Most Avtech highspeed pulse generators are intended for use with a 50 Ohm load. Some pulse generators are very intolerant of other load impedances. In general, the suitability of the pulse generators depends on:
 The pulse generator's rise time (t_{R}).
 The pulse generator's output impedance (Z_{OUT}).
Basic Rules
As a general rule, pulse generators with rise times of 2 ns or less must be used with 50 Ohm loads. They will not operate properly with non50Ohm loads. The datasheets for these instruments will note that the expected load is 50 Ohms. In some moderatelyfast situations, these instruments can still be used with non50Ohm loads if series or shunt resistance is added to the load to change the overall effective load resistance to 50 Ohms. However, for the very fastest applications (t_{R} ≤ 100 ps), a proper highbandwidth 50 Ohm load is required.
In contrast, most pulsers with rise times in excess of 10 ns are suitable for use with loads of 50 Ohms or higher. The datasheets for these instruments will usually note that the expected load is "R_{L} ≥ 50 Ohms". To minimize ringing, however, it may still be desirable to add series or shunt resistance to the load to change the overall effective load resistance to 50 Ohms. Alternatively, backmatching can be used to minimize ringing.
Most instruments with rise times in the range of 210 ns require 50 Ohm loads, either because the output circuit requires a 50 Ohm impedance to function at all, or because the reflections caused by a non50Ohm load would distort the signal too much. However, there is an exception to this  instruments with output impedances of 50 Ohms (Z_{OUT} = 50 Ohms) can drive non50Ohms loads reasonably well, because the 50 Ohm output impedance provides builtin transmissionline "backmatching"  that is, the reflections caused by the improper load termination are absorbed by the pulse generator, rather than being rereflected. The AVL2B (now discontinued) is the main example of this.
If you are unsure of the suitability of an instrument for your application, contact Avtech for clarification.
Problems with reflections, Medium Speed Pulsers (210ns rise time)
The AV1011B1B is a 100V pulser with 2 ns rise and fall times, and switchable output impedance. In the 100V range, the output impedance is low (near zero). In this mode, it will tolerate a moderate deviation from 50 Ohms, but since its output impedance is not 50 Ohms, reflections will occur on the output waveform because of the impedance mismatch between the 50 Ohm coaxial cabling and the non50Ohm load. The photos below show the AV1011B1B driving 40 Ohm, 50 Ohm, 100 Ohm, and 150 Ohms with a 100V pulse. These loads are connected to the AV1011B1B using an 18 inch (45 cm) length of RG174 coaxial cabling.
AV1011B1B, R_{LOAD}=40 Ohms. Slight distortion, rounded waveform. 50V/div, 20ns/div 

AV1011B1B, R_{LOAD}=50 Ohms. Best waveform, minimal ringing. 50V/div, 20ns/div 

AV1011B1B, R_{LOAD}=100 Ohms. Major ringing. 50V/div, 20ns/div 

AV1011B1B, R_{LOAD}=150 Ohms. Major ringing. 50V/div, 20ns/div 
Effect of Mismatched Loads on High Speed Pulsers (<2ns rise time)
The AVRE3B is a 100V pulser with 0.5 ns rise time and 1 ns fall time. The photos below show the AVRE3B driving 36 Ohm, 50 Ohm, 83 Ohm, and 115 Ohms with a +100V pulse. These loads are connected to the AVRE3B using a 6 inch (15 cm) length of RG174 coaxial cabling. The non50Ohm loads cause the pulser to operate improperly under some conditions (e.g., the fall time increases dramatically for R_{L} = 36 Ohms, and the pulse width increases for R_{L} > 50 Ohms), and reflections appear.
AVRE3B. 100V/div, 5ns/div. R_{LOAD}=36 Ohms. Severely degraded fall time. 

AVRE3B. 100V/div, 5ns/div. R_{LOAD}=50 Ohms. Best waveform, good rise/fall times. 

AVRE3B. 100V/div, 5ns/div. R_{LOAD}=83 Ohms. Reflections, fall time rounding, extended pulse width. 

AVRE3B. 100V/div, 5ns/div. R_{LOAD}=115 Ohms. Reflections, fall time rounding, extended pulse width. 
As the results show, the AVRE3B is only suitable for use with 50 Ohm loads.
Pulsers with 50 Ohm Output Impedance
As note above, the AVL2B (now discontinued) pulser has Z_{OUT} = 50 Ohms, which allows these pulsers to absorb reflections. (The AV1011B1B also has 50 Ohm output impedance in its 50V amplitude range.) The photos below show the AVL2BP driving 36 Ohm, 50 Ohm, 83 Ohm, and 143 Ohms with the amplitude nominally set to +320V. (The actual output amplitude increases for loads greater than 50 Ohms, due to the effect of the nonzero output impedance.) These loads are connected to the AVL2BP using a 12 inch (30 cm) length of RG174 coaxial cabling.
AVL2BP, R_{LOAD}=36 Ohms. Clean pulse, reduved amplitude. 200V/div, 10ns/div 

AVL2BP, R_{LOAD}=50 Ohms. Best waveform, minimal ringing. 200V/div, 10ns/div 

AVL2BP, R_{LOAD}=83 Ohms. Increased amplitude, minor ringing. 200V/div, 10ns/div 

AVL2BP, R_{LOAD}=143 Ohms. Increased amplitude, minor ringing. 200V/div, 10ns/div 
Simple Compensation Techniques
As seen above for the AV1011B1B, the ringing can become very large for non50Ohm loads. Fortunately, if the load impedance is well characterized, the user can add series or shunt resistance to change the total effective resistance of a non50Ohm load to 50 Ohms. For instance, if the load impedance is less than 50 Ohms, add a series resistance to boost the total resistance to 50 Ohms, as shown in the diagram:
If the load impedance is more than 50 Ohms, add a shunt resistance (i.e., in parallel):
Excellent results can be obtained by adding series or shunt resistance, as appropriate. Note, however, that the R_{S} resistor must be located very close to the load resistance (R_{L}), or new impedance mismatches will be introduced.
These techniques are not suitable for the very fastest pulse generators (i.e., t_{R} ≤ 100 ps, because it is very difficult to add the series or shunt resistance in a manner that does not degrade the performance of the circuits at these time scales. That is, the parasitic elements introduced by the extra circuit elements become significant at these bandwidths.
For more information about transmission lines and impedance matching, see Chapter 4 of "HighSpeed Digital Design: A Handbook of Black Magic".
Backmatching Techniques (t_{R} ≥ 10 ns)
Model AV1010B is an example of an instrument whose output impedance can be set to 50 Ohms, for backmatching purposes. Other pulsers can typically be converted to 50 Ohm output impedance by adding a series 50 Ohm resistance at the pulse generator output.
When the pulser has been configured to operate with 50 Ohm output impedance, reflections from a mismatched load will be absorbed by the output impedance. This is useful when driving high impedance loads, as shown below:
AV1010B. 50V/div, 100ns/div. Z_{OUT}=2 Ohms, R_{LOAD}=100 kilohms, 24 inches of coaxial cabling. Severe ringing. 

AV1010B. 50V/div, 100ns/div. Z_{OUT}=50 Ohms, R_{LOAD}=100 kilohms, 24 inches of coaxial cabling. Minimal ringing. 