Vcc of pin diode

4. Standard circuit models for DD and MD

Detector diod

typical microwave circuit with a detector diode

Signal goes to the diode by the posts or loop coupling, that are depicted as a transformer on the scheme. At the diode the signal distorts according to VCC and goes to the capacitor. The signal, read from the diode, will not be a fine sinusoid, because the amplitude during the negative half-cycle will be much less than the amplitude during the positive half-cycle. That is why the capacitor will be charging to a high level during the positive high-cycle and will not almost discharge during the negative half-cycle. Sinusoid signal is converted into constant voltage read from the load. So, if the input of the circuit is a high frequency modulated signal, we will get the low frequency component at the output. That is the principal of detection.

In fact, this is the half-cycle rectifier circuit.

Half-cycle rectifier current and voltage time diagrams

Examples of plugging of a DD into a coaxial transmission line

coupling loop

Circuit with loop-coupling

Loop-coupling is used to apply power to the diode. There is a locking capacitance in the line set up after the diode. Its function is separation of the microwave channel and the source of direct voltage with a load on high frequencies.

post

Circuit with a post

This construction differs from previous one by the presence of the inductance, which is used for closing of the diode's power circuit on low frequencies, while in previous construction the coupling loop is used for this.

Mixer diode

Simple mixer circuit

Microwave signal and heterodyne signal are applied to the diode using the same transmission line, that makes the operation quality much worse.

Mixer with a Schottky-barrier FET

To avoid this drawback the following circuit can be used. In this circuit a signal-heterodyne channels isolation is used.

Balanced mixer

Stable operation of the device is provided by symmetry of the circuit: the signal-heterodyne interaction is low. This helps to reduce he count of combination frequencies, decreases the influence of heterodyne noise and suppress output heterodyne oscillation (30..40 dB and more). Suppression level depends on symmetry of transformer windings, branch resistances equality and parasitic capacitance.

Double-balanced mixer

This circuit provides frequency conversion with a full signal-heterodyne channels isolation.

links: lecture materials on MWEl "Semiconductor devices"; S. Sze "Physics of Semiconductor Devices", 1984; guidelines for laboratory works on microwave electronics; Wikipedia

Task №2 Diodes with negative dynamic resistance

2.1. Draw drift velocity and static field distribution for two structures from the list of diodes with negative dynamic resistance (choose from the above list, one device should be made of Si and the other one of GaAs ). Present the device as a layer structure

2.2. Calculate generation frequency of IMPATT diode and GD with moving domain, if active length .

Given:

Find: 1. Drift velocity and static field distribution

2. (IMPATT diode and GD)

Solution:

1. I've chosen Gunn diode (GaAs) and IMPATT diode (Si)

I MPATT diode