An Affordable Harmonic Load Pull Setup
Focus Microwaves Inc.
Ville St. Laurent, Quebec, Canada
Since the early days of semiconductor amplifier design when bipolar transistors were used, it has been common knowledge that short-circuiting the second and third harmonic frequencies with the correct phase at the output of a transistor can improve the device's performance. Numerous simulations and experiments have confirmed that this improvement affects the gain and, in particular, the power-added efficiency (PAE) of the amplifiers being designed.
Several additional facts also have been confirmed: Harmonic tuning at 2f0 is important when the device is in saturation, the amplitude of the harmonic reflection factor is equal to 1, harmonic tuning at 3f0 has approximately 25 percent of the effect of second harmonic tuning, harmonic tuning effects depend on frequency for a given device, PAE improves 10 to 15 percent from 2f0 and another one to three percent from 3f0 tuning at the load, and linearity (intermodulation) improves 3 to 8 dB from 2f0 tuning at the source. In addition to the load and source impedance at fundamental and harmonic frequencies, these improvements also strongly depend on the transistor type, power saturation level, frequency and bias conditions.
These dependencies show that until the elusive universal nonlinear transistor model has been developed successfully, harmonic load pull will remain the accepted method for investigating and optimizing the harmonic terminations of high power amplifier circuits. To avoid parasitic oscillations, this type of a harmonic load pull system must provide high reflection inside the test bandwidth, high tuning resolution, high power handling and lowpass characteristics.
A Harmonic Tuning Solution Comparison
Several different methods are currently available for generating controllable harmonic loads and for testing devices under those conditions. Table 1 lists these methods and presents their advantages and shortcomings from technical, practical and economical standpoints. The data take into account factors such as performance, versatility, simplicity, compatibility with existing hardware, cost and availability. At this time, programmable harmonic tuners are the best overall solution for harmonic load pull.
|
Table I | |||
|
Technique |
Description |
Advantages |
Disadvantages |
|
Active harmonic load pull (active load) |
f0 into DUT; 2f0 and 3f0 extracted, amplified and fed back into DUT |
| G | = 1 at e; fast for single tone; extendable to high frequencies; integratable with network analyzers |
Marginal phase stability; power limits; slow for two-tone and modulated signals; high cost |
|
Active load pull (split signal Takayama) |
f0 fed into DUT and f0 , 2f0 and 3f0 via split path; combined at output and injected into DUT output |
Same as above |
Complex calibration; saturation plots a problem; power limits; slow for two-tones; high cost |
|
Harmonic fixture |
l /4 stubs used at various positions |
Low cost; easy to employ |
Crude method; very long; no corrections |
|
Wideband tuners and multiplexers (MUX) |
Frequency discriminator for f0 , 2f0 , and 3f0 with multiple tuners |
Independent tuning of harmonics; high power |
Low | G | (MUX loss); off-band reflections and DUT parasitic oscillations; limited bandwidth; complex MUX; high cost |
|
MUX + tuners + active modules (AM) |
Same as above but AM amplifies signal to compensate for MUX loss |
Same as above; | G | = 1 |
Complex MUX; DUT and AM parasitic oscillations (off band); power and limited frequency range; high cost |
|
Multifrequency/multistate electronic tuners |
Selection among > .5 million diode states to simultaneously correspond to required impedence z at f0 , 2f0 , 3f0 , | |
High speed (once calibrated) acceptable | G | |
Lengthy calibration; power limitations; limited isolation (not exactly tuned points); limited frequency range |
|
Manual harmonic tuners (model MHMT) |
= independent tuning at f0 , 2f0 , 3f0 using harmonic heads |
Low cost; high | G |; high power; versatile |
Limited isolation; iterative process; z(f0 ) requires adjustment when z(2f0 ) |
|
Programmable harmonic tuners (model PHT) |
Electromechanical version of the above with calibration and impedance back tuning |
High | G | (> .9); high power; no parasitic oscillations; independent harmonic tuning; extends existing load pull systems; compatible with transformers and transforming probes |
Gmax at f0 lower than fundamental tuners by .2 - .4 dB (can be compensated with AM) |
|
Programmable fundamental and harmonic tuners (model CCMT-2H) |
Same as above, but combined harmonic and fundamental tuning in a single housing |
Same as above; compact; lower loss and higher G at f0 |
Gmax at f0 lower than fundamental tuners by » 0.1 dB |
Harmonic Tuner Characteristics and Advantages
The models PHT and CCMT-2H programmable harmonic tuners use the same proven and dependable electromechanical components as the wideband computer-controlled microwave tuner (CCMT) of which nearly 400 units have been put into operation over the last 10 years. The selective high reflection at the harmonic frequencies is generated by harmonic heads developed using a proprietary design. The probes can be manufactured for harmonic frequencies between 400 MHz and over 60 GHz, a selection that is limited only by customer awareness and demand. Currently, the upper frequency is defined only by the use of 1.9 mm (V) connectors. (Extension to over 100 GHz using 1 mm connectors is envisaged.) The units presented in this article cover harmonic frequencies from 1.6 to 18 GHz and 3 to 33 GHz.
The nature of the harmonic probes allows high CW fundamental power to be injected without damage because the harmonic heads present low reflection at f0 and because no active semiconductor device (amplifiers and diodes) is used in the setup (except in the device under test (DUT)). The harmonic tuners may include one or two independent resonant heads. In calibration, each cavity moves a number of steps N(2f0) and N(3f0) to cover 360° at the corresponding frequency; the S parameters of the tuner are measured simultaneously at f0, 2f0 and 3f0 using a network analyzer and saved in binary harmonic calibration files. For higher accuracy, the user can select the number of calibration points. However, the software interpolates accurately between calibrated points.
An intelligent calibration algorithm allows construction of the combined {2f0 x 3f0} tuner matrix by measuring only N(2f0) + N(3f0) instead of N(2f0) x N(3f0) points. In the case of 20 steps, this calculation utilizes 40 points instead of 400, a time savings of 90 percent. In this way, the harmonic tuners are calibrated in a few (five to eight) minutes (depending on the frequency). The tuner calibration data are used to interpolate between calibrated points with a high accuracy of approximately 40 dB (depending on the frequency).
Using the calibration data, the measurement software can synthesize any harmonic tuning phase from 0° to 360°. The tuner sweeps the harmonic phase, generates output (harmonic phase) plots and searches for a maximum of any preselected parameter, such as Pout, gain, PAE, intermodulation, adjacent-channel power ratio and power output at 1 dB compression as a function of the harmonic phase. During all harmonic tuning operations, the fundamental and remaining harmonic impedances are automatically corrected to their original values. This capability makes the system extremely versatile
The Harmonic Load Pull Setup
The harmonic load pull setup is constructed using a network analyzer, test fixture (or probe station) and 
two programmable harmonic tuners (models 1816-2H, 3003-2H or 4006-2H). These tuners include fundamental heads from 1.6 to 18 GHz, 3 to 30 GHz and 6 to 40 GHz and harmonic heads from 3.2 to 18 GHz, 6 to 33 GHz and 12 to 44 GHz, respectively. Figure 1 shows the harmonic load pull setup.
The network analyzer serves as signal source and fast receiver and is used for DC biasing the DUT. A second synthesizer is required for intermodulation tests. The two signals are combined at port 1 of the analyzer and injected into a spectrum analyzer via an output directional coupler. The setup is calibrated using a power meter connected to port 1 of the analyzer for absolute power reference.
The system is controlled by an IBM PC equipped with a general-purpose 
interface bus and CC-3 tuner controller that can position and initialize two tuners with three axes each. The tuners may be calibrated in situ at f0 but must be replaced by a thru line for harmonic calibrations at 2f0 (or 3f0). Results of measurements made using this type of setup are shown in Figure 2.
Conclusion
Programmable mm-wave harmonic and fundamental/harmonic combination tuners that operate from 1.6 to 33 GHz have been presented. (Models are also available that cover the 0.4 to 60 GHz frequency range.) In addition, an affordable mm-wave harmonic load pull setup that uses two harmonic tuners and one network analyzer as a signal source and fast receiver has been presented. Additional information can be obtained from the company's Web site at http://www.info@focus-microwaves.com.
Focus Microwaves Inc.,
Ville St. Laurent, Quebec, Canada
(514) 335-6227.