SOFIE Signal Gain Analysis

Mark HervigGATS

SOFIE Analog Signal Path

Vin is the signal into the balance attenuator, after synchronous rectification:

Vin = VA/Dmax * margin

set margin to 1.2, so Vin = 3V * 1.2 = 3.6V

we’ll get Vw and Vs below 3V using the BA’s

for example to get Vexo = 2.95V requires BA = 0.82

The difference signal:

V = (Vin,w * BAw – Vin,s* BAs) * GV

Weak Channel Balance Attenuator attenuation = BAw

Vin,w

Strong Channel Balance Attenuator attenuation = BAs

Vin,s

Differential Amp gain = GV

A/D

14 bits

-3V to 3V

-213 to 213 counts

1 count = 366 V

Balance attenuator setting vs. V balance voltages

We can balance at various voltages to increase dynamic range in V

Little impact on Vw

These curves apply to all channels

Difference signal balance voltage, Vbal:

Vbal = (Vin,w * BAw – Vin,s* BAs) * GV

Solve for BAw

V precision vs. V gain and balance attenuator settings

count precision, CPV = (214 BAs GV)-1

These curves apply to all channels

V precision vs. balance attenuator setting

Count precision, CPV = (214 BA)-1

Lowering BA reduces precision

These curves apply to all channels

Recommended V Gain SettingsChannel, Target

V Gain

Required

V Gain

Recommended

V precision

Required / CBE 1-count

Altitude Range (km)

1. O3 1 3.3 1.0 10-4 / 2.2 10-5 60 - 99

2. PMC 74 300 1.0 10-6 / 2.5 10-7 Cloud

3. H2O 2 96 4.0 10-5 / 7.7 10-7 50 - 98

4. CO2 22 27 3.3 10-6 / 2.8 10-6 55 - 114

5. PMC 7 120 1.0 10-5 / 6.1 10-7 Cloud

6. CH4 30 202 2.5 10-6 / 3.7 10-7 40 - 95

7. CO2 30 36 2.5 10-6 / 2.1 10-6 64 - 121

8. NO 22 300 3.3 10-6 / 2.5 10-7 80 - 127

Assumes 14 bit A/D, -3 to 3V.V balance voltage was –2.5V for all channels.Upper altitude is where the 1st 10 count change occurs.Lower altitude is where V reaches 0.8214 counts.Note that the CBE 1-count precisions are not CBE system noise.Required V precisions are taken as the strong band requirements.Signals were based on atmospheric transmissions calculated using a climatology for summer at 60°N.The analyses that lead to the above results are shown on the following pages.

SOFIE spectral band specifications and channel S/N requirements

Band S/N* # Bits for S/N Range Required Physical Gain

On Diff Channel (Above 214 counts)

O3 strong 1.0104 214 (1.64 104) 2

O3 weak 1.0104 214 (1.64 104) 2

particle strong 1.0106 220 (1.05 106) 128

particle weak 1.0106 220 (1.05 106) 128

H2O weak 2.5104 214 (1.64 104) 2

H2O strong 2.5104 214 (1.64 104) 2

CO2 strong 3.0105 219 (5.24 105) 64

CO2 weak 3.0105 219 (5.24 105) 64

particle strong 1.0105 217 (1.31 105) 16

particle weak 1.0105 217 (1.31 105) 16

CH4 strong 4.0105 219 (5.24 105) 64

CH4 weak 4.0105 219 (5.24 105) 64

CO2 strong 4.0105 219 (5.24 105) 64

CO2 weak 4.0105 219 (5.24 105) 64

NO weak 3.0105 219 (5.24 105) 64

NO strong 3.0105 219 (5.24 105) 64

(Chad) SOFIE Radiometric Measurement End-to-End Required SNRs

Detector Band A

Detector Band B

PC Detector Channel Pair

Front End

PreAmp

G1

Chopped (1 kHz) Optical Input

G4

Data Acquisition

G2

LPF (2.15 kHz)

u1x1

14-bit ADC

MUX

Ground Processing(2 Hz Information Bandwidth,

3.14 Hz Effective Noise Bandwidth)

20 Hz Sampling Rate32 X Oversampling

Bit Resolution = 366uVLPF Settling to 12 RC Constants

BPF (1 kHz)

BPF (1 kHz)

Balance Attenuation

A1

Difference Amp

G3

Phase Reference (x3)

Optical Chopper (1 kHz)

Chopper Drive Control

Chopper Reference Signal Phase Control

1us Resolution

Balance Attenuation Controlled by Software

Phase Sensitive Detectors(Synchronous Rectification)

SWITCHING DEMODULATION

LPF (10 Hz)G =1

G = -1

DemodulatedSignal

BPF (1 kHz)

SWITCHING DEMODULATION

LPF (10 Hz)G =1

G = -1

SWITCHING DEMODULATION

LPF (10 Hz)G =1

G = -1

A2

Jumper

Jumper

Jumper

G5

G6

Phase Adjustment?

Phase Adjustment?

SOFIE PC Radiometric Electronics Overview

– Carrier Frequency = 1kHz, Modulation = 2 Hz, Effective Sync Rect Q = 500– Nominal Equal System-wide Phasing: Butterworth and Bessel Filters

1) O3 channel profiles

Balance V at –2.5V

BAs = 0.819

BAw = 0.613

GV = 3.3

V saturates at 60 km, or 0.8*214 counts

1) O3 Channel useful altitude

Useful altitude of V signals is determined by the V gain and balance attenuator settings.

Baseline altitude range will be determined by GV

Adjusting the BA settings changes the altitude range very little in this case

2) SW particle channel profiles

Balance V at –2.5V

BAs = 0.819

BAw = 0.814

GV = 300

V useable through typical PMC

3) H2O channel profiles

Balance V at –2.5V

BAs = 0.819

BAw = 0.812

GV = 96

V saturates at 50 km, or 0.8*214 counts

3) H2O Channel useful altitude

Useful altitude of V signals is determined by the V gain and balance attenuator settings.

Baseline altitude range will be determined by GV

We can adjust the BA settings to change our altitude range

3) H2O channel useful altitude

The V gain to saturate at 50 km altitude changes with V balance voltage

Decreasing the V balance voltage increases the dynamic range

With increased dynamic range, we can tolerate an increase in the V gain

The figure shows the V gain that saturates V at 50 km vs. Vbal

Decreasing Vbal allows us to get more precision and dynamic range

4) 2.8 m CO2 channel profiles

4) 2.8 m CO2 channel useful altitude

Useful altitude of V signals is determined by the V gain and balance attenuator settings.

Baseline altitude range will be determined by GV

Adjusting the BA settings changes the altitude range

5) IR particle channel profiles

Balance V at –2.5V

BAs = 0.819

BAw = 0.814

GV = 120

V useable through typical PMC

6) CH4 channel profiles

Balance V at –2.5V

BAs = 0.819

BAw = 0.816

GV = 202

V saturates at 40 km, or 0.8*214 counts

6) CH4 Channel useful altitude

Useful altitude of V signals is determined by the V gain and balance attenuator settings.

Baseline altitude range will be determined by GV

Adjusting the BA settings changes the altitude range

7) 4.3 m CO2 channel profiles

7) 4.3 m CO2 channel useful altitude

Useful altitude of V signals is determined by the V gain and balance attenuator settings.

Baseline altitude range will be determined by GV

Adjusting the BA settings changes the altitude range

8) NO channel profiles

Balance V at –2.5V

BAs = 0.819

BAw = 0.817

GV = 300

V never saturates, but the signal is dominated by atmospheric interference below 80 km.