NBSIR 74-377

PICOSECOND PULSE GENERATORS USING

MICROMINIATURE MERCURY SWITCHES

James R. Andrews

Electromagnetics Division

Institute for Basic Standards

National Bureau of Standards

Boulder, Colorado 80302

March 1974

Final Report

Prepared for

Department of Defense

Calibration Coordination Group

72-66

NBSIR 74-377

PICOSECOND PULSE GENERATORS USING

MICROMINIATURE MERCURY SWITCHES

James R. Andrews

Electromagnetics Division

Institute for Basic Standards

National Bureau of Standards

Boulder, Colorado 80302

March 1974

Final Report

Prepared for

Department of Defense

Calibration Coordination Group

72-66

U.S. DEPARTMENT OF COMMERCE, Frederick B. Dent, Secretary

NATIONAL BUREAU OF STANDARDS Richard W Roberts Director

TABLE OF CONTENTS

Page

I. INTRODUCTION 1

II. FLAT PULSE GENERATOR CONCEPT 5

III. PRELIMINARY WORK 6

IV. ALTERNATE SWITCH CONFIGURATIONS STUDIED 7

V. COMMERCIAL STRIPLINE SWITCH EVALUATION 8

VI. NBS STRIPLINE SWITCH 13

VII. CLASSICAL MERCURY SWITCH PULSE GENERATOR 16

VIII. CONCLUSIONS 17

REFERENCES 18

LIST OF FIGURES

Figure Page

2-1 Diode switch flat pulse generator 21

2- 2 Mercury switch flat pulse generator 21

3- 1 Basic SPDT microminiature mercury wetted switch 22

3-2 DIP IC mercury switch pulse generator 23

3-3 Transition time measurement set-up 24

3-4 Transient step response of measurement set-up. Horizontal scale is 100 ps/div. 24

3- 5 Pulse output from DIP IC mercury switch pulse generator.

Horizontal scale is 200 ps/div. 25

4- 1 Microminiature mercury switched mounted in a 14 mm

insertion unit 26

4- 2 Pulse output of a microminiature switch mounted in a 14 mm insertion unit. Horizontal scale is 100 ps/div.- 26

5- 1 Microminiature mercury switch in commercial strip line

package 27

5-2 TDR characteristics of commercial stripline microminiature mercury switch, (a) Port 2 to Port 1.

Cb) Port 2 to Port 3. 28

5-3 Fast pulse transmission characteristics of commercial

stripline microminiature mercury switch. (a) Input,

(b) Port 2 to Port 1. (c) Port 2 to Port 3. 28

iii
LIST OF FIGURES (continued)

Figure Page

5-4 Pulse output from commercial stripline microminiature

mercury switch. Ordinary delay line measurement set-up,

Fig, 3-3. Horizontal scale is 100 ps/cm 29

5-5 Pulse output from commercial stripline microminiature

mercury switch. Random sampling measurement. Horizontal scale is 0.6 ns/div. Data acquisition time of

120 min. 29

5-6 Transient step response of 70 ns superconducting delay

line. Horizontal scale is 20 ps/div. 30

5-7 Mercury switch transition time measurement setup using

a superconducting delay line 31

5-8 Pulse output from commercial stripline microminiature

mercury switch. Superconducting delay line measurement

setup, Fig. 5-7. Horizontal scale is 50 ps/div. 32

5- 9 Same as Fig. 5-8 except with replacement switch

element. Horizontal scale is 100 ps/div. 32

6- 1 Modified microminiature mercury switch. Center contact

and end terminal shorted with silver paint 33

6-2 NBS in-line stripline housing for a modified microminiature mercury switch 34

6-3 TDR characteristic of NBS stripline mercury switch

housing 35

6-4 Fast pulse transmission characteristic of NBS stripline

mercury switch housing 35

6-5 Pulse output from the NBS stripline mercury switch

pulse generator. Superconducting delay line measurement setup, Fig. 5-7. Horizontal scale is 100 ps/div.

and 20 ps/div. 36

6- 6 Mercury switch trailing edge transition observed on the opening of the switch. Horizontal scale is

1 ns/div. 36

7- 1 Classical reed mercury switch 37

7-2 3 mm coaxial housing for a classical reed mercury

switch 38

7-3 TDR characteristic of classical reed mercury switch in

3 mm coaxial housing. Vertical scale is 0.1 p/div.

Horizontal scale is 100 ps/div. 39

7-4 Fast pulse transmission through a classical reed

mercury switch in a 3 mm coaxial housing. Horizontal

scale is 50 ps/div. 39

7-5 Pulse output from classical reed mercury switch pulse

generator. Superconducting delay line measurement

set-up, Fig, 5-7. Horizontal scale is 100 ps/cm 40

iv
PICOSECOND PULSE GENERATORS USING

MICROMINIATURE MERCURY SWITCHES

James R. Andrews

Pulse generators have been built using microminiature mercury

switches. A commercial RF coaxial switch was also evaluated as

a pulse generator. A superconducting delay line (t^ = 18 ps,

t^ = 70 ns) and a sampling oscilloscope (t^ = 22 ps) were used to measure the generated pulse 10^-90% transition time. The best

result obtained was a transition time of 39 ps . Pulse amplitudes

were independently adjustable up to 50 volts. The microminiature

mercury switches in general were found to give very unreliable

operation.

Key Avords : ^lercury switch; picosecond; pulse generator; pulse

measurement; superconductivity.

I. INTRODUCTION

This is the final report on CCG project 72-66. The objective of this

project Av'as to construct a 7 mm reference waveform generator with a 50

picosecond (ps) or less 10%-90% transition time, one volt maximum amplitude

with a known uncertainty and source impedance. The generator was to use a microminiature coaxial mercury switch as the pulse source. Mercury wetted switches have been used for many years to generate

subnanosecond transition time (risetime) pulses [1,2]. The fundamental

mechanism of the mercury wetted contact is not fully understood as to

just what the ultimate switching time limitation is. The switching times

of present switches have been limited by parasitic reactances associated

with the switch package rather than the mercury -wetted contact. Some

me rcury -wetted switches are also capable of generating voltage pulses

of several kilovolts . The pulse amplitude is adjustable and independent

of the pulse waveform. The major disadvantage of the mercury switch is

its mechanical nature which inherently limits its switching repetition

rate to a few hundred hertz at most. Also due to its mechanical inertia,

friction, etc., it is not possible to trigger the switch closure with better

than a few hundred microseconds precision.
In 1968, Elliott [3] was able to achieve a 70 ps , 10%-90% transition

time with a classical mercury-wetted reed relay. In 1970 Meyer [4] reported

a self-sampling pulse measurement system with system transition time o£

18 ps . Meyer used a recently introduced microminiature mercury switch [5,6]

as his pulse generator. He reported a 12 ps transition time for the mercury

switch. Subsequently Meyer has reported more details of his work [7,8].

NBS Reference Waveform Generators use the band- limiting properties of

a lossy uniform transmission line [9,10]. The line is driven by an abrupt

step-like pulse. The pulse output of the line then closely approximates the

step response of the line which may be mathematically predicted using the

various physical and electrical parameters of the line. Deviations in the

driving waveform from an ideal step cause corresponding deviations in the

output from its ideal step response. The contribution of the driving source transition time is given approximately by the sum of the squares expression.

,. , .: . . = y t^.

r total ^ ri

t ^ . T is the transition time of the total system, while t . is the transir total ^ ' ri

tion time of an individual component.

The present NBS Reference Waveform Generators are lossy transmission

lines with ideal step response transition times in the range of 200 ps to

500 ps . They are driven by a commercial tunnel diode pulse generator with a transition time of the order of 20 ps . Based upon the above equation this

nonperfect dr''/''"'.ng ^unction introduces an error of 0.5"^ in the output transition time for a line with a 200 ps transition time. If one had a line with

a 50 ps transition time an error of 7% is introduced. Additional errors of

even greater importance are caused by the further imperfections of the waveshape of the pulse from the commercial tunnel diode generator. IVhen the

tunnel diode switches from its low to high voltage state, it does produce a

fast rising step-like transition (- 20 ps). However this transition is

superimposed upon a slow (1 ns) triggering pulse that distorts the baseline

and topline. In addition there are some m.ultiple reflections and ringing on

the topline that are excited by the fast transition. An additional problem

2
with the commercial tunnel diode generator is a long term sag in the pulse

topline. The sag amounts to 3% in the first 100 ns . These perturbations

are not completely removed by the band- limiting properties of the lossy

transmission line. They thus appear in the output and distort it from the

ideal, perfectly smooth, theoretical step response.

Due to the limitations of the present tunnel diode driving source and

the encouraging results reported by Meyer, this project was proposed to

develop a reference waveform generator driven by a microminiature mercury

wetted switch. The switch promised a 2:1 improvement in transition time,

elimination of the problem of the superposition of the triggering pulse on the step transition, and elimination of the long term sag.

Another independent benefit of the development of a fast mercury switch

pulse generator related to the CCG project 72-68, "Pulse Testing of RF and

Microwave Devices" [11]. The objective of this project was to measure the

attenuation versus frequency of coaxial attenuators using fast pulse

techniques and the fast Fourier transform (FFT) . The measurement system

uses a sampling oscilloscope and a minicomputer. To maximize the signal to

noise (S/N) ratio in the measurement, it is desirable to use a signal into

the sampling head of amplitude slightly less than the dynamic range (± 1 V)

of the instrument. Using a tunnel diode the pulse amplitude is fixed

C'^ 250 mV) . This is adequate as a signal to apply directly to the sampler.

However when this pulse is passed through a 40 dB attenuator the output

pulse applied to the sampler is only 2.5 mV. The noise level of the

sampler is of the order of 10 mV, thus a very poor signal to noise (S/N)

ratio results. The output amplitude from a mercury switch generator may be

independently adjusted by simply varying the power supply voltage. With

this it is possible to maintain the same S/N ratio for both the attenuator

input and output waveforms. The input waveform amplitude could be set up

for say 500 mV and measured. The attenuator would then be inserted. The

power supply dc voltage would then be increased precisely by the attenuator's nominal value. For example, for a 40 dB attenuator the dc

voltage would be increased from 500 mV to 50 V. These dc voltages can be

3
measured very accurately. The output from the attenuator would then still be

approximately 500 mV ; thus perserving the S/N ratio. The results of this

measurement and the FFT would then give the attenuator's deviation from its

nominal value

.

Several different housings for the microminiature mercury switch were investigated. They included: (1) DIP integrated circuit packages; (2)

7 mm coaxial line; (3) 14 mm coaxial line; (4) microstrip; and (5) stripline.

In most all instances, considerable difficulty was encountered in obtaining

suitable magnetic circuits for proper functioning of the switch.

The microminiature mercury switches have in this investigation been

found to give very unreliable performance. When this project was first con- ceived in 1970, switches were immediately purchased. At that time the

manufacturer was selling a high reliability series of switches. Those

switches gave uniformly good performance and reliability. On the basis of

our initial tests at that time we made a proposal to the CCG to support this

project. The initial batch of switches was soon exhausted due to repeated

use and also breakage during development of various coaxial housings. The

switch is very fragile. In the meantime the manufacturer discontinued the

high reliability series. Subsequent purchases of switches have not performed

well. They have given erratic behavior. Some have required rather strong

magnetic fields to activate. Many have exhibited contact bounce indicating

a dry metal contact and an apparent insufficiency of mercury. Many of

the switches that did not bounce were plagued by a contact resistance Rqj^

that varied wildly from one closure to the next. Variations in Rqj^ from

milliohms to 25 ohms were observed.

Several measurement techniques were used to measure the transition

times of the generated pulses. They included: (1) an ordinary delay

line and a sampling oscilloscope; (2) a random sampling oscilloscope;

(3) a pulse autocorre later ; and (4) a superconducting delay line and a sampling oscilloscope. The superconducting delay line gave the best

re suit s .

4
A superconducting 70 ns delay line system was assembled. With this delay

line and our 22 ps sampling oscilloscope we were able to observe the

mercury switch electrical transition time. The 10%-90l transition time

of the superconducting line and its associated input/output lines and

various connectors and adapters was 18 ps . Thus the measurement system

transition time was 28 ps

.

The objective of this project was achieved in one instance. The best

result obtained was a transition time of 39 ps . Pulse amplitudes up to

50 volts were possible and were independently adjustable.

II. FLAT PULSE GENERATOR CONCEPT

With conventional pulse generators it is difficult to obtain a pulse

with a topline that is known to be absolutely flat. One of the common

design defects is a varying load (pulse on-off) on the generator's power

supply. Depending upon the power supply's dynamic regulation some aberrations may appear in the generated topline and/or baseline. Other possible

sources of aberrations are parasitic reactances, charge storage times, etc.

associated with the active devices used in the generator.

One technique used by the author [12] to obtain a flat pulse consisted

of a simple diode switch, figure 2-1. Initially the switch S-^ (in reality

a PNP transistor operated in saturation or cutoff) is open. The constant

current I^ passes through the diode and R-^ producing the generator baseline

voltage -I R, , IVhen the switch S, is closed, the diode becomes reverse- ' o 1 1

' biased thus disconnecting I^ from R^. The output voltage thus changes

rapidly to its topline value of zero volts. The pulse generator system has

a source resistance equal to R-^. A constant current I^ is always drawn from

the independent power supply -V. Thus the topline and baseline determining

elements are completely free from power supply dynamic regulation problems.

The only uncertainty in this circuit is in the transition region due to the

switching transients of S-j^ and the charge storage time in the diode. By

using a Schottky diode the charge storage time may be minimized. With this

circuit very flat pulses are obtained 20 ns after the positive going transition.

5
To extend the flat pulse concept to the subnanos econd region the mercury

wetted switch was attractive. Figure 2-2 proposes such a flat pulse generator. Again the power supply (+V) supplies a constant current irregardless

of the switch condition (open or closed). With the switch open, the baseline is zero volts. When the switch closes, the equivalent generator topline

is IqR^* equivalent generator source resistance is R^,

" '

" ' III. PRELIMINARY WORK

When this project was first conceived in 1970, some microminiature

mercury switches were initially purchased. Basic switches, figure 3-1, and

switches packaged in a 14 pin DIP IC package along with their associated

electromagnet coils were obtained. Only top -of - the - line , high reliability

switches were purchased.

Immediately a simple generator was constructed using an IC packaged

switch. The circuit diagram of the generator is shown in figure 3-2. The

flat pulse concept of chapter II was utilized. The 1.5 Kfi resistor and the

+15V supply function as the constant current source I^. The IC switch is

driven directly by TTL logic square waves which was similar to [13].

This generator performed very reliably. It was possible to operate

this switch at repetition rates up to 350 Hz without drop-out, contact

bounce, or intermittent closures.

To evaluate the transition time characteristics of the mercury switch

pulse generators, the measurement setup of figure 3-3 was used. The delay

line is necessary because the generators do not furnish a pretrigger. The

oscilloscope must be triggered 70 ns prior to the event of interest with

at most a few picoseconds of jitter. It is impossible to use the driving

square wave as a trigger as the switches typically exhibit 200 ys of jitter

relative to it. The transient step response of this measurement setup is

shown in figure 3-4. This was obtained by pulsing the system with an ultrafast tunnel diode pulse generator (t < 23 ps) . The system transition

6
time (10^-901) is 154 ps , The overshoot and ringing is due to the phase

dispersion of the large diameter, semi-solid coaxial delay line [14].

Figure 3-5 is the measured leading edge of the pulse from the IC switch

generator. The transition time is 0.7 ns . The reason for the relatively

slow transition time is the unavoidable parasitic reactances associated with

the IC package. The switch element is surrounded by the electromagnet coils

which is a far cry from the ideal coaxial line or stripline geometry.

IV. ALTERNATE SWITCH CONFIGURATIONS STUDIED

After the very encouraging results obtained with the IC switch, efforts

were then directed toward using the basic microminiature switch element in

a coaxial holder. The objective was to eliminate the parasitic reactances

that were limiting the transition time of the IC packaged switch. By putting

the switch in a very clean, coaxial environment, we felt we might achieve

the 12 ps transition time reported by Myers [15]. A suitable holder was fabricated using precision 7 mm coaxial air line. However extreme difficulty was encountered in developing a suitable magnetic circuit to

reliably activate the switch. Numerous arrangements were tried without

success

.

After abandoning the 7 mm coaxial holder, efforts were directed toward

obtaining a workable magnetic circuit. A workable solution was obtained

by rotating a small bar magnet very near the microminiature switch. One

configuration used a small motor with a magnet attached and mounted on a printed circuit board. The switch was soldered by its wire leads onto a

50 ohm microstrip on the board. Transition times of the order of 500 ps were obtained due to the non-optimum mounting of the switch. With selected

switches, good switch closures were obtained. Due to mechanical considerations, vibrations, etc. the maximum repetition rate of this configuration

was rather low, typically 20 Hz or less.

7
In an attempt to improve the electrical environment for the switch, a modified 14 mm, coaxial insertion unit was devised, figure 4-1. The insertion unit slide-on cover was removed. An eight pole disc magnet was mounted on the shaft of a small variable speed hand drill. The magnet was placed in close proximity (4-8 mm) to the switch. This arrangement worked.

However, it was extremely critical in terms of magnet position and motor

speed to obtain a good switch closure. It did not have any long term

stability, i.e., it seldom functioned at one setting for longer than a minute .

' "'

'

Figure 4-2 shows the leading edge of the observed pulse. The observed

transition time is 0.15 ns . The waveform and transition time are almost

identical to that of the measurement system step response, figure 3-4. Thus

this generator's transition time is considerably less than 150 ps but no exact value can be assigned.

V/hile the 14 mm insertion unit was an improvement over the simple

circuit board, it was still not the optimum as shown by TDR and pulse transmission tests. The necessity of removing the outer cover of the insertion

unit raised the characteristic impedance of the unit to 65 ohms. In addition the very small diameter switch (O.D. 1 mm) inserted in the large

diameter center conductor (6.35 mm) created a large inductive discontinuity.

With the switch closed a pulse transmission step response (10%-90%) time of

59 ps was measured. Thus additional improvements were still called for

both in the coaxial housing and in the magnetic circuit.

;„,- •• V. CnMW.RCIAL STRIPLINE SWITCH EVALUATION

In 1972 the manufacturer of the microminiature mercury switches introduced a coaxial switch product line using the microminiature switch elements

[16]. Several of these coaxial switches were purchased for evaluation.

Included was a SPDT coaxial switch, figure 5-1, which was the top-of-theline product rated especially for the transmission of fast (t^ < 100 ps)

pulses. This switch was also rather expensive (= $115).

8
Although the switches were advertised as being coaxial switches, they

were not built in a true circular coaxial configuration. The microminiature switch element was mounted as the center conductor in a triplate stripline configuration. The input/output connections were made

through 3 mm SMA coaxial connectors. The two electromagnet switching coils were mounted on the outside and adjacent to one of the two ground planes [17].

To complete the magnetic circuit a small ferrite toroid was slipped over the

center of the microminiature switch element.

Pulse transmission and TDR tests were performed on this stripline

switch. For the purposes of the remaining discussion port 2 is defined as the center wiper of the SPDT configuration. Port 1 is defined as the arm

of the SPDT switch which has the above mentioned ferrite toroid slipped over

it. The remaining arm is defined as port 3. Figure 5-2 shows the TDR

characteristics of this switch. The TDR test instrument was looking into

port 2. The switch was connected to either port 1 or port 3 with the con- nected port terminated in a 3 mm , 50 ohm termination. The presence of the

ferrite toroid is definitely shown on the port 2 to 1 TDR trace by the

large negative dip.

From the TDR information one would intuitively reason that the transmission bandwidth from port 2 to 1 would be less than from port 2 to 3

.

Figure 5-3 demonstrates that such is indeed the case. The same sampling

oscilloscope and tunnel diode used in the TDR tests were used for the

transmission tests. The input transition time (fig. 5-3a) was 30 ps

.

For port 2 to 1 the observed transition time (fig. 5-3b) was 57 ps. Removing the input transition time a value of 49 ps is obtained for this

half of the switch. For port 2 to 3 the observed transition time was 52 ps

giving a value of 43 ps for the switch.

Problems were encountered in obtaining reliable operation of this

switch. The switch gave very erratic switch closures. It was plagued with

contact bounce symptomatic of dry contacts. IVhen the switch did close,

its contact resistance many times varied wildly from a few milliohms to as much as 25 ohms. To obtain a good mercury wetted contact closure it was

9
found necessary to adjust the electromagnet driving current square wave frequency to match a mechanical resonance of the switch. For this particular switch, this resonance was found to be in the vicinity of 140-

150 Hz. However, even operating at this frequency did not assure con- tinuously reliable closures as the switch was still prone to erratic

behavior. . The output pulse waveform from this switch when it was operated

as a pulse generator was measured with several different techniques.

They were: (1) an ordinary delay line setup, figure 3-3; (2) a random

sampling oscilloscope; (3) a pulse autocorrelator ; and (4) a superconducting delay line and sampling oscilloscope.

Using the same delay line setup as mentioned earlier, figure 3-3,

the result shown in figure 5-4 was obtained. The result is almost identical to that of the measurement system step response measured earlier,

figure 3-4. The conclusion from this first test was that the mercury

switch generator 101-90^ transition time was easily less than 50 ps

.

The second transition time measurement method used a random sampling

oscilloscope specifically constructed at NBS for this purpose [18,19].

Due to the random measurement process the probability P of obtaining a valid sampled data point is quite small.

P = t /T w

T is the period of the pulse generator. t is the time window of interest.

In this case the time window is positioned to include the fast rising,

leading edge transition. Although the random sampling time base con- tained a trigger arrival time prediction circuit (i.e., a phase locked

oscillator) , this feature did not appreciably increase P due to the

extreme pulse to pulse jitter (^'^ 200 ys) . This circuit did function

properly with better behaved waveforms.

The random sampling time base was capable of sweep speeds as fast

as 10 ps/div. Due to the inability of the trigger predictor circuit to

increase P, considerations of data acquisition time forced us to use a wider than desired time window of 6 ns (i.e., 0.6 ns/div.). Figure 5-5

10
is the result of this measurement. The data acquisition time for this photograph was 120 minutes. The transition time is seen to be less than 50 ps

.

A 30 ps feedthrough sampler was used. The long term timing drift for this

NBS time base was less than 15 ps/hr.

Measurements were attempted using the NBS pulse autocorrelator [20-23].

This instrument utilized a pyroelectric square law detector and a sliding

short. Its theory, construction and operation are fully described in

references [21,22]. It was not possible to obtain reliable, repeatable

measurements of the generator transition time. This was due to the erratic

performance of the switch. Fifty volt pulses were used. The pyroelectric

detector electrical output was a very low level signal. Phase-lock signal

detection amplifiers were required to recover the detector signal. The

erratic switch behavior seriously degraded the phase-lock system's performance. From the data obtained, it did however appear that the transition

time was less than 100 ps

.

The results of the measurements using the ordinary delay line, random

sampling, and the pulse autocorrelator were all inconclusive. They all indicated that the transition time was less than 50 ps but could not assign an exact value. The remaining alternative was to use a superconducting delay

line and our fastest sampling oscilloscope.

In 1960 Nahman and Gooch [24] first proposed the use of superconductivity as a means of drastically reducing the metal losses in a coaxial delay line. In 1968 Elliott [25] used a Pb-Nb superconducting line.

He obtained a 30 ps transition time with a 100 ns delay. The most recent

progress in developing high quality, miniature, superconducting coaxial

lines has been accomplished in Japan [26].

We have obtained a superconducting coaxial line similar to that re- ported by Nahman [26] in his figure 1, curve A. The inner and outer

conductors are lead plated copper. The dielectric is f luorethylene

propylene (FEP) with a diameter of 1.6 mm. A 15 meter length is used to

give a nominal delay of 70 ns . The line was wrapped into a 10 cm diameter

11
coil for immersion into a liquid helium bath. The line was fitted with

slightly modified 3 mm 9^1A connectors. The 50 ohm input and output air lines

for the cryostat were 7 mm O.D. , 50 cm long with precision 7 mm connectors

at each end. Silver plated, rhodium flashed, brass tubing was used to

minimize the ohmic losses and provide the maximum bandwidth. To minimize

the heat leak into the liquid helium, very thin wall (0.5 mm) tubing was

used. Precision 7 mm to 3 mm SMA adapters were used to connect the superconducting line and the input/output air lines.

This delay line gave excellent performance as shown by the pulse transmission test, figure 5-6. For these measurements a tunnel diode pulse

generator and sampling oscilloscope having 101-901 transition times of 15 ps

and 20 ps respectively were used. The observed system transition time is

31 ps . Thus the transition time of the total delay line package is 18 ps

.

The two input/output air lines and the two 7 mm/3 mm adapters together had

a transition time of 14 ps . TDR measurements of the two 3 mm connectors

showed series inductive discontinuities of 0.3 nH. These inductive discontinuities yielded a transition time of 9.3 ps. Thus, the transition

time of the superconducting line itself is deduced to be of the order of

8 ps or less

.

Swept frequency measurements were made of the insertion loss of the

delay line. At 1 GHz an insertion loss of 0.21 dB was obtained. Extrapolated to 1 km this yielded an attenuation coefficient of 14 dB/km. At

10 GHz a minimum insertion loss of 0.8 dB was noted with a 0.8 dB ripple.

The measurement setup shown in figure 5-7 was used. To avoid degrading the generated pulse by a trigger pickoff at the input to the delay

line, as is normally done, the trigger pickoff was placed in the charge

line 1 ns away from the switch. When the switch closed (2-3), a positive

pulse was generated at terminal 3 and propagated into the superconducting

line. A negative pulse propagated away from the switch back down the

charge line. This negative pulse was used to trigger the sampling

oscilloscope

.

. 12
Using the commercial stripline mercury switch the result shown in

figure 5-8 was obtained. The observed transition time is 48 ps . After

removing the transition times of the delay line and the sampling oscilloscope, a value of 39 ps was obtained for the mercury switch. This

compares closely with the 43 ps value obtained for pulse transmission

through the switch, figure 5-2.

The close correlation between the transition times obtained for pulse

transmission through the closed switch and operation as a pulse generator

lead to a significant conclusion. The basic physical mechanism of a mercury

wetted contact closure is thus shown to be considerably faster than 40 ps

.

The factor limiting the transition time of this particular switch is the

parasitic reactances associated with the switch and its housing.

Considerable difficulty was encountered in the above measurement due

to the erratic behavior of the switch. The switch would not operate con- tinuously long enough to obtain an X-Y recorder trace of the waveform. A

decision was then made to replace the switch element with a new one. The

nev/ switch element was mounted exactly as the old one. This switch was also

erratic but not as bad as the original one. However when this switch was operated as a pulse generator, the output waveform, figure 5-9, was deteriorated. The initial transition is comparable in speed ('^ 50 ps) to

figure 5-8 but the pulse top has a very long dribble-up.

VI . NBS STRIPLINE SWITCH

The transition time achieved with the commercial stripline switch

discussed in chapter V was encouraging. It was felt that we could improve

upon the stripline housing for the microminiature switch and thus obtain

a faster transition time.

The design of the commercial stripline housing had several shortcomings:

1, The SPDT configuration necessitated an abrupt right angle bend

in the signal path.

13
2. . The lanconnected portion o£ the center o£ the switch element appeared

as a short open-circuited transmission line connected to the

signal path.

3. The presence of the ferrite toroid in the signal path.

4. The switch element was soldered to the input/output 50 ohm striplines by wire leads welded to the switch. These wires introduced

additional series inductance.

To meet these shortcomings several steps were taken. For items 1 and

2 a SPST in-line configuration was chosen. The center contact and one end terminal were shorted together by coating half of the switch with con- ducting silver paint, figure 6-1.

For item 3, the ferrite toroid was eliminated. To replace the toroid

in the magnetic circuit, a small bias magnet was mounted on the opposite

side of the board from the driving electromagnetic coils. It was necessary to adjust the position of this magnet to obtain proper operation with

each particular microminiature switch element.

For item 4, the wire leads were removed. The microminiature switch

was held in place by two small sockets originally designed as printed

circuit sockets for individual transistor leads.

Figure 6-2 shows the NBS in-line stripline housing for the modified

microminiature mercury switch in both assembled and disassembled views.

The switch is mounted in the large opening in the two circuit boards.

The voids are then refilled with pieces of FEP and small strips of brass

to reestablish a uniform 50 Q characteristic impedance throughout the

switch housing. A 30 ps TDR instrument was used for this purpose.

Pulse transmission and TDR measurement results are shown in figures

6-3 and 6-4. After removing the input transition time, a 10^-90^ transition time of 34 ps was obtained for this switch housing. This is 9 ps

faster than the value obtained for the commercial housing. Even faster

housings would be possible to obtain with further effort. The attenuation

seen between the input and output waveforms, figure 6-4, is due to the

8 ohms series resistance introduced by the imperfectly conducting silver

paint on the switch.

14
Many problems were encountered in obtaining a good microminiature

mercury switch to use in this housing. The original stock o£ high

reliability switches had been depleted by the time this housing was

developed. This was due to their fragile nature and consequent breakage

in the design phases of this project. When a new stock of switches were

ordered, the manufacturer no longer sold a high reliability version. This

new batch of switches without exception gave very erratic behavior. The

manufacturer was contacted concerning the problem. He offered to sell

us a quantity of nonadvertised , specially selected switches that they

normally reserve for their products requiring their highest quality switches.

Some of these were purchased. They worked somewhat better than the

previous batch. They were still plagued by dry contacts and the necessity

to operate at a mechanical resonance.

The best switch available was assembled into the NBS stripline switch

housing. The output pulse waveform from this switch when it was operated

as a pulse generator was measured using the superconducting delay line

technique described in chapter V, figure 5-6. The results are shown in

figure 6-5. The 101-90% transition time is 80 ps

.

The best results obtained with the commercial switch, figure 5-7,

showed that for a properly functioning mercury wetted contact there was

a close correlation between the transition times obtained for pulse transmission through the closed switch and operation as a pulse generator.

The transmission results obtained with the NBS stripline indicate the

potential for obtaining generated pulse transition times of 34 ps or faster. The actual value obtained of 80 ps clearly indicates that the

mercury wetted contact was not functioning properly. This was also apparent from the erratic function of the switch.

An interesting effect was observed on the opening of the switch.

The pulse trailing edge transition exhibited a long slow ('^^ 4 ns) sag,

figure 6-6. This can be explained physically. As the solid metal sliding

contact starts to move away from the fixed contact, a steadily lengthening

filament of mercury maintains electrical contact. As time progresses,

15
the filament becomes longer and narrower thus increasing its resistance. This

resistance acts to steadily decrease the output voltage. Ultimately the filament breaks completely interrupting the output voltage.

- VII. CLASSICAL MERCURY SWITCH PULSE GENERATOR

The major goal of this project was to construct an extremely fast pulse

generator using the radically different microminiature mercury switch

element, figure 3-1. As a comparison, a pulse generator was also built

using a mercury switch of classical construction.

A classical reed mercury switch is shown in figure 7-1. It consists

of a fixed and a movable flat reed sealed in a glass envelope. A pool of

mercury is also sealed in the envelope. The mercury wets all of the metal

surfaces. The large pool serves to constantly replenish the mercury on the

contacts

.

A simple 3 mm coaxial housing for this switch was built, figure 7-2.

Individual transistor lead sockets were soldered to the long center pins of

the 3mm Sf4A jacks. The mercury switch was held in place by its wire leads

in the sockets. The outer conductor was a 6.35 cm length of 3.4 mm I.D.

brass tubing. The mercury switch made a slip fit inside this tubing. A

5 volt relay coil was then slipped over the outer conductor. This switch

could then be driven directly by a TTL power buffer (7440).

The switch housing was designed to provide a 50 ohm impedance up to

the switch package itself. It was impossible to maintain a 50 ohm impedance

through the glass enclosed portion of the switch. The minimum outer con- ductor diameter was fixed by the glass envelope diameter. In the flat

moving reed region, the reed dimensions are such as to cause a higher

characteristic impedance (> 50 Q). The mercury pool has a much larger

equivalent diameter and thus has a characteristic impedance considerably

less than 50 ohms. That such is the case is shown by the TDR measurement,

figure 7-3.

16
The large capacitive dip due to the mercury pool is seen to be the major

reactive discontinuity. It would be possible to reduce the magnitude of this

dip by carefully tailoring the outer conductor diameter to follow the geometry

of the mercury pool. However the shape of the mercury pool varies with each

individual switch and is also dependent upon the physical orientation of the

switch. Thus to optimize this a special housing would have to be designed

for each individual switch. B. Elliot [27] has done this and was able to

obtain a generated pulse transition time of 70 ps

.

With the non - opt imized 3 mm switch housing, figure 7-2, pulse transmission and pulse generation tests were performed. For pulse transmission a 10%-90% transition time of 0.10 ns was obtained, figure 7-4. The output

pulse waveform, figure 7-5, from this switch when it was operated as a pulse generator was measured using the superconducting delay line technique.

The 10%-90% transition time is 0.21 ns

.

No problems were encountered with this classical switch. It functioned

very reliably. It would switch at frequencies up to 180 Hz. It has been used

to generate 150 ns pulses of up to 25 volts without degradation.

VIII. CONCLUSIONS

Several conclusions can be drawn from this work:

1. The basic physical mechanism of a mercury wetted contact is

exceedingly fast (<< 40 ps)

.

2. Generated pulse transition times of 39 ps may be obtained from a commercial switch using a microminiature mercury switch element.

3. The reliability of the microminiature mercury switch element is

very poor.

4. The microminiature mercury switch element is totally unsuitable

for use by NBS in any standards such as the reference waveform

generator due to the poor reliability and erratic behavior.

5. For times greater than 500 ps the classical reed mercury switch

should be used.

8

17
REFERENCES

[I] D.A. Lewis and F.D. Wells, Millimicrosecond Pulse Techniques , Pergamon

Press, New York, 19 55.

[2] E.J. Martin, Jr., "Calibrated Source o£ Millimicrosecond Pulses,"

Electronics, April 17, 1959, pp. 56-57.

[3] B.J. Elliott, "Picosecond Pulse Measurement o£ the Conduction Current

Versus Voltage Characteristics of Semiconductor Materials with Bulk

Negative Differential Conductivity," IEEE Trans, on Inst. § Meas.,

Vol. IM-17, No. 4, Dec. 1968, pp. 330-332. '

[4] F. Meyer, "Impulsmessplatz Mit Einer Ziet auflosung Von 18 ps

,"

Electronic Letters, Vol. 6, No. 22, Oct. 29, 1970, pp. 703-704.

[5] E. Donath, "Logcell, a New Switch Element Based on Mercury Film

Mechanics ," Proceedings 1966 Electronic Components Conf., Washington,

D.C., lEEE-EIA, pp. 438-441.

[6] J. P. Breicker, "A Mercury Relay Which Operates in Any Plane," 17th

Annual National Relay Conference," Stillwater, Oklahoma, April 1969,

pp. 23-1 to 23-9.

[7] F. Meyer, "Ein Impulsme 3 platz mit einer Zeitauf losung von 18

Pikos ekunden , " Archiv fiir Elektrot echnik , 19 73 , pp. 237 -245.

[8] F. Meyer, "Analyse von Impulsf lanken mit einer Zeitauf losung von

12 Pikosekunden, " Arch. Elektron § Ubertragungs tech , Vol. 27, No. 1,

Jan. 1973, pp. 19-24.

[9] W.D. McCaa, Jr. and N.S. Nahman, "Generation of Reference Waveforms

by Uniform Lossy Transmission Lines," IEEE Trans. Inst. § Meas.,

Vol. IM-19, No. 4, Nov. 1970, pp. 382-390.

[10] N.S. Nahman, R.M. Jickling, and D.R. Holt, "Reference Waveform

Generation Using Debye Dielectric Dispersion," National Bureau of

Standards, Boulder, NBSIR 73-304, Dec. 1972.

[II] W.L. Cans and N.S. Nahman, "Pulse Testing of RF and Microwave

Components," National Bureau of Standards, Boulder, Interim Report

to CCG, July 1973.

18
[12] J.R. Andrews, T.R. Whittemore, and W.D. McCaa, Jr., "Survey of Present

Waveform Sampling System Limitations," National Bureau of Standards,

Boulder, Unpublished Report to CCG , Feb. 1972, pp. 44-47.

[13] R.F. Fullwood, "A Versatile Mercury Film Pulser," Nuclear Inst. § Methods, 87, 1970, pp. 149-150.

[14] C.H. Manney, Jr., The Frequency and Time Domain Transmission Responses

of Coaxial Lines Having Semi-Solid Dielectric Structures , Ph.D. Thesis,

Univ. of Kansas, Lawrence, April 1970.

[15] F. Meyer, op. cit., [4].

[16] W.A. Schilling, "Have You Tried Switching with Mercury Films?,"

Microwaves, Feb. 19 72 , pp. 46 and 4 8. .

[17] Ibid, figure 3.

[18] J.R. Andrews, "Random Sampling Oscilloscope for the Observation of

Mercury Switch Closure Transition Times," IEEE Trans. Instr. § Meas .

,

Vol. IM-22, No. 4, Dec. 1973, pp. 375-381.

[19] J.R. Andrews, "Random Sampling Oscilloscope Time Base," National

Bureau of Standards, Boulder, NBSIR 73-309 June 1973, Final Report

to CCG.

[20] R.A. Lawton and N.S. Nahman, "Pryoelectr ic Detector Application to

Baseband Pulse Measurements," Electronic Letters, Vol. 8, No. 12

June 15, 1972, pp. 318-320.

[21] R.A. Lawton, The Application of Pyroelectric Detectors to Baseband

Pulse Measurements , Ph.D. Thesis, Univ. of Colorado, Boulder, Aug. 1972.

[22] R.A. Lawton and N.S. Nahman, "Pyroelectric Baseband Pulse Autocorrelator," National Bureau of Standards, Boulder, Unpublished Final

Report to CCG, Oct. 19 72.

[23] R.A. Lawton, "Autocorrelation and Power Measurement with Pyroelectric

and Dielectric Bolometers," IEEE Trans. Inst. § Meas., Vol. IM-22,

No. 4, Dec. 1973, pp. 299-306.

[24] N.S. Nahman and G.M. Gooch, "Nanosecond Response and Attenuation

Characteristics of a Superconductive Coaxial Line," Proc. IRE,

Vol. 48, Nov. 1960, pp. 1852-1854.
[25] B.J. Elliott, op. cit., [3].

[26] N.S. Nahman, "Miniature Superconductive Coaxial Transmission Lines,"

Proc. IEEE, Vol. 61, No. 1, Jan. 1973, pp. 76-79.

[27] B.J. Elliott, op. cit., [3].

20
+v

-V

Fig. 2-1. Diode switch flat pulse generator.

+V

Fig. 2-2. Mercury switch flat pulse generator.

21
Fig. 3-1. Basic SPDT microminiature mercury wetted switch.

22
TTL

fX

n

-4 O—p ^^fpnt

/00SX.

5^^

» c

I 0—1 4-

I

I

Fig. 3-2. DIP IC mercury switch pulse generator.

23
Fig. 3-3. Transition time measurement set-up

f :

1 i

'

" '

'

i" i i .1 ;

'

(••-•

1

i

[ j

....

Fig. 3-4. Transient step response of measurement set-up. Horizontal scale is 100 ps/div.

2 4
Fig. 3-5. Pulse output from DIP IC mercury switch pulse

generator. Horizontal scale is 200 ps/div.

2S
Departme u cm I 4 D 0 lu

Department of Commerce, Boulder

Fig. 4-1. Microminiature mercury switched mounted in a 14 mm

insertion unit.

t

T

T

J.

.1,

!

, , , A. . ,

.

'\

]

j

..... , .

Fig. 4-2. Pulse output of a microminiature switch mounted in

a 14 mm insertion unit. Horizontal scale is

100 ps/div.

26
Fig. 5-1. Microminiature mercury switch in commercial strip

line package.

27
SOsv

Fig. 5-2 TDR characteristics of commercial stripline micro

miniature mercury switch, (a) Port 2 to Port 1.

(b) Port 2 to Port 3.

Fig. 5-3. Fast pulse transmission characteristics of commercial stripline microminiature mercury switch,

(a) Input. (b) Port 2 to Port 1. fc) Port 2 to

Port 3.

28
Fig. 5-4. Pulse output from commercial stripline microminiature mercury switch. Ordinary delay line

measurement set-up, Fig. 3-3. Horizontal scale

is 100 ps/cm.

Fig. 5-5. Pulse output from commercial stripline microminiature mercury switch. Random sampling

measurement. Horizontal scale is 0.6 ns/div.

Data acquisition time of 120 min.

29
30
Fig. 5-7. Mercury switch transition time measurement setup

using a superconducting delay line.

31
Fig. 5-8. Pulse output from commercial stripline microminiature mercury switch. Superconducting delay

line measurement setup, Fig. 5-7. Horizontal

scale is 50 ps/div.

Fig. 5-9. Same as Fig. 5-8 except with replacement switch

element. Horizontal scale is 100 ps/div.

32
Fig. 6-1. Modified microminiature mercury switch. Center

contact and end terminal shorted with silver paint.

33
34
Fig. 6-3, TDR characteristic o£ NBS stripline mercury

switch housing.

(a) Input

(b) Output

Fig. 6-4. Fast pulse transmission characteristic of NBS

stripline mercury switch housing.

35
Fig. 6-5. Pulse output from the NBS stripline mercury

switch pulse generator. Superconducting delay

line measurement setup, Fig. 5-7. Horizontal

scale is 100 ps/div. and 20 ps/div.

Fig. 6-6. Mercury switch trailing edge transition observed

on the opening of the switch. Horizontal scale

is 1 ns/div.

36
cm

ig. 7-1. Classical reed mercury sw

37
Fig. 7-2. 3 mm coaxial housing for a classical reed

mercury switch.

38
Fig. 7-3. TDR characteristic of classical reed mercury

switch in 3 mm coaxial housing. Vertical scale

is 0.1 p/div. Horizontal scale is 100 ps/div.

39
Fig. 7-5. Pulse output from classical reed mercury switch

pulse generator. Superconducting delay line

. . measurement set-up, Fig. 5-7. Horizontal scale

is 100 ps /cm.

40
NBS-1 14A (REV. 7-73)

U.S. DEPT. OF COMM. BIBLIOGRAPHIC DATA

SHEET

1. PUHI.K A I ION OR RI-.PORT NO. NBSIR 74-377

2. Gov't Accession

No.

3. Recipient's Accession No.

4. xn LI- AND SUin iTLl-: Picosecond Pulse Generators Using Microminiature

Mercury Switches

5. Publ ication Date March 1974

6. P(.' r t t)rm in^; t^r>;.ui i / .it 1 1 in ( oi.K 9 7 9 9 0 ( PY ' 7 ^ 272.50 (FY'74)

7. AU rHOR(S)

James R. Andrews

8. I'erlorming Organ. Report No. NBSIR 74-177

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

Washington, D.C. 20234

10. Proiect/ rask/Work linn No. FY'73 2725396

JCX / H ^ 1 ^ ~J O V \) 11. ' ontract./Grant No. DoD CCG 72-66

12. Sponsoring Organization Name and ( omplete Address (Street, City, State, ZIP)

Dept. of Defense/Calibration Coordination Group

c/o Mr. M.L. Fruechtenicht AMSMI-M, Bldg. 5435

U.S. Army Metrology & Calibration Center

Redstone Arsenal, Alabama 358 09

13. Type of Report & l-'erioLi C-ovcred Final Report FY'73

14. Sponsoring Ageniy ( ode

15. SUPPEEMENTARV NOTES

16. ABSTRACT (A 200-word or less factual summary of most significant information. If document includes a significant

bibliography or literature survey, mention it here.) Pulse generators have been built using microminiature mercury

switches. A commercial RF coaxial switch was also evaluated as a pulse

generator. A superconducting delay line (tj- = 18 ps, t^j = 70 ns) and a sampling oscilloscope (tr = 22 ps) were used to measure the generated

pulse 10%-90% transition time. The best result obtained was a transition time of 39 ps. Pulse amplitudes were independently adjustable

up to 50 volts. The microminiature mercury switches in general were

found to give very unreliable operation.

17. KEY WORDS fsix to twelve entries; alphabetical order; capitalize only the first letter of the first key word unless a proper name; separated by semicolons)

Mercury switch; picosecond; pulse generator; pulse measurement;

superconductivity

18. AVAILABILITY Unlimited

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