Astronomical Society of Victoria Inc.

Transcription

1 Radio Astronomy Section. 1.0 Background The natural phenomena of lightning generates electromagnetic waves over a broad spectrum of frequencies. A substantial component of the energy occurs in the spectrum that is normally considered part of the audio frequency range below 20 KHz. The spectrum covers the ELF (extremely low frequency) range from 30 Hz to 3 KHz and the VLF (very low frequency) range from 3 KHz to 30 KHz. However, since the energy is in the form of electromagnetic waves and not the movement of air, one cannot actually hear these phenomena by ear. A simple receiver can be devised which converts the ELF and VLF electromagnetic waves into sound. The receiver is in fact a very sophisticated audio amplifier, however instead of a microphone or other acoustic instrument at the input of the amplifier, a radio antenna is used. The ASV whistler receiver as described in this document provides a useful item of equipment which enables continuous study of the ELF/VLF radio spectrum. Lightning typically generates a short duration transient which is heard as a click in the receiver, refer to Figure 1. This is what we normally describe as static and in the receiver the sounds are named sferics which is an abbreviation of atmospherics. Local lightning strikes within a distance of a thousand kilometres range or thereabouts generate this kind of sound in the receiver. The phenomenon however does not end there. Over medium distances of a few thousand kilometres the transient pulse is stretched in time by the characteristics of the radio wave interaction between the earth and the ionosphere. This causes the higher frequencies to arrive at a remote receiving location earlier than the low frequencies. The resulting sound of such distant lightning strikes sound like water droplets or short plop sounds with a duration of about 5 to 10 milliseconds. These are named tweeks. A lightning strike may also initiate a radio wave which propagates out along the earth s magnetic field lines and travel many thousands or hundreds of thousands of kilometres out into space eventually returning back to the earth typically on the opposite side of the planet from where the lightning initially occurred. Over the long journey along the magnetic field line, the transient pulse is stretched even further, to the extent that an observer listening to the signal which returns to earth hears a tone with a descending pitch that is sustained over a period of one to several seconds, a truly astounding and surprisingly ethereal experience. This is what has been named a whistler. Figure 2 presents 3 examples of lightning phenomena graphically in the frequency domain versus time. The top and bottom of each trace has been shown as a thin line to illustrate the reduced level of intensity of the received signal above 10 KHz and below 300 Hz. This is both a function of the actual spectral energy of the lightning static as well as the characteristics of the receiver. As shown, a sferic is observed as a vertical line. This indicates a wide spectrum of radio frequency energy occurring in a short period of time. For the tweek the line has a hook feature towards the end of the event at the lower frequencies. Note also how the spectral energy also abruptly cuts off just below 2 KHz. This is a function of the earth to ionosphere propagation characteristics. There are many and varied sounds also associated with the way the electromagnetic waves of lightning propagate. Recurring whistlers in a sequence create a sound described as a chorus. Whistlers can also occur in the reverse pattern where the pitch begins at a low frequency and sweep up to a high frequency. 1

2 The whistler receiver is also sensitive to any form of low frequency electromagnetic waves. The beating wings of flying insects near the receiver generate weak but detectable waves which are heard as sound. Similarly, small animals near the receiver will generate sounds as the animal or indeed, the person walks past the receiver. An unfortunate reality with this type of receiver is the fact that it is susceptible to the low frequency field created by the AC power mains. This appears as a loud hum in the receiver and in extreme cases simply swamps the faint sounds of the whistler phenomena. To avoid or minimise the impact of the AC mains the best location for a whistler receiver is to be as far away from the power grid as possible. The author of this document has experimented with a with a portable whistler receiver and found the impact of the AC mains grid was nonexistent on the Bogong High Plains near the mountain ski resort of Falls Creek in Norther n Victoria. The ASV s property the LMDSS at Heathcote does not quite meet this criterion but has proven to be satisfactory. It is however necessary to locate the actual receiver remotely from the audio interface within the radio astronomy equipment laboratory. Therefore the whistler receiver constructed for the ASV consists of 3 essential components. The remote receiver unit, the base unit (inside the RAS laboratory) and a transmission link between the remote and base unit. The remote unit is an e-field or electric field receiver. The receiver s antenna is a vertical whip and is very short in terms of wavelength compared to the wavelengths of the audio frequencies being received which may be as long as 1 million metres. Therefore the receiver is responding predominantly to the electric field of the electromagnetic wave. Figure 1. Lightning static signals Amplitude Vs Time Figure 2. Lightning static signals Frequency Vs Time 2

3 2.0 Description Figure 3: Photograph of the ASV Whistler Receiver Remote Unit (Left), Base Unit (Right). 2.1 General Figure 3 presents photographs of the whistler receiver remote unit and the base unit which is installed in the RAS laboratory at the LMDSS. 2.2 System Components In Figure 4 the whistler receiver system components are presented. Details of the cabling and connectors for the complete system installation are also presented in Figure 17. The remote unit e-field receiver utilises a short vertical whip antenna. To reduce or eliminate the receiver from being impacted by the AC main field present at the LMDSS, the e-field receiver is installed approximately 45 meters away from the shipping container radio astronomy laboratory. The transmission link between the e-field receiver and the base unit is realised by a conventional twisted pair telephone cable. This link effectively provides a balanced transmission path whereby common mode signals that are induced into the line from the AC mains field are cancelled out. 3

4 At both ends of the transmission link the signals are coupled to the transmission line via audio transformers. Additional components are provided in the interface units at each end of the transmission line to filter out higher radio frequency signals. The interface unit at the e-field receiver end needs to be located at least a few meters away from the vertical whip antenna to ensure common mode AC mains hum in the transmission line is not coupled into the antenna. Therefore the line interface unit is installed in a waterproof enclosure near the whistler receiver remote installation. A coaxial cable is connected between the remote unit and the line interface unit. The transmission link runs partially underground and partially above ground between the remote unit and the RAS laboratory. The line interface unit at the RAS laboratory end is located in the entrance area of the shipping container and on the outside of the gland plate. From the line interface unit a coaxial cable connects the transmission line to the gland plate. On the inside of the screened room part of the shipping container the whistler receiver base unit is located. The input to the base unit is via a coaxial cable to the appropriate socket on the gland plate. The base unit may be used to directly monitor the signals being received by the whistler receiver remote unit (e-field receiver) using the built-in loudspeaker. The output of the base unit is connected to the audio input of a computer and the sound is streamed live onto the internet. 2.3 Remote Unit Circuit Description Refer to the circuit diagram provided in Figure 5. The circuit board component layout is shown in Figure 15 and a photograph of the internal wiring of the remote unit is shown in Figure 11. The e-field receiver uses an N-Channel Junction FET (Field Effect Transistor) TR1 as the input stage. A High input impedance is achieved by biasing the gate of TR1 with a resistance of 80 Meg Ohm. Input protection against static charge and radio frequency is achieved by the use of D1 and D2 and the 470 pf capacitor from TR1 gate to ground. TR1 provides approximately 19 db of voltage gain with a very low level of residual noise generated. The output of TR1 is coupled to a passive low pass filter realised by 2 ferrite pot-core inductors and 4 capacitors in a Pi configuration. The low pass filter attenuates signals above 10 KHz to ensure the commercial and military communication signals from 15 KHz and above do not overload the receiver. Following the low pass filter a low noise operational amplifier IC1 provides additional gain which may be adjusted from a gain of 2 (6 db) up to a gain of 1000 or 60 db. This stage is configured as a non-inverting amplifier so that there is an overall phase inversion between the e-field receiver input and the audio output. This aids in general stability and noise reduction. The output of the op-amp is connected to the chassis socket and ultimately is connected via a coaxial cable to the transmission line interface unit. The low frequency response of the e- field receiver is attenuated below 300 Hz by careful selection of the inter-stage coupling capacitors. This also aids in reduction of the impact of any residual AC mains signals. The remote unit overall frequency response may be seen in figure 8. The remote unit is powered by a pack of 10 nickel metal hydride rechargeable batteries which deliver a total of 12 volts DC with a capacity of 2000 milliamp hour. The battery pack is trickle charged using the small 5 W solar panel via a 90 Ohm resistor. The maximum charging current is approximately 54 milliamps. The remote unit enclosure is fully sealed and is waterproof and airtight. To prevent the potential build-up of gas in the event of the batteries overcharging, the enclosure is provided with a pressure relief valve. 4

5 For safety reasons the high capacity battery power is connected to the e-field receiver circuit via a built-in fuse which will fail for a circuit current exceeding 100 milliamps. 2.4 Base Unit Circuit Description Refer to the circuit diagram provided in Figure 6. The circuit board component layout is shown in Figure 16 and a photograph of the internal wiring of the base unit is shown in Figure 12. The base unit provides additional audio amplification as well as the features of audio peak level clipping and loudspeaker monitoring. The base unit has a built-in AC mains power supply. The AC power input is not switched and relies on the AC outlet switch to provide that function. The IEC socket contains a built-in fuse. The power transformer also has a built-in thermal fuse in the event the transformer is overloaded and experiences an excessive temperature rise. The AC power is rectified using a bridge rectifier with diodes D3 to D6. The output of the rectifier is approximately 16 V DC which is regulated to 10 V using IC3. The regulated power operates the operational amplifiers of IC1 and IC2 and the unregulated voltage supplies power to the speaker power amplifier IC4. At the input to IC1 additional filtering is provided to reduce high frequencies above 50 KHz. The op-amp IC1 provides adjustable gain from 0 to a maximum of 10 times (20 db). The output of IC1 is connected to the input of Line output amplifier IC2 and the input of the speaker power amplifier IC4. In this way the adjustment of the line output level and the speaker level are independent. IC2 has an adjustable gain from 0 to 1 (0 db) and is provided to optimise the signal level interface into the following PC soundcard interface. IC1 uses 2 diodes D1 and D2 in the feedback path to provide the feature of signal peak clipping. When the peak clipper switch is activated the output of IC1 is limited to a maximum signal swing of approximately 1.4 V peak to peak due to the action of the diodes clipping the signal waveform. The transfer function of the circuit with the peak clipper in action is shown in figure 10. As can be seen, for input signals above -10 db the output level is contained below a maximum of - 4dB and is effective over an input level range of more than 40 db. The peak clipper introduces distortion to the signal but does provide a means of effectively amplifying weak signals at the expense of attenuating loud signals. The peak clipper function may prove to be advantageous when listening to the natural phenomena of lightning static. To obtain the optimum performance from the peak clipper, the gain of IC1 needs to be adjusted to a suitable level to obtain the right balance between amplification of the weak signals and minimising the overall level of distortion to the loud signals. The operation of the whistler receiver can be monitored using the built-in audio power amplifier IC4 which is connected to the built-in speaker in the base unit. The speaker volume is adjustable. The audio power amplifier may be deactivated which is the normal state of the base unit when left unattended. The signal present at the speaker volume control is provided at a 3.5 mm jack socket at the rear of the base unit. The line output is provided at the rear of the base unit via an RCA socket. The overall frequency response of the base unit may be seen in Figure Line Interface Circuit Description Refer to the circuit diagram provided in Figure 7 and the wiring photographs in Figure 13 and 14. The twisted pair transmission line is terminated at each end using the line interface circuit. 5

6 Both circuits are essentially the same however different cable connectors are used on the interface enclosures. Transformers T1 and T2 are conventional audio transformers with the nominal impedance ratio of 500 Ohm to 8 Ohm, however in this application they function more simply as isolation transformers. The transmission line connection of both transformers includes radio frequency chokes and bypass capacitors to ensure stray RF fields are greatly attenuated and do not couple into either the remote or base unit of the system. 3.0 Operation The whistler receiver remote unit is activated by the on/off switch located at the base of the remote unit box. A red LED will be illuminated when the remote unit is active. The solar panel will ensure the internal battery pack in the remote unit remains charged. The low current consumption of the remote unit means the unit should continue to operate 24 hours per day for many weeks even if the solar panel charging levels are limited due to seasonal or weather conditions. The receiver base unit requires an AC mains supply to operate. Once plugged into an outlet the base unit is activated by the power on/off switch on the front panel. When active, a green Led will illuminate on the front panel. The input signal from the transmission link is connected via a patch cord between the gland plate and the base unit jack socket located on the lower left side of the base unit box. The input level is adjusted by the input gain control knob on the front panel. This adjustment may be used to overcome any small loss of signal from the remote that many occur in the transmission link. Up to 20 db gain is provided to increase the overall sensitivity of the system should that be required. The base unit has a built in loudspeaker to monitor the remote receiver and for demonstration purposes. The loudspeaker amplifier is activated by the speaker on/off switch on the front panel. The loudspeaker volume is adjusted by the speaker volume knob on the front panel. Normally when unattended the loudspeaker is switched off. This also reduces the base unit current consumption. The line output of the base unit is located on the rear of the box. This is provided by an RCA socket. The line output is connected to the line input of a PC in the RAS laboratory. The computer then streams the received signals from the whistler receiver live onto the internet. The line output level may be adjusted by means of the line output level knob located on the front panel. A 3.5 mm jack socket is provided on the back panel. This socket is connected to the speaker volume signal however the output is still present even with the speaker output switched off. A peak clipper feature is provided as a means of limiting the audio output level. The feature is activated by the peak clipper on/off switch. The peak clipper is a simple circuit which is a hard limiter design and introduces distortion to the source signal when in operation. This feature may require careful adjustment and experimentation to provide a useful increase in the functional range of sensitivity of the whistler receiver at the expense of distorting high level sferics. When active, the clipping level is adjusted by manipulation of the input gain control using the loudspeaker to monitor the result. Once a suitable clipping level is achieved, the line output level is readjusted to the required level for the live streaming. 6

7 4.0 Performance Specification 4.1 Remote Unit Sensitivity <5 uv for 10 db signal to noise ratio Gain 60 db (adjusted, maximum 65 db) Audio Bandwidth 340 Hz 12.8 KHz (-3 db) Response at 19.8 KHz -28 db Signal to Noise Ratio 52 db for V rms output. Input 50 Ohm terminated Input Impedance 80 Meg Ohm Output Impedance 100 Ohm Power Supply Voltage 12 V nominal, 16 V with fully charged batteries Current Consumption 6.1 ma quiescent 4.2 Base Unit Gain to Line output 21 db (Input gain and line output level set to maximum) Audio Bandwidth 50 Hz 11 KHz (-3 db) Signal to Noise Ratio 50 db for V rms output Input Impedance 10 K Ohm Line Output Impedance 100 Ohm Loudspeaker Power Output: 1.5 W into 4 Ohm less than 10 % THD Input Gain Setting for 0 db Output Line Level Setting for 0 db Gain Peak Clipper Output Limiting 446 millivolt rms (-4 db) Internal Power Supply Voltage 10 V Current Consumption 20 ma quiescent AC Power 240 VAC, 100 ma fuse 4.3 Line and Interface Unit Transformer ratio 62.5:1 (500 Ohm: 8 Ohm) RF low pass filter > 500 KHz Total Line Loss 5 db (100m twisted pair plus line interface at each end) Total Line Frequency Response See Figure Alignment Alignment of the whistler receiver is not necessary. The only internal adjustment is a gain trimpot within the whistler receive remote unit. This has been pre-set to provide an overall gain of 60 db for the remote unit. If the trimpot is adjusted to maximum (fully clockwise) the overall gain of the remote unit will increase to 65 db. This should not be necessary and may only result in degrading the receiver signal to noise performance. 7

8 Figure 4: ASV Whistler Receiver System Components Diagram 8

9 Figure 5: ASV Whistler Receiver Remote Unit Circuit Diagram 9

10 Figure 6: ASV Whistler Receiver Base Unit Circuit Diagram 10

11 Figure 7: ASV Whistler Receiver Transmission Link Circuit Diagram 11

12 Figure 8: ASV Whistler Receiver Remote Unit Frequency Response 12

13 Figure 9: ASV Whistler Receiver Base Unit Frequency Response 13

14 Figure 10: ASV Whistler Receiver Base Unit Peak Clipper Response 14

15 Figure 11: ASV Whistler Receiver Remote Unit Internal Wiring Photograph 15

16 Figure 12: ASV Whistler Receiver Base Unit Internal Wiring Photograph 16

17 Figure 13: ASV Whistler Receiver Remote Line Interface Internal Wiring Photograph Figure 14: ASV Whistler Receiver Base Line Interface Internal Wiring Photograph with Board View 17

18 Figure 15: ASV Whistler Receiver Remote Unit Circuit Board Layout 18

19 Figure 16: ASV Whistler Receiver Base Unit Circuit Board Layout 19

20 Figure 17: ASV Whistler Receiver Cabling and Connectors Diagram 20

21 Figure 18: ASV Whistler Receiver - Line and Line Interface Frequency Response 21