The Lightning Stroke Simulator is a replacement for the older Antenna Array Signal Simulator that provides artificial signals in lieu of actual signals yielded by the Antenna Array.  It is useful for bench testing of the GP-1 system Interface and Display when Antenna Array signals are not available or are otherwise unsuitable.  The Stroke Simulator is an enhancement over its predecessor, with better approximations of return and intracloud stroke waveforms, the allowance for continuous bearing simulations over about a 400-degree range, and the provision for continuously adjustable signal levels from 0 to 0.5 and 0 to 5.0 peak volts in either polarity. The capability of making small, continuous adjustments in both bearing and magnitude is often found helpful in trouble-shooting hardware and software problems. The new device  is more expensive than the earlier unit because its improved performance depends, in large part, on use of several costly 4-quadrant analog multipliers.

The Simulator provides the three basic signals that are yielded by the Antenna Array from actual lightning: the e-field (voltage field) signal as from the pair of large  horizontal plates and the pair of b-field (magnetic field) signals as from the North-South and the East-West antenna loops. Information about the signal bearing is based on the relative magnitude and polarity of the pair of b-field signals according to the sine and cosine of the bearing.

The appropriate signals are generated in three steps or stages implemented by three separate circuit boards contained in the Simulator cabinet. The basic return stroke and intracloud stroke waveforms are generated at either a 1 Hz or 5 Hz frequency on the Waveform Generator board. Because all three signals, e-field and the pair of b-field signals, have the same temporal wave shape,  this one source serves the three output signals.  The bearing information for the pair of b-field signals is provided by the Sine, Cosine Generator board. This board generates a pair of dc signals whose magnitude and polarity are according to the sine and cosine of the desired bearing selected by the pair of panel-mounted potentiometers, the coarse and fine Bearing controls, R1 and R2 on the Simulator cabinet and the Sine, Cosine Generator board. Gross adjustment over a 400 (-20 to 0 to 360 to +20)-degree range is made by the R1, coarse-Bearing dial, and fine adjustments, over +/- 10 degrees, are made by the R2, fine-Bearing dial. The third, Multipliers, Output Set board combines the signals from the above two boards and allows various selections of: stroke type, Return or Intracloud (S4); stroke polarity, Normal or Inverted (S3); stroke output signal levels, Lo or Hi; and Output level (S5 and R3) front panel controls. A fourth circuit board holds the Power Supply components. All the operating controls are mounted on the Cabinet front panel, while AC power and the output signals are accessed through rear panel connectors.

Wave form generation starts with a 5-Hz clock pulse created by U1, a LM555 timer (i), divided by 5 by U2 (j), a 7490 decade (divide by 2 and divide by 5) counter, and combined in U3, a 74157 data selector. U3 is operated by S2 to select which of the two waveform frequencies is desired. The panel-mounted LED indicates the generation of each pulse. The descending limb of the pulse (i or j), selected by U3, triggers LM555 timer, U4, to generate a 10-microsecond pulse (k). The descending limb of this pulse, in turn, triggers another 10-microsecond pulse by LM555 timer, U5. This second pulse is then inverted and slightly attenuated (l) by U6, an LM318 op amp. Generation of the intracloud waveform, a bipolar wave, begins by combining the first, and second inverted, 10-microsecond pulses by op amp, U7 (m). Some rounding of the square-topped peaks also occurs in this stage by limiting the high frequency gain with the feedback 47-picofarad capacitor. Additional shaping and level setting is provided by the resistor-capacitor network coupling the bipolar pulse to op amp U8. The output of U8 is the finished, but inverted, intracloud waveform (o). This pulse is inverted by op amp, U9, to provide the normal polarity intracloud wave form (n). Both intracloud waves (n and o) are sent on to the Multiplier board. The return stroke waveform is derived from the inverted 10-microsecond pulse (l) from U6 coupled through a diode to an RC network that slows the waves trailing edge decay to zero (p). This more slowly decaying pulse is then further shaped by the U10 op amp by slowing its rate of rise and rounding the peak as was done earlier by U7. A slight undershoot in the trailing portion of the wave is created by the high-pass RC network in the output path of U10 that also includes a level-adjusting pot. Voltage follower, U11, provides the inverted finished return stroke waveform (r) to the Multiplier board while the inverter, U12, provides the normal return waveform (q).

The dc voltages representing the sine and cosine of the selected bearing are provided by the Sine, Cosine Generator board. The process is begun by generating a pair of 100-Hz sine waves, separated in phase by 90 degrees, by LM741 op amps U1 and U2. The output of U1 is taken as the "sine" waveform (a) and the output of U2 as the "cosine" waveform (b). It is important that this pair of waves be reasonably pure and undistorted. This high quality is partly assured by careful adjustment of the 5k level-setting pot.  The pair of waves (b and a) are sent on to the LM4066 analog switch, U9.  The cosine wave (b) is buffered by follower, U3, then converted to an approximate unipolar square wave by U4 and further squared up (c) by Schmitt trigger, U5d. The frequency of this square wave is then divided by two (d) by the 7474 flip-flop, U6b. This 50-Hz "clock" square wave from U6b (d) is thus phase-locked to the pair of 100-Hz sine and cosine waves (a and b), whose periods are about 10 milliseconds, at the input of analog switch, U9. The trailing, descending edge of the clock pulse triggers the LM555 timer, U7, to generate an adjustable-width square wave (e). The width is adjustable by the pair of panel-mounted Bearing potentiometers, R1 and R2, over a range of about 2 to 15 milliseconds. Therefore, the trailing edge of the pulse may be set at any phase point well over a full period of the pair of waves, (a and b). The trailing edge of the adjustable width pulse from U7 triggers the second LM555 timer, U8, to generate a 100-microsecond "sample" pulse (f). The sample pulse is level- and polarity-set by the three transistors to then momentarily close analog switch, U9. The output of U9 is then a pair of 100-microsecond pulses that are the sample magnitude and polarity of the sine and cosine wave pair at the moment the trailing edge of the adjustable-width Bearing pulse (e) is generated by U7. The pair of pulses, whose magnitude and polarity represent the sine and cosine of the selected bearing, are then converted into dc by track-and-hold followers, U10 and U11. Their outputs (g and h) are then the dc equivalents of momentary values of cosine wave (b), and sine wave (a), respectively. This pair of dc signals (g and h), representing the selected bearing, are then sent on to the Multiplier board.

The final three output signals are created by combining the previous boards' signals in the Multiplier, Output Set board and their type, polarity, and amplitude selected.  The four 1- or 5-Hz waveform signals, normal (o) and inverted intracloud (n), and normal (q) and inverted return (r), from the Waveform Generator board, are supplied to the LM4066 analog switch, U1. U1 is operate by panel switch, S3, to select either the pair of normal waves (o and q) or the pair of inverted waves (n and r). The pair of waves selected by U1 and S3 are then sent to the second LM 4066 analog switch, U2, which chooses the desired wave type, intracloud or return, with the aid of panel-mounted switch, S4. The output of U2 is now the selected wave type and polarity at a fixed magnitude and polarity. This signal is then supplied to the pair of AD633 four-quadrant analog multipliers, U3 and U4, as the "x" input. The dc cosine value bearing information (g) from the Sine and Cosine Generator board is connected to the "y" input of U3, and the dc sine value (h) is connected to the "y" input of U4. The resulting output from this pair of multipliers is the selected wave type of magnitude and polarity in accordance with the sine and cosine of the selected bearing, but at fixed magnitude. The outputs of U3 and U4 are next connected to the "x" input of the two MPY634 four-quadrant multipliers, U6 and U7. The "y" inputs to this pair of multipliers are a dc voltage adjusted by the panel-mounted output-level potentiometer and switch S5 (t).  To ensure the pair of resulting product (see example) b-field signals are identical in peak magnitude (i.e., that the NS peak magnitude when a north bearing is selected is the same as the EW peak magnitude when an east bearing is selected), the resistors marked with an asterisk, connected to the "sf" (Scale Factor) terminals of U6 and U7, may need small adjustment (see below). Low output impedance drive for the signal pair, NS (i.e. cosine b) and EW (sine b), is by the two pair of emitter followers, Q3-Q4 and Q5-Q6. The e-field output signal, phase-locked to the pair of b-field signals, is adjusted in level from the same panel-mounted output level potentiometer by way of the third MPY634 four-quadrant multiplier, U5. Its "x" input is directly from the U2 output and is of the selected wave type and polarity, while the "y" input is the same output level voltage from R3 (t). Thus, the output level front panel dial simultaneously sets all three levels of the e-field, NS and EW signals. The same low impedance source is established for the e-field output by the emitter follower pair, Q1-Q2.

The Power supply is a modified from the Jameco JE 215 kit. The kit circuit board is discarded and replaced by one constructed from the artwork shown.

Simulator construction is quite ordinary and carried out in a manner similar to that described for the Preamplifier-Integrator portion of the Antenna Array. All four circuit boards are available from FAR Circuits. See the References section for details. It is suggested that, first, jumpers be placed on the component side of all four boards as shown by the jumper placement figures for the Sine, Cosine Generator, Waveform Generator, Multipliers, Output Set, and Power Supply boards. That can then be followed by the remaining component installation on the Sine, Cosine Generator, Waveform Generator, Multipliers, Output Set, and Power Supply boards. Before applying power to the boards, carefully check for errors, inter-trace solder bridges, and reversed polarity installation of diodes and polarized capacitors. Neither power rail-to-ground resistance should be less than 1000 ohms. The Sine, Cosine Generator board and the Waveform Generator board can be tested independently. Instructions for adjustment of on-board trimmers is given in the schematic legends and described below. Proper function of the simulator will be suggested by obtaining the several waveforms shown in the accompanying figures. Testing of the Multiplier, Output Set board  can only be done when the two previous boards are properly functioning and connected. Again, obtaining the waveforms, similar to those shown for the NS output channel example will suggest proper behavior.

Final calibration can be performed after general performance checks display all the example waveforms and the 100-Hz sine and cosine wave signal levels have been properly set by the 5k trimmer on the Sine, Cosine Generator board. The signal levels from the Waveform Generator board should be set close to the desired 4-volt peak by the two 500-ohm trimmers, but will be accurately adjusted later.

The bearing span 10k trimmer on the Sine, Cosine Generator is adjusted for an approximate 400-degree coarse Bearing control (R1) range as follows. Connect the external trigger input of your oscilloscope to waveform sample point (d) (pin #9 of U6b) on the Sine, Cosine Generator board. Set the trigger slope of the oscilloscope to negative-going so that the trigger occurs on the descending limb of Reference wave (d). Adjust the time base of the oscilloscope to 2.5 milliseconds per centimeter, or as close to that as allowed by your instrument. If your oscilloscope is a two-channel type, then connect one channel input probe to waveform sample point (b) of the same circuit board to obtain a stable 2-1/2 cycles, or so, of the cosine wave as seen in the top waveform. Channel vertical sensitivities should be close to 2 volts/centimeter. Connect the other channel probe to waveform sample point (f) to obtain a very narrow pulse, again as seen as sample (f), somewhere along the horizontal axis of the screen. Next, set the fine Bearing control (R2) to its mid range. Now, rotate the Coarse Bearing control (R1) to bring the narrow pulse (f) as far as it will go to the left side of the screen. Particularly note its position relative to the left-most positive peak of Cosine wave (b). It should be very close to the peak, preferably slightly to the left. Next, rotate the Coarse Bearing control (R1) to move pulse (f) as far to the right side of the screen as possible. It should now be positioned very close to, preferably slightly to the right of, the second positive Cosine wave (b) peak. After noting the position of pulse (f) relative to the two positive (b) peaks, adjust the 10k max bearing span trimmer until, at the two extremes of the coarse Bearing control, pulse (f) moves from about 5% interpeak span to the left of the first Cosine wave peak to about 5% to the right of the second Cosine wave peak. This adjustment will allow the coarse Bearing control to span more than a full 360 degrees by about +/- 20 degrees. If only a single channel oscilloscope is used, then simply move the input probe between points (b) and (f) noting their horizontal positions when following the above process. This completes the bearing span trimmer adjustment.

The three output multiplier gains must be equalized by selecting the proper resistor values, somewhere between 3.6k and 5.1 k, connected between pins #4 (Scale Factor) of  U5, U6, and U7 and the -12 volt rail, of the Multipliers, Output Set board. To make these selections, do the following: Initially, make sure there are resistors of about 4.7k connected at the above three positions. Place 100-ohm resistor loads on all three outputs, E-field, NS and EW. Adjust the Output level control to near its maximum clockwise position, and set S3 to select a Normal stroke and S4 to select a Return stroke. Connect the oscilloscopes external trigger to sample point (j) on the Waveform Generator board to trigger on the negative-going, trailing edge of the clock pulse. Connect an oscilloscope probe, whose channel sensitivity is about 1 volt/centimeter, to the EW output and rotate the coarse Bearing control. You should see a wave, similar to the (q) or (r) return stroke waveforms, that changes magnitude and polarity as you rotate the control. Next, carefully rotate the Bearing control until the EW wave peak falls from a positive value to zero near one extreme of the Bearing control. The fine Bearing control can be helpful here. This bearing is now to the north. Move the probe to the NS output. You should now see a wave similar to (q) and of a peak magnitude close to +5.0 volts. Bring the peak magnitude of the NS output to exactly 5.0 volts by a slight adjustment of the Set Return level 500-ohm trimmer (between U10 and U11) on the Waveform Generator. Switch S4 to select the intracloud wave. You should now see a wave very similar to the (n) intracloud waveform, with a positive peak magnitude close to 5.0 volts. Bring the peak to exactly 5.0 volts by adjusting the Set IntCld level trimmer (between U7 and U8) on the Waveform Generator board. This completes final adjustment of the two-level set trimmers on the Waveform Generator board. It also determines that the Scale Factor resistor connect-to pin #4 of U6 is the correct value.  Return S4 to the Return stroke position. Connect the oscilloscope probe to the E-field output. The screen should display a positive polarity wave, identical in shape to the previous NS wave and close to 5.0 volts peak magnitude. Because the E-field signal level does not need to track exactly with the NS and EW levels, its Scale Factor resistor connected to pin#4 of U5 will need no change unless there is a substantial difference from 5.0 volts. If the wave is of smaller magnitude than 5 volts, then replace the resistor at pin #4 of U5 with a smaller value. Conversely, if the E-field peak magnitude is greater than 5 volts, then increase the value of the Scale Factor resistor. This resistor should be some value between 3.6k and 5.1k. Return the oscilloscope probe to the NS output and note that the peak magnitude is still at +5.0 volts. Rotate the Bearing controls until the NS wave falls to exactly zero. Move the probe to the EW output. If a negative-going wave, similar to the (r) return stroke waveform is seen, then replace the oscilloscope probe back to the NS output and rotate the Bearing controls to find the other NS zero output. Replace  the probe to the EW output and find a positive polarity return stroke wave similar to the (q) return stroke waveform and of a peak magnitude close to 5.0 volts. As described for the E-field Scale Factor resistor, select a value for that connected to pin #4 of U7 to bring the EW peak value to exactly 5.0 volts.

The coarse Bearing control dial can be calibrated by noting the relative magnitudes and polarities of the NS and EW outputs.  Set the fine Bearing control to mid scale and do not readjust during the coarse Bearing control calibration. North is when the NS output is at its positive maximum and the EW output is exactly zero. Northeast is simulated when both the NS and EW outputs are positive and of exactly equal magnitudes. East is simulated when the NS signal is exactly zero and the EW signal is positive and of maximum magnitude. Continue this process around the complete span of the coarse Bearing control. To calibrate the Output level control, restore the Bearing control to exactly north and connect the oscilloscope probe to the NS output. Position S5, Hi-Lo level switch, to Hi and rotate the Level control toward its maximum extreme position and note the position when the NS peak output is at exactly 5.0 volts. Next, rotate the Level control until the NS peak is at 4.0 volts. Continue this process for 3, 2, 1, and 0.5 volts. When S5, the Hi-Lo level switch, is placed in the Lo position, the same Level control marks will represent 0.5, 0.4, 0.3, etc. output volts.

The calibration of the Lightning Stroke Simulator is now complete. It may be used in all future GP-1 calibration and testing as described in the use of the older Antenna Array Signal Simulator, but with the luxury of continuous adjustment for any bearing and any magnitude from 0 to 5 volts.