The L-band receiver is calibrated by comparing the system power output with and without a calibrated noise source. The noise source, also called a "noise diode," can be remotely switched on and off in order to mark the telescope output data with a known value. A TTL (Transistor to Transistor Logic) controlled coaxial switch controls the output of the noise diode. The output of the switch is attenuated to bring the level of the noise source signal down to a level comparable with the noise power output of the receiver and injected with a quarter-wave dipole mounted in the L-band feedhorn. A PC generates the TTL signals.
The dish reflects a wide range of wavelengths. Therefore, a large range of signal frequencies is reflected into a wave-guide called a feedhorn. The feedhorn is an open-ended cavity that permits standing waves from signals of particular wavelengths to be formed. Due to the nature of this cavity, the feedhorn acts as a preliminary filter, screening out some signals of unwanted frequencies. A probe is placed in the feedhorn at an antinode (antinode - a region of maximum amplitude) position of the standing wave pattern. There, the signal creates a current in the probe proportional to the intensity of the wave. The L-band feedhorn is a pyramidal horn that stands about two meters tall. It consists of several parts: the horn itself, a matching section, and a wave-guide to coaxial adapter. A calibration source antenna also extends into the interior of the horn. Finally, a plexi-glass cover keeps the weather out &ldots; rain, sleet, snow, etc.
A device used to boost the strength of an electronic signal. It operates over a particular band-pass containing the desired radio frequency and amplifies only those input signals with frequencies contained in its range. Receivers for radio astronomy must generate the lowest possible noise to obtain the maximum sensitivity. It is noteworthy that the first RF amplifier noise is usually the predominant noise source in the receiver system. Therefore, maximum effort is spent on minimizing the noise contribution of the RF amplifier. As some of the electronic devices used to amplify or mix radio frequency signals generate less noise when cooled, radio astronomy receivers are normally refrigerated to very low temperatures. The electronic devices most often used at the input of modern radio astronomy receivers are High Electron Mobility Transistors (HEMT) amplifiers. These devices can be cooled to cryogenic temperatures (~15 Kelvin, -258º Celsius) to obtain the lowest possible noise. For higher frequencies, up to hundreds of GHz, the input signal is first down converted to a lower frequency by a mixer, and then amplified by a cryogenically cooled HEMT amplifier. In modern millimeter wave receivers, the mixer is usually a Superconductor Isolator Superconductor (SIS) junction cooled at liquid Helium temperatures (4 Kelvin, -269º Celsius).
This component keeps reflections from the RF filter from going back to the feedhorn.
The function of a filter is to pass a desired band of frequencies with minimum attenuation and to suppress all other frequencies. The radio frequency filter is the first filtering step in the RF system and serves as an image and interference rejection device.
The RF amplifier does what its name suggests. A device used to boost the strength of an electronic signal. It operates over a particular band-pass containing the desired radio frequency and amplifies only those input signals with frequencies contained in its range.
This component provides a computer controlled reference signal, generally around 10 MHz.
This component is a variable radio-frequency generator that is adequately stable. Because the frequency of the oscillator signal can be arbitrarily chosen, the intermediate frequency can also be chosen. Phase Lock Loop Oscillator's (PLLO) are designed to operate in the range from 1.5 to 1.6 GHz (in this example receiver). Conversion of the received energy to a lower frequency, where it can be amplified more efficiently than at the signal frequency, is a basic principle of the superheterodyne receiver. An essential component in a superheterodyne receiver is a tunable oscillator.
In most radio receivers, ours included, a process called mixing takes place to transform the incoming radio frequency (RF) energy to a lower frequency. Mixing creates a signal which retains all of the information contained in the high frequency spectrum, but is easier to work with. Mixing is accomplished by combining the incoming RF with another signal of known frequency from the local oscillator (LO). What results is the difference between the two. The lower frequency signal is known as the intermediate frequency (IF). The formula for determining the IF (in this example receiver) is as follows: RF- LO = IF but could also be RF + LO = IF depending on the design of the receiver.
IF amplification is done in several stages. One amplifier is put before the IF filter, however there can be several stages of amplification after the filter, depending on the level requirement at the signal processing equipment.
In this example receiver the IF filter could be one of two filters 150/10 or 155/110 (Center Frequency/Band Width). The band-pass would be centered at 150 or 155 MHz and the bandwidth would be a total of 10 or 110 MHz wide depending on the filter selected.
Types of Filters. Filters are classified into three groups:
A low-pass filter is one in which all frequencies below a specified frequency called the cut-off frequency are passed without attenuation. Above the cut-off frequency there is attenuation, which changes with frequency in a way that is determined by the network design.
A high-pass filter is just the opposite: in it there is no attenuation above the cut-off frequency, but attenuation does occur below its cut-off.
The third type is the band-pass filter. In this type there are two cut-off frequencies, one at the upper-frequency edge and one at the lower edge. Frequencies on both sides of the pass band are attenuated.
The IF amplification is done in several stages to optimize signal level.
The Complex Mixer is made up of a power splitter to split the IF signal, a hybrid to split the LO's and to shift one signal 90 degrees out of phase, two mixers to mix the unshifted and the 90 degree shifted LO's with the IF. This works with the dual channel base band filters and the PC to separate the side bands and mix the signal down to base band.
These base band filters are dual channel, adjustable, low-pass active filters. They operate just like the low-pass filters previously mentioned.
Analog-to-digital conversion is an electronic process in which a continuously variable (analog) signal is changed, without altering its essential content, into a multi-level (digital) signal. In other words, this device takes an incoming analog waveform (series of sine waves) signal and converts it to a digital (sequence of numbers) signal.
The PC provides all computer control and data collection capabilities for the receiver.