Zonge GDP-32 - Synchronizing Timing Circuits

GDP-32
II
INSTRUCTION MANUAL
May 2002 Section 6, Page 14
6.2 SYNCHRONIZING TIMING CIRCUITS
FREQUENCY ADJUST AND SYNCHRONIZING OPERATIONS
The timing circuits of the XMT-32 and the GDP-32
II
are comprised of an ultra-stabilized
4.980736 MHz oscillator and a digital counter chain providing a binary sequence of frequency
divisions. Frequencies on the XMT-32 Transmitter Controller are selectable from a push-button
panel switch. Frequencies on the GDP are selectable from a menu screen under the control of
the on-board computer.
To ensure that the timing edge of the transmitter controller (i.e. the transmitter output) and the
receiver waveforms coincide exactly throughout the day, two adjustments must be made:
First, the two 4.980736 MHz crystals must be adjusted to EXACTLY the same frequency
to prevent the phase drift.
Second, the counter chains in the XMT-32 and GDP-32
II
must be reset so that the
switching edges of both occur simultaneously (i.e. the waveforms are synchronized.)
The GDP uses the counter waveform to determine the polarity of the transmitted waveform and
the on/off cycle in the case of time domain. Both of these adjustments require that the
synchronization cable be connected between the XMT-32 and GDP-32
II
.
The synchronization cable connects the GDP-32
II
’s 4.980736 MHz oscillator output to the XMT-
32 phase detector. The detector displays the frequency difference between the two oscillators on
the XMT-32 phase meter. Each complete swing of the meter represents one cycle of 4.980736
MHz or 0.2 microseconds (µsec) of time shift.
The following table shows the phase shift in milliradians (mr) per meter swing that occur at
various frequencies, where:
Period = 1/Frequency
Period/mr = Period/(6.283*1000)
Freq
Period Period/mr mr/swing
8192 Hz
122.0µ
µµ
µsec 0.019µ
µµ
µsec/mr
10.5mr/swing
512 1953.0 0.31 0.62
64 15.6msec 2.5 0.08
8 125.0 19.9 0.01
1 1.0sec 159.0 0.00125
0.125 8.0 1.3 msec/mr 0.00016
The above list shows the importance of accurate frequency adjustments for the acquisition of
accurate high frequency phase measurements. The frequency match is obtained by performing a
Frequency Adjustment as outlined in the section below. Typically, the frequency should be
adjusted so that meter drift is less than one division over a 30 second interval. Drift rates of this
magnitude yield phase drifts of less than one hundredth of the above values.
The synchronization cable also connects and enables the counter chain reset circuitry in both the
GDP-32
II
and XMT-32.
Refer to Figure 6.3 and 6.4a,b for a pictorial description of the frequency adjust and
synchronization operations.