Q. How critical is LO level in a mixer application?
A. The purpose of the LO signal is to switch the mixer diodes on and off. Mixer manufacturers select fixed LO levels so they can test and specify at each particular level. In practice, however, LO drive can vary with minimal effect on mixer performance. For example, a Mini-Circuits SRA-1 unit is specified at +7 dBm LO level; if the LO drive was only 3 dBm perhaps a 0.5 dB increase in conversion loss might take place. Or, a high-level SRA-1H might experience a 0.3-dB increase in conversion loss if LO level was +14 dBm instead of the specified +17 dBm LO. The curve below illustrates how conversion loss varies with LO power for a Level 7 mixer.
Q. I notice Mini-Circuits offers many series of mixers, such as the ADE, MCA1, SIM, SRA, SYM, TFM, ZX05, etc. including over 450 models. Why so many models?
A. Mixers are used in systems covering communications, weapons, test instruments, radar, data transmission and countless other applications. A mixer manufacturer can opt to produce a dozen models and thus, in a sense, force the design engineer to compromise specific system needs by offering a rather narrow selection. Mini-Circuits approach is to offer a wide variety so the designer can buy just what is needed, and thus neither compromise system performance nor pay for more than is really needed. That’s why Mini-Circuits offers more off-the-shelf models than all other manufacturers combined. But that’s not enough. As needs change, new models will become available. For example, the recent demand for higher density packaging has been filled by Mini-Circuits surface-mount mixers such as ADE, MCA1, and SIM series, more than doubling packaging density on a PC board. Finally, although we list over 450 off-the-shelf models, we can supply a great number of variations (such as connector configurations, tighter specs, etc.) at our customers’ request.
Q. What’s the difference between “Level 7”, “Level 10”, and “Level 27” mixers?
A. The different levels of mixers are determined by the LO drive required for a particular application. Thus, each level of mixer has been optimized for a given LO drive, and will offer low distortion even at the maximum input level specified for the mixer. The higher the level of mixer, the lower the distortion expected. It is obvious that the highest level mixer could satisfy most requirements. However, high LO drive would be necessary and the cost of a level 27 mixer is higher than a lower level mixer. Thus, rather than over-specify, select the level of mixer that will optimize your system.
Q. I don’t have enough LO drive even for a level 3 mixer. What can I do?
A. Level 3 mixers, although specified for +3dBm LO, will operate well with LO drive as low as 0 dBm. However, there is some degradation: about 0.5 to 1.5 dB additional conversion loss among various Level 3 models. Also, the upper frequency limit might be slightly reduced and there may be increase in two-tone, third-order IM.
Q. I am operating over a wide bandwidth and my LO drive level changes by about 3 dB. How should I take this into account when I select a mixer?
A. The preferred approach would be to determine your lowest LO drive level over the band and then select a mixer for this drive level. Remember that as LO level increases over the nominal (to +10 dBm for a level 7 mixer), conversion loss remains flat and thus optimum performance will be maintained and not degraded.
Q. From my requirement for intercept point, I have selected a Level 13 mixer. Can I apply a +7dBm LO drive and expect the high performance specified in the data sheets?
A. No. Performance to the specifications take place when the mixer diodes are driven fully “on” by the LO drive. Therefore, when LO drive is insufficient, the diodes operate on a lower-current portion of their I-V characteristic curve. The resulting increase in diode non-linearity increases distortion of the output signal. That is why Mini-Circuits offers so many different mixer levels, so that the user can choose optimally.
Q. I need a mixer with flat conversion loss over my frequency range spec. How do I go about selecting a model?
A. Conversion loss over the mid-range response of a mixer is usually flat. So select a mixer that has a mid-range response covering the frequency range you need. Also, the VSWR of the mixer (especially the RF and IF ports) should be checked in order to minimize mismatch errors which could compromise the conversion loss flatness.
Q. I expect wide temperature extremes for my latest RF design project and mixer conversion loss vs. temperature concerns me. What advice can you offer?
A. For extremely cold environments (down to -55°C), to maintain flat conversion loss, pick a mixer that operates a minimum of two octaves below your lowest frequency of operation. For example, if your application involves operation at 30 MHz, select a mixer that extends to 7.5 MHz or lower. Why? The permeability of the ferrite core in the mixer transformer will drop at reduced temperature, resulting in higher losses or frequency fall-off at the low end of the band. In extremely high-temperature environments, the mixer diodes will be the predominant factor affecting conversion loss. Diode impedance will change and cause mixer mismatch to upset flat conversion loss. So, select a mixer whose upper frequency is an octave higher than that of your application. For example, if you are concerned with high-temperature performance at 500 MHz, select a mixer good up to 1000 MHz (such as the SRA-2CM or TFM-2).
Q. Do you supply a mixer with a specified maximum VSWR?
A. Generally, a maximum VSWR spec is not given in the mixer data sheets. You can find typical performance data of mixers including VSWR on our website. As a general rule, try to select a mixer whose mid-range covers your frequency band of interest. If you need to specify a maximum VSWR requirement, Mini-Circuits can screen and test to meet your needs.
Q. My application involves maximum RF input signal levels of -20 dBm. I notice the 1 dB compression point specification of many mixers is at +1 dBm and higher. Should this spec be of concern to me?
A. Yes, if two-tone, third-order IM is critical or if receiver response to unwanted signals is important. The 1 dB compression point is related to the intercept point, which is a “figure-of-merit” of the linearity or distortion characteristic of a mixer. It also indicates the susceptibility of the mixer to unwanted signals.
Q. I need a mixer that nominally operates at + 10 dBm LO drive but requires protection against sudden surges of RF input power as high as 200 mW. What mixer should I select?
A. To avoid damage from a 200 mW input signal, choose a Level 17 mixer. Reason: a Level 17 mixer contains eight diodes, and is rated at 200 mW maximum RF power. But, there is a penalty. For minimum distortion, you’ll require +17 dBm LO drive, which may be higher than your original intent. Or, you can operate at a lower LO drive and accept an additional conversion loss of 1.0 to 1.5 dB.
Q. l have an application requiring an RF frequency response from 100 Hz to 10 MHz. I don’t see any mixers specified with an RF this low. What can I do?
A. One of the characteristics of double-balanced mixers (DBM) is that for most models IF response extends down to DC. All ports of the DBM are balanced and isolated from each other. Therefore, if the high-frequency response of the IF port is sufficient to handle the RF signal in your application, simply connect the RF input signal to the IF port and take the IF output from the RF port. Make sure the RF frequency response of the mixer will accommodate the IF frequency requirements, since in this configuration the IF (formerly the RF) does not go down to DC.
Disadvantages? Yes. Generally, the linearity characteristic will not be as good as with the signal connected in the conventional manner. Also, there is a danger that transients at the RF port (previously the IF port) can damage the diodes internal to the mixer.
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