step recovery diodes

8/14/2019 Step Recovery Diodes

1/3

Step Recovery Diodes

Application Note

Application

Notes

94 MicroMetrics, Inc. 136 Harvey Road, Building C, Londonderry, NH 03053Voice: 603-641-3800, Fax: 603-641-3500, Internet: www.micrometrics.com, E-mail: [email protected]

0.1

0.1

1

10

100

200

1.0 10 20

VALUEOFJUNCTIO

NCAPACITANCEat-6VOLTS

(pF)

OUTPUT FREQUENCY(GHz)

Max.Power(X

c=10-30)

Max.Efficien

cy(Xc-60

)

.0110pS

100pS

1nS

10nS

100nS

0.1 .1.0 10 .20

MAXIMUMSNAPTIME

(Ts)Nano

seconds


SuggestedTsRange

Ts< 1

Fout

Maximum Value Ts

Figure 1. Capacitance Vs. Output Frequency

Figure 2. Snap Time Vs. Output Frequency

Step recovery diodes, or SRDs, are usedas high order multipliers and will multiply

as high as 20x when used as a comb gen-erator. They depend on extremely fastrecovery time, often referred to as snaptime to generate pulses rich in harmon-ics. Proper filtering and pulse shaping pro-duces the familiar output of sharp, fastrise time pulses.

By storing charge during the positive halfof an input sinesoidal signal and thenextracting that charge during the nega-

tive going half cycle, a current pulse witha rise time equivalent to the snap timeof the diode is generated in the impulsecircuit of the multiplier. Output filteringthen selects the desired harmonics for theapplication at hand.

Unlike C-Swing multipliers which utilizethe non-linearity of the capacitance-voltage curve to generate harmonics, andare most useful as low order, high power

multipliers, SRDs are most useful as highorder, low power multipliers. Becausethey can be made with low capacitanceand low parasitics, they are most useful atthe higher frequency ranges and will workwith outputs as high as Ku band.


2/3

Step Recovery Diodes Application Note

95MicroMetrics, Inc. 136 Harvey Road, Building C, Londonderry, NH 03053Voice: 603-641-3800, Fax: 603-641-3500, Internet: www.micrometrics.com, E-mail: [email protected]

Application

NotesSelection of SRDs

In selecting a diode for comb generatoroperation, the following criteria should bedetermined:

Input Frequency

Output Frequency Bandwidth Efficiency

Once these values have been determined,the proper diode can be selected, as wellas the proper value of the bias resistor.

The major requirement for an efficientmultiplier diode is that its lifetime is longenough that the reverse current can reach

a peak before the diode snaps back toits high impedance state.

Diode Current

Diode

Current

Time

Ts

20% IrPeak

80% IrPeak

+

-

Figure 3. Ideal Lifetime

Diode Current

Short Lifetime

T1Adequately Long Lifetime

T2

Snap

1RP

1RPT

Figure 4 Snap Time (Ts) Definition

Input Frequency

The minimum required lifetime for a giveninput frequency can be calculated by:TLx FIN = 10 Min. However, a value of20-30 is considered best in most applica-

tions with the exception of bandwidths of10% or greater, where a value closer to 10will minimize difficulties of input matching.

Output Frequency

Transition time, or snap time as we havebeen using, is the key determinant foroptimizing output of a multiplier circuit,i.e.: Ts < 1

Fout

.0110pS

100pS

1nS

10nS

100nS

0.1 .1.0 10 .20

MAXIMUMSNAPTIME

(Ts)Nanoseconds


SuggestedTsRange

Ts< 1

Fout

Maximum Value Ts

Maximum and suggested values of snaptime (Ts) forSRD varactors vs. multiplier output frequency in GHz.

Output frequency also controls the choice

of junction capacitance. A multiplier willwork best if the output reactance isapproximately 50 ohms at the self biaspoint established by the bias resistor.(Usually very close to the specified capaci-tance at -6 volts.) Cj = 1

2 f X c

Normally a reactance value between 30and 60 Ohms is adequate for mostmultiplier applications, however sincepower handling is dependent on junctionarea, the larger the junction the better, soa reactance of 30 to 40 Ohms should bethe goal.

Continued on next page.


3/3

Step Recovery Diodes

Application Note (continued)

Application

Notes

96 MicroMetrics, Inc. 136 Harvey Road, Building C, Londonderry, NH 03053Voice: 603-641-3800, Fax: 603-641-3500, Internet: www.micrometrics.com, E-mail: [email protected]

Output Power

The output power required will determine

the breakdown voltage needed and thethermal resistance of the diode. Thebreakdown voltage must be high enoughso that the peak RF voltage plus the biasvoltage will not cause the diode toavalanche. If this occurs, the diode willbecome quite noisy and could burn out.The required breakdown voltage can beestimated by:

where the constantk = 0.8 for multiplication < 4

= 1.1 for multiplication = 4= 1.5 for multiplication > 4

The thermal resistance must be lowenough to keep the junction temperaturebelow 150C during operation (all powernot converted to harmonics may beassumed to be dissipated in the diode)

jo = 150C - TA(power in) - (power out)

where TA is the ambient temperature.

Bandwidth

Step recovery diodes are inherentlynarrow band due to the spike responsenature of the input impedance. For appli-

cations requiring bandwidths of 5-10% orgreater, a low order multiplier utilizing aC-Swing type diode is recommended. If abroadband comb generator is required,the input and output filters should be keptas simple as possible.

Bv = k 2 Po(Fin x C

t-6)

Efficiency

The principle contributor to decreased

efficiency in multiplier circuits used tobe the cutoff frequency of the diode.Today, however, most modern SRD andmultiplier designs have cutoffs in the200 - 300 GHz range which is more thanadequate for most multipliers up to Kuband. Efficiency for most multiplier circuitscan be approximated by the expression:Efficiency 1 where n > 3

nFor doublers, efficiency as high as 65%

can be achieved. For triplers, efficiency ashigh as 50% can be achieved.

step recovery diodes

Documents