Download System Level Review

Initial General Selection Process for the Single-Balanced Mixer Project
Updated 4/21/2012
Introduction
The block diagram below shows the elements required for realizing the single-balanced mixer to
be designed and built. It consists of several discrete components which either have to be
purchased or included as part of the printed substrate. The items which will, or could potentially
be purchased as components are RF SMA connectors, the RF splitting element, the diodes, and
the IF low-pass filter. Each of these elements will be evaluated and chosen for the initial design
(to be simulated). Any of these selections may be modified as a result of the design and
simulation phase.
For this project it was decided that the initial design approach will consist of a substrate and will
make use of as many surface mount commercially available components as possible.
Purchased components will include SMA connectors, the RF Splitting Element, and the Series
Diode Pair. The LPF was also considered for purchase, but was too expensive in low quantity
and will be realized as a lumped element design on the substrate (see further detail in the LPF
section). A final iteration of the product could include the RF splitting element as well and
potentially save cost.
Substrate Material
Introduction: As discussed in the introduction it was decided to purchase the majority
of the RF elements (bandpass filters, diodes, coupler, connectors). Several elements
will need to be incorporated into the substrate, including interconnecting transmission
lines, the low-pass filter, the RF chokes and the connector launches. It is expected that
the substrate should be able to consist of a single layer of dielectric with conductor
layers on top and bottom.
Choices: The initial selection must be made between soft-board (printed circuit board
materials) and ceramic. Potential substrate materials are as listed in the table below.
Parameter
Dielectric Constant
Loss Tangent
Cost
Type
Coefficient of Thermal Expansion (X, Y,
Z)
RO4003C
3.38 +/- 0.05
0.0021
Low
Ceramic &
Glass
11,14, 46
ppm/deg C
RT Duroid 5880
2.20 +/- 0.02
0.0009
High
Glass & PTFE
CF (DLI)
25 +/- 2
0.0003
Med
Ceramic
31, 48, 237 ppm/deg
C
9, TBD, TBD ppm/deg
C
Trade-offs: Since this substrate will not contain much critical circuitry, its RF
performance should not greatly impact the mixer final performance. In fact, standard
low-cost printed circuit board materials such as FR-4 could be considered as the final
production version material. It was decided that the prototype should use a better RF
performance material to minimize risk at this time.
The ceramic material will allow for shorter and narrower lines (due to higher dielectric
constant), improved thermal conduction (heat sinking) but is more complex to process
(takes considerably longer), and more difficult to solder components to (better suited for
conductive epoxies or wire-bonding).
The Rogers materials (RO4003C and RT Duroid 5880) have lower dielectric constants
and poorer thermal conductivity, but are much easier (quicker) to process. The
RO4003C material is well-suited for solder applications and its thermal expansion
properties are an excellent match for standard surface mount packages as well as
plated-through hole reliability. The RT Duroid 5880 is not as well suited to soldiering as
its thermal expansion is less conducive to larger surface mount packages and it is very
poor for plated-through hole reliability due to its z-axis expansion not being closely
matched to copper.
Final Selection: RO4003C was chosen because it is an inexpensive, reasonably low
loss material that is well suited for this application. Its loss is acceptable, particularly due
to the fact that limited circuitry will be included in the substrate itself (use of purchased
components). This material is also better suited to soldering of components due to its
thermal expansion properties being better matched to ceramic and other RF surfacemount components. Finally, this material (or the 5880) is much easier to process (much
quicker) than Ceramic with fabrication times in days instead of weeks.
Selection of the RF Splitting Element
For this evaluation, 90-degree and 180-degree hybrid couplers were considered. Each element
offers particular advantages/disadvantages depending on the desired performance and
operating conditions of the mixer. The two most significant specification trade-offs influenced by
the RF splitting technique are RF/LO VSWR (Return Loss) and isolation between the RF and
LO ports. These are detailed further.
The Target RF Splitting Element Specifications are as follows:
Parameter
Frequency Range
Characteristic Impedance
VSWR
Insertion Loss
Isolation (RF/LO & Diode 1/Diode 2)
Amplitude Balance
Phase Balance
Packaging
Finish
Specification
0.8 - 1.2 GHz
50 ohms nom.
1.2:1 max.
0.25 dB max.
20 dB min.
+/- 0.25 dB
90 +/- 3.0 deg.
Surface-mount solderable
Tin-Lead, Tin, or Gold
90-degree hybrid coupler:
Description: See the diagram below. Signals present at either the RF or LO ports will
be equally split to the Diode 1 and Diode 2 ports, with a 90-degree phase difference
between them.
VSWR: The 90-degree hybrid will provide good RF and LO input port VSWR’s due to it’s
the 90-degree phase balance.
The reflection coefficient at either the RF or LO ports is given by:
]/2
Assuming the two diodes and the interconnecting transmission lines are well-matched to
each other, the resulting RF and LO reflection coefficients should be very low. As is
discussed in the diode selection section, the diodes to be chosen are packaged together
(on the same piece of semiconductor) and should be optimally matched.
RF & LO Isolation: The isolation between the LO and RF ports is given by:
Isolation (dB) = 20 Log10 {2/[
}
Thus, the isolation will depend upon the absolute match of the diodes to 50 ohms. The
impedance of the diodes are dependent on frequency and LO drive levels, so this needs
to be a consideration in the design and selection.
Insertion Loss: The 90-degree hybrid coupler will have about the same loss as the
baluns and will be the lower loss than the 180-degree hybrid coupler.
Realization/Size/Cost Considerations: T
Pros & Cons: Pros - RF and LO port VSWR, insertion loss and size. Cons - RF/LO
Isolation.
180-degree hybrid coupler:
Description: For the 180-degree hybrid coupler, signals present at either the RF or LO
ports will also be equally split to the Diode 1 and Diode 2 ports, or 3dB lower (plus
insertion losses) from the input signal. The split signals will also have a 180-degree
phase difference between them. The 180-degree hybrid is not a readily realized
structure in a surface mount component. It could be printed as a “rat-race”
implementation, but this would be very large. To build this function using components a
90-degree hybrid coupler can be either purchased and then the additional 90-degree
phase shift can be either printed as additional transmission line (narrow-band) or use of
a printed phase shift element, such as a Schiffman. A Shiffman consists of a 90-degree
coupled section (180-degrees in total length) in one leg of the hybrid coupler and 270degrees of transmission line in the other. This produces a wider bandwidth 90-degree
shift (flatter response).
VSWR: The 180-degree hybrid will not provide good RF and LO input port VSWR’s due
to it’s the 180-degree phase balance. The reflection coefficient at either the RF or LO
ports is given by:
]/2
Even though the diodes are well-matched to each other, they would need to be wellmatched to 50-ohms for low reflection coefficients and thus low VSWR’s at the RF and
LO ports. This is expected to be difficult to maintain over input frequency LO power level
ranges (drive level for the diodes), and so needs to be considered in the design
(selection).
RF & LO Isolation: The isolation between the LO and RF ports for the 180-degree
hybrid is given by:
Isolation (dB) = 20 Log10 {2/[
}
Thus, the isolation will depend upon the relative match of the diodes to each other over
all operating conditions. As is discussed in the diode selection section, the diodes to be
chosen will be packaged together (on the same piece of semiconductor) and should be
closely matched, thus the RF to LO isolation should be good.
Insertion Loss: Due to the additional line-length and coupled section required for the
Schiffman phase -shifter, the 180-degree hybrid coupler will have the greatest insertion
loss of the 3 elements considered.
Realization/Size/Cost Considerations: The 190-degree hybrid is not readily available
as a stand-alone surface mount component, so it must be realized using a 90-degree
hybrid and a phase shifter (eg. Shiffman).
Pros & Cons: Pros - RF/LO Isolation. Cons - RF and LO port VSWR , insertion loss
and size.
References:
1) Anaren Application Note: “Balanced and Double-Balanced Mixers”
Final RF Splitting Element Selection:
For this purpose the 90-degree hybrid coupler was considered the most appropriate since it
provided the simplest implementation while also theoretically being able to meet all of the
required specifications. For this project it was decided that the initial design approach will utilize
a Xinger (Anaren) XC0900A-03 as it provides good isolation, VSWR phase and amplitude
balance over the desired frequency range. It is also well-suited for mounting on RO4003
material.
Note: A balun could also be considered, should the coupler be problematic after final diode
selection. Baluns could provide the same isolation benefit of the 180-degree coupler, but
improved VSWR as the balun could potentially be more closely matched to a particular diode.
However standard product offerings will be more limited.
Parameter
Frequency Range
Characteristic Impedance
VSWR
Insertion Loss
Isolation
(RF/LO & Diode1 /Diode 2)
Amplitude Balance
Phase Balance
Packaging
Finish
Specification
0.8 - 1.2 GHz
50 ohms nom.
1.25:1 max.
0.25 dB max.
XC09000A-03
0.8 - 1.2 GHz**
50 ohms nom.
1.20:1 max
0.20 max
20 dB min.
20 dB min.
+/- 0.25 dB
+/- 0.20 dB
90 +/- 3.0 deg.
90 +/- 2.0 deg.
Sur-mount solder
Surf-mount solder
Tin-Lead, Tin, or Gold Tin-Lead or Tin
Note: **The catalog part is only specified from 0.81 - 1.0 GHz. Discussions and
wide-band data from Anaren result in the specifications shown above.
Switching Element
Parameter
Specification
Breakdown Voltage - VBR
2 V min.
(tested at IBR = 100 uA)
Forward Voltage - VF
0.20 V min. (tested at IF = 10 mA)
1
Delta Forward Voltage - ΔVF 20 mV max. (tested at IF = 10 mA)
Capacitance - CT
1.5 pF max.
Dynamic Resistance - RD
15 ohms max.
Configuration
2 Series connected diodes
Packaging
Sur-mount package
Lead Finish
Tin-Lead, Tin, or Gold
Notes: 1 Delta voltage is difference between diodes in the same package
Schottky-Surface Barrier Diodes:
Introduction: Schottky diodes are the preferred device used in mixer applications.
Their non-linear behavior produces a desired mixing product when utilized correctly.
Additionally, they are cost effective and can be simply implemented within a typical mixer
circuit.
Description: The Schottky junction is composed of a single semiconductor (either ptype or n-type) and a single metal layer. This allows forward current flow ofrom
semiconductor to metal for n-type or hole flow from semicaonductor to metal for p-type.
There are no minority carriers, so once the driving signal changes polarity, recovery time
is nearly instantaneous, which results in a high switching speed.
Barrier Height: The Schottky-barrier diodes can operate over a wide input signal power
range. The input power level at which the diode can operate depends on the barrier
height. The forward voltage of the schottky junction in general can be much lower than a
typical junction diode which allows it to be a more sensitive detector. A lower barrier
height diode are most practical for small signal level applications and are good for low
LO levels, but aren’t as good for detecting large signals as well as higher barrier height
diodes if a high dynamic range and large LO are needed.
Silicon vs. GaAs: Silicon schottky diodes have cutoff frequencies that are good through
the Ku-band. GaAs devices have a lower Rs which translates to higher cutoff
frequencies. GaAs diodes have a high 1/f noise, which can adversely affect a low IF.
Final Schottky-Barrier Diode Selection:
The Avago HSMS-2822 silicon Schottky-barrier diode was selected. It comes in a series
pair SOT package which can be easily solder surface mounted to the substrate. The
advantage that it has over purchasing separate diodes is that both diodes are from the
same wafer of silicon which eliminates any problems with the diodes not being matched.
It also comes in the series configuration which makes layout and soldering of the board
easier. It has a low forward voltage, a high reverse voltage, low reverse leakage, low
total capacitance, and low dynamic resistance that are suitable for the mixer application
that is being constructed.
As a risk reduction, several other competing diodes will be purchased as well, and may
be utilized if needed.
Parameter
Specification
HSMS-2822
Breakdown Voltage - VBR
2 V min.
15 V min.
(tested at IBR = 100 uA)
Forward Voltage - VF
0.20 V min.
0.5 V min.
(tested at IF = 10 mA)
1
Delta Forward Voltage - ΔVF 20 mV max.
15 mV max.
(tested at IF = 10 mA)
Capacitance - CT
1.25 pF max.
1.0 pF max.
Dynamic Resistance - RD
15 ohms max.
12 ohms max.
Configuration
2 Series diodes
2 Series diodes
Packaging
Sur-mount package
SOT-363 Pkg
Lead Finish
Tin-Lead, Tin, or Gold Tin-Lead or Tin
IF Port Low-Pass Filter (LPF)
Introduction: The function of the LPF at the IF port is to attenuate the higher-order
spurious signals generated by the RF and LO frequencies. These harmonic signals will
be at higher RF frequencies as described in the next section.
Discussion of Mixer Spurious Products:
Will add more detailed discussion of mixer spurious products later.
Definition: The LPF will have a passband frequency of DC - 200 MHz (minimum) and
attenuate signals by at least 20 dB above 1600 MHZ (min RF + LO product). Because
the desired rejection frequency is far beyond the passband a gentle roll-off could be
tolerated, resulting in a low-order filter (several sections).
Parameter
Passband Frequency Range
Characteristic Impedance
Passband Insertion Loss
Passband Flatness
Rejection
VSWR
Specification
DC - 200 MHz
50 ohms nominal
0.25 dB max.
0.3 dB
20 dB min. above 1.8 GHz
1.2:1 max.
Choices: The filter could either be purchased as a component, printed as a distributed
element or realized as a lumped-element design. A search of potential purchase filters
resulted in several issues. The first being limited number of standard, off-the-shelf filters
with the desired (or near) specifications. The second was cost and/or availability; the
purchase restriction of minimum quantities or not available in stock made this an
unattractive solution. So this was eliminated from consideration, leaving distributed or
lumped-element implementations.
Distributed LPF:
Description: The distributed configuration would consist of multiple coupled quarter
wave sections.
Pros & Cons: On the surface, this could potentially be the lowest-cost implementation,
however because the LPF is at such a low frequency, the quarter-wave sections would
need to be quite long and would take up an inordinate amount of substrate area, adding
significantly to the size of the mixer and probably the cost. Because of this a design will
not be investigated further.
Lumped-element LPF:
Description: The lumped-element configuration would consist of discrete inductors and
capacitors which can be easily surface mount soldered onto the substrate.
Pros & Cons: After investigating availability and prices it was found that high frequency
capacitors and chip inductors are readily available and pennies each. This
implementation will (design) will be detailed further at a later date.
Final LPF Selection:
As stated, the Lumped-element approach will be chosen. Design of a low-order filter will be
included as part of the design of the mixer.
RF Chokes
These will be included shortly.
SMA Connectors
The SMA connectors will be used for the RF Input, LO Input and IF Output ports. The
connectors will need to meet the following specifications:
Parameter
Specification
Frequency Range
DC to 1.5 GHz min.
Characteristic Impedance
50 ohms
VSWR
1.10:1 minimum
Finish
Gold plated (for soldering)
Interface
SMA Female Jack
Termination
End-Launch PCB Mount (Solder)
A search for connectors which meet the above specifications yielded a availability from a large
number of suppliers. For this project, connectors stocked at DLI will be used. These are
Gigalane PAF-S05-007 connectors with the following specifications.
Parameter
Frequency Range
Characteristic Impedance
VSWR
Finish
Interface
Specification
DC to 1.5 GHz min.
50 ohms
1.10:1 minimum
Gold plated (for soldering)
SMA Female Jack
PAF-S05-007
DC to 6.0 GHz
50 ohms
TBD
Gold
SMA Female Jack
Termination
End-Launch PCB Mount
End-Launch PCB Mount