SOLRAS (An acronym for SOLar RAdio Spectrograph) is a very low cost swept frequency radio-telescope that can be used to observe metre-wavelength radio emissions from the Sun.
SOLRAS employs recycling of domestic electronic equipment to achieve very low cost implementation of an an instrument useful for space weather monitoring and forecasting purposes.
The heart of SOLRAS is the radio frequency (RF) section of a standard video cassette recorder (VCR) or domestic TV receiver. The lifetime of a VCR is normally determined by failure in the mechanical section which provides cassette load/unload functions and tape threading and transport. Even when the VCR is unusable for normal purposes, the electronics modules, including the RF section, are usually still in good condition. The power supply in the VCR is also suitable for supplying all of the power requirements of SOLRAS.
Apart from the RF section and power supply, a digital to analog converter and an analog interface are required to allow interfacing of the RF section to a recycled personal computer (PC) such as a 386 with a VGA display. The digital-analog converter accepts output from the PC printer parallel port and generates an analog sawtooth waveform which is used to sweep the RF section over most of the VHF band. The analog interface accepts the demodulated signal output from the RF section and converts it for input to the games port of the PC. This in effect functions as a low cost analog to digital converter to provide the PC with a digital representation of the radio telescope signal intensity output.
The RF section comprises a tuner followed by an intermediate frequency (IF) amplifier and signal demodulator. The RF tuner typically covers three bands, VHF Lo (~45 to 100MHz). VHF Hi (120-220MHz) and UHF (500 - 900MHz). Only the two VHF bands are used in SOLRAS.
Band selection is accomplished electronically by asserting the appropriate voltage on one of the three lines. Tuning within a band is achieved by a variable voltage, typically 2 to 28V. The high end of the voltage range corresponds to the lower frequencies within the band and vice versa.
The operation of SOLRAS is controlled by the PC. Two of the control bits of the parallel port (LPT1) are used for bandswitching. A sweep is commenced by first asserting the low band bit. A value of 0 is then output to the eight data bits of the parallel port. The external DAC accepts this value and produces an output of 28V which is fed to the tuning input of the TV tuner. The CPU then reads a byte from the X-component of the joystick #1 input on the games port. This corresponds to the signal intensity fed to the tuner by the antenna at the lowest frequency of the low band.
The value at the output of the parallel data port is then incremented by one and the signal level is again sampled from the games port. This process is repeated until the parallel port is outputting a data value of 255. At this time the system is receiving on the highest frequency of the low band.
The low band bit is then deasserted and the high bit asserted. This band is then swept and sampled in the same manner as for the low band.
Note that the sweep may be linear or logarithmic as required, a simple look-up table providing the conversion between a simple 0 to 225 count and the actual data value output to the parallel port and thus to the DAC which provides the tuning voltage.
The signal levels collected during the two band sweep sequence are displayed immediately on the PC monitor as a colour coded (intensity) frequency versus time display, and are also stored to the hard disc in a day archive file. The total time to complete the dual band sweep varies according to the PC used, but should lie between 3 and 6 seconds. Sampling of the games port is the limiting process in the cycle.
The simplest antenna useful for SOLRAS is a dipole suspended in the shape of a V with an approximate right angle at the vertex (see diagram below). The V-shape broadbands the antenna making it acceptable for the swept frequency radiotelescope. The feed impedance at the vertex is approximately 75 ohms, and in the absence of a 1:1 balun (balanced to unbalanced) may even be fed directly with RG-59 or similar coax. This also matches the feed impedance of the tuner.
Improvements to the simple arrangement described above are first, the provision of an appropriate balun and secondly the inclusion of a small wideband preamplifier between the antenna and the tuner. The gain of this preamplifier should lie between 5 and 20dB. If the RFI environment is severe, only a small amount of additional gain will be possible before spurious cross/inter-modulation effects - signals produced in the RF section raise the noise floor and obscure small solar signals. Quiet locations can benefit from the maximum 20dB gain.
Alternative antennas which provide additional (passive) gain are the horizontal semi-bicone (from 3-6dB gain over the 'V') and the log-periodic (7 to 13dB gain over the 'V'). The latter antenna requires a mount and mechanism to track the sun (at least to within about 15 degrees).
The RF section is 'recycled' from a domestic VCR or TV receiver. Some familiarity with these devices is required to identify and extract the required modules and identify the required I/O points on the modules.
To be suitable for the SOLRAS application the RF tuner must be of the voltage-controlled tuning (VCT) type. All VCR's use VCT and all but the oldest TV's do also. The only TV receivers not suitable are those with a rotary/mechanical channel selector. Later TV sets employed either push-button tuning or processor controlled tuning. Both these types employ tuners with VCT.
Many VCR's are built in a modular fashion with separate printed circuit boards (PCB) for separate functions. In such a case it may be possible to extract a single PCB which contains the tuner (a shielded rectangular shaped box) together with the IF amplifier and video demodulator. Some other functions may also be included.
It is necessary to identify and isolate the
The power supply is normally located on a single board. Older VCR's used linear power supplies (with a mains power transformer) and these are usually considerably more reliable (although somewhat less efficient) than the later switching power supplies.
It is normally only necessary to identify the power cable from the power supply PCB to RF PCB, and cut all the rest. However occasionally, one power line may be routed through another PCB, and this must then be run with a separate bridging wire. Typically voltages are +12V for most of the circuitry, sometimes a second +12V line, and +50V, which is fed to a regulator on the RF board to provide a highly stable +30V for tuning purposes. If a microprocessor is present on the board, a +5V line or so may also be provided. This line will generally not be required. It is best to remove voltage supply to any digital controllers to minimise RFI generation in the final layout.
The DAC and bandswitch unit (to be constructed) require both a +12V and a +30V power line, and the VCR power module can also be used for this purpose.
The purpose of the sweep generator is to supply a tuning voltage to the RF tuner that produces a ramp from 2 to 28 volts and thus causes the tuner to sweep across the tuning range of the selected band. The actually voltage ramp is output in digital form as a binary number from 0 to 255 from the parallel printer port (LPT1) of the computer (PC). This voltage is applied to the sweep generator which is a digital to analog converter (DAC). The binary number is thus converted to an analog voltage.
The DAC uses the eight TTL outputs of the printer port to switch eight transistors off or on. These in turn apply either 0 or 12 volts to a resistor ladder network whose output is summed by a 741 operational amplifier. There are two variable controls in this DAC that need to be set up to produce the correct voltage output. This process is described in a latter calibration section.
The prototype DAC was contructed on a copper strip clad bakelite board (Veroboard type). The resistors of the ladder network (50k and 100k ohms) must be of 1% tolerance or better, to ensure adequate accuracy of the voltage output. Almost any switching type NPN transistors should be suitable.
This board requires both a +12 volt and a +30 volt supply from the power supply unit (from the VCR).
The bandswitching circuity is also included on this board, and consists of two switching transistors that convert the PC TTL signals to a form suitable for switching the band (VHF-Lo and VHF-Hi) of the RF tuner. The UHF band is not used in this application.
The analog interface is a very simple analog to digital converter which accepts the analog video output of the RF/IF board and puts it into a form that can be read by a PC Games port. The joystick input of the games port essentially converts a resistance in the range 0 to 100k ohms into a binary number from 0 to 255. The interface transistor looks to the PC as such a variable resistance, whose value is roughly proportional to the input voltage. The proportionality is not linear, but then neither is the receiver output. It can be used as a relative value, or it can be calibrated if a good quality signal generator is available. However, this is not necessary.
It is necessary however, to set up the interface in accordance with the output values that it will receive from the RF board. This process is described later.
The power for this interface is supplied from the games port of the PC (5 volts).
The eight data outputs of register one are used to tune the radiospectrograph across the frequency band. These are fed to the digital to analog converter interface as previously described to produce a voltage that varies from 2 to 28 volts. This is applied to the tuning pin of the RF tuner.
The two least significant bits of register 3 are used to select either low band (VHF-Lo) or the high band (VHF-Hi) of the RF tuner. Asserting bit 0 selects VHF-Lo, while asserting bit 1 selects VHF-Hi. Note that both bits 0 and 1 must not be asserted together. Also realise that the hardware does an inversion of the values presented by the software to these two register bits.
Calibrating the DAC
A small QBASIC program that cycles the parallel port data byte from 0 to 255 can be used to set the max and min voltages output by the digital to analog converter (sweep generator). The code is listed below.
'File name DACTEST.BAS 'DAC test 'produces a sawtooth ramp dly = 0 'increase to increase step delay PRINT "Press ESC to end test" DO FOR i% = 0 TO 255 'value for DAC OUT 888, i% 'output to LPT1 GOSUB Delay 'wait a while NEXT i% 'increment number LOOP WHILE INKEY$ <> CHR$(27) 'press escape to exit END Delay: FOR zz = 0 TO dly NEXT zz RETURN
This program must be run in DOS mode (not in a DOS window under WINDOWS). Only in this mode are you sure of direct access to the parallel port without delays.
With the parallel port connected to the DAC and power applied, connect the output of the DAC to an oscilloscope. Activate the program and observe the oscilloscope trace. It should be a sawtooth waveform. Adjust both the gain and the offset controls in the DAC to ensure that the waveform ramps between 2 volts minimum and 28 volts maximum. The DAC is then calibrated.
Calibrating the Analog Interface
The next major calibration is the analog interface. The voltage applied to the input of this circuit from the VCR RF section will depend on the nature of the intermediate frequency (IF) amplifier and the subsequent demodulator/detector. If this is a really old unit it will probably employ several discrete transistor amplifying stages to achieve the required high gain. The detector in these cases is usually a discrete germanium diode. When there is no input to the RF section (eg antenna terminals terminated by a 75 ohm resistor) the output from the diode detector will be approximately zero. As the input signal increases, the output voltage will rise to maybe a volt or two.
A more modern VCR will usually employ an integrated circuit amplifier as both the IF amp and demodulator. These often have a resting voltage well above zero. The table below indicates the output DC level for the prototype SOLRAS as a function of signal input to the antenna. This was measured at a frequency of 70 MHz.
Input (microvolts) Output (volts) 0.3 6.71 1 6.75 3 7.00 10 7.81 30 8.12 100 8.20 300 8.21 (Note: system saturation at ~ 200 uV)Note that the system is quite non-linear.
To set up the analog interface to make maximum use of the 8 amplitude bits available, another small QBASIC program can be used.
'Filename STIKTEST.BAS 'Testing calibration of games port joystick input SCREEN 7 'large characters for visibility DO 'set up an infinite loop a = STICK(0) 'sample joystick A - X input LOCATE 12, 20 'then display in the PRINT a 'middle of the screen LOOP WHILE INKEY$ <> CHR$(27) 'ESCape to exit loop END
The first control to adjust is the Hi Set variable resistor. This should be set with no input connected to the analog interface, and with the sensitivity control set to maximum resistance and the Lo Set control turned to maximum (ie moving arm set to ensure maximum resistance to ground/common (0V). When these conditions are met, the Hi Set control should be adjusted until the reading on the screen (with the above program activated) shows about 250.
The output of the RF section (video) should now be connected to the analog interface, and the antenna input should be terminated with a 75 ohn resistor (ie no signal input). With the sensitivity control set to about the half way travel, the Lo Set control should be adjusted to give a reading of about 5 on the screen. If this cannot be achieved with the Lo Set control alone, the value of the Sensitivity control should be reduced (ie greater sensitivity).
The receiver now needs to have a very strong signal injected at the antenna, one large enough to achieve saturation. This can most easily be done with a signal generator, but a nearby transmitter in the frequency range may also suffice. The receiver may have to be tuned. This can be done by adding a few lines of code to the above program to ask for a manual input (0 to 255) and outputting this number to the DAC which of course must then be connected to the RF tuner.
With a saturation signal at the input the Sensitivity control can then be adjusted to give a number in the region of 220 to 240. Note that this will affect the zero signal number, as the Lo Set and Sensitivity controls interact and affect both the zero setting and the sensitivity. It is thus necessary to go back and forth several times between zero and full signal to ensure an optimum setting of each control.
Testing the Band Switch
The function of the band switch can be tested by another simple QBASIC program.
'Filename BANDTEST.BAS 'BandSwitchTest SCREEN 7 'to display large characters DO FOR i% = 0 TO 7 'cycle lower 3 bits through all values LOCATE 12, 20 'print value PRINT i% 'in the middle of the screen OUT 890, i% 'and output value to register 3 of LPT1 SLEEP 3 'wait 3 seconds on each value NEXT i 'increment to next value LOOP WHILE INKEY$<>CHR$(27) 'ESCape to exit loop ENDWith the program running, the voltage at the input of the bandswitch transistors can be measured. If the collectors of the transistors are connected to the RF tuner, the voltage at these points can be monitored as well.
An Amplitude-Frequency Display
A final small QBASIC program can be used to perform an end to end test of the spectrograph. This will display an amplitude versus frequency graph for one selected band. All boards should be connected and powered for this test. The antenna should also be connected at this stage.
'Filename SPECTEST.BAS 'Spectrum Test SCREEN 9 '640 x 350 pixels DO 'do continuing frequency sweeps CLS 'clear screen PRINT "SOLRAS SWEEP TEST" PSET (50, 300) 'set intial graph point OUT 890,1 'select frequency band ( 1 or 2 ) FOR i% = 0 TO 160 STEP 2 'step through frequency range OUT 888, i% 'output frequency value to DAC a% = STICK(0) 'sample analog interface LOCATE 3, 40 'print out values PRINT USING "Freq=### Ampl=###"; i%; a%; LINE -(i% + 50, 300 - a%) 'draw graph NEXT i% 'increment frequency SLEEP 2 'wait two seconds between sweeps LOOP WHILE INKEY$ <> CHR$(27) 'ESCape to exit program END
The frequency band to be swept is set by the OUT 890,b command where b must have the value 1 or 2. The loop with index i% then sweeps the frequency range within this band. The amplitude sampled by the games port (JYA-X) at this frequency is then both printed and plotted on the screen.
Note that the frequency values are limited to 160. This is because the frequency becomes so non-linear above 160 that little frequency range is covered above these values. The frequency is also incremented in steps of 2. This is because of the large bandwidth (about 5 MHz) used in conventional TV. Incrementing by 1 would result in a vast oversampling of the frequency domain.
The frequency spectrum should change a little from sweep to sweep, especially on the high band (VHF-Hi) as this spectrum is host to intermittent aviation and public service transmissions.
The screen display on startup looks like the image below.
The first box of data on the top right hand side of the screen displays the hour angle, declination, azimuth and elevation of the Sun at the current time (if the site latitude and longitude have been entered correctly, and if the PC clock has been set to the correct local time and the correct time zone has been set). The second box of four parameters at the top far right shows, from top to bottom, the time in seconds to collect the current data record, the min and max values of the current data record, and the number of six second intervals since midnight. These numbers are useful in troubleshooting problems.
A QBASIC display program RASDISPA.BAS may be used to display data from the files archived by SOLRAS.BAS.
An example of the output of this program is shown below.
The lower band of this display shows a classic sequence of solar radio burst activity that accompanies a moderate solar flare. At 0902 local time there is a short vertical spike (called a type 3 burst), which is produced when a stream of relativistic electrons (ie electrons travelling at a substantial fraction of the speed of light) travels up through the solar corona. This is followed by a type 2 burst (sloping feature) from 0904 ot 0911 (with some activity out to 0918), which is indicative of a shock wave moving through the corona. The slope of this feature can be used to obtain an estimate of the speed of this shock wave.
The horizontal features at around 60 and particularly 90 MHz are due to interference from the 2nd and 3rd harmonic of the controlling PC internal clock (which had a fundamental frequency of 33 MHz). This is due to lack of adequate shielding in the prototype spectrograph.
A Visual Basic display program has also been written, and this provides a better dynamic range to display all possible 256 amplitude values. The 3 source files to run this may be downloaded from Vrasdisp.frm, Vrasdisp.vbp, and Vrasdisp.vbw.
An example output from this viewer is shown below for the same date and time as for the QBASIC viewer above.
The intermittent activity seen in the high band around 125 MHz is due to local aviation transmissions.
Australian Space Academy