Time- -Domain Measurement Method to Domain Measurement Method to Time Guard Against Preamplifier Saturation Guard Against Preamplifier Saturation April 10, 2012 Matthew Jackson
Outline • Background • Amplifier Saturation • 1 dB Compression Point • Testing with a Preamplifier • Saturation Analysis Methods – Attenuator Check – Band-pass Filter • Time-Domain Measurement Method • Conclusion • Questions 2 2012
Background • Ambient noise requirements – 6 dB below limit (CISPR 2008) • Problems: – Stringent limit – Large AF/cable loss – Noisy measuring instrument • Solutions: – Quieter measuring instrument Use a low noise – More efficient antenna preamplifier – Shorter cable or low-loss cable 3 2012
Amplifier Saturation • Amplifier linear dynamic range P out (dB) = P in (dB) + Linear Gain (dB) • Max output voltage limited by supply voltage Amp Input Amp Output 1 0.8 0.6 0.4 0.2 0 -0.2 -0.4 -0.6 -0.8 -1 Clipping To avoid saturation, the input signal to the amplifier must be low enough to keep the amplifier in its linear dynamic range. 4 2012
1 dB Compression Point 1dB Compression: 1 MHz Amplifer Output Theoretical 1dB Down 20 P out 1 dB = P in 1 dB + Linear Gain – 1 dB 15 10 5 P out (dBm) 1 dB Compression Point 0 -5 -10 -15 -40 -39 -38 -37 -36 -35 -34 -33 -32 -31 -30 -29 -28 -27 -26 -25 -24 -23 -22 -21 -20 -19 -18 -17 -16 -15 -14 -13 -12 -11 -10 P in (dBm) Gain = 27 dB 5 2012
Testing with a Preamplifier • Gain (G) used as a correction factor E f = V f + AF + C 1 + C 2 + C 3 – G • Preamplifier saturation = nonlinear gain • Nonlinear gain < G • Control SW corrects V f with G • Report lower emission levels! 6 2012
Saturation Analysis Methods • Attenuator Check – Consecutive scans w/ different input attenuation – Compare magnitude responses – Difficult to evaluate BB disturbances • Band-pass Filter – Install at preamplifier input Band-pass – Remove out-of-band content Filter – Need to know when/where to use – Drawbacks -f 3 -f 2 -f 1 f 1 f 2 f 3 f • Impedance matching • Insertion loss 7 2012
TD Measurement Method • Signal Propagation • E-Field Incident Wave Voltage α = α 0 A 0 0 φ 1 j α 1 α 1 α = = * A e k , k 1 , ... , N k k N 2 ∑ α N α N * = + π + φ v ( t ) A A cos( 2 f t ) 0 k k k = k 1 -f N … -f 1 f 1 … f N f Saturation is not always caused by one dominate signal; 8 many small signals can combine to produce a large v(t) 2012
TD Measurement Method • Step 1 : Calculate P in (dBm) P in = P out 1 dB – G + 1 – S cf • Where: P in = Max input (in dBm) that will not saturate the preamplifier P out 1 dB = Rated output power at 1 dB compression G = Rated gain (in dB) S cf = Saturation correction factor Output (dBm) 8 • Miteq AM-1300 Example • P out 1 dB = 8 dBm S cf • G = 27 P in • P in = 8 – 27 + 1 – S cf -18 = -18 – S cf Input (dBm) 9 2012
TD Measurement Method • Step 2 : Convert P in to a peak voltage (V pk ) ( ) ⎟ ( )( ) ⎛ ⎞ P 10 = ± ⎜ V Z 0 . 001 10 in pk ⎝ ⎠ • Where: V pk = Max input (in Volts) that will not saturate the preamplifier P in = Max input (in dBm) that will not saturate the preamplifier Z = Preamplifier input impedance • Miteq AM-1300 Example • P in = -18 – S cf Let: Z = 50 Ω ; S cf = 3 dB = -21 • V pk = ± 19.9 mV 10 2012
TD Measurement Method • Step 3 : Correct for cable loss • Equation: Where: V limit = Adjusted max voltage limit V pk = C 2 , C 3 = Max cable loss (in dB) V ( ( ) ) limit + C C 20 10 2 3 11 2012
TD Measurement Method • Step 4 : Select an oscilloscope – Input sensitivity of at least 1 mV/div – BW equal to upper frequency of preamplifier range > 2 ∗ f max – Sampling rate: f s • Step 5 : Adjust oscilloscope 1. Set input impedance to 50 Ω 2. Turn ON max and min measurements 3. Turn ON measurement statistics 4. Trigger on rising and falling edges 5. Turn ON infinite persistence 6. Limit input bandwidth (optional) 12 2012
TD Measurement Method • Step 6 : Setup equipment & measure emissions • Adjust scope horizontal/vertical scales to monitor v(t) • Measure for 5-10 minutes, or until no higher levels • Repeat for each antenna and antenna polarization • Repeat for each DUT orientation and DUT mode 13 2012
TD Measurement Method Tektronix DPO7254, 2.5 GHz, 40 GS/s 14 2012
TD Measurement Method • Step 7 : Compare measurement to V limit Max/Min > Preamplifier will No V limit ? NOT saturate Yes Preamplifier will saturate 15 2012
Conclusion • TD Measurement Method – Preamplifiers have a max input voltage – Exceeding the max input voltage → saturation – Find the max permissible input voltage – Use an o-scope, and measure the input voltage – Verify the input voltage < max input voltage • Advantages – Theoretically sound – Eliminates BB disturbance uncertainty – Comprehensive check – Conservative evaluation – Simple setup 16 2012
2012 Questions 17
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