Wideband matching design of RF power amplifier
In many remote communication, radar or test systems, the transmitter amplifier is required to operate in a very wide frequency range. For example, working in multiple times even dozens of times. This requires a wideband matching design for radio frequency power, and broadband power amplifier has some significant advantages. It does not require tuning resonance circuits, which can realize a quick frequency and a wide-mode signal spectrum. Broadband match is a brief referusion of broadband impedance, is the main circuit of broadband radio frequency and power transmission system. The wideband matching is to match the input of the RF power amplifier, and the output achieved impedance is matched to realize the power amplification transmission in the broadband. Therefore, the design of the broadband impedance matching network is the main task of broadband radio generation design. Coaxial cable impedance converter referred to as the coaxial converter, achieving effective broadband matching, which provides a wide frequency bandwelling condition for RF power amplifiers. Coaxial converters have a high power capacity, bandwidth and shielding properties, can be widely used in HF / VHF / UHF bands.
1 program design
Coaxial converter and its combination are a broadband impedance matching network with high impedance transformations, which can effectively match the lower input impedance or output impedance of the RF power amplifier to 50 Ω. Coaxial converter design is used to use 1: 1 amplified form, 1: 4 amplification form and its combination.
1.1 Coaxial Converter Principle
The coaxial converter is composed of a coaxial cable or coaxial cable on the ferrite core on the ferrite core, which is generally referred to as "Baren". The structure of "Baren" is shown in FIG. 1 (a), and its equivalent circuit is shown in Figure 1 (b).
The coaxial converter is in a centralized parameter and the distribution parameter. Therefore, at a low frequency end, its equivalent circuit can be described in a conventional low-frequency transformer characteristic, while at a higher frequency, it is a transmission line that characteristic resistance is ZO. The advantage of the coaxial converter is that the parasitic hutcaps determine its characteristic impedance, and in a conventional discrete winding transformer, the contribution of parasitic capacitance on frequency performance is negatively functioning.
When RS = RL = ZO, "Barron" can be considered to be 1: 1 impedance converter. Coaxial converters must be two points in design: the matching relationship between source impedance, load impedance and transmission line impedance; the input and output should be applied in a prescribed connection and grounding mode. In most cases, the cable length cannot exceed one-third of the wavelength. In order to ensure that the low frequency response is good, there must be a winding length, which can estimate the length of the winding required when the frequency is high and the frequency low end can be estimated according to the following empirical formula.
At high frequency:
LMAX ≤ 18 O00N / FH (cm). (1)
In the formula, FH is the operating frequency (MHz); n is a constant, generally takes about 0.08.
At low frequency:
LMIN ≥ 50RL / [(1 + U / UO) × fl]. (2) In the formula, FL is the operating frequency (MHz); U / UO is the relative magnetic permeability of the magnetic core.
The effect of the magnetic core can be reacted with an equivalent inductance, and the equivalent inductance determines the size of the frequency band low-segment reflection amount, calculated:
L = UR N2 (S / J) (S / J) (3), L is inductive value (h); UR is a relative magnetic permeability; UO = 4πx 10-7; s is the area of the magnetic ring; J is average electric length N is the number of coils.
In order to avoid deterioration of the high segment of the frequency band, the inductance value cannot be greater than the actual needs, and its empirical formula is:
L = 4 (r / wmin) (4)
In the formula, R is an input impedance of an intermediate frequency band; Wmin is a angular frequency.
1.2 1: 4 coaxial converter design
1: 4 The coaxial converter consists of two coaxial cables equal to the length, and the structure is shown in FIG. 2 (a). 1: 4 Coaxial transducer horizontally rotates 180 ° as 4: 1 coaxial converter.
Ideally 1: 4 Coaxial converter input, output impedance matches, input, output impedance equal to its feature impedance ZO, the equivalent circuit model is shown in 2 (b).
The transformation ratio of its source impedance ZG and load impedance ZL is: zg / zl = zin / zout = (ZO / 2) / (ZO + ZO) (5)
2 and formula (5) show that the impedance transformation of the coaxial converter is equal to the ratio of the input impedance and the output impedance. The input impedance of the coaxial converter is equal to the parallel, output impedance of the coaxial cable, the output impedance is equal to series of coaxial cable feature impedance.
1.3 Concentrated parameter components matching design
Since the characteristic impedance of the impedance converter transmission cable is a real number, the input impedance and output impedance of the radio frequency power amplifier are generally a complex impedance. Therefore, the input impedance of the RF amplifier tube is required to realize a conjugate matching of the source impedance or load impedance, thereby achieving power transmission. The complex impedance can be expressed in series with resistance and reactance or in parallel with resistance and reactance. The method of achieving impedance matching with centralized parameter components is that the resistance parallel electricity is reduced, and then the electroplating resistance to defect the imaginary portion, reaches 2 pure resistance matching; when the matching is not a pure resistance, the capacitance of the centralized parameter can be used. Inductance to offset and absorb the method of complex the impedance impedance to achieve the realization of the complex impedance.
2 Key technical issues to be solved
2.1 Low-frequency gain suppression
The gain of the radio frequency amplifier decreases with the increase of frequencies, in general, each additional frequency, the gain drops about 3 dB. In the narrowband circuit, the case where the gain increases with the frequency of the frequency can be ignored, but in the multirade circuit, the pressing of the low frequency gain must be considered. The solution is to use the resistance negative feedback network. The resistance negative feedback network is used to suppress the smooth radio and high gain characteristics on the low frequency. The smaller the resistance value, the larger the suppression smooth effect. With the high frequency band gain, 100 ~ 200 N resistance is used, the gain of the low frequency segment is reduced to greater than the reference gain 2 to 3 dB.
2.2 Coaxial Cable Features Impedance Choice
The coaxial "Barron" completes the conversion of balance to imbalance, and is generally selected from the 50 n characteristic impedance. 1: 4 Coaxial converter cable requires the size of the source or load resistance, the calculation formula is as follows:
ZO = (4R) 2/25 (Ω). (3)
In the formula, ZO is the cable characteristic impedance; R is the source or load resistance.
2.3 Clearing heat dissipation and power verification
In the output matching network, when the coaxial converter is transmitted, the magnetic core will accumulate more heat due to circuit loss, which in turn causes the magnetic core temperature to rise, and the magnetic thermal magnetic permeability is lowered. , Affect the low frequency response of the coaxial converter. The solution is to take good heat dissipation measures to the core, and the core is directly fixed to the metal heat dissipation floor with a heat transfer.
The choice of core material is very important. To get high inductance values, high magnetic cores must be used; in order to select a suitable ferrite core for coaxial converters, you need to know the saturated magnetic flux of the core and it. Nonlinear characteristics. When the transmission power is large, the power capacity of the magnetic ring must be verified. This is due to the magnetic flux of the magnetic ring, magnetic saturation occurs when the power is large, so that the equivalent electricity value is lowered, and the power is not present. The general rules of the coaxial transducer magnetic saturation are the more serious the frequency, so the power check is carried out at low frequencies.
3 design instance
According to the needs of the project, the use of coaxial converter broadband matching technology is designed with a multi-times high-power amplifying circuit, covering civilian and military frequency bands, with a frequency range of 20 to 500 MHz. Power tube uses a balanced N-channel enhanced radio frequency amplifier of the dual die structure BLF574. Designed for the output power of 350 W, the power gain is greater than 16 dB, the frequency range high HF to UHF is widely wideband power amplifier. The input and output impedance of the 225 MHz frequency left and right devices are inductively, input impedance Zs = (3.2 + J2.5) Ω, output impedance ZL = (7.5 + J4.0) Ω.
3.1 Enter the matching network
The BLF574 has a considerable input capacitor, in order to provide the device input on multiplier broadband matching, the BLF57 must consider the effect of the luminous capacitor in the frequency high-end, and the intermediate frequency and lower frequency are high-value input impedance. Influence. The input matching network is designed to set the 4: 1 coaxial converter of the 2-level cascade, complete the 16: 1 impedance transformation, close the 5O Ω standard impedance matching close to 3 Ω, this value also passes simple series microproofline and parallel capacitor Convert the input resistance of the device. Level 1 4: 1 Coaxial converter cable Select UT - 047 -25, characteristic impedance ZO = 25 Ω, cable length 45 mm. Compensate the magnetic core of the low frequency response 2861002402, the initial magnetic permeability UI = 125. The second stage 4: 1 coaxial converter cable Select UT - 043 -L0, the characteristic impedance ZO = 10 Ω, the cable length 45 mm, the core compensates for the low frequency response to the magnetic core. 2861002402. The input matching network is shown in Figure 3.
3.2 Output Matching Network
The output matching network is designed to be 1: 4 in the form of a coaxial transducer level. 1: 4 Coaxial converter cable Select UT - L4L-L5, characteristic impedance ZO = 15 Ω, cable length 68 mm. Compensate the magnetic core of the low frequency response 2661540202, the initial magnetic permeability UI = 125. The coaxial "Barron" completes the conversion of balance to unbalanced output, coaxial "Baren" cable selection UT - 141, characteristic impedance ZO = 50Ω, cable length 68 mm. The matching resistance is: r = (25 × 15) 1/2 / 4 = 4.8 Ω, which needs to be converted into the output resistance of the device via a simple series microproofline and a parallel capacitor. The output matching network is shown in Figure 4.
3.3 Software Simulation and Test Verification
3.3.1 Software Simulation
The input impedance and output impedance of the power amplifier tube are each assumed to be variable impedance with frequency variations, and the impedance matching method is performed according to the broadband network impedance approximation matching method, and the software tool ANSOFT-SeeNade 8.7 is used to establish coaxial impedance converters respectively. For the input and output broadband matching network of the model, the matching port is the standard 50 Ω feature impedance, the matching target is the input or output port voltage stationary wave ratio VSWR ≤ 2: 1. Using the frequency parameter scanning curve, the length of each coaxial cable is adjusted, the length of the characteristic impedance, the length of the series microstrip line and the widening capacitor is adjusted to the ideal standing wave-wave one frequency characteristic curve.
3.3.2 Test Verification
Testing the actual circuitry based on the above design, in the 20 ~ 500 MHz band, input echo loss ≤1.95: 1, when the power is 10 W, the output power of the amplifier. 1882.350 W. The test results show that the amplifier has a good performance, and the designed coaxial converter matching network satisfies broadband matching and power requirements.
RF power amplifier
Our other product:
