Test of PAF Backend Amplitude and Noise Linearity
Date: June 8-9, 2015
Authors: A. Roshi, B. Shillue, B. Simon, S. White
Purpose: To investigate the PAF analog link, downconverter, and data acquisition gain linearity, linear
dynamic range, and consistency of noise temperature measurements at different signal levels. This
investigation seeks to shed light on the January 2015 GBT test in which the beamformed Tsys/h had a
positively correlated dependence on calibration source flux density.
First Test: Linearity
Test Setup:
CIAO 40 dB
amplifier
Noise Source
HP 83640A Signal
Source
GBT Receiver Box
Cross dipole
element
X
1.55.0.5 GHz
Cplr 10
dB
LNA
Analog
F/O Xmtr
BPF
Analog
F/O Rcvr
BYU PAF Downconverter
1.2 GHz
1.9 GHz
700 MHz
400/5 MHz
2.81/0.42 MHz
HPF
LPF
LPF
BPF
BPF
LO1 1600-2300 MHz
LO2 396 MHz
Windows computer
12-bit ADC
Figure 1- Test Setup for Linearity Measurement
Data Storage
The test setup was as shown in Figure 1. The PAF receiver was cold, turned on, and pointed into the
hot load at the OTF test facility in Green Bank. In this configuration, the signal source was set to -50
dBm and 1635 MHz, and LO1 and LO2 were set so the tone fell in the middle of the 420 kHz wide IF.
Also, the noise source attenuator was adjusted to values between 0 and 30 dB.
Figure 2- Normalized power transmitted through the backend is plotted against attenuator value
(stars). Solid line shows values with linear gain term removed.
Normalized power measured through the digital backend is plotted against the input attenuator value.
The stars show the measured power at 1635 MHz (bandwidth ~ 100 Khz) from all the 38 channels. A
tone at frequency 1635 MHz was injected along with the noise source used for linearity measurement.
The plotted power values are normalized with respect to the tone power. We use the tone signal to
estimate the gain. The measured power after correcting for the gain and removing the expected increase
in power is shown by continuous lines. As can be seen the gain estimate using the tone signal
removes the change in gain well beyond the 1 dB compression point. In the figure below we show the
gain corrected value with y-range +/- 1 dB. The variation in gain corrected values are in the range -0.5
to 0.2 dB up to the 1 dB compression point, indicating the tone signal can be used to remove gain
variation in the system.
Figure 3- Closeup of power variation with linear gain removed
Second Test: Receiver Temperature
Test Setup:
GBT Receiver Box
1.55.0.5 GHz
X
Cplr 10
dB
LNA
Analog
F/O Xmtr
BPF
Analog
F/O Rcvr
BYU PAF Downconverter
1.2 GHz
1.9 GHz
700 MHz
400/5 MHz
2.81/0.42 MHz
HPF
LPF
LPF
BPF
BPF
LO1 1600-2300 MHz
LO2 396 MHz
Windows computer
12-bit ADC
Data Storage
Figure 4- test setup for Receiver Noise Temperature vs attenuation test
We measured the receiver temperature of channel Y1 by measuring the hot/cold load noise power ratio
at two locations:
a) at the output of the fiber link (before the downconverter) with a 20 MHz filter, and
b) after the BYU down-converter and digital backend
The measurements at the fiber link output were made with spectrum analyzer near 1635 MHz and with
resolution bandwidth of <= 1 MHz. The measurement through the digital backend was made at 1635
MHz with a bandwidth of ~100 Khz. The receiver temperature calculated using these measured noise
powers and assuming Thot = 300K and Tcold = 8K.
Attenuator value
Trec (K) @ Fiberlink output
0dB 1dB 2 dB 3dB
19.2 18.5 19.2 19.9
Trec (K) @ BYU downconverter/digital backend
22.3
24.1 24.5 24.0
The noise temperature does not show a strong or significant variation with attenuator setting, so we
conclude that neither the system linearity nor the presence of RFI is causing noise temperature
variations. However, the receiver temperature measurement made through the digital receiver on the
average is about 4 K higher. This seems to indicate that something in the backend is adding significant
noise to the PAF system.