by Douglas Grosch
Quadrature hybrids are useful for countless applications, including balanced amplifiers, IQ/IR/SSB mixers, reflectionless filters, and reflective phase shifters. Marki Microwave now offers quadrature hybrids in connectorized module and bare die form factors. The bare die products (MQH and MQS) cover frequencies from 2-42 GHz (as of 2019) and enable a variety of new applications in chip and wire circuit designs.
MQH vs. MQS
One common question we received regarding our MMIC 90° hybrids is the difference between the MQH quad hybrid and the MQS “90° splitter” series. MQH devices are reciprocal, so any port can be used as an input. However, for the MQS series only ports 1 and 2 can be used as an input. Injecting a signal into ports 3 or 4 will present a phase balance walk-off on the output ports. For some applications where all that is required is to split or combine a signal in quadrature, such as with an SSB or IR mixer, this hybrid would work well. However, this can be a problem for applications involving reflected signals like a balanced amplifier or reflectionless filter since the reflected signal cannot be properly terminated in the isolated port. It is also a problem for applications which require reciprocal hybrids such as Butler Matrices.
Figure 1: MQS-0218CH phase balance for each port
The MQS devices clearly have some limitations that prevent them from being usable in certain applications, so why make them? The MQS designs allow for broader bandwidths than similar sized quad hybrids. The MQS-0218 would not have be realizable on a MMIC platform as a quad hybrid with the amount of phase and amplitude balance achieved. They’re useful if the device doesn’t need to be reciprocal, like when paired with our IQ mixers to make a broadband IR/SSB mixer with high image rejection.
Quad Hybrid Application 1: Reflectionless Filter
When identical reflections are presented to the output ports of a quadrature hybrid, the two reflections combine in phase at the isolated port and out of phase at the input port. This is an important property that allows the first application, a reflectionless filter. In this application two reflective filter die are used as the load between two matched quad hybrids as below:
Here is the MQS on it’s own:
Here is a simulation using the SNP files provided on the website and accounting for bond wire inductance. The simulation predicts a return loss better than 10 dB from around 2-18 GHz, as expected. The tradeoff is an increase in insertion loss and reduction in return loss in the passband.
Finally here is the measured insertion loss and return loss from the chip and wire assembly:
This shows that the goal of reducing broadband return loss can be achieved at the expense of higher insertion loss, more ripple in the passband, and return loss in the passband. While this was a somewhat trivial example, it is possible to create more complex reflectionless filter structures using these basic building blocks.
Quad Hybrid Application 2: Balanced Amplifier
In a balanced amplifier, as in a reflectionless filter, the poor return loss of an amplifier is compensated for with a quadrature hybrid. Additionally the output power of a single amplifier can be increased (but only if the insertion loss of a single quad hybrid is less than the 3 dB improvement in output power).
Below is a balanced amplifier simulated using two distributed amplifiers and two MQS-0218 hybrids. Gain is on the top and return loss is on the bottom. The yellow trace is input return loss and blue is output.
Below are the plots of the return loss for the amplifiers for comparison.
As expected, the return losses are significantly improved, particularly the output return loss above 15 GHz. The cost is paid by the gain, which is reduced by the insertion loss of the quad hybrids (especially in the middle of the band). Not shown is the output power, which will increase by approximately 1-2 dB (3 dB minus the insertion loss of one of the quadrature hybrids).
Quad Hybrid Application 3: Reflective Phase Shifter
Unlike in the previous applications, reflective applications only work well with a quadrature hybrid (not a 90˚ Splitter/Combiner). In these applications a signal is reflected off of two identical structures (typically a PIN diode) and the output signal is collected at the isolated port. In this case the desired signal is deliberately reflected from the short circuit, and the phase delay is implemented on the line using a PIN diode. The two lines must be perfectly matched or the signal will be reflected to the input and not the output port.
The below measurements show a reflective phase shifter using a MQH-0517 quad hybrid and a pair of PIN diodes in series with a short. The upper left plot shows the insertion loss of the system under the two states and the bottom plot shows the phase shift. This circuit uses diodes to reflect the signal back into the hybrid. DC voltage is applied to the hybrid to turn the diodes either on or off, creating two states of load impedance on opposite side of the Smith chart.
In the test unit, there’s a phase walk-off, so there’s probably a path length difference or a load mismatch. No impedance matching was added for the diodes, but the phase walk-off may be corrected by one. While there is some ripple in the phase and some variance in the insertion loss, the bandwidth is a massive 4-18 GHz, so a variable phase shifter across this bandwidth is an interesting circuit.
These simple application examples are intended not to show the state of the art, but to demonstrate roughly what is possible using the MQH and MQS series of quadrature hybrids and quadrature splitters. These bare die offer excellent phase and amplitude balance over unprecedented bandwidths in a small form factor. By integrating them with the MMIC or devices of your choice it is possible to realize novel circuits that shatter your own performance barriers.