Performance

Presented by:Chris Brinton

Advisors:Dr. Allen KatzDave McGee

The College of New Jersey (Ewing, NJ)Linearizer Technology, Inc. (Hamilton, NJ)

Agenda1. What is Reverse IMD?2. Observation test system3. Amplifier measurements and data interpretation4. Mitigating the phenomenon5. 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

FinIMD

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 IMD1 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

• 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

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.