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RF Test Methods

RF Test Methods. Types of RF Test Methods. Functional test: check if all subsystem and the overall system is functional Specification-based test: check if every specification is met Observer based test Statistical-based test Structural test Correlation-based test Defect-oriented test

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RF Test Methods

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  1. RF Test Methods

  2. Types of RF Test Methods • Functional test: check if all subsystem and the overall system is functional • Specification-based test: check if every specification is met • Observer based test • Statistical-based test • Structural test • Correlation-based test • Defect-oriented test • Built-in test (BIT), built-in self-test (BIST)

  3. Temperature Temperature Z Z Sensor Sensor 0 0 Power measurement: calorimeter • Tap into an RF signal • Measure heat transfer from Z0 to environment • Calculate power dissipation on Z0 RF Power RF Signal Calorimeter

  4. Power measurement: spectrum analyzer • Use a local oscillator • Mix with RF and down convert to IF • Amplify IF • Band pass filter with sweeping center freq • Measure power in passing band by • Amplify pass-band signal • Rectify to positive only • Average to get average amplitude • Measure pseudo DC and display

  5. Input attenuator IF-tracking filter Log amplifier IF amplifier Mixer Detector Input signal Pre-selector filter Video filter Local oscillator Sweep generator Display Reference oscillator Ask senior engineer to teach you how to use it Ask vendor to provide training

  6. Power sensor: zero-IF with sampling • Use a local oscillator • Direct conversion to low frequency • Low pass filtering • Use ADC to convert to digital domain • Use DSP to analyze power

  7. Mixer LPF amplifier Input signal ADC RF LNA Input attenuator Local oscillator Reference oscillator

  8. -20 -20 Sample Rate Fs = 40 MHz Sample Size N = 4096 fIF = 5 MHz M = 511 -40 Sample Rate Fs = 40 MHz Sample Size N = 4096 fIF = 3 MHz M = 307.2 -40 -60 Decibel Decibel -60 -80 -80 -100 -120 -100 -140 16 16 20 20 8 8 12 12 14 14 18 18 4 4 6 6 10 10 2 2 Frequency [MHz] Frequency [MHz] Not phase noise It’s noncoherency Phase noise

  9. Gain G’A RF Connector RF Connector Input RF line Output RF line Source Meter DUT Interface Board Input loss output loss operating loss GA

  10. D1 D2 D2 Power [dBm] T1 Time [ms] T2 T3 Time-dependent power Mask from standards

  11. Intermodulation measurement • User two tones close to each other, f_RF • HDs are at high freq: 2f_RF, 3f_RF, … • 2nd IMD is at DC and near 2f_RF • All even IMDs lead to DC and even HD freq • 3rd IMD is at f_RF and 3f_RF • Odd IMDs are at f_RF and odd HD freq • IMD3 is most important for interference • IMD2 is most important for DC leakage

  12. Two tone test spectrum with harmonic distortions and intermodulation distortions Pin1 Pin2 S Pimd2 Phd2 Phd3 Pimd3 Pimd3 w1 – w2 w2 – w1 w1 3w2 w2 2w2 3w1 2w1 2w1 – w2 2w2 – w1

  13. OIP2 Second Order Intercept Point Third Order Intercept Point OIP3 saturated output power 1 dB compression point Output Power [dBm] fundamental Response 2nd Order Response 3rd Order Response Slope = 1 Slope = 2 Slope = 3 IIP3 IIP2 Input Power [dBm] Noise floor

  14. Linear Time-Invariant systems Linear Time-Invariant Memory-less, or static 3rd order polynomial approximation adequate for weak nonlinearities, eg. LNA

  15. Nonlinearity issues • Harmonic distortions • Gain compression • Desensitization and Blocking • Cross Modulation • Intermodulation distortions • Effect of feedback on nonlinearity

  16. Harmonic distortions Harmonic distortion uses single tone b0 b1 b2 b3 DC component (offset) Fundamental (amplitude influenced by nonlinearity) Harmonics (not of major concern in a receiver – as they are out of band)

  17. Advantages of differential circuits Even harmonics can be suppressed by using differential mode (as in Differ. Amplifier): In practice, only partial suppression, mismatch corrupts symmetry between the two signal paths

  18. Gain compression • As A ↑, b1/a1A ↓ • When b1/a1A = - 1 dB, A = “1-dB compression point”

  19. Desensitization and Blocking Gain of desired signal is reduced by interferer signal. Blocking: b1 reduces to zero

  20. If the interferer is amplitude modulated: A2cos(w2t) = A2(1+m(t))cos(w2t) Where m(t) is the message being transmitted. The cross modulation component becomes: The component that can be heard in our channel is: When both channels have the same modulation index:

  21. Two-tone Distortion • 3rd Order Intermodulation Intercept Point (IP3) If a1>>a3

  22. 3rd Order IM Intercept Point (IP3) Pout fundamental Pout 3rd IM power PY Pin Pin PIIP3 PX If PX is the minimum receivable input power, and if the corresponding output PY is not to be exceeded by IM3, then Pin must stay below the above expression

  23. IM2 • 2nd order intermodulation (IM2) can be similarly defined and computed • Main cause: • Coupling from and to LO (self mixing) • Distortion in amp and mixer • Mismatch in differential topologies • Main effect is on DC • Can be minimized with fully differential circuit

  24. G1=a1 G2=b1

  25. In general:

  26. Comparison of these points AHIP3 A Ac.m AIP3 A-3-dB A-1-dB ABlock PIIP3= 10*log(3) + Psig+HD3/2

  27. HW • Derive the -3 dB desensitization point expression • Derive the 3rd harmonic intercept point expression Here assume differential structure and only keep one nonlinear term.

  28. HW • Assume single ended structure. Only keep linear and 2nd order term. Derive the 2nd intermodulation intercept point expression.

  29. Dynamic Range • Could be measured at either input or output • Could be based on various nonlinearity measures Examples:

  30. OIP2 Second Order Intercept Point Third Order Intercept Point OIP3 saturated output power 1 dB compression point Output Power [dBm] fundamental Response 2nd Order Response 3rd Order Response Slope = 1 Slope = 2 Slope = 3 IIP3 IIP2 Input Power [dBm] Noise floor

  31. -1 dB compression point test • Go-no-go test: apply Pin, check if Po > OP1dB 1dB OP1dB Output Power (dBm) Output Power (dBm) Test-point Linear region (slope = small-signal gain) Input Power (dBm) IP1dB

  32. 1dB IteratedP1dB Output Power (dBm) Output Power (dBm) nth Test-point Linear region (slope = small-signal gain) 6 8 1 7 4 5 n 2 3 IP1dB Input Power (dBm) Finding -1 dB compression point • Binary search

  33. 1dB IteratedP1dB Output Power (dBm) Output Power (dBm) nth Test-point Linear region (slope = small-signal gain) 2 n 1 3 IP1dB Input Power (dBm) Finding -1 dB compression point • Newton search

  34. Compression 1dB InterpolatedP1dB Output Power (dBm) Output Power (dBm) Test-points Linear region (slope = small-signal gain) Input Power (dBm) IP1dB Finding -1 dB compression point • For multi-site testing: sweep input power, measure output power, interpolate to find 1dB

  35. Ramp power sweep output powercurve 1dB Actual output power curve input power waveform Power actual power Fitted power Test-points IP1dB Time [s] Gains – 1 dB line I/O gain

  36. Gain[dB] straight-line approx. nom. Gain Gnom 1 dB Gk-1 G1dB Gk error Pin,k-1 Pin,k P1dBest Input Power[dBm] Pin,nom P1dB Interpolate on Gain vs input curve

  37. Constant step power sweep Fitted output powercurve 1dB Actual output power curve input power waveform Power actual power Fitted power Test-points Time [s] IP1dB

  38. Book Example 13-7 Small signal gain previous measured to be 19.6 dB,  -1 dB point = 4.176 dBm Does not seem right. Small signal gain seem to be > 19.6 dB.

  39. -2 -4 -6

  40. HW • In the previous discussion, the -1 dB compression point is defined as the gain compression point. That is when the output fundamental component gain is reduced by 1 dB. • In actual measurement, it might be more convenient to measure the total output power instead of the output fundamental component. The expected total output power is input power time small signal gain. • The practical -1 dB compression point is then the point when the output power is 1 dB below the expected total output power. • In the simplified differential case, derive the expression for the -1 dB compression point. This involves solving a quadratic equation. Show that if approximate with a linear equation, your answer is close.

  41. HW optional • Show that only two point I/O power measurements are needed in order to correctly test the -1 dB compression point • Assume a1, a3, and A-1dB are unknown • Assume you have two pair of measurement available: {A1, Po1}, and {A2, Po2}. • Solve for A-1dB in terms of {A1, Po1}, and {A2, Po2}. You can assume that A1 and A2 are close to A-1dB.

  42. HW optional Develop a strategy for multi-site test with only two I/O power measurements • Assume you have N devices of the same product from the same lot. Their a1 and a3 will have small random mismatches. Suppose these random mismatches will cause the -1 dB point to scatter with s (0.5 dB for example) standard deviation. • What values of A1 and A2 should be selected that will work well for all N devices? • Use these two input power for all N devices, obtain 2N measurements of output powers, and compute the -1 dB point for all N devices.

  43. HW optional Now consider the situation when we have lot to lot changes. In the previous case, the mean value of the N devices are assumed to be the same as the nominal values. But as time goes by, the mean value may change. (slowly drifting as process ages, and then a jump back to near nominal, and drifting again, or a jump away indicating bad things happening…) • Assume the mean values are random in a range (may take a uniform distribution, could be -1.5 dB to +1.5 dB). The N devices have normal distribution with these means. The standard deviations are as before. • What value of A1 should be selected for the first power measurement? • Based on the first measurement, the mean error in power is obtained. Based on this, select a second power A2. • Use these 2N measurements of output powers to compute the -1 dB point for all N devices.

  44. HW optional Generalize the above to the following two cases: • Differential structure with both 3rd and 5th order nonlinearities • Single-ended structure with both 2nd and 3rd order nonlinearities. • In these cases, you need to perform 3 I/O power measurements • Argue why these two cases may be sufficient for real applications.

  45. HW • Create or borrow an LNA. • Under nominal condition, estimate -1 dB point • Create 8 copies, introduce random mismatches and PVT variations • Apply your test method to test -1 dB point for all 8 LNAs. • Based on that test, determine amplitude of a two tone test, and test IIP2, IIP3, and DR.

  46. Sensitivity Psig: Input signal power PIN: Input noise power The minimum required signal power at the input terminal in order for the circuit to achieve a specific SNR at the output, which determines the Bit Error Rate (BER)

  47. Example: GSM receiver • Baseband required SNR • 6 dB for static conditions • 9 dB with fading • Channel bandwidth typically 170kHz • 10log(170000)=52.3 dB • GSM specification required sensitivity: • <= -- 102 dBm

  48. Noise temperature Rs RL=Rs Vs I V RL But noise free Maximum delivered power:

  49. Noise measurement Thot, Tcold RF- IN RF- OUT Power Meter Noise Source DUT No,hot, No,cold Noise source generate two noise power, Thot, Tcold Power meter measure two output noise powers: No,hot, No,cold

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