Constant Impedance, Constant Phase Pin Diode With Attenuator

The PIN diode current controlled resistor is especially useful at high frequencies. The long carrier lifetime of these diodes makes them act as pure resistances to RF signals while remaining normal diodes to DC. Intermods are theoretically nonexistent at frequencies well above cut-off because the devices are no longer square law. A PIN diode attenuator, then would be especially desirable at higher frequencies such as in the region of 300 MHz. However, a single conventional PIN diode attenuator causes the phase to be shifted almost 80.degree. at maximum attenuation. The effects of a phase shift with attenuation can be detrimental. To keep the phase shift to a minimum and increase attenuation, a novel double .pi. was devised. The phase shift with attenuation was kept at a minimum over the range of 0 to 40 dB attenuation.

A constant impedance, constant phase PIN diode attenuator is provided. The diodes are arranged in a double pi pad arrangement. Resistors are connected in parallel to the series diodes of the pad to provide the resistance value necessary for the desired amount of attenuation and to limit the phase shifting effects of the diode junction capacitance on the total impedance. A control voltage is applied simultaneously to the double pi pad arrangement for varying the attenuation.

In the single FIGURE of the invention there is shown a schematic for the constant impedance, constant phase PIN diode attenuator.

The single FIGURE shows in schematic form the circuit which makes use of the fact that the PIN diode acts like a variable resistor at frequencies above approximately 100 MHz while compensating for the detrimental effects of the diode junction capacitance on phase with a resistor in shunt with the series diode. The RF resistance of the PIN diode is dependent upon the bias current, with typical resistance varying from approximately 10,000 ohms at 0 mA to 1 ohm or less at 30 mA bias.

The diodes are arranged in a double .pi. pad arrangement which is two pads sharing diode 3. It is noted junctions 30-34 are used as tie points. Input terminal 25 receives an RF signal and output terminal 26 provides the variable attenuated signal. Half of the attenuator, the left side, simplifies the bias description. There are actually two DC current loops, one through resistor 9 and diode 2, the other through resistor 8, diode 3, and diode 1. Both loops share resistor 6. In the "off" state, the control voltage at terminal 15 is 0 and diode 2 is back-biased, having a value of 10 K ohms. Resistor 11 in parallel with diode 2 gives the effective resistance value necessary for a 20 dB pad, thus limiting the effect of the junction capacitance of the diode on the total impedance. This keeps the phase shift due to the shunt RC combination to a minimum. Resistor 8 and resistor 9 set the current through diode 1 and diode 3 for the proper 20 dB pad shunt values. As voltage is applied to the control voltage point, current flows through diode 2 and more voltage is dropped across resistor 6, effectively reducing the bias of diode 1 and diode 3. This causes diode 2 to decrease in resistance and diode 1 and diode 3 to increase. Finally, as the control voltage increases, the drop across resistor 6 is so large that diode 1 and diode 3 are back-biased and are effectively 10 K ohms each. Diode 2 is approximately 12 ohms at this point. For the combined double .pi., the current through diode 3 is always twice that through diode 1 and diode 5. The two sections (left and right) are limited to 20 dB attenuation per section by resistor 11 and resistor 12, which is mentioned, keeps the phase shift low by keeping the shunt RC impedance highly resistive.

Resistors 9, 10, 13 and 14 are for RF isolation. The values of resistors 6, 7, and 8 must be calculated for the particular PIN diodes to be used. This may be aided by graphs of diode RF resistance versus bias current. The values for resistors and capacitors for HP 8052-0012 PIN diode chips are as follows: for capacitors 15-24, each one is 100 pF; for resistors 6 and 7, each is 6.7 K ohms; for resistor 8, the value is 14.2 K ohms; for resistors 9 and 10, each value is 1 K ohms; for resistors 11 and 12, each value is 306 ohms; and the value for each of resistors 13 and 14 is 100 ohms.