RF Power amplifier: two tone testing

Two tone testing is used most often to test the linearity of a RF power amplifier ( or other amplifiers for that matter). The technique is fairly simple in principle but can have its little “gotchas”. First off, choice of frequencies. If you have 2.5 Ghz S band RFPA you are going to test, what frequencies do you use? Use frequencies close to 2.5 Ghz, say a few Mhz above and below or whatever provides the test results accurately ( simulations can be used to do this).Then the second issue is how do you generate the two tones to be input into the amplifier and not generate significant other IM products that will certainly destroy the test. ( A handy rule of thumb is to keep these input IMPs at least 6 dB below the test tones). Simple things like this can cause a real headache. My take is that if you are testing a 2.5 Ghz amplifier then you should place the two tones close to that frequency. Drive the amplifier so that the output at the tone will be close its rated output. Then measure using a good spectrum analyzer. ( Somewhat of a problem if you are a small company or hobbyist). Generally what will happen is that if the two tones are separated by f0 then the strongest two products will appear at f0 below the lower test tone and f0 above the higher test tone. Obviously there will be many more IM products that will be generated.

Industry standard  measures the ratio of the weakest desirable output (they both should be the same) to the strongest undesirable frequency (closest to one of the desirable frequencies) as a ratio in decibels (dB) and calls this number the IMD value.  However the American Radio Relay League refers to the undesirable strongest output
to the peak power in both desirable outputs resulting in a number that is 6dB larger than industry numbers.
Signal processing Group Inc., design and delivers analog and RF/Wireless ASICs and modules.


RF Power Amplifiers: Noise Power Ratio, NPR

Noise power ratio or NPR is a measure for a RF power amplifier ( among other analog circuits) that is usually used in the case of multicarrier power transmission using RFPAs. In order to understand NPR, it is instructive to basically describe how it is measured. First a white noise source is used and its output is passed through a band pass filter in the frequency band of interest. This represents multiple carriers with random amplitudes and phases. Then the resulting filtered signal is passed through a narrow, steep notch filter which is tuned to the frequencies of interest at the current measurement. First the output of this filter is passed through the DUT ( device under test) and the noise power is measured in the notch by using a narrow band receiver. Then the notch filter is bypassed and the noise power is again measured in the frequencies where the notch is using the narrow band receiver. The ratio of the two readings in dB is the NPR.

SPG designs, develops and delivers custom analog and RF/wireless asics and modules.

RF Power Amplifier Design: Conjugate matching and load line matching

A power amplifier device is characterized for a maximum operating current and a maximum operating voltage ( also see the safe operating areas of the device). A load line match is simply using the calculation: Vmax ( operating)/Imax(operating). This is the impedance that needs to be matched to if you want maximum performance out of the device. This is referred to in many books as the load line match. See also the paper in this blog on RF power amplifier design for a discussion of the load line. ( Go to www.signalpro.biz, go to complementary menu item, go to more reports, select RF Amp 1 paper), Conjugate match on the other hand refers to matching of the real parts of the generator and load with the reactance tuned out. See the book at :https://www.amazon.com/VSWR-Impedance-matching-techniques-electronic/dp/1490902813 for more details on this and other matching topics.

SPG designs, develops and delivers custom analog and RF/wireless asics and modules.

LC balun calculator

A LC balun balun calculator is now available for interested users on the Signal Processing Group Inc.’s website under the complementary menu item. Please scan through the items listed until you can select your choice. For background on LC baluns please read the post on LC baluns on the SPG blog. Use search to find it.

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Balun design: A simple LC balun

One of the simplest baluns can be designed using L and C elements as shown below:

The design equations have been embodied in a Javascript calculator that will be provided in this blog for free download in the next post. Check back in a day or two to download the calculator. The input of this circuit is a balanced waveform and the impedance is set by the values of the L/C combinations. The output is another balanced signal with the required output impedance also calculated using the input and output impedances. This circuit may be implemented using discrete elements for use as a impedance transformer or a balun.

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Balun typical s parameters and their interpretation

The s parameters of a balun ( three port) are:

s12 = -s13 = s21 = -s31 and,

s11 = -infinity.

What does this mean in terms of conceptual understanding? The interpretation of these s parameter statements are:

  1. A balun is a three port device with a matched input and differential outputs.
  2. A balun is a three port power splitter
  3. The two outputs from a  balun are equal in amplitude but opposite in phase.
  4. The voltage of one balanced output is the negative of the other balanced output.
  5. The input is matched to the input transmission line.
  6. The usual impedance is 50 Ohms. But it could be something else.
  7. A balun is a reciprocal device. Its output and inputs may be exchanged

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Transmission line balun. What is it?

Simple baluns ( balanced-unbalanced) at lower frequencies use flux coupled transformers.They are generally limited to lower frequencies below 1 GHz. The reason is that at high frequencies the dipoles in the magnetic core cannot switch quickly enough, causing loss in the balun’s coupling between windings.

Parasitic capacitance between the windings leads to high frequency signals that pass  directly to ground without coupling through the metallic core. Magnetic cores also have a large loss tangent. This causes high signal losses at microwave operation.

As a result of these effects, capacitance coupled transmission line baluns were invented. A transmission line balun is a set of coupled transmission lines with one end grounded.  This type of coupling induces equal and opposite signals in both lines.

As a result of using a transmission line to connect to ground, the transmission line allows differential operation. Common forms of transmission line baluns include the 1:4 impedance ratio Ruthroff balun and 1:4 Guanella balun.

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An example program plan for ASIC/IC development

A good program plan can save a lot of time and expense in the development of ASICs/ICs as well as help towards a first pass success. We at Signal  Processing Group Inc., have prepared an example of a plan based on an actual development, Please access this plan at www.signalpro.biz/progplan.pdf. Please note that contact points have changed. Please call 866-487-1119 for quick response to your requests or questions. Or contact us by email at spg@signalpro.biz.

Solid State Power Amplifiers ( SSPA) for small satellite constellations.

It appears that the exploration of and the colonization of planet Mars will require a multitude of communication satellite constellations, The present effort is gearing up using application specific cubesats and similar technology. One of the key elements in these small satellites is the Solid State Power Amplifier- the SSPA. Of necessity these amplifiers have to be very efficient and consume very little space. They will be have to be robust and may need multi band capability. Please contact us for more on this important topic. Visit our website for more informative articles and white papers.