Page 498 - Instrumentation Reference Book 3E

Digital frequency and period/time-interval measurement 481

Table 20.9 UK time broadcasts
GBR 16kHz rcidiated.from Rugby (52 22' 13" N 01 ' 10' 25" W)
Power: ERP 65 kW
Transmission modes: AI, FSK (16.00 and 15.95kHzf. and MSK (future)
Time signals: Schedule (UTC) Form of the time signals
5255 to 0300 A 1 type second pulses lasting 100 ms,
0855 to 0900 lengthened to 500ms at the minute
1455 to 1500 The reference point is the start of carrier rise
2055 to 2100 Unintermpted carrier is transmitted for 24 s
There is an interruption for maintenance from 1000 to 1400 from 54 m 30 s and from 0 m 6 s
every Tuesday DUT 1 :CCIR code by double pulses
MSF 60 kHz rndiotedfiarn Rugby
Power: ERP 27 kW
Schedule (UTC) Form of the time signals
Continuous except for an Interruptions of the carrier of 100 ms for the second
interruption for maintenance pulses and of 500ms for the minute pulses. The
from 1000 to 1400 on the first epoch is given by the beginning of the interruption
Tuesday in each month BCD NRZ code, 100 bitds (month, day of month,
hour, minute), during minute interruptions BCD
PWM code, lbit/s (year, month. day of month, day
of week, hour, minute) from seconds 17 to 59 in
each minute DUT1:CCIR code by double pulses
The MSF and GBR transmission are controlled by a cesium beam frequency standard. Accuracy t2 x

20V rms. The trigger circuit has a trigger level crystal and are suitable for use in seven- to nine-
which is either set automatically with respect to digit instruments.
the input wave or can be continuously adjusted over The microprocessor provides control of the
some range. The triser circuit generally employs counting operation and the display and post-
hysteresis to reduce the effect of noise on the wave measurement computation.
form as shown in Figure 20.76(a), although this can Conventional frequency counters count the
cause errors in time measurement, as shown in number of cycles, ni, of the input waveform of
Figure 20.76(b). frequency, A, in a gating period, tg, which corre-
The quartz crystal oscillator in a frequency sponds to a number of counts, no=, of the 10-MHz
counter or universal counter timer can be uncom- crystal oscillator. They have an uncertainty corre-
pensated, temperature compensated, or oven sta- sponding to fl count of the inpat waveform. The
bilized. The frequency stability of quartz relative resolution is given by
oscillators is affected by aging, temperature, var-
iations in supply voltage, and changes in power Relative resolution
supply mode, Le., changing from line-frequency
supply to battery supply. Table '0.10 gives com- Smallest measurable change in
parative figures for the three types of quartz measurement value
oscillator. The uncompensated oscillator gives Measurement value
sufficient accuracy for five- or six-digit measure-
ment in most room-temperature applications. The and for the measurement of frequency is thus
temperaturscompensated oscillator has a tem-
perature-dependent compensating network for 1 1
- f-
frequency correction and can give suficient accu- * Gating period x input frequency -
racy for a six- or seven-digit instrument. Oven- t,
stabilized oscillators maintain the temperature
of the crystal typically at 70 f 0.01 "C. They gen- In order to achieve measurements with good rela-
erally employ higher mass crystals with lower tive resolution for low-frequency signals long
resonant frequencies and operate at an overtone gating times are required. Reciprocal frequency
of their fundamental frequency. They have better counters synchronize the gating time to the input
aging performance than the other two types of waveform, which then becomes an exact number