Helping to Eliminate the Mix-up over Mixers
As Custom MMIC releases its first subharmonic mixers, we in the Applications Group thought it was important to explain some of the benefits of working with these products over fundamental mixers, particularly when mixing at very high frequencies.
First, we should take a minute to explain these two types of mixers. A fundamental mixer combines an input RF signal, typically received via an antenna or a prior mixing stage and multiplies it with a fixed local oscillator (LO) signal. The LO signal is typically much higher in power level, as this signal provides the energy for the mixer to combine the two signals in an efficient fashion. The output of the mixer at the third, or intermediate frequency (IF), port of the mixer ideally contains a signal with two new frequencies at the sum (f LO + f RF ) and difference (f LO – f RF ) of the RF and LO frequencies. In the real world, other signals appear at the IF port such as LO leakage, RF leakage, and spurs, all of which are at varying power levels.
A fundamental mixer offers an efficient solution at low frequencies where it is relatively easy to
generate the LO signal. However, at higher frequencies (say greater than 10 GHz), producing a high-quality, high power LO signal can be difficult as well as expensive. One solution is to generate an LO at half the required frequency and then use a multiplier to convert the signal to the desired LO frequency.
A passive multiplier is one solution, though this approach also requires a buffer amplifier on the output to increase the LO signal strength, since passive multipliers can be quite lossy. Alternatively, an active multiplier can be employed, though this approach requires a DC voltage and may still require an additional output buffer amplifier. As a result, the multiplication method, while effective, requires additional components between the LO synthesizer and the mixer which take up valuable board space and can increase the complexity of the design.
A more elegant solution is to use a subharmonic mixer such as Custom MMIC’s new CMD303 (Ku-band) and CMD310 (Ka-band). These components still perform the same basic mixing function as a fundamental mixer, however they work directly with the LO signal at half the frequency. In addition, both the CMD303 and CMD310 contain an efficient on-chip LO buffer amplifier so the required LO power level is considerably lower than that needed to drive a passive fundamental mixer. Another benefit of a subharmonic mixer is the even harmonics of the provided LO signal have very high rejection at the RF and IF outputs, simplifying any LO rejection filtering requirements. Finally, no additional components are necessary aside from a bypass capacitor, thus making Custom MMIC’s subharmonic mixers easy to implement and easy to save board space.
Please check out our mixer offerings and stay tuned for new wideband mixers, I/Q mixers and subharmonic mixers coming soon.