M1 is an RF buffer amplifier, while M2 and M3 provide the switching function needed for mixing action. Another observation is that the DC term in the bracket means that the RF signal is not suppressed. Upon mixing with the RF signal, it will create the upper and lower sidebands around the LO and its odd harmonics. The mixer output wave can be modeled as 2Įquation 1 simply states that the RTZ square waveform consists of only odd harmonics of the LO. For SBM, the waveform for the LO can be modeled as a return-to-zero (RTZ) square waveform. The most widely used technique is to have the LO signal turn on and off the switch in the RF path. There are many ways to implement a mixer. The IC implementations of the SBM and DBM are presented first.
1 They are the foundation for sub-harmonic mixers (SHM). The standard single-balanced mixer (SBM) and double-balanced mixer (DBM) are well known and well documented. This article targets the IC implementation of SHM in consumer markets. Much research has been done to explore the possibility of using SHM for consumer radio transceivers such as WLAN and cell phones. It is typically implemented with discrete anti-parallel diode pairs. Traditionally, SHM is used for upper microwave and millimeter-wave ranges, where it is difficult to get frequency sources above 20 GHz. The article ends with a discussion of SHM enhancement techniques. Next, four SHM topologies are discussed in detail. This article starts by briefly reviewing the standard Gilbert cell mixer. These problems can be greatly reduced by using a sub-harmonic mixer (SHM) topology. It mixes with the LO signal itself to create a DC offset at the baseband. The LO leakage finds its way into the RF path.
The root of the DC offset and poor IM2 problems is due to the fact that the RF signal and the LO are at the same frequency and the RF-to-LO isolation provided by the mixer is never infinite.