DATA RECONSTRUCTION 39 
An important factor which affects the performance of systems which 
include a first-order data hold is the phase shift. As stated before, 
feedback control systems generally are low-pass, and instability is the 
result of excessive lagging phase shifts in the control loop. It is noted 
that the average phase shift of the zero-order hold is approximately half 

Fie. 3.10. Component functions of first-order data hold. 


a 
T 
Frequency, radians per sec. 



tan-1 (2 
Ang G, (jw) aa 
Fig. 3.11. Amplitude and phase frequency response of first-order data hold. 
the average phase shift of the first-order hold. For instance, at a fre- 
quency of 27/T radians/sec, the phase shift of the former is —z radians, 
while that of the latter is (—2r + tan! 27) radians, or about —280°. 
For this reason, the inclusion of such a data hold in a feedback control 
system generally complicates the problem of stabilization, so first-order 
holds are not commonly employed in practical feedback control systems. 
