Abstract: Now many units are engaged in low noise amplifier (LNA) project work, moreover requests LNA to have the low power input, sometimes the power input even is smaller than - 60dBm. In view of such LNA, if wants to survey its four S parameter accurately, will become very difficult. But establishes the network analyzer each output port’s power, the intermediate band affable attentuator as well as the high accuracy calibration reasonably, may survey LNA accurately four S parameter. This paper take Anjielun the PNA-X network analyzer as an example, narrated how to enhance LNA the measuring accuracy.
Key word: Vector network analyzer, low noise amplifier, S parameter, measuring accuracy
1. Low noise amplifier characteristic and application
LNA mainly uses in the weak signal the enlargement, enlarges the antenna from the airborne receive to the weak signal, the noise reduction disturbance, supplies the message data which the system demodulates needs. To the LNA overriding demand is: Small noise factor (NF), namely LNA itself produces the noise power is small, the noise is the limit weak signal detection fundamental factor, any weak signal may enlarges after LNA examine theoretically, therefore examination ability is decided by the signal-to-noise ratio; The high gain, has the good smoothness the high gain not only to be possible effectively the signal amplification, moreover may reduce the subordinate noise the influence; The great dynamirange, gives an input signal change the scope not to have the distortion; Matches well with the supply oscillator, front end this LNA usually is the radio frequency passive filter, this kind of filter’s transmission characteristic is sensitive to its load, therefore needs to have the outstanding input output reflection loss, moreover the LNA misalignment causes the third-order intermodulation distorts is also an important target.
LNA widely applies in receiving systems and so on microwave communication, microwave measurement, radar, is in the receiver circuit’s first active circuit, the input termination RF filter, the output termination mirror image suppression filter or the direct connection mixer, its major function is comes from antenna’s microvolt level voltage signal to carry on the enlargement. The range coverage is far, the coverage area is big as well as the low distortion and so on has become Radar, E/W, Satellite and the GPS system’s universal pursue, this proposed to system’s receiving sensitivity a higher request, we knew, the system receiving sensitivity’s formula is as follows:
Is obvious by the above equation, in each kind specific (band width, demodulates S/N already decides) in the wireless communication system, can enhance the sensitivity the key aspect to reduce receiver’s noise factor NF effectively, but decided that receiver’s noise factor’s key component is in receiver most front end LNA, therefore how the accurate survey LNA each kind of target parameter is especially important.
2. Calibration principle
The calibration goal is to eliminate the system error which in the test system exists. Must realize that to the calibration is in itself also one kind of test procedure, namely carries on the survey with the network analyzer to the known high accuracy parameter’s standard calibration, the calibration parameter data which in the network analyzer test’s result and the system saves carries on compared to rightly, between two groups of data has the error inevitably, these errors are because network analyzer’s system error causes, thus gain network analyzer’s system error. These errors will be eliminated in following measuring process, obtains finally is measured component’s measurement result.
This year the calibration fundamental type has the single port calibration, the twin port calibration, the normalized calibration also has the enlargement mode response calibration which just promoted (Enhanced Response Calibration). Regarding the amplifier survey, we need to survey the forward gain frequently, the input end lose, exit losses and reverse isolation, therefore needs the twin port calibration. The twin port error model is as follows:
The above 12 system errors, may obtain through the twin port calibration. After the calibration, to was measured that carries on the survey, the measuring process obtains four to survey S parameter S11m, S21m, S12m and S22m. Four twin port error correction formula which based on shown in Figure 3, eliminates 12 system errors, calculates the actual need to measure finally four S parameter S11a, S21a, S12a and S22a.
Figure 3 four formula simplification is:
Note: E12 represents 12 system errors, S parameter subscript a is the Actual actual value, m is the Measure observed value.
Conclusion: Each actual S parameter is four tests the S parameter and 12 system error’s function. Therefore, if wants to obtain the high accuracy the S parameter measurement result, must guarantee that four test the S parameter the measuring accuracy and 12 system error’s accuracy.
3. Network analyzer system structure
Must want to gain the high accuracy measurement result, must understand network analyzer’s system structure very clearly. Anjielun newest network analyzer PNA-X system structure as shown in Figure 4
When forward survey, B is tests the receiver, A is reflects the receiver, R1 is refers to the receiver; When reverse survey, A is tests the receiver, B is reflects the receiver, R2 is refers to the receiver. Two 35dB attentuators are the receiver attentuators, uses for to avoid the high efficiency causing the receiver to compress; Two 65dB attentuators for forward and reverse source attentuator, changes the port output scope. Corresponds each port to have a Bias-T cocurrent in the rear panel to refer to the input port, provides the direct-current signal to the amplifier.
Four S parameter definition is as follows:
Forward: S11=A/R1, S21=B/R1 Reverse: S22=B/R2, S12=A/R2
4. Traditional calibration and test
The supposition low noise amplifier input level request for - 60dBm, the reverse isolation is 40dB, the work frequency band from 1.8 GHz to 2.0 GHz.
In the ordinary circumstances, engineer establishes the network analyzer: The outset frequency is 1.8 GHz, the termination frequency is 2.0 GHz, the power for - 60 dBm, the band width is 10kHz. After completing the establishment, the connection electron calibration according to shown in Figure 5 (also to be possible to use machinery calibration) to carry on the twin port calibration. Then according to shown in Figure 6 connects the amplifier, carries on the survey, test result as shown in Figure 7. May see, the test result vibration was big, presented the burr, this was in the practical application cannot accept. 
5. Has the problem to the traditional test in the analysis and the solution
1) The calibration power level is quite low
The calibration gains the high-accuracy survey result precondition, if the alignment precision is bad, is absolutely impossible to obtain the quite high measuring accuracy, must therefore enhance the calibration as far as possible the precision. Above talks about the calibration is in itself also one kind of measuring process, namely uses standard calibration survey network analyzer own system error.
Anjielun the PNA-X interior supply oscillator’s power range or higher (maximum power output is decided from - 30dBm to 13dBm by frequency band), because PNA-X has the 65dB source attentuator, therefore the power level is lowest may arrive at - 95dBm. If the manual establishment attentuator is 30dB, PNA-X source output scope for from - 60dBm to - 17dBm.
A use network analyzer very important spot, if the network analyzer attentuator is invariable, after the calibration, the change amount of power, basically does not affect the measuring accuracy. Therefore when calibration, the power may establish as - 20dBm, but is not - 60dBm, like this may increase the alignment precision. After the calibration completes, establishes the power as - 60dBm, is advantageous satisfies LNA the test condition.
After completing the twin port calibration, through connection. The power for - 60dBm with - the 20dBm calibration error to for instance shown in Figure 8. 
2) The PNA-X port 2 outputs are low
Under the PNA-X default pattern, the port 1 and the port 2 powers are the coupling conditions, therefore port 2 outputs also for - 60dBm. Because the calibration is 2 port calibrations, even if on the screen does not test the S12 isolation, the network analyzer backstage is also surveying S12, because of according to Figure 3 formula or the reduction formula, amplifier S21a needs S12m. The network analyzer when tests S12m, because the port 2 power outputs for - 60dBm and the isolation are 40dB, arrives at the port 1 power for - 100dBm, the coupling wall which weakens after port 1 directional coupler’s 15dB arrives at a receiver’s power again for - 115dBm. - 115dBm close receiver low chirp, therefore the S12m measuring accuracy is bad, thus causes four actual S parameters the test precision to be bad.
Network analyzer’s two port power may establish as the non-coupling condition, is also the port 2 powers may not be dissimilar with the port 1 power establishment. We may establish port 1 output - 60dBm, port 2 output 0dBm, like this may guarantee S12m the measuring accuracy, thus causes 4 S parameter measuring accuracy to enhance greatly.
3) When calibration the band width value is big
Because the calibration is to obtain network analyzer’s system error, therefore calibrates time, the band width suggestion establishment is 100Hz, after completing the calibration, to raise the test speed, may enhance the band width to 10kHz or 1kHz, such change will not change the calibration obviously the condition and the influence test result.
After solving the above three problems, carries on the calibration and the survey, measurement result as shown in Figure 9, may see the vibration and the burr phenomenon disappears, the measurement result is quite ideal. 
6. Summary
The modern LNA design target is getting better and better, the outstanding LNA performance presented the very big challenge to the traditional parameter measuring technique, but through establishes the network analyzer as well as the optimized calibration process reasonably, may obtain the high measuring accuracy.
Reference
1st, Wang Guobin and so on, in superconductivity receiver low radio frequency low temperature low noise amplifier development, Applications of Electronic Technique, 2007.8
2nd, Agilent AN 1287-3 Applying Error Correction to Network Analyzer Measurements
