**VectorAnalyzer60**

1/23/07

Impedance,
reflection, return loss, VSWR. All are ways
of measuring the same thing, and each is useful in its own right. But if you want to measure any of them at RF
frequencies you need a “network analyzer”.
This has nothing to do with common networks, but is rather a way of
measuring the response of a device under test (“DUT”) to an input signal.

The simplest
method is to measure amplitude (magnitude) only—that’s a scalar network
analyzer. The fancier way is to measure
both magnitude and phase—that’s a vector network analyzer. Given an input of an arbitrary magnitude and
phase, what is the magnitude and phase of the output? That’s what a vector network analyzer tells
you. From that information, you can
determine impedance, return loss, VSWR…. lots of good info.

As an extension of
Scotty’s Spectrum Analyzer,
Scotty Sprowls is developing a vector network analyzer (VNA). It includes a digital phase detector intended
to work at 10.7 MHz, or whatever final IF the builder chooses. I modified that phase detector to use higher
speed logic and to include a few other tweaks, resulting in a phase detector
which will work from LF to 60 …80 …100 MHz.
Actually, the LF still requires some work, but it will be somewhere near
100 kHz.

If you precede the
phase detector with a reflection/transmission bridge, and add a log amp to
measure amplitude, you get a very functional VNA. It requires an external signal generator as
the signal source, but the advantage is that it is computer/processor/calibration
free, so it can be used on its own as to provide readouts of magnitude and
phase to an ordinary voltmeter—or a panel meter, if you want to be fancy. I call it the VectorAnalyzer60, though it has
decent functionality to 100 MHz.

Photo 1 shows the
original prototype.

**Photo 1—VectorAnalyzer60
Prototype**

Output to DVM is via
binding posts, which can be replaced

by a digital panel
meter. Blue knob selects phase or

magnitude output. SMA connectors are reflection port and

transmission signal
output. Toggle switch selects reflection
or

transmission
measurement. External signal input and
power

input are on far side.

First, the
basics. If you want to measure
reflection/return loss/VSWR, you stimulate the DUT and measure the “response”,
which comes back as a reflection into the same port that provided the
stimulus. The VectorAnalyzer60 measures
the response as “return loss”, from which you can calculate (or let your
spreadsheet calculate) reflection coefficient, impedance, or VSWR. If the DUT has an input and an output, like
an amplifier, the return loss can be measured for either the input or the
output.

If you want to
measure how the output of the DUT responds to the input, you measure the
transmission characteristics, which requires that the VectorAnalyzer60 stimulate
the DUT input and the measure the DUT output.

As far as
schematics and construction details, I am still working on that, as the device
tested here is a prototype. But you can
download a test and schematic
of the digital phase detector.

Accuracy

A VNA must compare
the magnitude and phase of a test signal to that of the response signal, so it
needs magnitude accuracy and phase accuracy.
For the most common measurements of return loss—those between 0 and 30
db, the VectorAnalyzer60 has worst case accuracy of about 0.5 db and 3 degrees
without calibration. Return losses of 30-40 db may have another 1 db and a few
more degrees of inaccuracy. It is normally not necessary to measure return
losses beyond 40 db. (For reference,
note that most commercial “50-ohm” coax cables do not have return losses better
than 30 or 35 db.)

For transmission
losses, the phase accuracy is about the same but the magnitude accuracy is
likely about 0.1 db for measurements between 0 and 20 db, and 0.5 db beyond
that. In most situations, the accuracy for
both return and transmission losses will be better than those worst case
numbers, especially below 30 MHz.

The digital phase
detector itself is accurate to better than ¼ degree over the full range, and
about 0.1 degree below 20 MHz.

The accuracy of
the instrument can be improved by calibrating it with an open, short and 50-ohm
load (known as “OSL” calibration), and adjusting all subsequent
measurements. BUT MY GOAL IS TO MINIMIZE
CALIBRATION. I am working out a
calibration-by-spreadsheet method, but I want the VectorAnalyzer60 to be
accurate enough to be used in most situations with no calibration.

Actual
Measurements

The most basic
thing a VNA must do is measure return loss, which is another way of measuring
the impedance of a load. Its accuracy is
based primarily on two things: (1) the
measured return loss of a 50-ohm load should be infinite (right!), and (2) the
measured return loss of an open (i.e. no DUT) should be 0 db @ 0 degrees, and
of a short should be 0 db @ 180 degrees (or -180 degrees—you have to get used
to those being the same thing).

So here is how the
VectorAnalyzer60 does on return loss (looking at magnitude only):

**Figure
1—Return Loss accuracy**. Remember that the
convention is for return loss

to be specified as a
positive number. A return loss of 50 db
means the reflected

power is -55 db of the incident
signal, or about 1/500. That’s tiny. So while the

50-ohm DUT should have an
infinite return loss, having a 55 db return loss

is darn good. The more the DUT deviates from 50 ohms, the
more consistent

the return loss
measurement over frequency.

So we can see that
the VectorAnalyzer60 has very high directivity, on the order of 55 db. That means it sees a “ghost reflection” of
-55 db when there should be no reflection.
That is very minor, and not enough to have a significant effect on
normal measurements of return loss.

The whole concept
of return loss is designed to be very sensitive to changes when the DUT is near
50 ohms and less sensitive to changes in DUTs far from 50 ohms. The entire range from infinite db to 40 db is
devoted to DUTs from 49 to 51 ohms. VSWR
is a similar concept, but has less emphasis on the near-50-ohm range. For many purposes, everything in the 40-60
ohm range is treated as being as good as 50 ohms, and the concern is with more
serious deviations, and that is where it makes sense to use VSWR. Figure 2 expresses the same results as Figure
1, except in terms of VSWR rather than return loss.

**Figure
2—VSWR accuracy.** If you prefer VSWR to return loss, here it
is.

Measurement accuracy is
very good, with the biggest errors being at high

VSWRs at high frequency.

Figure 2 shows
that the accuracy of the VectorAnalyzer60 looks even better when viewed as
VSWR, because the effects of small deviations around 50 ohms are no longer
exaggerated.

The second
requirement for accuracy of a VNA is that an open and short DUT should both
have 0 return loss (i.e. 100% reflection), but the phase of the short
reflection should be the negative of that of the open. So the phase difference between the two
should be 180 degrees. Figure 3 shows the
actual numbers for the VectorAnalyzer60.

**Figure 3—**Phase of open vs. short reflection. Should ideally be 180 degrees.

With a max error of about
3 degrees, there is potential for 3 degrees

of error for measurement
of DUTs which are very high or very low

impedance. That’s not bad, and if we want to improve it
we can calibrate.

Again, the small
deviations shown in Figure 3 can be taken care of by calibration, but our goal
is to avoid the need for calibration in all but the most sensitive measurements. One trick to avoid the effects of the
open/short difference is to make measurements of high impedances relative to
the open measurement, and low impedances relative to the short measurement. (I’m not going into the details of the
measurement process in this overview.)

OK, let’s measure
something real here. Attach a 24” piece
of 50-ohm coax cable to the VectorAnalyzer60, leave the other end of the cable
unterminated, and measure the reflection.
The results are in Figure 4.

**Figure 4—**Phase of reflection of 24” unterminated
coax. Error is

multiplied by -10 to make
it visible on this scale, and is based

on deviation from linear
fit, since the phase should ideally be

proportional to frequency. Maximum error is about 1 degree,

and results either from the
VectorAnalyzer60 or the coax itself.

So the measurement
of reflections looks very good. But we
can also measure transmission. Figure 5
shows the measurement of the phase of the signal transmitted by a 15”
coax. The numbers were derived from
comparing the phase of a 24” cable to that of a 9” reference cable.

**Figure 5—**Phase of transmission of 15” coax. The phase changes

linearly with frequency,
as it should, with a minor glitch somewhere

in the area of 48 MHz. The phase at any given frequency should be

15/48 of the phase in
Figure 4, since the coax here is shorter and

the signal is making only
a one-way trip.

Let’s throw some
capacitance in the mix. I measured the
return loss of a precision 47.5 ohm resistor in parallel with a 1% 62pf
capacitor, both surface mount components mounted directly on an SMA connector
attached to the VectorAnalyzer60. See
Figure 6.

**Figure 6—**Return loss of parallel combination of
62pf 1% capacitor

and 47.5 ohm 0.1% resistor. Increased magnitude/phase

error at RL>30db is due
to the fact that no correction is

made for directivity, but
the results are still very good.

Now let’s try a
capacitor with leads, which as we know create some parasitic inductance. Don’t ask why, but I have a 0.5%, 0.01107 uf
capacitor, which is about 1” long and has ½” leads. One lead is attached to the VectorAnalyzer60
and the other is grounded. See Figure 7
for the results.

**Figure 7**—Actual vs. measured phase of precision
capacitor. Adjusted line assumes 1” 120 ohm transmission
line on each end of ideal capacitor

to account for the leads
and body length. Measurements here are
relative

to the phase of a short,
which is 180 degrees.

As can be seen
from Figure 7, the actual performance of the capacitor deviates substantially
from theory, unless you adjust the theory to account for the leads and the body
length of the capacitor. With a very
crude adjustment, the results are very good.

Conclusion

The VectorAnalyzer60
provides very respectable measurements of return loss and transmission characteristics
to at least 60 MHz, without the need for a microprocessor or computer to make
calibration adjustments. If calibration
is needed, it can be accomplished with a spreadsheet, still in progress.

For additional use
of the VectorAnalyzer60, see the test results
of some homemade “hardline” coax.