INTRAPULSE FREQUENCY MODULATION AND THE AN-FPN/64 TRANSMITTER Dr. Paul Johannessen Andrei Grebnev Erik Johannessen Megapulse, Inc. Presented at ILA 30 - St.Germain-en-Laye
IFM CHALLENGES Narrow band Loran transmitting antenna: 625 ft. TLM: F = 1.7 kHz @ - 3 dB, or Q = 60 - if not re-tuned synchronously with modulated signal, losses are inevitable Solutions : - increase transmitter power to compensate for losses, or - re-tune transmitting antenna
(cont'd) IFM CHALLENGES Modulation schemes Phase sweeping - requires continuous change of signal's phase in a predetermined manner and, - transmitting antenna has to be re-tuned synchronously to avoid loss in signal amplitude - implementation is a real challenge Frequency jumping - instant increase/decrease of antenna inductance or capacitance will result in frequency jump and linear change of signal's phase - any required phase can be achieved at desired time by selecting one of predetermined frequencies - relatively easy to implement in AN/FPN 64
AN/FPN-64 OPERATION - four Drive Half Cycles (DHC's) are delivered to Coupling Network and Antenna circuit during first 20 usec - tuned Coupling/Antenna circuitry resonates at 100 kHz - changing the amount of energy in DHC's allows for precise generation of Loran-C signal Lt 7.45uH La - addition of more DHC's will result 255uH Ct in overall change of signal shape C1 L1 0.34uF 1.34uF 1.893uH T1 Ca Rt 0.01uF 5 TBswich Ra 2.5 Vtb Tailbiter Output Network AN/FPN-64 Coupling Network and Antenna
SINGLE LEVEL IFM IMPLEMENTATION Simplified IFM schematic V6 V7 V8 V9 S2 S3 S1 S4 C6 L3 L4 C5 Lt 0.2uF 12.63uH 12.63uH 0.2uF La 255uH IFM Switches Ct C1 L1 T1 Ca Rt 0.01uF TBswich Ra 2.5 Vtb AN/FPN-64 Coupling Tailbiter Output Network Network and Antenna - Two capacitors and two inductors are added in series with antenna: closing the capacitor switch (S1 or S2) results in lower frequency; closing the inductor switch (S3 or S4) results in higher frequency
SINGLE LEVEL IFM IMPLEMENTATION Frequency and Phase Shift 102.5 to 97.5 kHz time (usec) Int. 1 - S1-S4 opened; Int. 2 - S3 closed; Int. 3 - S1and S2 closed; Int. 1 Int. 2 Int. 3 at the end of Int. 3 all switches opened F (kHz) 102.5 kHz 100 kHz 97.5 to 102.5 kHz 60 160 ~ 500 time (usec) 97.5 kHz Int. 1 - S1-S4 opened; Int. 2 - S1 closed; Int. 3 - S3 and S4 closed; at the end of Int. 3 all switches opened Phaze [ ] 0 90 0 time (usec) - 90
SINGLE LEVEL IFM IMPLEMENTATION Modulated vs unmodulated signals 0.8 Normalized Antenna Current 0.4 Modulated Unmodulated 0 -0.4 -0.8 20 60 100 140 180 Time in usec
SINGLE LEVEL IFM IMPLEMENTATION Modulated vs unmodulated signal spectrum 0 0 -10 -10 -20 -20 Modulated Modulated -30 -30 Unmodulated Unmodulated Attenuation (dB) Attenuation (dB) -40 -40 -50 -50 -60 -60 -70 -70 50000 50000 60000 60000 70000 70000 80000 80000 90000 90000 100000 100000 110000 110000 120000 120000 130000 130000 140000 140000 150000 150000 Frequency (Hz) Frequency (Hz) (102..5 kHz from 60 to 160 usec; 97.5 kHz from 160 to 500 usec)
MULTI LEVEL IFM Proposed Modulation Scheme 270 96.70 kHz 225 98.88 kHz 180 101.15 kHz 135 103.53 kHz 90 phase (deg.) 45 0 -45 -90 -135 -180 -225 -270 30 140 250 470 time (usec)
MULTI LEVEL IFM Advantages of the Proposed Scheme The beginning of all modulation patterns from 0 phase eliminates "time shifted" pulses which in turn minimizes the effect on legacy receivers Longer 1st modulation window - 110 usec vs 80 usec - allows for longer "averaged" pulse, which will make search and acquisition by legacy receivers more reliable All required phase magnitudes are achieved by using only four frequencies (by making modulation windows lengths equal) Easier to implement in Solid State Transmitter because all switches are identical
MULTI LEVEL IFM Possible Signal Shape Enhancements More energy in the tail 0.8 of the pulse without adding more HCG's (16 HCG transmitter) Normalized Antenna Current Modulated 0.4 DHC's re-distributed: Unmodulated 0 from: 6-6-2-2 to: 5-5-1-1-2-2 -0.4 -0.8 150 50 250 350 450 Time in usec
MULTI LEVEL IFM Possible Signal Shape Enhancements More energy in the tail 0.8 of the pulse with addition of 8 HCG's (16 HCG transmitter). Normalized Antenna Current Modulated 0.4 Additional DHC's are applied at 65, 70, 80 and 85 usec to produce Unmodulated 0 a flat "top" of the pulse -0.4 -0.8 150 50 250 350 450 Time in usec
MULTI LEVEL IFM Possible Signal Shape Enhancements More energy in the tail 0.8 of the pulse with addition of 8 HCG's (16 HCG transmitter). Normalized Antenna Current Modulated 0.4 Additional DHC's are applied at 40, 45, 50 and 55 usec, resulting Unmodulated 0 in more energy not only in the tail but max. radiated power -0.4 -0.8 150 50 250 350 450 Time in usec
MULTI LEVEL IFM Averaged Signal Envelope Shapes Ideal Pulse (no modulation) 700 Average Pulse (Megapulse) with 8 added HCG's 600 and decreased Tailbiter resistance (from 4 to 1 ohm) Current in Amps 500 Average Pulse (USCG) 400 300 200 100 20 40 60 80 100 120 140 150 Time in usec
DESIGN OF IFM SWITCH IFM Switch Simplified Schematic General Principle of Operation SCR Gate signal is applied i A At the peak of antenna current, A fully rectified voltage V CD is applied D1 D2 to inductors L S1 and L S2 , causing the current i SCR to increase rapidly C D (5 usec) to its peak value i L L This peak value is greater than peak antenna current, thus causing D3 D4 the diode bridge to be a short i SCR B circuit for the antenna current I A SCR current decreases to 0 at appr. A (to Switch Control Circuit) 500 usec, thus turning the SCR off L S1 L S2 SCR
DESIGN OF IFM SWITCH IFM Switch Simulated Waveforms Antenna Current i A i L IFM Inductor L Current SCR Current i SCR 20 60 100 140 180 Time in usec
DESIGN OF IFM SWITCH IFM Switch Tests
DESIGN OF IFM SWITCH IFM Switch Tests
DESIGN OF IFM SWITCH Status of Work Single HCG Test Transmitter, Coupling Network and Antenna Equivalent (including IFM additional inductance) were built and fully tested IFM Switch Engineering Model was connected and a first set of tests were performed IFM Switch performed precisely as predicted by Spice simulations Additional work is under way - DC power supply is being integrated with the Switch to enhance the performance
CONCLUSION Megapulse modulation scheme can provide required phase shifts. Antenna retuning circuitry allowing for minimal future changes in AN/FPN-64 transmitter was evaluated and tests are under way. Additional work is required to evaluate spectrum effects/constraints.
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