Select the proper mixer for your needs. There are hundreds of models available. Under-specify and face marginal performance, over-specify and pay for more than you need. Here’s the proper approach.
Mixers are abundant in electronic systems ranging from inexpensive consumer products to sophisticated military hardware. You’ll find them in entertainment equipment as well as communications gear, test instruments, radar units and countermeasure systems. Mini-Circuits offers over 400 different off-the-shelf mixer models with thousands of variations available. Different connector configurations, tighter specs, and reliability testing are available on request. Non-optimum mixer selection bears a penalty: under-specify and face marginal performance; over-specify and pay extra for unnecessary performance characteristics.
Making the Right Choice
Simply stated, there are three basic steps:
- Deciding on a surface-mount, connector, or plug-in version.
- Selecting the mixer “0Level”, which is the LO (local oscillator) drive power in dBm required for the application.
- Picking a model that extends over the frequency range involved.
The sequence of these steps follows the manner in which mixer information is accessed when selecting “Frequency Mixers” in the “Product Selection” menu on the Mini-Circuit website. A general presentation will be given for each of these steps, and then pertinent technical considerations will be discussed in more detail.
At the start it is important to understand your specific needs, so it is strongly recommended that you organize the requirements for your application and put them in writing. Ascertain what frequency ranges (LO, RF, and IF) are involved, and the LO drive you have available. Some applications might specify other factors, such as the acceptable amount of harmonic distortion (caused by mixing of LO and single-tone RF harmonics) and/or two-tone, third-order intermodulation (IM) distortion.
Determine the Type of Package and Connections: Surface-Mount, Plug-In, or With Coaxial Connectors
Generally, the user has evaluated the system requirements and will know beforehand whether the mixer is to be mounted on the user’s PC board or needs to have coaxial connectors. Mini-Circuits has three types of packages available. Users typically install surface-mount components via re-flow soldering, and many of such products are aqueous-washable. All plug-in (through-hole) mounting units have at least four pins to ensure a mechanically rugged connection to the user’s PC board. Some mixers are made with hermetically sealed ceramic diode “quads”. Various off-the-shelf mixers are available with BNC or SMA connectors, and specials can be provided with N or TNC connectors. Many connectored units are available with an optional mounting bracket, as shown on the data sheets. Please describe your connector and bracket requirements when ordering.
Determine the LO Level Required
In applications that specify distortion at a particular value of RF input signal power, such a specification might be the governing factor in determining the choice of mixer level; i.e. Level 7, level l0, level 17, etc. As a first-order guide for diode mixers, the LO power should be 10 dB greater than the highest input signal level anticipated. Mini-Circuits high-IP3 mixers utilize field-effect transistors, and among most of those mixers it is sufficient to choose one with LO power 3 dB less than the RF power.
In many cases these first-order guidance numbers may be modified. The trade-off is generally between performance and cost. Keep in mind that it is desirable to select the lowest level mixer that will meet the application because it will be more economical to use, and it should result in the least amount of LO leakage within the user’s system.
Mini-Circuits offers a wide range of mixers of different levels, from 3 to 27. Referring to Figure 1, select the mixer level for your requirement. For active mixers, which incorporate an amplifier in one of the internal signal paths, as well as for image reject mixers, see the individual data sheets for this information.
Determine the Frequency Range Required and Select a Model
Mini-Circuits offers extremely wide-band mixers with a considerable amount of frequency overlap between models. Determine the required frequency range of operation. Then select the mixer whose frequency range specification best “straddles” the range you need. For optimum performance, it is good practice to select a mixer whose mid-band frequency range (identified by lower-case letter “m” in the specifications) covers the intended operation. Aside from that, all Mini-Circuits mixers perform to specifications with good margin over their entire frequency range. Furthermore, many users operate them outside the specified band with good results, and can still expect to receive high-performing mixers with unit-to-unit repeatability.
1 dB Compression Point is Key to Choosing Mixer Level
What does the 1 dB compression point signify? As RF input level is increased, IF output should follow in a linear manner. Eventually, however, IF output increases at a lower rate until the mixer IF output becomes nearly constant.
The RF input power at which the IF output power deviates from linearity by 1 dB is termed the 1 dB compression point. Figure 1 displays the RF input level at 1 dB compression for each level of mixers of the different types. The 1 dB compression point is useful in comparing dynamic range, maximum output, and two-tone performance of various mixers. It is a basis on which the user can chose mixer level.
The 1 dB compression point, which is one definition for the high end of a mixer’s linear range, is relatively simple to test. It is therefore included on data sheets supplied by Mini-Circuits and by other mixer manufacturers. But, in some applications mixer linearity may be specified otherwise, in accordance with the intended application. For a receiver as an example, two-tone, third-order intermod may be critical. For video applications, percent distortion or intermodulation intercept point may be specified, and in an attenuator measurement system, compression at a given RF input power might be important. A systems engineer needs a convenient means of relating the criterion for a particular application to the published 1 dB compression point. The following linearity criteria will now be discussed in that regard.
- Maximum RF input power anticipated.
- Percent distortion.
- Intermodulation intercept point.
- Two-tone third-order intermodulation, dBc, at a given RF input in dBm per tone.
- LO drive power available.
- No particular spec to meet.
Maximum RF Input Power Anticipated
If you know this number, simply select the lowest level mixer whose RF input level at 1 dB compression exceeds your requirement for RF input power. For example, if the maximum encountered RF level is +5 dBm, select a level 13 mixer, rated at +9 dBm typical RF input for 1 dB compression.
The percent distortion is usually specified in terms of voltage. Thus, a 0.1% distortion figure means 0.999 of the desired voltage appears at the mixer output. Next, convert this voltage ratio to a power ratio in dB by squaring and taking ten times the log. The resultant figure is the amount of compression for the specified RF input level. Now, this must be extrapolated to 1 dB compression for a corresponding RF input level. As a rule of thumb, extrapolation may be achieved by assuming a linear relationship in dB, between compression and RF input level. Thus, a ten-times increase in compression corresponds to ten-times increase in RF input level.
Let’s illustrate with the 0.1 percent distortion example, and say it is specified at -10 dBm RF input. The 0.1 percentage corresponds to a relative 0.999 voltage output; squaring this yields 0.998 power ratio or -0.009 dB. We therefore know that 0.1% distortion at -10 dBm input means the maximum allowable compression is 0.009 dB (stated as a positive number). Extrapolating, using the rule-of-thumb relationship, we state that 0.09 dB compression corresponds to an RF of 0 dBm, and 0.9 dB compression corresponds to a maximum allowable RF input level of 10 dBm. So, which mixer is appropriate? A level 17 mixer, with +14 dBm RF input level at the 1 dB compression point.
Intermodulation Intercept Point
This is a figure of merit relating to the level of intermodulation products generated by a mixer, amplifier, or other mildly nonlinear device. An intercept point can be defined for a second-order, third-order, fifth-order, or other product. For a diode mixer, as a rule-of-thumb, two-tone third-order intercept point (IP3) is approximately 15 dB above the 1 dB compression point. For a FET mixer, the rule-of-thumb is 10 dB. So, if the intercept point value is given, merely subtract the corresponding number of dB from it and pick a mixer having at least that value of 1 dB compression point.
Two-Tone, Third-Order Intermodulation
For a mixer, two-tone, third-order intermodulation intercept point (IP3) is usually specified with respect to the RF input. Intermodulation (IM) level on the other hand, is measured at the output, either as power in dBm or as dBc relative to the desired IF signal. To find input IP3 given the output IM power in dBm, first calculate output IP3 as:
IP3OUT (dBm) = |IM3OUT (dBm) – POUT (dBm)| / 2 + POUT (dBm)
In this formula POUT is the power of the desired IF output signal, IM3OUT is the power of the third-order intermodulation product in dBm, and |IM3OUT – POUT| is the absolute value of the difference in dB between the third-order IM product at the output and the desired IF output.
If output IM is given in dBc, the formula is:
IP3OUT (dBm) = |IM3OUT (dBc)| / 2 + POUT (dBm)
To convert the output IP3 obtained from these formulas to input IP3, add the conversion loss value (a positive number, in dB).
Once the output intercept point is calculated, simply subtract 15 dB or 10 dB (see paragraph 3 above) to find the 1 dB compression point, and select among mixers having that value of 1 dB compression point or higher.
LO DRIVE POWER AVAILABLE
The LO drive is critical since the function of the LO drive is to switch the mixer diodes fully on and off for lowest distortion. So, for optimum performance, select the mixer level to match the LO drive. Mini-Circuits conveniently identifies its mixer levels by the LO drive requirement; thus a level 7 mixer refers to a LO drive level of +7 dBm. If there are constraints on the LO power available, select the closest mixer level that is lower than the available LO power. For example, if +12 dBm LO drive is available, select a level 10 mixer.
No Particular Spec to Meet
Suppose no linearity spec or criterion is given for you application. Choose a level 7 mixer. Why? Because it is the most popular and offers the widest choice of models at lowest cost.
Frequency Range is the Final Deciding Factor
Once the package style and mixer level have been decided you may find a wide variety of models from which to choose. You may notice models with overlapping frequency ranges. Why? So you can choose the optimum mixer for your requirement.
Frequency specs for RF and LO on most of the mixer data sheets are given in three ranges. The lower frequency range, L, covers the lowest specified frequency to one decade (or in some cases, one octave) higher. The upper frequency range, U, starts one octave below the highest frequency. Mid-range, M, covers the frequency range between L and U. For example, a mixer specified from 0.5 to 500 MHz could offer a low-frequency range of 0.5 to 5 MHz, an upper frequency range of 250 to 500 MHz, and a mid-range of 5 MHz to 250 MHz. Try to select a mixer with the highest low-frequency limit for which the mid-range matches your application. Be sure that the IF frequency range covers your requirements, including whether you need it to go down to DC.
What About “Specials”?
When a particular model has been selected the user must determine if the model will meet the electrical performance criteria the user has generated. All MCL mixers are characterized, and a significant number of performance curves and tabulated data describing the models are given on the data sheets and the “View Data” pages on the website. The performance curves on the data sheets describe typical performance. Sometimes the system designer requires higher isolation, lower conversion loss, temperature tracking of conversion loss, unit to unit tracking of conversion loss, selection of higher order harmonic products, less than 100 micro-volts of phase-detector DC offset, larger specified bandwidths, etc. Mini-Circuits offers these special performance criteria and many more. Our high volume production enables us to select units to customer specifications at extremely low cost. Mini-Circuits maintains a highly documented system to handle specialized customer requirements.
In summary, contact us about “specials” and we’ll do our best to satisfy your needs without excessive cost or delivery date extension. Sometimes, “special” means shipping a quantity of mixers within a very short time span; since Mini-Circuits is the largest manufacturer of mixers in the world, you may be pleasantly surprised by the quick response. Try us.
Reference the articled titled “Frequently Asked Questions Relevant to Selecting a Mixer,” for more information.
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