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 / RO4003C / RT Duroid 5880 / CF (DLI)Dielectric Constant / 3.38 +/- 0.05 / 2.20 +/- 0.02 / 25 +/- 2
Loss Tangent / 0.0021 / 0.0009 / 0.0003
Cost / Low / High / Med
Type / Ceramic & Glass / Glass & PTFE / Ceramic
Coefficient of Thermal Expansion (X, Y, Z) / 11,14, 46
ppm/deg C / 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 surface-mount 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:
ParameterSpecification
Frequency Range0.8 - 1.2 GHz
Characteristic Impedance50 ohms nom.
VSWR1.2:1 max.
Insertion Loss0.25 dB max.
Isolation (RF/LO & Diode 1/Diode 2)20 dB min.
Amplitude Balance+/- 0.25 dB
Phase Balance90 +/- 3.0 deg.
PackagingSurface-mount solderable
FinishTin-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 270-degrees 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 well-matched 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.
ParameterSpecificationXC09000A-03
Frequency Range0.8 - 1.2 GHz0.8 - 1.2 GHz**
Characteristic Impedance50 ohms nom.50 ohms nom.
VSWR1.25:1 max.1.20:1 max
Insertion Loss0.25 dB max.0.20 max
Isolation
(RF/LO & Diode1 /Diode 2)20 dB min.20 dB min.
Amplitude Balance+/- 0.25 dB+/- 0.20 dB
Phase Balance90 +/- 3.0 deg.90 +/- 2.0 deg.
PackagingSur-mount solderSurf-mount solder
FinishTin-Lead, Tin, or GoldTin-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
ParameterSpecification
Breakdown Voltage - VBR2 V min.(tested at IBR = 100 uA)
Forward Voltage - VF0.20 V min.(tested at IF = 10 mA)
1Delta Forward Voltage - ΔVF20 mV max.(tested at IF = 10 mA)
Capacitance - CT1.5 pF max.
Dynamic Resistance - RD15 ohms max.
Configuration2 Series connected diodes
PackagingSur-mount package
Lead FinishTin-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 p-type 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.
ParameterSpecificationHSMS-2822
Breakdown Voltage - VBR2 V min.15 V min.
(tested at IBR = 100 uA)
Forward Voltage - VF0.20 V min.0.5 V min.
(tested at IF = 10 mA)
1Delta Forward Voltage - ΔVF20 mV max.15 mV max.
(tested at IF = 10 mA)
Capacitance - CT1.25 pF max.1.0 pF max.
Dynamic Resistance - RD15 ohms max.12 ohms max.
Configuration2 Series diodes2 Series diodes
PackagingSur-mount packageSOT-363 Pkg
Lead FinishTin-Lead, Tin, or GoldTin-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).
ParameterSpecification
Passband Frequency RangeDC - 200 MHz
Characteristic Impedance50 ohms nominal
Passband Insertion Loss0.25 dB max.
Passband Flatness0.3 dB
Rejection20 dB min. above 1.8 GHz
VSWR1.2:1 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:
ParameterSpecification
Frequency RangeDC to 1.5 GHz min.
Characteristic Impedance50 ohms
VSWR1.10:1 minimum
FinishGold plated (for soldering)
InterfaceSMA Female Jack
TerminationEnd-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.
ParameterSpecificationPAF-S05-007
Frequency RangeDC to 1.5 GHz min.DC to 6.0 GHz
Characteristic Impedance50 ohms50 ohms
VSWR1.10:1 minimumTBD
FinishGold plated (for soldering)Gold
InterfaceSMA Female JackSMA Female Jack
TerminationEnd-Launch PCB MountEnd-Launch PCB Mount