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Domain Measurement Method to Guard Against Preamplifier Saturation Guard Against Preamplifier Saturation

April 10, 2012 Matthew Jackson

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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

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• Ambient noise requirements

– 6 dB below limit (CISPR 2008)

• Problems:

– Stringent limit – Large AF/cable loss – Noisy measuring instrument

• Solutions:

– Quieter measuring instrument – More efficient antenna – Shorter cable or low-loss cable

Use a low noise preamplifier

Background

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Amplifier Saturation

• Amplifier linear dynamic range
• Max output voltage limited by supply voltage
• 1
• 0.8
• 0.6
• 0.4
• 0.2

0.2 0.4 0.6 0.8 1

Amp Input Amp Output

Clipping

To avoid saturation, the input signal to the amplifier must be low enough to keep the amplifier in its linear dynamic range.

Pout (dB) = Pin (dB) + Linear Gain (dB)

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1 dB Compression Point

1dB Compression: 1 MHz

• 15
• 10
• 5

5 10 15 20

• 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

Pin (dBm) Pout (dBm) Amplifer Output Theoretical 1dB Down

Gain = 27 dB Pout 1 dB = Pin 1 dB + Linear Gain – 1 dB

1 dB Compression Point

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Testing with a Preamplifier

• Gain (G) used as a correction factor
• Preamplifier saturation = nonlinear gain
• Nonlinear gain < G
• Control SW corrects Vf

with G

• Report lower emission levels!

Ef = Vf + AF + C1 + C2 + C3 – G

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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 – Remove out-of-band content – Need to know when/where to use – Drawbacks

• Impedance matching
• Insertion loss

f f1 f2 f3

• f1
• f2
• f3

Band-pass Filter

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TD Measurement Method

• Signal Propagation
• E-Field Incident Wave Voltage

f f1 … fN

• fN … -f1

α0 α1 αN α1

*

αN

*

N k e A

k

j k k

, ... , 1 , 2 1 = =

φ

α A = α

Saturation is not always caused by one dominate signal; many small signals can combine to produce a large v(t)

∑

=

+ + =

N k k k k

t f A A t v

1

) 2 cos( ) ( φ π

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TD Measurement Method

• Step 1: Calculate Pin (dBm)
• Where:

Pin = Max input (in dBm) that will not saturate the preamplifier Pout 1 dB = Rated output power at 1 dB compression G = Rated gain (in dB) Scf = Saturation correction factor

• Miteq AM-1300 Example
• Pout

1 dB = 8 dBm

• G = 27
• Pin

= 8 – 27 + 1 – Scf = -18 – Scf

Pin = Pout 1 dB – G + 1 – Scf

Input (dBm) Output (dBm)

• 18

8 Scf Pin

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TD Measurement Method

• Step 2: Convert Pin

to a peak voltage (Vpk )

• Where:

Vpk = Max input (in Volts) that will not saturate the preamplifier Pin = Max input (in dBm) that will not saturate the preamplifier Z = Preamplifier input impedance

• Miteq AM-1300 Example
• Pin

= -18 – Scf Let: Z = 50 Ω; Scf = 3 dB = -21

• Vpk

= ± 19.9 mV

( )( )

( )⎟

⎠ ⎞ ⎜ ⎝ ⎛ ± =

10

10 001 .

in

P pk

Z V

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TD Measurement Method

• Step 3: Correct for cable loss
• Equation:

( ) ( )

20

3 2

10

C C pk limit

V V

+

=

Where:

Vlimit = Adjusted max voltage limit C2 , C3 = Max cable loss (in dB)

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TD Measurement Method

• Step 4: Select an oscilloscope

– Input sensitivity of at least 1 mV/div – BW equal to upper frequency of preamplifier range – Sampling rate: fs > 2∗fmax

• 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)

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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

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TD Measurement Method

Tektronix DPO7254, 2.5 GHz, 40 GS/s

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TD Measurement Method

• Step 7: Compare measurement to Vlimit

Max/Min > Vlimit ? Preamplifier will saturate Preamplifier will NOT saturate No Yes

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• 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

Conclusion

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Questions