The higher in frequency you go, the harder it is for a connector to find a mate.
The key to a successful connection is finding a good mate. As it turns out, finding a mate may be more difficult at millimeter-wave frequencies.
Before we talk about connections, let’s consider the block diagram of a transceiver operating at millimeter-wave frequencies. The implementation issues in the physics mean that a different approach is required, at least for now, because the capacitance of a tiny metal plate or the inductance of a bond lead have an impact that goes up at least linearly with frequency. So, it is important to keep this in mind as a design consideration.
Figure 1. Significant elements in a block diagram of high-frequency, wide-bandwidth radios.
In the red box on the left-hand side, we've got a phased array antenna. Since we are running at higher frequencies, we are very likely to be using beam-steered arrays. Beams are formed by shifting the phase of the signal emitted from each radiating element to provide constructive or destructive interference, which steers the beams in the desired direction.
Next is the transmit/receive portion of the block diagram with up and down converters.
Moving to the right, in the red block in the IFIQ section, we move into the world of quadrature mixing where we are doing frequency conversion. Doing frequency conversion using this 90-degree difference means that you have a single-sideband mixer, so you directly get image suppression on the IF side. Due to this benefit, it's a commonly used technique.
Figure 1 is representative for both backhaul and user equipment, because even on the user equipment side, it is likely that the devices will try and make use of diversity reception.
Let’s come back to the challenges of capacitance and inductance from above. As old-school as it may sound, impedance matching in these circuits is critical. To get designs at these frequencies working well, you must pay close attention to capacitive and inductive tuning. This is some of the hard work required to make wide-bandwidth, high-frequency radios operate. While the components may be “hidden” inside the RFIC, we still see the higher levels of integration to account for the extremely small dimensions required by these frequencies.
Depending on how highly integrated, when we look at those phased array antennas, there’s an increasing chance we're not going find connectors anymore because the extremely small size of the components makes the notion of a “connector” geometrically impractical. The higher in frequency we go, the smaller the dimensions and the more likely that we won't find a connector to mate with. The growth of this connector-less interface is the heart of over-the-air (OTA) test.
This is yet another example of the ways radio development at millimeter-wave frequencies requires extra care and attention.