Welcome to Issue #9 of the Radio-Sky Journal
Febuary 2008
Copyright © 2008 by Radio-Sky Publishing
All rights reserved.
Contents:
Jupiter Hits Bottom in 2008
AD8302 RF/IF Gain and Phase Detector
Amateur Does Digital Correlation
ADC News
SkyPipe to FITs
SpectraCyber /VLF Telescope UDS Beta for SkyPipe
More New UDS Support
Radio Astronomy Teachers Notebook
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Jupiter Hits Bottom in 2008
Jupiter is lower in sky for northern latitude observers than it has been in 11
years. Far northern latitude
observers will be most severely affect by this shift in Jupiter's declination. The maximum elevations reached by Jupiter for an observer
at latitude +50 this January will be as low as 17 degrees. The situation will slightly improve over the rest of the season.
This will be challenging. Why? Because it hard to point a high frequency (below
30 MHz) antenna towards these low angles. Ground reflections tend to place
the beam higher in the sky. To reduce this effect you must raise the
antenna to a greater height above ground.
Richard Flagg has prepared the chart above to illustrate the situation for the new edition of the
Radio
Jove Antenna Manual. The elevation of Jupiter at transit is shown for four
northern latitudes. The manual includes suggestions for modifying the basic Radio Jove dual dipole antenna to catch Jupiter
at these lower elevations. Changes include raising the antenna and adding a 135 degree phasing line.
See the retrofit
manual for modifying existing Jove dual dipole antennas. This would be a
good time to own a Yagi type antenna mounted on a tall tower, but most of us
will have to just try to get our dual dipoles up higher to achieve the lower
beam elevation.
One helpful aspect of the current Jupiter season is that it occurs while the
Sun is still in quiet mode at the beginning of a new cycle of sunspots. This
means the ionosphere will generally remain less refractive of radio signals from
space and Jupiter's weak signals stand a better chance of being heard because of
this.
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AD8302 RF/IF Gain and Phase Detector
Analog Devices produces several integrated circuits which should be of interest to amateur radio astronomers. One device, the AD8302, has the
potential for replacing a large number of individual components in an interferometer.
>
In a simple interferometer we utilize signals from two antennas, usually separated by many wavelengths. Of prime importance
is the difference in phase between the two signals. We won't go into the details of interferometer theory here, but you can find a simple, non-mathematical
explanation in William Lonc's "Radio Astronomy Projects" book. Suffice it to say that depending on the separation between the antennas,
and the angle at which the source appears in the sky, the waves from the cosmic source will arrive at the antennas at slightly different times. This
difference in arrival times means that there will be a "phase" difference between the signals that when charted can tell us something about the
cosmic sources position and size.
There are several ways that we can extract this phase information from the signals arriving from the two antennas. The signals can simply
be added together or can be multiplied together with the appropriate circuitry. Care must be taken in all parts of the interferometer to make
sure that the phase relationship is not transformed or destroyed by asymmetries in the two branches of the interferometer leading from
the point of signal combination to the antennas. The AD8302 can be thought of as a black box that is capable of providing an accurate
measurement of the difference in phase between the two signals applied to separate inputs. You can see how it might be of value to the amateur
radio astronomer.
Before we get to excited we should note that the AD8302 is not capable of performing its magic on the tiny un-amplified signals coming directly
from the antennas. The usable input range for the AD8302 is from -60dBm to 0dBm. The signals extracted from the antennas are going
to need 80 dB or more of amplification before they can be combined in this device. The signals will need to pass through several stages of
amplification and bandpass filtering prior to being applied to the AD8302 input ports. One of the beautiful things about the AD8302, however, is
that it allows the signal levels of the inputs to vary by a wide range and still present an accurate phase measurement. Thus we can theoretically
be less concerned about the absolute gain in each arm of the system than we would be in most other simple interferometers. The AD8302 is
usable up to 2.7 GHZ!
The figure above shows the basic configuration for the chip when measuring signals from two separate sources. The inputs are on the
left and outputs on the right. Pin 13 produces a voltage that is proportional to the
magnitude ratio of the two inputs. Ideally, this would remain constant in the interferometer application. Pin
9 outputs a voltage that is proportional to the phase difference of the two input signals.
My initial experiments with the AD8302 provided no surprises. The chip seemed to work as promised
even though I was a bit sloppy with the construction of the test setup. I used a power divider from to
split a signal from a 144 MHz signal source. In the line going to one of the inputs I used a "line stretcher"; a brass tubing coaxial
line. Changing the length of the line stretcher produced a smooth change in the voltage measured on pin 9. Unfortunately, the device
is available only in a tiny surface mount package. I had to hack a circuit board that I had from a DDS project to make a breadboard
on which to solder to the TSSOP device.
Download the datasheet from the Analog Devices website at: http://analog.com
. There is a part number search box on their home page. In the datasheet you will find a couple of other interesting
applications (not surprisingly, they don't really mention interferometry). These include a setup for measuring input phase
and gain of a device such as an amplifier. Another circuit is a reflectometer to measure the vector reflection coefficient of an
arbitrary load. There are probably many other possible ways this device could be used in an observatory. Please write and tell
me what you come up with.
Note: If you use this device in a simple interferometer, you will lose the amplitude component of the measurement. You will probably
not be able to see the characteristic rise and fall of the signal level as source passes through the beam pattern of the telescope.
Still, it will be an interesting experiment.
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Amateur Does Digital Correlation
As explained in the article above, interferometry involves comparing signals from two antennas. There are several ways the
comparison can be made. The methods most commonly used by amateur radio astronomers involve adding or multiplying the signals in
an analog device such as combiner or mixer. We use these methods partially because they are easy to implement using common devices.
More and more often, professional observatories use devices called digital correlators in their interferometers. Using digital
techniques, it becomes possible to combine signals that were recorded on tape from two widely separated observatories. This is the heart
of Very Long Baseline Interferometery (VLBI). The wide separation of observing antennas (the baseline) provides interferometers
with very high resolution. Digitized telescope data can also be processed by various programs to improve the quality of the
measurements.
Now that home computers have become very powerful and inexpensive, it becomes quite reasonable that VLBI could be accomplished by
amateur radio astronomers. This challenge has been taken on by some amateurs in the European Radio Astronomy Club (ERAC). The
first step towards amateur VLBI is to develop a digital interferometer that works with signals from local antennas. Marko Cebokli has
done just that with his "SImple Digita Interferometer" or SIDI. Read about this interesting project at:
http://lea.hamradio.si/~s57uuu/astro/sidi1/index.htm
Much still needs to be done. In order to combine signals recorded from widely separated locations, the problem of having extremely well
synchronized local oscillators must be addressed. Professional observatories use expensive atomic clocks. There is talk in the
amateur community of using GPS derived oscillators or broadcast radio signals to synchronize observatories. Whatever the solution
turns out to be, it will be a great day for amateur radio astronomy when those first VLBI fringes are published.
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ADC News
In a past issue we discussed the problems with sample timing on multitasking operating systems such as Windows and Linux. The
solution has always been to move the timing away from the computer and into an external device. By using a microcontroller
with an ADC,
we can rely on the crystal controlled heartbeat of this device to sample data evenly over time. Data sent to the computer serial
port is buffered by the computer, so even if the computer "gets busy" doing something else, the data will be preserved in the
buffer and read into the strip chart when the CPU has time to do so.
A new ADC system is being developed that will monitor an external pin that can be supplied with a timing pulse. This could be from
a very stable external oscillator or the one pulse-per-second (PPS) output of a GPS. This input signal may used to initiate the
collection of a sample or "train" of samples using the crystal controlled sampling of the microcontroller.
Experiments using a PIC microcontroller suggest that it is possible to ensure
that the sample is collected with an accuracy of about 10 milliseconds using
this system. With the GPS marking the start of each second two remote
observers can be synchronized to within this 10 millisecond window.
Correlations can be made in the time domain at more than a power of 10 better
accuracy than when relying on the internet clocks. This means that there
can be more certainty that two distant observers are seeing the same event.
If you already own a MAX186 ADC from Radio-Sky, you won't have to throw it away and start again. The future also brings an
adapter board that will provide you with the same timing and serial port capabilities.
Keep an eye on the website.
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SkyPipe to FITS Conversion Tool
Hannes Mayer is one of the most prolific amateur scientists I know. He has his hands in numerous areas from radio astronomy to
plants from the bryophyllum group. No doubt Hannes (or Captain as he is reverently
called) picked up on all of the clamor for a way to get SkyPipe files into FITS format. FITS is the semi-universal file format for astronomical images and other types
of data. FITS is used by most professional observatories and can accommodate data of numerous dimensions. FITS readers exist
for most computer platforms.
Hannes has created a JAVA utility to convert SkyPipe data format files, (*.spd) to FITS format. You can find the utility on the
web at: http://www.jupiterradio.com/jove-spd2fits.php
Look around his web pages for many more great tools, information pages, and projects. Thanks Hannes for your support of our efforts.
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Spectra-Cyber and 40 kHz VLF Telescope UDS for Radio-SkyPipe
If you own one of Radio Astronomy Supplies Spectra-Cyber
receivers or one of their 40 kHz receivers, you can now feed it's output in continuum mode to Radio-SkyPipe.
(I believe this interface will also work with the Ultra-Cyber receivers but I
have not had this confirmed.)
Some time back I announced the User Data Source project which opened the possibility of feeding most any type of time/value data
into Radio-SkyPipe. If you have forgotten what this is all about see: http://www.radiosky.com/skypipehelp/UDS_model.html .
Basically,
a UDS is a small program that translates information into a form that
Radio-SkyPipe can accept. These are available free to Pro license holders
of Radio-SkyPipe.
The Spectra-Cyber operates in a frequency sweeping or fixed frequency "continuum mode". It is the second of these modes that the UDS
utilizes. A small control panel appears when the UDS is executed allowing you access to the various computer controlled functions
of the receiver. Of course all of these controls may not make sense to the VLF
receiver, but the interface still works for that device.
Since the UDS uses TCP to talk to SkyPipe, there must be TCP installed on the SpectraCyber PC for this to work. It also requires that own a
Pro Version of SkyPipe.
1. Download and run the UDS installation on the SpectraCyber PC.
http://radiosky.com/skypipe/SpecCyberUDS.exe
2. Configure Radio-SkyPipe to talk to the UDS:
Under Options / Data Source select CH1 to use UDS CH1 as its data source. Select the UDS configuration button and point to the IP and port that you
also configure the UDS to use. If running SkyPipe on the PC that hosts
SpectraCyber, use the loopback IP of 127.0.0.1 for both the UDS IP and SkyPipe UDS communication options. This usually works. If running the
UDS and SkyPipe on the same PC, browse to the UDS executable at the top of this panel and tell
SkyPipe to start the UDS automatically. Save it all.
3. Open the UDS program and make sure the local IP it binds to matches the IP you set SkyPipe to look for the UDS on. If you are using something
other than Com Port 1 for the SpectraCyber, put that in. The Gain, Offset, etc. controls should immediately affect the SpectraCyber. Set that up and
apply and save everything as defaults.
4. Start a SkyPipe chart and see what happens.
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Other New UDS software for Radio-SkyPipe
If you a Pico AD-10 or AD-12 ADC you can use the new Pico UDS software available in the UDS zip package.
New also is expanded support for Metex brand PC linked multi-meters. Previously only the
Metex-11 was supported. You can now configure
the baud rate to support other Metex products like the Metex-22. Though I
don't have one to test with, after reviewing the documentation for the Metex
3640, I believe the UDS will work with this meter also. If you have a Metex
PC linked meter and it does not work with this UDS, let me know and I will try
to get it working for you.
Experimental UDS modules have been released for the continuum output of the SDR-14 software defined radio from
http://rfspace.com .
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Radio Astronomy Teacher's Notebook Reprinted
Honestly, I was going to retire this books several times. I
was tired of running to the printer to have a few copies made at a time
for a very dear price. But the orders kept trickling in as people
are hungry for beginner type radio astronomy information. This book
contains some simple projects. Some of the technology is a little
old, but principles are still valid. So I made some revisions and had 250
copies printed of the 160 page book thinking that this might last a
lifetime. The price went down a bit too. You can find it on the website.
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