less the 5 milliseconds to allow for the length of the received seconds pulse.

With a sweep rate of 1 ms/division, for example, greater resolution can be realized by measuring the second zero crossover point of the 5 ms received tick. Although the leading edge of the seconds pulse as broadcast from these stations is "on time", coincident with the NBS or CHU standard, it is difficult to measure because of the slow rise time at the beginning of the burst and distortion due to propagation. For this reason, the second zero crossover should be used. The second zero crossover of the WWV or CHU pulse is delayed exactly 1000 microseconds and the WWVH crossover is delayed 833 microseconds as shown in figure 5.19. This is called the cycle correction.

As an example, assume an operator at a distant receiver location is interested in comparing his time to that of WWVH. The propagation and receiver time delays were measured as 11.7 milliseconds and 300 microseconds respectively. Since the total delay is 12.0 milliseconds (11.7 ms + 0.3 ms), the oscilloscope sweep rate was set at 2 ms/ division for a total sweep time of 20 ms-slightly greater than the propagation delay + receiver delay + 5 ms total. The second zero crossover of the tick was observed and measured 12.5 ms after the sweep was triggered by the WWVH clock.

Note that if a receiving station is located at a distance greater than 3,000 km (1863 miles) from the transmitter, the propagation time delay will exceed 10 ms. (The radio path delay works out to be about 5 microseconds per mile. At 1863 miles, the delay would be at least 9.315 ms. It is usually greater than this due to the fact the HF radio signals bounce off the ionosphere.) This forces use of a scope sweep time of 2 ms/ division and lowers the measurement resolution. The next section describes a method of measurement to overcome this difficulty.

D. Delayed Triggering: An Alternate Method that Doesn't Change the Clock Output

To improve the resolution of measurement, the oscilloscope sweep must be operated as fast as possible. The user does have an option: He can generate a trigger pulse independent of his clock. He then positions the pulse for maximum sweep speed and makes his measurement. But then he must measure the difference between his clock and the trigger pulse. Note: This can be accomplished by using an oscilloscope with a delayed sweep circuit built in or with an outboard trigger generator. The latter method is discussed here, but the delayed sweep scope could be used as discussed in Chapter 4. Reference to the instrument manual will aid in using that technique.

On a typical digital delay generator, a delay dial indicates the delay between the input local clock tick and the output trigger pulse. If the user already has a variable rate divider to produce delayed pulses, a time interval counter can be used instead of the delay generator. In either case, the amount of trigger delay must be accounted for in measuring the total time delay (TD) of the received tick with respect to the local master clock.

Measurements should be made at the same time every day (within ten minutes) for consistent results. A time of day should be selected when it is approximately noon midway between the transmitting station and the receiver's location. For night measurements, a time should be chosen when the midpoint is near midnight. Measurements should not be made near twilight.

The equipment can be connected as shown in figure 5.20. A commercially available

the nearest one-tenth of a millisecond and added to the trigger delay, resulting in an approximate total time delay. If the local clock 1 pps time is exactly coincident with the UTC(NBS) seconds pulse, the total measured time delay will be approximately equal to the sum of the propagation delay time, the receiver delay time (typically 200 - 500 microseconds), and the cycle correction (1000 microseconds for WWV or CHU and 833 microseconds for WWVH).

To further increase the resolution of delay measurement, the oscilloscope sweep rate can be increased to 0.1 ms/division (100 microseconds/division) and the trigger pulse from the generator adjusted to be approximately 500 microseconds less than the total delay time previously measured. At these settings, the second zero crossover of the tick will be somewhere near the midscale of the oscilloscope face.

The vertical centering of of the sweep should be rechecked and centered if necessary. The tick is measured to the nearest 10 microseconds (figure 5.21). The result should be within 100 microseconds of the result obtained at the 1 ms/division sweep rate. If the result of this measurement falls outside this tolerance, then the procedure should be repeated again by measuring the total time delay at a sweep rate of 1 ms/ division. To obtain the time, the equations described earlier should be used.

with more accuracy. The exposures are made when consistently strong and undistorted ticks appear on the oscilloscope. To determine the time, the average of the second zero crossover point of the tick is measured using the same procedure explained above.

In making measurements using this technique, an oscilloscope camera using selfdeveloping film is mandatory. The camera shutter is placed in the time exposure position so that it can be opened and closed manually. The lens opening of the camera, the oscilloscope trace intensity, and the scale illumination must be determined by experiment. Refer to your camera manual.

One of the previously described procedures is followed to obtain the seconds tick. At a sweep rate of 1 ms/division, the shutter is opened before the sweep starts and closed after the sweep ends. This is repeated each second until five overlapping exposures are completed (figure 5.23). The pictures should be taken when the ticks begin to arrive with the least distortion and maximum amplitude.

This procedure can also be used at a faster sweep rate of 100 microseconds/division with the second zero crossover point appearing approximately at midpoint of the trace. (One complete cycle of the tick should be visible-figure 5.24.) Overlapping exposures of the ticks are taken and an average reading is obtained from the photograph.

In every case, the decimal equivalent of a BCD group is derived by multiplying each binary digit times the weight factor of its respective column and then adding the four products together. For instance, the binary sequence 1010 in the 1-2-4-8 scheme means (1 x 1) + (0 × 2) + (1 × 4) + (0 x 8) = 1 + 0 + 4 + 0 = 5, as shown in the table. If fewer than nine decimal digits are needed, one or more of the binary columns may be omitted.

In the standard IRIG-H code, a binary 0 pulse consists of exactly 20 cycles of 100-Hz amplitude modulation (200 milliseconds duration), whereas a binary 1 consists of 50 cycles of 100 Hz (500 milliseconds duration). In the WWV/WWVH broadcast format, however, all tones are suppressed for 40 ms while the seconds pulses are transmitted.

Because the tone suppression applies also to the 100-Hz subcarrier frequency, it has the effect of deleting the first 30-millisecond portion of each binary pulse in the time code. Thus, a binary 0 contains only 17 cycles of 100-Hz amplitude modulation (170 milliseconds duration) and a binary 1 contains 47 cycles of