Placement of the Measurement Centers

Chapter 4. Sampled-Line Network Analyzer Theory

4.1 Placement of the Measurement Centers

Given that the sampled-line analyzer is a type of six-port, the best way to examine its operation is to consider the placement in the w-plane of the measurement centers w1 , w2 , . . . , W(n- 2 ) where n is the number of detectors used.

Figure 4.2 shows the theoretical model of the sampled-line analyzer used to calculate the positions of the measurement centers and scale factors. It is assumed that the detectors do not load the transmission line at all. The B's are the electrical lengths of the various sections of transmission line, and the detectors are numbered as shown. Only four detectors are shown here, though there may in general be many more. Detectors 3, 4, and 5 are assumed to have the uniform .\/6-spacing, and detector i has some arbitrary placement on the line. No attenuator is placed between the line and the device under test for this analysis.

4

3 5

z

r

©

^{I ,}

^{I I ,}

^{, I···}

^{I ~}

f4-8i -I

14 85 I

1. 83 I

.~

14 84 I

Figure 4.2 Model of the ideal sampled-line at a single frequency. The detectors have infinite impedance. Their response is the square of the magnitude of the voltage at the point of connection on the line.

As in chapter 3,

( 4.1) (4.2) ( 4.3) The values of (1

= 1/IKl

^{2 and w1}

=

-L/ K are to be found.

The voltage measured by a given sampler is the sum of the left and right travelling voltage waves on the line at the point where the sampler is connected.

Associating this complex voltage with a single travelling wave value, bi, may seem odd, but it is easy to see that such an assignment is valid. The sampler could be replaced with an ideal voltage amplifier which would sample the voltage on

the line and produce a travelling wave at its output with an amplitude equal to the sampled voltage. Keeping this in mind, then, expressions for b3 , .•• , b₆ can be written:

b4 = b2(l

+

re-j284 )

b3 = b2(l

+

re-j283 )e-j(84-83)

b5 = b2(l

+

fe-j2Bs)e-j(84-Bs) b6 = b2(l

+

fe-j2B6)e-j(84-86)

Substituting ( 4.4--4.6) in ( 4.1) gives

(4.4) (4.5) (4.6) (4.7)

(1

+

re-j285)e-j(84-85) = I<(l

+

re-j283)e-j(B4-83)

+

L(l

+

re-j284) (4.8)

e-j(84-85)

+

re-j(B4+Bs) = J{ e-j(B4-83)

+

I<re-j(B4+83)

+

L

+

Lre-j284(4.9)

Since ( 4.9) must be true for all values of

r,

it can be rewritten as two equations:

When these two equations are solved for Kand L they give

and thus

J{ = sin(84 - 85) sin(84 - 83) L = sin(83 - 85) sin( 8_{3 -} 84)

L sin(83 - 85)

W1

= - - =

J{ s.in(84 - 85) 1 sin²(84 - 83) (i =

IKl

² = sin²(84 - 85)

(4.10) (4.11)

( 4.12) ( 4.13)

( 4.14)

( 4.15)

An identical analysis for detector i, placed an electrical distance of ()i from the load gives

sin(B3 - Bi)

W(i-4) = . _sm (B _{4 -} () ) _i ^(4.16) ( 4.17) Finally, from ( 4.4-4.6), the expression for w = b₃/b₄ can be written:

( 4.18) Figure 4.3 shows these results graphically. The circles of constant II'I show how the bilinear transform maps the I'-plane to the w-plane. First, I' = 0 is mapped tow= e-j(B⁴-Ba). Since detectors 3 and 4 are assumed to be >../6 apart, the zero reflection coefficient point maps to e-i 7r /3.

The

r

⁼ 0 point maps to a unity-magnitude w for any diode spacing. The spacing determines the angle of the resulting w. This must be the case since, when I'

=

0, there is no standing wave on the sampled-line, and all detectors observe signals of the same magnitude. The ratio of samples at any two points on the line then has unity magnitude.

The II'I = 1 circle is mapped to the real axis of the w-plane. This can be seen by substituting r

=

ei<I> into ( 4.18). The result is

cos(B3 - </>/2)

w= cos(B4 - </>/2) (4.19) This equation shows that w -+ 0 when there is a standing wave null at sampler 3, and w -+ oo when there is a null at sampler 4, as must be the case for the sampled-line.

Completing the picture, it is observed from equations ( 4.1;1) and ( 4.lG) that all the measurement centers, w1 , •.. , W(n- 2 ), lie along the real axis as well. This

1=j

lm(w)

f'=1

^w₁ ¹

=-j

Re(w)

111=0.3 111=0.4

111=0.5

111=0.6

Figure 4.3 The sampled-line analyzer maps the f-plane to thew-plane as shown here.

can also be seen by examination of the physical configuration. From equa- tion ( 4.3) it is seen that w = w₁ is the point at which P5 goes to zero. On the sampled-line, a zero detector output indicates a standing wave null. Only unity-magnitude I''s produce nulls, and the corresponding w lies on the real w-axis, as noted above.

So the ideal sampled-line network analyzer transforms the

r

plane such that the region of

lfl >

1 is mapped into the upper half of thew-plane (Im w

>

0), and

!fl <

1 is mapped to the (Im w

<

0) region. This has the advantage that when measuring passive circuits, only the data from three detectors need be used to find the value of

r.

These data give two circles, which will intersect at two points.

The ambiguity in the value of w is resolved immediately, however, since only one of these intersections, the one with Im w ::; 0, corresponds to a realizable

r.

^As

noted in chapter 3, this makes the sampled-line network analyzer a "five-port"

network analyzer.

Having the measurement centers lie along a line in the w-plane is a dis- advantage. If it is not known whether the measured

r

has a magnitude greater than or less than one, then the ambiguity cannot be resolved from the diode data directly. Losses in the line and reflections from the diodes do cause the centers not to lie exactly along a line, but the scatter is usually small and cannot be used reliably to resolve the ambiguity. In future versions, a method of resolving the ambiguity will need to be incorporated. Having one diode upstream of the others (toward the generator) with an attenuator between it and the others would be one possibility. This diode would give a center above the real axis of thew-plane.

There is another problem, with the sampled-line analyzer as shown in fig- ure 4.2. Its measurement accuracy for

!fl

~ 1 is bad. Reexamining Figure 4.3, for a value of w near the real axis, all the circles resulting from the measured data intersect at very oblique angles. Thus, a small error due to circuit noise in

one of the circles' radii results in a large error in the determination of the point of intersection.

This problem is reduced by placing an attenuator between the device under test and the sampled-line as noted above. The results are shown in figures 4.4 and 4.5. These show contour plots of error sensitivity in the I'-plane. For three ,\./6-spaced diodes, define Rer

=

J(v1,v2,v3 ) and Imr

=

g(v₁,v2,v3 ) where v_{1 ,}

v2, and V3 are the observed detected voltages. Then the quantity plotted in the following figures is

( 4.20) These plots are in the r-plane. The value of w, with noise added, is found and then transformed into the r-plane. It is interesting to note how the trans- formation process symmetrizes the error around the origin of the I'-plane. The effect of the attenuator is interesting, as well. The error function is bowl-shaped, rising steeply at the edges. Adding attenuation between the analyzer and the device under test degrades measurement accuracy slightly at the minimum of the bowl, but it flattens out the bowl, improving the accuracy near the edge.

An optimum value for the attenuator attached to the sampled-line has not been found. Other considerations come into play, such as A/D quantization error when the transformed circle becomes too small. Experimentally, values ranging from 3 to 6 dB have been found satisfactory with various versions of the sampled-line analyzer.

N

Figure 4.4 Sensitivity of the calculated

r

to noise in the power detectors over the f::::; 1 region with a 0.5 dB attenuator between the sampled-line and the device under test.

Figure 4.5 Sensitivity of the calculated f to noise in the power detectors over the r::; 1 region with a 3 dB attenuator between the sampled-line and the device under test.