Quadrature oscillator circuitry (800; 900), comprising: first (1; 1E) and second (1; 1E) differential oscillator circuits, each differential oscillator circuit comprising first and second tail nodes, a pair of output nodes (12, 22), and a pair of parallel current paths (10, 20) which extend in parallel between its first and second tail nodes via its output nodes (12, 22), respectively, and each differential oscillator circuit configured to generate a pair of differential oscillator signals (I+, I-; Q+,Q-) at its respective output nodes (12, 22); and a cross-coupling circuit connected to cross-couple the differential oscillator circuits, wherein: for each of the differential oscillator circuits, a first tail current path connects the first tail node to a first voltage-source node and a second tail current path connects the second tail node to a second voltage-source node, one of the first and second voltage-source nodes for connection to a high-voltage source and the other of the first and second voltage-source nodes for connection to a low-voltage source; each of the differential oscillator circuits comprises a common-mode voltage node at which a common-mode voltage signal is produced, the common-mode voltage node either being an intermediate node of a potential divider connected between the parallel current paths of that differential oscillator circuit or being a node of the second tail current path of that differential oscillator circuit; and the cross-coupling circuit comprises, for at least one or each of the differential oscillator circuits, a current-control device (34; 802, 804) which forms part of the first tail current path of that differential oscillator circuit and is configured to control the current flowing along that first tail current path based on the common-mode voltage signal of the other one of the differential oscillator circuits to cause the common-mode voltage signals of the respective differential oscillator circuits to be substantially in antiphase. first (1; 1E) and second (1; 1E) differential oscillator circuits, each differential oscillator circuit comprising first and second tail nodes, a pair of output nodes (12, 22), and a pair of parallel current paths (10, 20) which extend in parallel between its first and second tail nodes via its output nodes (12, 22), respectively, and each differential oscillator circuit configured to generate a pair of differential oscillator signals (I+, I-; Q+,Q-) at its respective output nodes (12, 22); and a cross-coupling circuit connected to cross-couple the differential oscillator circuits, wherein: for each of the differential oscillator circuits, a first tail current path connects the first tail node to a first voltage-source node and a second tail current path connects the second tail node to a second voltage-source node, one of the first and second voltage-source nodes for connection to a high-voltage source and the other of the first and second voltage-source nodes for connection to a low-voltage source; each of the differential oscillator circuits comprises a common-mode voltage node at which a common-mode voltage signal is produced, the common-mode voltage node either being an intermediate node of a potential divider connected between the parallel current paths of that differential oscillator circuit or being a node of the second tail current path of that differential oscillator circuit; and the cross-coupling circuit comprises, for at least one or each of the differential oscillator circuits, a current-control device (34; 802, 804) which forms part of the first tail current path of that differential oscillator circuit and is configured to control the current flowing along that first tail current path based on the common-mode voltage signal of the other one of the differential oscillator circuits to cause the common-mode voltage signals of the respective differential oscillator circuits to be substantially in antiphase. for each of the differential oscillator circuits, a first tail current path connects the first tail node to a first voltage-source node and a second tail current path connects the second tail node to a second voltage-source node, one of the first and second voltage-source nodes for connection to a high-voltage source and the other of the first and second voltage-source nodes for connection to a low-voltage source; each of the differential oscillator circuits comprises a common-mode voltage node at which a common-mode voltage signal is produced, the common-mode voltage node either being an intermediate node of a potential divider connected between the parallel current paths of that differential oscillator circuit or being a node of the second tail current path of that differential oscillator circuit; and the cross-coupling circuit comprises, for at least one or each of the differential oscillator circuits, a current-control device (34; 802, 804) which forms part of the first tail current path of that differential oscillator circuit and is configured to control the current flowing along that first tail current path based on the common-mode voltage signal of the other one of the differential oscillator circuits to cause the common-mode voltage signals of the respective differential oscillator circuits to be substantially in antiphase.