Under certain circumstances, erroneous time milliampere values may be obtained from the Dynalyzer Display. This is primarily due to the way the initial conditions of the trigger circuit are set in the Percent Mode. By using the Preset trigger level, maximum trigger accuracy will result. Comparison with the Dynalyzer II will be made. High frequency generators may also pose special problems.
The Dynalyzer III has two trigger circuit modes. First there is a PRESET mode. This is quite simple. The trigger circuit will start when there is the first crossing of the trigger source with its level. When using KV trigger, the user would, for example, dial in a fixed KV value that might be 75% of the expected KV value. If the exposure was 125 KVP and 300 mA then the KV trigger value chose would be 94 KVP. 0.75 x 125 KVP = 93.75 KVP or 94 KV selected.
Figure 1
When the exposure ends, and the microprocessor has determined that there is not another pulse in the sequence, as happens in a single phase generator, the exposure time from the first to the last crossing of this threshold is saved, and so is the corresponding MAS data. The milliamps, or average anode current is calculated from
mA = MAS/ TIME.
If the KV value is changed, the trigger level is unaffected. If the KV is lowered below the preset trigger level, the system may not trigger at all. If milliampere triggering were used, then a milliampere value would be entered.
With external trigger, only preset level triggering is used. An error will be displayed if percent trigger is selected. External trigger is used in the Dynalyzer calibration procedure. It also allows the user to set up some custom measurement procedure. Dial in a trigger level corresponding to the trigger source. For a 5V TTL signal, set the dial for "30" = 3.0 volts.
PERCENT triggering is a carry-over from the Dynalyzer II. The concept behind the percent mode was quite simple. Initially, the Dynalyzer is reset. The trigger level is quite low. Any percentage of zero is zero. There is a low default value of trigger level in the percent mode. An offset is added to prevent the system from triggering on noise. When the first exposure is made in a Dynalyzer II, an automatic gain control amplifier adjusts itself to pass the KV waveform through at a fixed amplitude. The trigger waveform is normalized to 10 volts peak. The trigger level is therefore 0 to 10 volts, with 10 volts being 100%. The first exposure primed the system, and the data was discarded. This is because the AGC amplifier gain distorted the waveform on the first exposure. If there is leading edge overshoot, the AGC amplifier will lock onto the highest value.
1st Exposure 2nd
Exposure
AGC distorts first pulse Undistorted
AGC Amplifier of Dynalyzer II % trigger
Figure 2.
In a Dynalyzer II, if there is a sequence of exposures, each at a higher voltage or current than the previous one, the trigger level would rise, but in proportion to the preceding exposure. This system would at least insure that the Dynalyzer II would trigger. The actual trigger level in this sequence would be lower than expected.
If the exposure sequence was decreasing in KV or mA values, then the trigger level would stay fixed (except for leakage effects), at the highest value. The trigger circuit would not know that the KV values were dropping. Thus the reference level would be constant and the waveform would be getting smaller.
How to miss an exposure when KV decreases
Dynalyzer III % Trigger Mode
Figure 3.
It is easy to miss the exposure if the trigger level was high, and the next exposure in the sequence was low. The AGC amplifier would not reset itself, as it only follows increasing signal levels.
The Dynalyzer III is smarter. With a microprocessor, it is possible to calculate a new percentage trigger level from exposure to exposure. The waveform is not processed. The trigger level is supplied by a DAC. The front panel trigger level control indirectly generates the trigger level. Its voltage is read by the analog circuit board and the CPU computes the DAC controls. The first exposure is still in error, as the trigger level is suppressed to an artificially low value, with 100% equivalent to 20 KV, and the range of 25% to 2% is 5 KV. After the first exposure, the trigger level is set to the calculated level. If there is a sequence of exposures, the trigger level will always be a percentage of the preceding exposure. If the steps between exposures are small enough, the system will always trigger.
A low trigger level will usually have a longer exposure time. If the cable lengths are long, and the mA values low, there will be significant discharge time of the cable capacitance.
Effect of cable discharge at low trigger levels
Figure 4.
The Dynalyzer instructions recommend using 75% trigger level for 3 phase equipment and 10-20% for single phase equipment. Our experience shows that when using mA trigger, try to keep the trigger level as high as possible in three phase applications. The higher the trigger level, the closer to the correct mA will be indicated. Experiment... you will find the best correlation of the mA value, especially in three phase or constant potential type equipment. The mA waveform rises and falls faster than the KV waveform. At 20% of the voltage, the mA may be at 80% of final value because x-ray tubes are generally emission limited.
Certain high frequency generators have a slowly rising leading KV waveform to allow the internal stabilization circuitry to operate. Under certain load conditions, the ripple on the leading edge is large enough to cause a Dynalyzer III display to appear to lock up. This is because each trigger pulse causes an interrupt to the CPU, and the display cannot handle too many interrupts. GiCi has developed a modification for these displays. We would be happy to install it on request when your equipment is sent in for calibration.