Overview of CustomRF's Capabilities
| This example, a 3D EMF
simulation in CST Microwave Studio, shows how the
impedance, the near-field and the far-field gain
pattern of a monopole WiFi antenna are affected by a
nearby obstruction. |
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| Antenna
Unobstructed |
Antenna
Near Obstruction |
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E-Field
Magnitude in YZ-Plane
 |
E-Field
Magnitude in YZ-Plane |
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Far-Field
Antenna Pattern
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Far-Field
Antenna Pattern
 |
| WR42 waveguide is
recommended to be used in the 17.6 - 26.7 GHz frequency range, but,
with a cut-off frequency of 14.058 GHz, it could be used at lower
frequencies, albeit with somewhat degraded performance such as higher
dispersion. In this case WR42 with a 17 - 22 GHz operating band is
desired. The launch is from a .141 semi-rigid coaxial cable. CustomRF
optimized a launch with a single probe as shown below. |
Coaxial
to
WR42 Waveguide Transition, Single Probe |
Return
Loss, Coaxial to WR42 Waveguide Transition, Single Probe |
| This
return loss is
too poor for the application. CustomRF
reoptimized the structure with two #0-80 tuning screws incorporated and
thus significantly improved the return loss. The improved structure
and return loss are shown below. |
Coaxial
to WR42
Waveguide Transition, With Tuning Stubs |
Return
Loss, Coaxial to WR42 Waveguide Transition, With
Tuning Stubs |
| This example, a 3D EMF
simulation in CST
Microwave Studio, shows the isolation between 2 critical points on a
2-sided
PCB. Port 1 is connected to the power amplifier, Port 3 to the low
noise receiver of a transceiver, so that S31 is the transmit to receive
(TX/RX) isolation. Also shown is how the isolation can
be significantly improved by surrounding each via between top and
bottom side with a
"via fence" that connect the top and bottom-side ground planes. These
via fences inhibit signals that otherwise propagate between the ground
planes much like inside waveguide. |
|
PCB Without
Via Fence |
PCB with
Via Fence |
TX/RX
Isolation |
TX/RX
Isolation |
| Radio Frequency Identification
(RFID) is being used for many purposes, including to keep track of
patients in hospitals. In the United States passive RFID tags
in the UHF band operate in the band around 915 MHz. At this
frequency, a small tag can easily be detuned in the presence
of certain matter, including water.
The human body consists mostly of water, so performance of passive tags
is a concern. In this example we show simulation results of a typical
flat RFID tag in both free space and when it is placed on top of water. |
|
Antenna
Structure In Free Space |
Antenna Structure On Water |
Return
Loss, Free Space |
Return
Loss, On Water |
Antenna
Pattern, Free Space |
Antenna
Pattern, On Water |
| It can be seen that the
proximity of
water has a severe impact on return loss and on antenna gain. |
|
| The output spectrum of a digital-to-analog
converter (DAC) inherently contains not only the fundamental band but
alias bands as well. For this reason a reconstruction filter is used to
capture
the desired band. This is often the fundamental band, but in some cases
an alias band is preferred (digital upconversion). Then the
reconstruction filter must be a bandpass filter. This is shown below as
implemented to generate a 60 MHz IF for a radio transmitter. |
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| The output spectrum is not inherently flat as desired but has a roll-off, the voltage magnitude of which follows the shape of a sin(x)/x, or sinc(x), function, wherein the nulls are at multiples of the 80 MHz sample frequency. This is shown below. | |
 Note:
Desired alias is spectrally inverted. This is pre-compensated in the
baseband.
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| This
roll-off could
be pre-equalized in the digital baseband section before the DAC,
but that would require additional DAC resolution, thus
detracting from its dynamic range. To avoid this it was decided to use
analog compensation. The reconstruction filter had been slated to be a 3-pole Chebychev filter, with a topology as shown below. |
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| The
spectrum at the
upconverter input then appears as shown below. |
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| While
the resulting IF spectrum is not perfectly flat, this
filter nevertheless saves 9 dB in DAC dynamic range. |
| An OFDM signal
according to IEEE 802.11a, 54 Mbps, has a peak to average power ratio
of
approximately 22 dB. For this reason it is very susceptible to gain
compression in any active stage, such as an amplifier. The level of the
signal passing through such needs to be well below the 1dB compression
point or the modulated carrier will experience spectral regrowth and so
may not meet the specified spectral mask, and the receiver will get bit
errors. This
example shows simulation results, done in Mathcad, of the effect of an
amplifier operating 3 dB and 6 dB below its 1 dB output compression
point. The block diagram is shown below. |
|
OFDM
Compression Simulation Block
Diagram |
|
| An ideal OFDM
generator produces 256 OFDM symbols, each consisting of 48 64-QAM
subcarriers + 4 pilots as specified in 802.11a. This signal is used to
modulate a 60 MHz IF, which is then applied to a unity-gain amplifier
with gain compression per the Rapp model. The input spectrum, with no
gain compression, and the output spectrum, when operating 3 dB and 6 dB
below the 1 dB compression point, are shown below. |
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| It can be seen
that spectral regrowth increases noticeably as the level gets closer to
the 1 dB compression point. At the same time there is an increase in
the constellation error, or error vector magnitude, as shown below. |
|
Constellation
Without Compression |
Constellation With Compression |
| The simulated constellation
error is -21.6 dB with a 3 dB and -25.6 dB with a 6 dB backoff from
the 1 dB compression point, so with this amplifier 6 dB backoff is
required to meet the -25 dB constellation error required per
IEEE 802.11. In practice, with most amplifiers the backoff must
be 8 dB or greater because there are other contributors to error
(thermal noise, phase noise from the synthesizer, clock
synchronization errors, etc.). |
|
CustomRF, Inc. | 28 Lowell Drive | Stow | MA | 01775 | 978.637.2070 | info@customrf.com