Big Dish Telescope: Radio Receivers
Front-End Receiver Components
The front-end
components of the radio receiver system are those
at the "front end" of the signal path -- where incoming radio waves are
initially received -- and which are often mounted in front of the dish in
prime-focus telescopes like
ours. These components include feeds and waveguides to
receive the signal collected and focused by the reflector, active antenna
elements in which the radio waves induce alternating currents, a low-noise
radio-frequency amplifier, and, in many cases, a frequency downconverter that
reduces the received signal frequency to a lower
"intermediate
frequency" that is more manageable by subsequent system components. Radio
astronomy signals are incredibly faint, so the
initial amplifier stage is vitally important for enabling their detection.
In modern receiving systems, the front-end components are often combined in a
single unit called a low-noise amplifier
(LNA),
low-noise block-downconverter
(LNB),
or LNB + feed (LNBF).
Most of the following system characteristics describe available front-end
units. A few relate to the size of the reflector
dish.
Band (IEEE) |
Front-End Unit(s) |
Received Spectral Range |
Intermediate Freq. (GHz) |
Bandwidth (GHz) |
Beam Width (FWHM) |
Gain (dB) |
Brand |
Model |
Type |
Wavelength (cm)
| Frequency (GHz)
| Amp. |
Directive |
Total |
X - Ku |
Chaparral, Pauxis |
Universal Single |
LNB |
2.35 - 2.56 |
11.700 - 12.750 |
1.100 - 2.150 |
1.050 |
0.516 - 0.562° |
58 |
50.6 - 51.4 |
108.6 - 109.4 |
X |
2.56 - 2.80 |
10.700 - 11.700 |
0.950 - 1.950 |
1.000 |
0.562 - 0.615° |
49.8 - 50.6 |
107.8 - 108.6 |
S - C |
Chaparral, SmartSat |
MicroPac |
LNBF |
7.14 - 8.82 |
3.400 - 4.200 |
0.950 - 1.750 |
0.800 |
1.57 - 1.94° |
65 |
39.9 - 41.7 |
104.9 - 106.7 |
L (line) |
NooElec |
SAWbird +H1 |
LNA+ filter |
20.82 - 21.80 |
1.376 - 1.441 |
-- |
0.065 |
4.57 - 4.79° |
40 |
32.0 - 32.4 |
72.0 - 72.4 |
L (wide) |
GPIO Labs |
Ultra+ GNSS |
LNA+ filter |
17.65 - 27.27 |
1.100 - 1.700 |
-- |
0.600 |
3.88 - 5.99° |
40 |
30.1 - 33.8 |
70.1 - 73.8 |
Notes on the front-end table:
- A band graph shows where these received
frequencies fall on the broader electromagnetic radiation spectrum.
- Wavelengths presume electromagnetic radiation in a vacuum.
Propagation speed and wavelength are reduced in receiver components, feed
lines, etc. (frequencies are unaffected).
- Relevant IEEE band frequency ranges are L=1-2 GHz,
S=2-4 GHz, C=4-8 GHz, X=8-12 GHz, and Ku=12-18 GHz.
- The X-Ku LNBs observe one of two sub-bands at a time, controlled
by injection of a 22 kHz tone. Although they cover parts of both the X and
Ku IEEE bands, these LNBs are often called "X-band" or "Ku-band" units for
brevity.
- The S-C LNBFs include scalar rings that help reject ground
radiation from outside the dish area "seen" by the feed. They are often
called "C-band" units for brevity.
- The narrow-band LNA+filter is a single unit specifically designed
for hydrogen 21cm
line observations. It does not come with its own antenna or feed, so
we are considering other options, including
making
our own.
(This is not strictly a front-end component, since it is separated
sufficiently from the resonant element to be placed behind the dish, but
it performs a similar function as other LNA and LNB units.)
- The broad-band LNA+filter are separate units for L-band continuum
observations that, like the narrow-band LNA+filter, do not include a feed,
and do not need to be in front of the dish.
- The bandwidth shown here is what the front-end units deliver to
later stages of the receiver chain, but it is the capacity of these later
stages that dictates how much signal bandwidth can be viewed or recorded at
any given time.
- Beam Width is
the angular
resolution, or smallest detail that can be measured with the
telescope, defined as the full width at half maximum (FWHM) of the main
beam sensitivity pattern. Here we have assumed optimal dish illumination
with a 10 dB edge taper (10% sensitivity at the dish edges), for which
the FWHM = 1.15 rad *
λ / D, with D = 300 cm being the dish
diameter and λ the wavelength.
The actual beam FWHM may differ from the 10 dB taper case assumed here and
will be measured during commissioning.
- Gain
is a measure of signal strength increase, usually defined as a factor
(e.g., output power / input power), but often expressed logarithmically
in decibel (dB) units,
where GdB = 10 log10 Gfactor
(or inversely, Gfactor =
10 (GdB / 10), so
that GdB = 40 implies Gfactor =
10,000).
Here we give the amplifier gain of the LNA/LNB stage listed by the
manufacturer and the maximum directive gain
or directivity
of the telescope's main beam. The latter is found as Gdir = 4
π / Ω, where the
beam solid angle
is Ω = π θ2 / (4 ln(2)) for
a circular
Gaussian beam, and θ is the beam FWHM in
radians (for simplicity,
we assume a radiation efficiency of 1, with negligible ohmic losses in
the antenna and feed). Thus, all else being equal, doubling a telescope's
dish diameter D would increase its directivity by a factor of 4, a
gain improvement of 6 dB. Note that while gain factors multiply
together, gains in decibels add instead.
- Sensitivity to specific radio sources is not yet determined.
The noise temperatures of our S-C LNBs are listed as 17 K by the
manufacturer. We do not plan any active cooling of the front-end receiver
electronics.
Back-End Receiver Components
Back-end components of the receiver system are those toward
the "back end" of the signal path, also usually located in back of the dish,
or even at some distance from it. They accept the amplified and often
down-converted signal from the front-end components, sent over a coaxial
cable, and perform whatever additional operations on the signal are required.
These operations would include additional amplification, demodulation,
digitization, and display or recording of the resulting output signal. In
many modern systems, all these functions are handled by
a software-defined
radio (SDR), which can be either a standalone unit or one connected to a
computer running specialized software. Our system uses the latter. There
are many options for this. Back-end configurations we have tested to date
include the following.
Brand |
Model |
Frequency Range (MHz) |
Maximum Bandwidth (MHz) |
Bit Depth |
Software |
Platform |
Computer |
RTL-SDR Blog |
RTL2832U V3 |
0.5 - 1750 |
2.4 |
8 |
LabVIEW |
Raspbian |
Raspberry Pi + Macbook |
SDR # |
Windows |
PC laptop |
Airspy |
R2 |
24 - 1750 |
10 |
12 |
SDR # |
Windows |
PC laptop |
SDRplay |
RSPdx |
0.001 - 2000 |
10 |
8 - 14 |
SDRuno |
Windows |
PC laptop |
Notes on the back-end table:
- The models shown here are a small subset of available SDRs
(e.g., see
Wikipedia list). Subject to cost and other constraints, we may
consider other models providing greater bandwidth or other capabilities.
- The frequency range is applied to whatever input signal is passed
down from the front-end receiver stages. This could be either an
intermediate frequency after down-conversion or the original received
frequency if no conversion is applied (for example, in L-band).
- The Airspy R2 does not cover frequencies as low as the other
SDRs, but this is not important for GHz radio astronomy (its coverage
can be lowered with
the Spyverter
frequency up-converter if desired).
- The maximum or instantaneous bandwidth is the largest frequency
range that the system can process and display in real time
via fast
Fourier transforms of the incoming waveform. Smaller bandwidths can be
selected to reduce CPU loading. This bandwidth is the "window" into the
larger band made available by the front-end receiver stages (see above) and
can be "tuned" around within that range.
- The bit depth is the number of bits used by
the analog-to-digital
converter (ADC) to encode the analog signal. Greater bit depth enables
more faithful representation and better dynamic range to distinguish
between strong and faint signals. (For example, 8 bits allows
28 = 256 different levels of signal strength to be captured.)
The RSPdx SDR has greater bit depth for narrower bandwidth settings.
- A LabVIEW interface to the RTL2832U SDR was developed to run on a
Raspberry Pi 3B board computer that could be controlled over a wireless TCP
connection by a Macbook Pro laptop. Off-the-shelf software (SDR #, SDRuno)
is currently being used for the more sophisticated SDR models while other
options are explored (e.g., GNU
Radio).
- Spectral channel sampling across the instantaneous band is being
investigated. In principle, the number of channels' worth of information
available to be recorded per second should be approximately the
instantaneous bandwidth divided by the number of bytes per channel (1 byte
= 8 bits). However, the details of how this is handled depend on the SDR
software.
Signal Generators
Although the radio telescope is designed only to receive signals, not to
transmit them, signal generators are still required for some functions.
- A 22 kHz tone generator is needed to select the upper sub-band in
the Ku-band LNB (see above). This is under current investigation.
- A radio-frequency (RF) signal generator is also very useful for
testing receiving system components, not unlike a calibration lamp for an
optical telescope. An inexpensive unit off eBay tunable between 0.14 and
4.4 GHz enabled direct bench and field tests of both L- and C-band
receivers, and its 3rd harmonic allowed testing Ku-band reception.
- For rudimentary tests of SDR functionality, FM broadcast stations
can also be received using, e.g., whip antennas shipped with the RTL2832U
SDR kit, although almost any conductor will do for a strong enough station.
Indeed, if RF shielding is inadequate, the RTL-SDR FM Trap Filter is sometimes
necessary to block such stations from contaminating the IF signal path.
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