LO reference system PDR
Dick Plambeck, 12 Jan 2004
Overview
The LO reference system distributes three reference signals from the control building over optical
fiber to each antenna:



1100-1260 MHz, tunable
10 MHz, fixed (+ 1 pps clock tick)
50 MHz from lobe rotator
To allow for subarrays, the 1100-1260 MHz reference tone may be derived from any of three
independent frequency synthesizers.
To track variations in the propagation delay through the optical fibers, an echo of the 1100-1260
MHz tone is returned from the antenna to the control building over a 4th optical fiber, and
linelength receivers in the lab monitor the roundtrip phase.
Design goals:
1. Minimize phase shifts due to temperature changes or flexure of the fibers.
2. Allow for up to 6 dB excess loss in the fiber, beyond the 3 dB loss expected.
3. Fiber components should contribute minimal additional phase noise to the local oscillators.
4. All boxes with photodiodes should provide a monitor voltage proportional to the
photodiode current, to help diagnose problems with excess loss in the fibers.
Lab components
Figure 1 shows the 1100-1260 MHz distribution system in the lab. Three synthesizers are
provided. Generally one will be used for OVRO+BIMA, another for the SZA, and the third for
tuning or other tests on selected antennas.
Each synthesizer is connected to an LO reference distribution box (Figure 2). This box
amplifies the synthesizer signal and splits it to provide 16 outputs at a level of approximately +13
dBm appropriate to drive the optical transmitters. In addition, each reference distribution box
includes an offset oscillator, phaselocked to frequency (synth + 10 MHz), which is needed by the
linelength receivers; this offset signal also is split 16 ways for distribution to the linelength
receivers. Finally, each LO distribution box incorporates a reference linelength receiver which
directly measures the phase of the outgoing synthesizer frequency.
The outputs of the LO reference boxes are connected to the laser transmitters and linelength
receivers in a coax patch panel. In the future we may be able to replace this with an RF matrix
switch.
Optical transmitter/receiver boxes contain the actual laser transmitters and linelength receivers.
The optical transmitters are Fiber-Span AC223T-1.3 lasers, one per antenna. These operate at
1310 nm and have typical laser output powers of 2 to 4 mW. A linelength receiver is mounted
next to each laser so that the fibers carrying the outgoing and echo reference signals to each
antenna may be kept physically close together. The linelength receivers provide 50 Hz sine waves
to a/d converters in the National Instruments 6031E i/o card; the phases of these low frequency
sine waves change as the delay through the fibers changes with temperature or flexure.
A low frequency synthesizer and splitter box provide a common 9.99995 MHz reference tone to
all the linelength receivers.
An RF multiplexer (Agilent 34970A with 34905A plugin) and counter (Agilent 53181A) are
used to check the frequencies of all the synthesizers and offset frequency generators. The
multiplexer and counter are controlled through an IEEE 488 bus.
Not shown in Figure 1 are the 10 MHz and 50 MHz distribution systems. The 10 MHz
distributor box converts the 10 MHz reference tone to a chain of pulses. A 1 pps clock tick is
used to excise one pulse each second. The pulse train is sent to an Ortel 3120A laser (1310 nm,
approx 1 mW output power). The laser power is split 16 ways in an optical power splitter and
transmitted to the antennas on singlemode fiber.
The ~50 MHz signals from the lobe rotators are coupled to other singlemode fibers for
distribution to the antennas.
Fiber distribution
Figure 3 shows the fiber distribution system. Both the 1100-1260 MHz outgoing signal and its
echo are handled by a single patch panel, since it is important to keep these fibers physically close.
The 10 MHz and 50 MHz signals may be handled either together in another patch panel, or
separately in two additional patch panels.
An optical loss of about 3 dB is expected for each fiber:
 5 FC-APC connectors, 0.35 dB loss each
 2 km fiber, 0.35 dB/km at 1310 nm
 perhaps 10 splices at 0.1 dB each
Antenna electronics
Three boxes on each receiver – the LO terminator box, the 10 MHz receiver, and the 50 MHz
receiver - are used to regenerate the phaselock reference signals at each receiver. These boxes are
described in separated documents.
10 MHz
10 MHz
buffer
110V
8
Agilent 34970A
with 34905A RF
multiplexer
110V
Agilent
53181A
freq counter
chan 2
chan 1
10 MHz
selected
freq
110V
110V
fiber patch cords
110V
f1
LO ref
synthesizer
no. 1
1100-1260
MHz
LO ref
distribution
box no. 1
16
8
f1 +
10 MHz
16
8
optical trx/
rcvr box
no. 1
(ants 1-8)
LO ref out
8
echo back
8
10 MHz
future
matrix
switch
110V
110V
110V
f2
LO ref
synthesizer
no. 2
LO ref
distribution
box no. 2
16
f2 +
10 MHz
16
LL ref 2
(50 Hz)
8
route
coaxes
manually
for now
8
110V
optical trx/
rcvr box
no. 2
(ants 9-16)
LO ref out
8
echo back
8
LO ref
fiber
patch
panel
110V
110V
f3
LO ref
synthesizer
no. 3
LO ref
distribution
box no. 3
9.99995 MHz
16
LL ref 3
(50 Hz)
8
optical trx/
rcvr box
no. 3
(ants 17-24)
LO ref out
8
echo back
8
fiber
9.99995 MHz
splitter box
fiber
distribution
rack
8
3 TTL signals: offset freq phaselock OK
3 analog levels: AGC OK
3 analog levels: 50 Hz ref
110V
TRACEWELL
PCI bus
IEEE-488
8
f3 +
10 MHz
9.99995 MHz
110V
low
frequency
synthesizer
16
NI 6031E
multifunction
I/O card
24 analog levels: 50 Hz linelengths
24 analog levels: linelength rcvr AGC
24 analog levels: linelength rcvr opt pwr in
Ethernet
100BaseT
linelength
computer
Figure 1. 1100-1260 MHz distribution in the lab.
LO ref distribution box
to RF mux/ freq counter
ZC16PD
-24-SMA
ZHL-2-12
C3203-20
1100-1260 MHz
+13 dBm
+28 dBm max
C3203-20
16-way splitter
12+1 dB loss
ZC16PD
-24-SMA
10 MHz
+7 dBm
offset
oscillator
+6 dBm
+7 dBm
C3203-20
12+1 dB loss
reference LL
rcvr
9.99995 MHz
0 dBm
phaselock
status
50 Hz
16-way splitter
15 outputs
+15 dBm
max to
Fiber-Span
lasers
15 outputs
-7 dBm to
linelength
receivers
-35 dB
AGC
level
Figure 2. LO reference distribution box.
rcvr
LO ref
out
FC/APC connector
echo
weathertight
box on side of
antenna
patch
panel
antenna
fiber
splice tray
az wrap
lab
underground cable
Figure 3. Fiber distribution.
elev
wrap