NBSIR 75-827

REPORT ON NBS DUAL MIXER TIME DIFFERENCE SYSTEM (DMTDj

BUILT FOR TIME-DOMAIN MEASUREMENTS

ASSOCIATED WITH PHASE 1 OF GPS

David W. Allan

mi

Time and Frequency Division

Institute for Basic Standards

National Bureau of Standards

Boulder , Colorado 80302

January 1976
I
NBSIR 75-827

REPORT ON NBS DUAL MIXER TIME OIFFERENCE SYSTEM (DMTD)

BUILT FOR TIME-DOMAIN MEASUREMENTS

ASSOCIATED WITH PHASE 1 OF GPS

David W. Allan

Time and Frequency Division

Institute for Basic Standards

National Bureau of Standards

Boulder , Colorado 80302

January 1976

O w ^ ^4

U.S. DEPARTMENT OF COMMERCE, Rogers C. B. Morton, Secretary

James A. Baker, III, Under Secretary

Dr. Betsy Ancker-Johnson , Assistant Secretary for Science and Technology

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Acting Director

REPORT ON NBS DUAL MIXER TIME DIFFERENCE SYSTEM (DMTD) BUILT FOR TIME-DOMAIN MEASUREMENTS ASSOCIATED WITH PHASE I OF GPS

David W. Allan

Based on a previous work reported at the 1975 Frequency Control Symposium,

the National Bureau of Standards was asked to build a Dual Mixer Time

Difference (DMTD) measuring system. This report includes the design, con- struction, and testing of this DMTD system in fulfillment of this request.

The precision of time difference measurement with this system was shown to

be about 0.1 picosecond and the accuracy about 10 ps; similarly, the

frequency stability precision was shown to be described by

a^(T) = 10"^^ r~\ 0.1s < T < lO^s and equal to 10'^^ at t equal about

half a day.

The request for this DMTD system was for measuring the clocks that will

go on board the satellites for Phase I of the Global Positioning System

(GPS) program. The DMTD system described in this report is the only

system that can easily meet all of the time-domain measurement require- ments for this program.

KEY WORDS: Frequency measurement; Frequency mixing; Global Positioning

System; Isolation amplifiers; Low-noise amplifiers; Precision frequency

measurements; Precision time measurements; Time difference measurements; Time-domain frequency stability; Time interval.

Having gained additional insights from the prototype DMTD system reported on at the 1975

Frequency Control Symposium we made appropriate changes in designing and building a DMTD system to fulfill a request for a time-domain measuring system to be used in Phase I of GPS. These included: a pair of NBS designed isolation amplifiers for splitting the signal from the common oscillator, the signal from the common oscillator is fed through isolation transformers

to the isolation amplifiers; an improved low-pass filter following each mixer; an additional

and lower noise stage of amplification following each low-pass filter, better bandwidth control included in the 2nd stage of amplification; a driver to feed each pulse isolation trans- former for an improved pulse shape; the ability to use or not use the phase shifter in one leg

of the system; both electric and magnetic shielding properly displaced to reduce capacitive

coupling, and non-battery operation using well isolated, regulated, and decoupled DC power

supplies.

A local subcontractor was sought and obtained to do the layout, assembly, and overall con- struction.

Attached are the block and circuit diagrams of the DMTD system as prepared by the subcontractor (No. 75008-Al through 75008-A6). The phase shifter in A-1 has 100 n input impedance

and 10 dB insertion loss. The inputs to the mixers, should not exceed % 3 mW. The low-pass

filter (LPF) has a roll-off at about 10 kHz. The bandwidths f^ of the amplifiers are ganged

and discretely variable (1, 3, 10, 30, 100, 300, 1000 Hz). The isolated output pulses cor- responding with the zero crossings of the beat frequency have about 4 volts output into a high impedance with 20 ns rise time. On sheet 1 of A-2 test point 1 (TP-1) should read 600 mV peak-to-peak for near optimum signal-to-noise performance out of the mixers. The common oscillator input shown in A-4 should not exceed 1 mW (0.6 V peak-to-peak at input of isolation

amplifiers A-5); the amplifier will distort at higher drive levels.
II. Testing and Measurement of DMTD System Noise

A 10.23 MHz quartz crystal oscillator lent to NBS for testing was used to drive both

mixer one and two to estimate the system noise. The oscillator output was insufficient to obtain optimum signal-to-noise conditions, but adequate for testing. The R-port of each mixer

was driven with about 0.6 volt peak-to-peak and the L-port with about 0.36 volt peak-to-peak

giving a mixer output of only 170 mV peak-to-peak, i.e., the signal-to-noise could be about

3 or 4 times better with higher drive levels.

The common oscillator employed was a low-noise synthesizer. One can graphically see the

effects of common oscillator noise by tuning the phase of the two beat signals to near zero. The

attached sheet headed with the date 8 October 1975 page 1 is a copy of the lab notes during the

testing. The left column of data are all taken with the bandwidth, f^, set to 1 kHz and the

beat frequency at 10 Hz -- giving a time difference measurement every 0.1s. The At indicates

the degree of synchronization of the phases of the beat signals, and it is obvious that as they are moved from near zero phase difference toward large phase difference the phase noise in- creases considerably. The right column shows the same effect only with a 10 Hz bandwidth.

The DMTD system noise was measured by setting the phase difference between the beat signals near zero to reduce the effects of common oscillator noise and then using a computing counter to measure the fractional frequency stability in terms of o^{t)

for various bandwidths and beat frequencies. These results are summarized in the attached o^ir)

vs T diagram. Also shown on this diagram are some values translated from some data taken

in the frequency domain. These data were obtained by removing both covers to the instrument

so that a lead could be attached to test point 2 {TP-2 in drawing A-2) on mixer 1 and then

later on mixer 2. A signal from the 10.23 MHz test oscillator was fed into the mixer port through

the phase shifter and also into the common oscillator port; the phase was shifted to obtain quadra

ture (0 volts out of the mixer). The signal from TP-2 was fed into a low-noise amplifier and

into a spectrum analyzer to deduce the frequency domain stability S^(f) of the phase

for this much of the circuit, which should be the critical noise contributor. The re- sults using mixer 2 were essentially the same. Some 60 Hz sidebands were present at a level

of -116 dB, but these apparently disappear when the shielding lids are put back, because such

-12

a level would give a (t= Is) = 7 x 10 , but instead we obtain a 1 second stability better

1 3 " than 1 part in 10 , and with no apparent 60 Hz present when the beat frequency is set to that

value in order to filter it out if it were present. Attached also is a sheet headed with the

date 8 October 1975 page 2 which shows the time difference stability over a couple of minutes

with f^=]Hz and with a 1 second beat, 1/t. A slight phase drift is observed. A test was conducted from a cold start of the instrument to ascertain the long-term time difference stability of the unit. During the first hour it drifted '\; 50 ps and then in an over- night run the phase stayed within a peak-to-peak deviation of 10 ps with ambient temperature

fluctuations of about ± 2C°. This would correspond to a worst case one day sample time

fractional frequency departure of about one part in 10^^.

2
III. Usage of the NBS DMTD Instrument

Attached are four sequential photographs of the instrument: 1) Front panel; 2) Top view

with electric shield cover removed; 3) Top view with electric and magnetic shield covers

removed; and 4) Close up of contents in magnetic shield compartment. The instrument is broadband 1 MHz - 60 MHz) except for the phase shifter which covers ± 1 MHz about 10.23 MHz. One may conveniently choose whether or not to use the phase shifter. If used it provides a con- venient method of setting the output time difference to any desired point, and if the com- mon oscillator noise is excessive one can set the beats in phase (AT - 0), so that one obtains a reduction of the noise contribution coming from the common oscillator. The phase

shifter has the disadvantage that it has 10 dB insertion loss.

The mixer inputs go directly to the low noise Schotcky barrier diode double balanced

mixers and optimally should be driven with about 1 to 3 mW. CAUTION: excessive drive levels

(maximum current 40 mA or about 2V ) on the mixers will burn them out. About 1 volt peakrms to-peak input is a good operating point. The common oscillator port can be driven up to about 0.6 volts peak-to-peak ('v .3 mW) before the isolation amplifiers, which feed the other

ports of the mixers, start to distort.

The measurement bandwidth f^ should always be greater than or equal to the beat frequency v^. It is sometimes convenient to change the bandwidth and see its effect on the

stability to get an idea of the kind of noise process that may be dominant. If the bandwidth is set too small, the stability determined will be too good for sample times

less than 1/f^

The output pulses have about a 20 ns rise time and about 4 volts height into a high impedance. The steepest slope and ideal trigger point is a + slope at about + 1 volt. The pulses will

work into a 50 load with the amplitude and rise time degraded by about a factor of 2. These

output pulses are respectively tied to the phases of oscillator 1 and oscillator 2, and the time

difference > is given byx(i) between the zero crossings of these two oscillators for the i^*^ measurement

0 0

where T-] - T^ = AT is the actual time difference read on a time interval counter with T-j connected to the start jack and T^ to the stop jack, where (t> is the total phase difference

between the two halves of the dual mixer time difference system--most of which will be that

due to the phase shifter, and where Vj^and are the nominal beat frequencies and carrier

frequencies respectively. As an example suppose that AT = 25.3us for the i^^ measurement

and = 10 Hz and = 10 MHz, then the time difference of osc 1 - osc 2 = 25.3 ps

+

2jiy

• Suppose, for the (i + l)^*^ measurement, we get 25.5 ps + —> which is 100 ms

"^0 ^0 later, then the average fractional frequency difference over that interval is given by:

3
y(i x(i+1) - x(i ) (2)

100 ms

^

The stability may be computed from the frequency or the time data as follows:

M-1

i = l M-1 or -^i [x(i+2) - 2 x(i+l) + x(i)]^

i = l

(3)

(4)

where there are M+1 time readings in a sequence. If one wishes to take the square root and

compute direct from the AT readings of a time interval counter then:

^ yiTM-l) T V.

M-1

i = l

[at (i+2) - 2 AT(i+l) + AT(i)]^ \

1/2

(5)

where x (the interval between time difference measurements) can be any integer multiple of

l/v)j^; i.e., one does not have to measure at every cycle of the beat frequency. Equation 5 can be approximately summarized for the measurement noise of the DMTD system as:

0.1 ps

, 0.1s < T < 10s

with only a slight degradation for longer times due to apparent temperature induced phase

drifts and for shorter times due to white noise as the bandwidth f^^ is opened up.

IV. Maintenance

The unit should be free of need for any maintenance procedure. The OP-05 operational

amplifiers were DC balanced during the NBS testing phase, and their specifications of

S.ByV per month should be low enough to give no difficulty in the foreseeable future unless

the ultimate finesse is wanted from this DMTD instrument. These operational amplifiers are zeroed by simply grounding the input test point, and while monitoring the output test point

of that operational amplifier adjusting the trim pot associated with it for zero volts DC output.

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8 October 75

DMTD test of system noise as a function of common oscillator rejection

10.23 feed through (j)-shifter to mixer 1 and split before ^-shifter to mixer 2 Same levels as yesterday. Set the beat = 10 Hz

;. T

= 0.1s and plotted the phase for various shifts

or At readings on counter. Chart speed = 10 cm/mi

n

Sensitivity 1 ps per cm

^^t ^2 ys

T 0. 1 >

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8 October 75

Same oscillator drive levels and condition as before. At % few ys to reduce common oscillator noise contribution to test measurement system

noise and phase drift.

Warm up phase drift was about 50 ps over the first hour; but stabilizing

to a peak of about 10 ps thereafter in an overnight run with + 2° C ambient temperature fluctuations.

13


16
17
18
NBS-1 14A (REV. 7-73)

U.S. DEPT. OF COMM. BIBLIOGRAPHIC DATA

SHEET

1. PUBLICATION OR REPORT NO. NBSIR 75-827

2. Gov't Accession

No.

3. Recipient's Accession No.

4. TITLE AND SUBTITLE

Report on NBS Dual Mixer Time Difference System (DMTD)

Built for Time-Domain Measurements Associated with

Phase I of GPS

5. Publication Date January 1976

6. Performing Organization Code 277.04

'-t^^'^W. Allan

8. Performing Orean, R^ort No. NBSIR 75-82/

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

10. Project/Task/Work Unit No. 2774246

11. Contract/Grant No.

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

Same as Item 9

13. Type of Report & Period Covered

14. Sponsoring Agency Code

15. SUPPLEMENTARY NOTES

16. ABSTRACT (A 200-word or less (actual summary ol most si^ilicant ir\(ormation. l( document includes a signidcant

bibliography or literature survey, mention it here.) Based on a previous work reported at the 1975 Frequency Control Symposium,

the National Bureau of Standards was asked to build a Dual Mixer Time Difference

(DMTD) measuring system. This report includes the design, construction, and

testing of this DMTD system in fulfillment of this request. The precision of

time difference measurement with this system was shown to be about 0.1 picosecond

and the accuracy about 10 ps; similarly, the frequency stability precision

was shown to be described by

^ 10"^^ T~\ 0.1s < T < 10"^s and equal to 10'^^ at t equal about half a day.

The request for this DMTD system was for measuring the clocks that will go

on board the satellites for Phase I of the Global Positioning System (GPS) program.

The DMTD system described in this report is the only system that can easily meet

all of the time-domain measurement requirements for this program.

17. KEY WORDS (six to twelve entries; alphabetical order: capitalize only the first letter of the first key word unless a proper name; separated by semicolons) Frequency measurement'. Frequency mixing; Global Positioning

System; Isolation amplifiers; Low-noise amplifiers; Precision frequency measurements;

Precision time measurements; Time difference measurements; Time-domain frequency

stability; Time interval.

18. AVAILABILITY Unlimited 19. SECURITY CLASS

(THIS REPORT)

21. NO. OF PAGES

For Official Distribution. Do Not Release to NTIS UNCLASSIFIED

20

1 Order From Sup. of Doc, U.S. Government Printing Office Washineton. D.C. 20402. SD Cat. No. C13

20. SECURITY CLASS

(THIS PAGE)

22. Price

1 Order From National Technical Information Service (NTIS)

Springfield, Virginia 22151 UNCLASSIFIED

$3.25

USCOMM-OC 29042-P74
1