Nutrientes y requerimientos

rela tiv ely low f o r w a r d voltage and a high forward

current. C o n v e r s e l y for the reverse bias state, the

diode requires a h i g h reverse v o ltage and low reverse

current. A combin a t i o n of these spec ifications plus

the addition al requir e m e n t of rapid switching times between the two s t a t e s introduce complexities in the driver design.

Fig. 4.8 sh o w s the circuit d i a g r a m of the

design ed driver. It uses a simple pre- amplifier

stage and can sw itch the p.i.n. dio d e between a fixed reverse bias v o l t a g e and a foward bias current wh os e mag ni tude can be adjusted by the resistor R 3 . Th e operation of t h i s driver is su ch that a logic •1 * ap pli ed to its input terminals turns transistor Q 2 "on" and the p.i.n. d i o d e will be connected to the

re ver se bias voltage through R 5 . D u r i n g this state

transist or Q 3 is non conducting. T h e value of R 2

chosen is such that t h e r e will not be enough voltage

to turn Qj on. A log i c 'O' applied at the input

terminals switches o n transistor Q j , thereby creating a path from the v o l t a g e supply to the diode through R 5.

To ensure that Q 2 is turned off during the forward biased state, a di o d e Dj was inserted in the circuit and its polarity is s u c h that it achieves this objective

in this bias state only. T r a n s i s t o r Q 2 also serves to

amplif y the current t h r o u g h Ri,, thus providing a more rapid removal of the i-region charge in the p.i.n. diode.

6 6

67

The transis tor s us ed are type Q D G - B F Y 5 1 . Important

cons id erati on made in the selection of t h e s e transistors

include the maximum permis s i b l e bias voltage. The

transistor should have a breakdown v o l t a g e of more

than twice the m a x i m u m bias voltage. In high

reliabi lit y applications, the use of t o r o i d a l coil in pl ac e of the output transistor is recommended. This should serve as an energy storing a n d discharging

device for the p.i.n. diode. Apart from reducing cost,

this also increases reliability.

4,4 D I O D E MEASUREMENT

4.4.1 Measurement technique: Early methods of m e asuring p.i.n. diod e parameters have been reported, the simplest of wh ic h is a low frequency bridge m e a s u r e m e n t where

the package parameters are ignored. For m o s t micro-

wave applications, the package reactance o f the diode influences the diode p e rformance and hen c e can not be ignored pa rt ic ularly if the operating f r e q u e n c y is

more than 2 . 0 O H z . Refinement in diode m e a s u r e m e n t s

began with the work of G e t s i n g e r 6 which u s e s resonant technique in calc ulating the equivalent c i r cuit

parameters of the device. This method is claimed to

give accuraci es b s t ts r than three percent. W i t h the

advent of the computer corrected network analyzer,

63

The pro cedure used he re involves characterizing the embedding network first in or der to account for all pa ras iti c elements arising from the mounting

arrangement. Then the equivalent ci rcuit elements of

the di od e were determined by measuring the loss p r o d u c e d by the test fixture and the di ode and de d u c i n g the values of the elements usi n g graphical

techniques. The same equivalent circuit was used for

the t w o diodes.

A measurement jig was built to ho use the device. The sub str ate s were fabricated from 0 . 1 59cm thick p o l yg uid e dielectric ma terial (er = 2.32) with a one

ounc e copper cladding. The input and output ports

have 50 ohms characteristic impedance lines. To

en su r e adequate grounding of the planes, conductive epo xy was used to glue aluminium planes to the unetched

co pp e r face of the d i electric material. The jig is

shown In fig. 4.9.

Th e scattering parameters of the Jig without the diode in position were then me a s u r e d ov er the frequency range 0.4-4.0 GHz in steps of 100 MH z usi n g the computer

co rr ect ed network analyzer. Th e results are shown in

fig. 4.10. Th e results show that the jig has a minimum

tr ans mis sio n coefficient |S2 xIm of 0.985 and a maximum refl ect ion coefficient | S n | m of only 0.02 throughout

the ope ra tin g band. T h e s e are equivalent to an

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