of the two diodes in each pair are connected via an associated rf choke and resistor circuit e g l10 l11 and l12 tuned to different self resonant frequencies and resistor r5 to restrict forward bias current to control terminals b c and d in the control unit 16 and having associated capacitors c4 c5 and c6 the inductor l5 is switched by means of four p i n diodes d9 d10 d11 d12 connected in series the diodes d9 d10 being connected together in series for conduction in one direction and in series with the diodes d11 d12 for conduction in the opposite direction the junction of the two groups of diodes is connected via rf chokes and a resistor to terminal e for inductor l6 the portion l6a is shunted by diodes d13 to d16 arranged similarly to the diodes d9 to d12 whilst the inductors l6b l6c are shunted by diodes d17 to d20 again arranged similarly to the diodes d9 to d12 these separate sets of diodes having separate switching circuits but with all the diodes d13 to d20 connected to a single control terminal f the six inductors l1 to l6 have magnitudes in a binary series and the associated switches thus provide 64 possible different.