M A U R Y M I C R O W AV E

C O R P O R A T I O N
July 2009

A Comparison of Harmonic Tuning Methods for

Load Pull Systems
Author: Gary Simpson, MSEE Director of Technical Development in Engineering, Maury Microwave Corporation

Editor's Note: For advances in updated harmonic tuning techniques since this paper was written, please refer
to 5C-081 "Cascading Tuners For High-VSWR and harmonic Load Pull"

Abstract: There are three methods of harmonic tuning that have been offered commercially for load pull systems
with passive automated tuners. Each method has strengths and weaknesses. This paper will examine the three methods
and compare the relative advantages and disadvantages of each.

Introduction Harmonic load pull consists of tuning the source and/or

load impedance at a harmonic frequency (F2 or F3) while
Load pull is widely used to characterize devices for power measuring device performance at F0. Harmonic load pull
amplifier applications because it accurately shows what can especially affect efficiency and linearity. The effect of
the device will do under actual operating conditions. The harmonic tuning depends strongly on the fundamental
measurement is done with bias, RF power, and impedance load pull tuning as well as the device type, operating point,
conditions which are applied in the final application; so drive power, and other factors.
there are no data extrapolation or model validity issue.
An example of harmonic tuning on power added efficiency
Fundamental load pull consists of tuning the source and is shown in Figure 1 (Eff). The efficiency is measured at
load impedances at the fundamental frequency of operation F0 while the load impedance at F2 is varied. The result
(F0). The fundamental impedances are the most important is a 16% change in efficiency, caused by changing only
by far, and that is why fundamental load pull, its accuracy the harmonic load impedance, keeping the fundamental
and repeatability, are most critical. impedance constant.

Figure 1. Power Added efficiency Contours vs. Second Harmonic Load Impedance.

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Description of Harmonic Tuning Methods
The three harmonic tuning methods considered here are summarized in Table 1 below:

Table 1. Harmonic Tuning Methods Summarized

Harmonic Tuning Method Description

Triplexer Use filters to separate the fundamental and harmonic signals so that they may be tuned
independently. A triplexer has a low pass filter for F0, a band pass filter for F2, and a
band pass or high pass filter for F3. a block diagram with source and load harmonic
tuning is shown in Figure 2.

Stub Resonator Use open stubs, quarter-wave at the harmonic, connected to the center conductor with
a sliding contact. A block diagram with load harmonic tuning is shown in Figure 3.
• The F3 stub is closest to the DUT, intending to reflect all F3 signal and pass the
F0 signal.
• The F2 stub follows the F3 stub, intending to reflect the F2 signal and pass the
F0 signal.
• A normal tuner is used to tune the F0 impedance.
• Dual quarter-wave stubs are typically used, since single stubs are very ineffec-
tive.

Cascaded Tuner Use two cascaded tuners with 625 states each, producing nearly 400,000 avail-
able impedance states with the combination. With this many states, multiple states
will produce any specified F0 impedance but will have a variety of F2 impedances.
Second harmonic load pull, for example, consists of measuring a set of states with
approximately constant F0 impedance.

TUNER POWER
F3 METER
T
R
I
P TUNER
L POWER
F2 METER
E
X
E
RF TUNER R
BIAS DUT TUNER POWER
SOURCE F0 BIAS METER
F0

Figure 2. Load Pull Setup with Triplexer Load harmonic Tuning.

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 RF TUNER TUNER POWER
SOURCE BIAS F0 DUT F3 STUB F2 STUB BIAS
RESONATOR F0 METER
RESONATOR

Figure 3. Load Pull Setup with Stub Resonator Load Harmonic Tuning.

RF TUNER TUNER TUNER POWER

SOURCE BIAS F0 DUT F0 F0 BIAS METER

Figure 4. Load Pull Setup with Cascaded Tuner Load Harmonic Tuning.

Comparison of Harmonic Tuning Methods

The Key parameter is HIGH ISOLATION between the fundamental and harmonic tuning. It is critical since the purpose
of harmonic tuning is to separate the effects of tuning F0, F2, and F3.

Table 2. Harmonic Tuning Methods – Advantages vs. Disadvantages

Harmonic Tuning Method Advantages Disadvantages

• Excellent Tuning Isolation • Small insertion loss in front of all three tuners results
Triplexer • Change Bans Easily (no disassembly, no tuning, no in slightly reduced matching range.
re-calibration.
• Non-contacting Tuners
• Harmonic impedance may be swept around edge
of Smith chart, or over entire chart
• Available to Millimeter Wave Frequency Range

Stub Resonator • No Loss in Front of F3 Tuner • Poor Tuning Isolation

• Small insertion loss in front of F0 and F2 tuners
results in slightly reduced matching range.
• Very narrow band
• Complex to change bands
– Disassemble Tuners
– Adjust Inter-stub Spacing
– Retune with VNA
– Re-characterize
• Sliding contact oxidizes or wears out, and is eas-
ily damaged
– Result: Poor Repeatability
– Result: Worse Isolation
• Limited to edge of Smith chart only – no gamma
control

Cascaded Tuner • No special hardware required • Poor Tuning Isolation

• Operates over full tuner BW with no setup changes • Limited tuning range with electronic tuners
• No loss in front of any tuner
• Harmonic impedance may be swept over the entire
Smith chart

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Trade-Off Criteria 5. Ease of changing tuning bands: Both the Triplexer method
Each method has trade-offs, but some parameters are much and the Stub Resonator method are band limited. The
more important to the RF designer than others. Cascaded Tuner method has an advantage here, because
no hardware needs to be changed. The Triplexer method
1. Tuning Isolation: The most important difference
also has an advantage here, since it allows quick and
between the three methods: Fundamental tuning is
easy changing of measurement bands.
far more sensitive than harmonic tuning, so if a large
harmonic impedance change causes even a small F0 The Stub Resonator method requires the following lengthy
impedance change, the results are confused. It becomes and complex procedure to change the measurement
unclear whether the device performance change is due band:
to the harmonic tuning or the unintentional F0 tuning a. Disassemble the test setup to remove the harmonic
change. Therefore, harmonic tuning must have almost tuners.
ZERO AFFECT on the Fundamental impedance. The
b. Disassemble both the F2 and F3 tuners, and reassemble
Triplexer method is the only approach with good tuning
them with new sets of resonators inside the units. Since
isolation.
the parts are delicate and easily damaged, this must
2. Insertion loss at the fundamental frequency: This will be done very carefully.
reduce the available matching range of the tuner at F0.
c. Place the extra resonators into safe storage. This
The F0 matching is critical because a true match to the
is critical to avoid damaging the critical exposed
DUT is desired. However, if the loss is low, this is not
parts.
usually a practical problem. Both the Triplexer have a
small loss in front of the F0 tuner, either the triplexer or d. Calibrate a Vector Network Analyzer, and use it to
the harmonic tuner. Therefore, the Triplexer method and manually tune the resonator pair spacing. When
the Stub Resonator method are about equal in this. finished, lock them down to hold that position. This
must be done for both the F2 and F3 tuners – a total
3. Insertion loss at the harmonic frequency: This will reduce
of four resonators – each time.
the matching range at the harmonic frequency. This is
much less important than the correct matching at the e. Reinstall the tuners back into the test setup. This
fundamental frequency for two reasons. must be before re-characterization if the in-situ cal
a. The harmonic tuning is much less sensitive than the is to be done. Otherwise, this step would follow the
fundamental, so getting close is generally sufficient. characterization.

b. In most power amplifier applications, there is no intent f. Use the load pull software to run a complete tuner
to actually deliver power at the harmonic frequency, characterization of the reassembled tuners. This must
but rather reflect it all back to the DUT. The question be done for both the F2 and F3 tuners.
is to find the optimum reflection phase to get the best This is a disadvantage of the Stub Resonator method.
performance at the fundamental. A small loss does
6. Sliding Mechanical Contacts: This can seriously degrade
not hinder this.
performance over time. It is due to contact wear, contact
Although the Stub Resonator has slightly less loss, mostly oxidation, and cumulative damage from band-changing.
at the 3rd harmonic, the Triplexer loss is small enough It can lead to intermittent contact and repeatability
that there is no practical effect. problems, vibration problems (especially for on-wafer
4. Bandwidth: The Stub Resonator method with dual stubs applications) and increased insertion loss of the tuner.
works over a very narrow bandwidth, so may sets of The result is loss of calibration validity and accuracy. The
resonators and a lot of band-changing may be required. Triplexer method and the Cascaded tuner method have
The Triplexer method is better, since the limitation is been implemented with non-contacting or solid state
to avoid overlapping F0, F2, or F3 which are far apart. hardware, so only the Stub Resonator method has this
Therefore, the Triplexer method easily covers major bands problem.
of interest without band-changing. This is a disadvantage 7. Harmonic tuning over the entire Smith chart: This allows
of the Stub Resonator method. contours vs. harmonic tuning to be drawn, providing

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 better insight into DUT operation and sensitivity. is not real difference from the basic load pull system
However, in many applications, this is not critical, since without harmonic tuning. All three methods are equal
the most important information comes from tuning in a on this.
ring near the edge of the Smith chart. Both the Triplexer
method and the Cascaded Tuner method have a small Triplexer Performance
advantage here. The Triplexer method has major advantages over the
other methods, but it depends on the performance of the
8. Availability at higher Frequencies: As technologies triplexer. The two most important parameters are isolation
evolve, the trend is to move higher in frequency. The and insertion loss.
sliding contact of the Stub Resonator is a major limitation,
Because of the wide frequency separation between the
because the quarter wave stubs become physically small
fundamental and harmonic bands, the isolation is easily
compared to the transmission line and contact size, so
achieved. Also, since the only isolation effect of interest
the stubs lose the ability to resonate. Both the Triplexer
is return loss isolation, which uses a two-way path, the
method and the Cascaded Tuner method easily extend
effective isolation is double the one-path value. Typical
into the millimeter wave frequencies. This is only a
return loss isolation is well over 100 dB.
problem for the Stub Resonator method.
Insertion loss is important to keep a high matching range.
9. Sensitivity to out of band oscillation: This question is
Typical insertion loss of 0.2 to 0.3 dB maintains a high
sometimes asked, mainly because a high reflection at
matching range, making the Triplexer method very effective.
low frequencies could cause an oscillation with high
Typical triplexer performance is shown in Figure 5, with
device gain. However, the Triplexer and Stub Resonator
the three paths overlaid.
both go to 50 Ohms below the operating band, so there

Figure 5. Typical Insertion Loss and Isolation of the Triplexer (a = Fundamental Path, b = 2nd Harmonic Path, c = 3rd Harmonic Path).

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Analysis of Harmonic Resonator Pull- using harmonic resonators.
ing on Fundamental Impedance It is important to note that the same developers equate
Since automated tuner systems rely on presenting a stable harmonic resonator isolation with di/triplexer isolation.
and repeatable impedance, Maury has chosen the di/ They are not the same, since, with a di/triplexer, the signal
triplexer method for harmonic tuning, due to the very is attenuated twice. Hence, a di/triplexer with an isolation
high isolation between the fundamental and harmonic o5, say 35 dB, actually has 70 dB of harmonic isolation, due
impedances. Uncontrolled perturbation of the fundamental to the round-trip isolation being 2x the one-way isolation.
impedance due to harmonic tuner pulling results in a sig- The same is not true for a harmonic isolator, however.
nificant increase in fundamental impedance uncertainty,
Figures 6a-6c show the actual fundamental impedance for
which is equivalent to adopting a low performance VNA
22 dB and 35 dB, respectively, versus varying harmonic
calibration.
impedance phase. Also shown are typical uncertainty
ranges for 3.5 mm SOLT VNA calibration and 7 mm TRL
To examine the effect of poor harmonic resonator isolation calibration. In addition, limits are shown for the worst-case
on fundamental impedance, consider the effect of varying 70 dB isolation afforded by Maury’s diplexer method.
the phase of a perfect reflection at the second-harmonic on
a fundamental tuner presenting 1 W. This tuning scenario There are several immediate consequences of the poor
would be consistent with, for example, characterizing a harmonic resonator isolation. First, as Figure 6a shows,
transistor for GSM operation. Typical harmonic resonator the resultant fundamental impedance pulling exceeds by
isolation specifications of 22 dB and 35 dB are assumed, a wide margin the uncertainty associated with a 7 mm TRL
which represent worst-case isolation without fundamen- calibration. Thus, left uncorrected, the harmonic resonator
tal correction and worst-case isolation with fundamental method is similar to performing tuner characterization with
correction, respectively. These numbers were recently a SOLT calibration, something that should be avoided at
published by developers of an automated loadpull system all costs. For reference, typical uncertainty for a 3.5 mm

Fundamental Impedance Pull Due to 22 dB Resonator Isolation Fundamental Impedance Pull Due to 22 dB Resonator Isolation
Typical Cal Uncertainty Using 3.5mm SOLT at 2 GHz Typical Cal Uncertainty Using 3.5mm SOLT at 2 GHz
Typical Cal Uncertainty Using 7mm TRL at 2 GHz Typical Cal Uncertainty Using 7mm TRL at 2 GHz
Fundamental Impedance Pull Due to 70 dB Diplexor Isomation Fundamental Impedance Pull Due to 70 dB Diplexor Isomation
1.5 1.5

1.4 1.4

1.3 1.3
Actual Tuner Impedance Magintude (Ohms)

Actual Tuner Impedance Magintude (Ohms)

1.2 1.2

1.1 1.1

1.0 1.0

0.9 0.9

0.8 0.8

0.7 0.7

0.6 0.6

0.5 0.5
0 50 100 150 200 250 300 350 0 50 100 150 200 250 300 350
Phase Angle of Second-Harmonic Resonator (Degreees) Phase Angle of Second-Harmonic Resonator (Degreees)

Figure 6a. Figure 6b.

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SOLT calibration is also shown in Figure 6a. Also shown • The harmonic resonator method for harmonic tuning,
is the fundamental uncertainty exhibited by the Maury under worst-case corrected conditions, is equivalent
di/triplexer method, showing that even at the worst-case to adopting a VNA calibration method with higher un-
isolation of 70 dB that uncertainty is dominated by VNA certainty than TRL, which virtually all automated tuner
calibration uncertainty. system developers agree is the preferred method.
Fundamental Impedance Pull Due to 22 dB Resonator Isolation
Figure 6b shows the resultant fundamental impedance Typical Cal Uncertainty Using 7mm TRL at 2 GHz
pulling with a harmonic resonator isolation of 35 dB. Even Fundamental Impedance Pull Due to 70 dB Diplexor Isomation

in this case, with fundamental correction, the uncertainty 1.02

exceeds typical 7 mm TRL calibration performance.

1.015
Figure 6c shows figure 6b with the y-axis magnified to
show the details of di/triplexer fundamental impedance
1.01
pulling. As clearly illustrated, even at a worst-case isolation

Actual Tuner Impedance Magintude (Ohms)

of 70 dB, the resultant fundamental impedance pulling is
far less than the uncertainty of a typical 7 mm TRL calibra- 1.005

tion. Hence, the Maury di/triplexer method is completely

transparent. 1.0

• Maury has adopted the di/triplexer method of har- 0.995

monic tuning due to its superior isolation, resulting
in nonexistent fundamental impedance pulling. Iso-
0.99
lation far exceds the ability to even measure it using
conventional VNA methods.
0.985

• The accuracy expected from using a high-performance

calibration method, like TRL, is maintained, with the 0.98
0 50 100 150 200 250 300 350
Maury di/triplexer harmonic tuning method. Phase Angle of Second-Harmonic Resonator (Degreees)

Figure 6c.

Summary of Comparison
1. The Triplexer method excels at the most important Maury Microwave selected the
factor for accurate measurements, which is the tuning Triplexer method because of the major
isolation. The small loss causes slightly less tuning range advantages over the other methods.
at the 3rd harmonic, but this has no practical effect on
the measurement. This method trades off an unimportant
factor to gain the critical tuning isolation factor. Due to advances in harmonic tuning
2. The Cascaded Tuner method is the only one that does algorithms and the implementation
not require band limited hardware. However, it has very of new software features, MMC's
poor tuning isolation.
method of cascaded tuners for
3. The Stub Resonator method has slightly less loss at the
harmonic tuning has become a viable
3rd harmonic, but this does not have much practical
measurement advantage. This small advantage is and recommended option and has
overwhelmed by the poor tuning isolation, complexity overcome any and all issues related
in changing bands, and degradation with use due to the to tuning isolation.
sliding mechanical contacts.

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