INTRAPULSE FREQUENCY MODULATION AND THE AN-FPN/64 TRANSMITTER Dr. - - PowerPoint PPT Presentation

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

Lt 7.45uH Ct 0.34uF Rt 5 C1 1.34uF L1 1.893uH Vtb TBswich La 255uH Ca 0.01uF Ra 2.5 T1

AN/FPN-64 Coupling Network Tailbiter Output Network and Antenna

• 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

• f Loran-C signal
• addition of more DHC's will result

in overall change of signal shape

SINGLE LEVEL IFM IMPLEMENTATION

Simplified IFM schematic

La 255uH Ca 0.01uF Ra 2.5 Lt Ct Rt V8 T1 C5 0.2uF C6 0.2uF L3 12.63uH L4 12.63uH S1 S2 S3 S4 V7 V6 C1 L1 Vtb TBswich V9

IFM Switches

AN/FPN-64 Coupling Network Tailbiter Output 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 kHz 97.5 kHz F (kHz) 100 kHz 90

• 90

Phaze [ ]

60 160 ~ 500

• Int. 1
• Int. 2
• Int. 3

time (usec) time (usec) time (usec)

102.5 to 97.5 kHz

• Int. 1 - S1-S4 opened;
• Int. 2 - S3 closed;
• Int. 3 - S1and S2 closed;

at the end of Int. 3 all switches opened

97.5 to 102.5 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

SINGLE LEVEL IFM IMPLEMENTATION

Modulated vs unmodulated signals

Modulated Unmodulated 20 60 100 140 180 0.4 0.8

• 0.4
• 0.8

Time in usec Normalized Antenna Current

SINGLE LEVEL IFM IMPLEMENTATION

Modulated vs unmodulated signal spectrum

50000 60000 70000 80000 90000 100000 110000 120000 130000 140000 150000

Frequency (Hz)

• 70
• 60
• 50
• 40
• 30
• 20
• 10

Attenuation (dB)

Modulated Unmodulated

50000 60000 70000 80000 90000 100000 110000 120000 130000 140000 150000

Frequency (Hz)

• 70
• 60
• 50
• 40
• 30
• 20
• 10

Attenuation (dB)

Modulated Unmodulated

(102..5 kHz from 60 to 160 usec; 97.5 kHz from 160 to 500 usec)

MULTI LEVEL IFM

Proposed Modulation Scheme

103.53 kHz 101.15 kHz 98.88 kHz 96.70 kHz 45 90 135 180 225 270

• 270
• 225
• 180
• 135
• 90
• 45

30 140 250 470

time (usec) phase (deg.)

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

50 150 250 350 450 0.4 0.8

• 0.4
• 0.8

Time in usec Normalized Antenna Current

Modulated Unmodulated

More energy in the tail

• f the pulse without

adding more HCG's (16 HCG transmitter) DHC's re-distributed: from: 6-6-2-2 to: 5-5-1-1-2-2

MULTI LEVEL IFM

Possible Signal Shape Enhancements

0.4 0.8

• 0.4
• 0.8

Time in usec Normalized Antenna Current

Modulated Unmodulated

50 150 250 350 450

More energy in the tail

• f the pulse with

addition of 8 HCG's (16 HCG transmitter). Additional DHC's are applied at 65, 70, 80 and 85 usec to produce a flat "top" of the pulse

MULTI LEVEL IFM

Possible Signal Shape Enhancements

0.4 0.8

• 0.4
• 0.8

Time in usec Normalized Antenna Current

Modulated Unmodulated

50 150 250 350 450

More energy in the tail

• f the pulse with

addition of 8 HCG's (16 HCG transmitter). Additional DHC's are applied at 40, 45, 50 and 55 usec, resulting in more energy not

• nly in the tail but max.

radiated power

MULTI LEVEL IFM

Averaged Signal Envelope Shapes

20 40 60 80 100 120 140 150 100 200 300 400 500 600 700 Time in usec Current in Amps Average Pulse (USCG) Average Pulse (Megapulse) with 8 added HCG's and decreased Tailbiter resistance (from 4 to 1 ohm) Ideal Pulse (no modulation)

DESIGN OF IFM SWITCH

IFM Switch Simplified Schematic

General Principle of Operation

L

D1 D4 D2 D3

L S1 L S2

SCR

(to Switch Control Circuit)

i L i SCR i A

A C D B

SCR Gate signal is applied At the peak of antenna current, fully rectified voltage VCD is applied to inductors LS1 and LS2 , causing the current iSCR to increase rapidly (5 usec) to its peak value This peak value is greater than peak antenna current, thus causing the diode bridge to be a short circuit for the antenna current IA SCR current decreases to 0 at appr. 500 usec, thus turning the SCR off

A

DESIGN OF IFM SWITCH

IFM Switch Simulated Waveforms

Antenna Current i A IFM Inductor L Current

i L

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.