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Figure 1. - Block Diagram 3. Description of Hardware for Phase-Coherent Holography Figure 1. - Block Diagram
3. Description of Hardware for Phase-Coherent Holography
This section describes the choice of hardware for the holography system that will be used
to measure the surface profile of the MMA 12 meter antenna. The hardware required for phase
coherent holography consists of a beacon transmitter operating at 92.4 GHz and a prime focus
dual channel receiver. A simplified diagram of the system in shown in Figure 1.
The transmitter is located at a distance of 300 meters from the antenna and it is mounted
on top of a 61 meters tower. The signal from the transmitter is received via two feeds. One direct
from the transmitter using a 10 cm diameter feed (reference feed) on the front of the receiver; the
other from the main feed that is used to illuminate the main reflector in a conventional way. The signals from the two feeds are down converted to a 340 MHz IF using common local
oscillators and are sent to the antenna receiver cabin via coaxial cables. Further down conversion
in the holography receiver back-end converts the two signals to a nominal 21.4 MHz frequency
suitable to be applied to the HP 8508A Vector Voltmeter. The voltmeter detects the two signals
and generates two analog outputs, one is the amplitude of the main channel signal, the other is
the phase difference between the reference and main channels. These two analog outputs are then
sent to A/D converters and the converters outputs are sent to the antenna M&C bus through an
interface box.
The basic hardware requirements are:
Phase accuracy:
< 0.3 degree RMS
Amplitude accuracy:
< 1%
Dynamic range:
$ 43 dB
SNR
$ 40 dB
Channel cross-talk:
> 100 dB
Data rate:
> 100 samples/second
Interfaced to the antenna M&C bus.
Phase accuracy, dynamic range and SNR values are calculated for a pathlength error of
10 microns and
8 = 8.2 mm.
The holography system design described in this document is based on a system built by
the Smithsonian Observatory to measure the SMA antennas at the Haystack test site. The SMA
holography system (Zhang, 1996) easily achieved phase accuracy of 1 degree and lab test result
shows measurement accuracy to the level of 0.1 degree.
3.1 Transmitter Location and Electronics
The transmitter signal source is a Gunn oscillator operating at a frequency of 92.4 GHz.
A simplified block diagram of the transmitter is shown in Figure 2. Two main circuits, phase
lock and automatic gain control (AGC), provides frequency and amplitude stability. Also, the
AGC system provides a convenient way for verifying if the holography receiver signal levels are
not saturating by inserting a calibrated power attenuation value in the signal path. Auxiliary
circuitry provides remote monitor and control of all important functions.
The transmitter box will be installed on or near the top of a 61 meters (200 ft) tower
erected 300 meters away from the MMA antenna at the VLA test site or on the 12 meter Kitt
Peak site for pre-testing the holography system performance. Under calm or low wind conditions,
the tower will provide a stable platform for the beacon transmitter. A drawing of this
arrangement is shown in Fig. 3. The choice of tower height is based on price, easy of assembly
and stability problems. The cost estimate shown in Figure 3 doesnt include assembly of the
tower. This cost can be as high as $30k for a 91 meters (300 ft) guyed tower. Figure 2
The enclosure that contains all transmitter components is weather proofed and
temperature controlled in order to withstand extreme weather conditions when mounted on the
tower.

Specifications:
Frequency:
92.4 GHz
Frequency stability:
# ±5Hz/day @ 92.04 GHz
Output power:
10mW at feed input (nominal)
Output power stability:
< 0.1%
Feed horn:
conical horn (1.63cm dia.), WR-10 waveguide, 21dB gain
All waveguide components are WR-10 size (75-110GHz band). Gunn
Oscillator
Power
Detector
92.4 GHz
Gunn
Bias
Isolator
Coupler
Coupler
AGC
Feed Horn
Monitor
&
Control
Electronics
Telemetry
Tx/Rx
Level
Modulator
10MHz
OCXO
9.22
GHz
200MHz
Lock
Refer.
Harmonic
Mixer
Weather Proof/Temperature Controlled Enclosure
Radio or
Optical Fiber
Data Link
LO Ref.
F = (9220MHz x 10) + 200MHz
10mW min.
Power
Supplies
TX
Lock Box
AGC
Tripl.
IF processor
Detector
Frequency/Phase Lock Box
Figure 3: 92.4 GHz Transmitter Block Diagram
3.1.1 Transmitter Phase Lock System
This system uses phase lock loop that locks the output frequency of a Gunn oscillator to a
low phase noise, high stability reference signal employing conventional heterodyne technique.
A part of the 92.4 GHz signal is downconverted to an IF near 200 MHz by mixing it with a
harmonic N (N = 10) of the output frequency of a phase locked oscillator at 9.22 GHz. The IF
and LO signal paths are separated in the triplexer.
The IF signal is amplified, filtered and adjusted to a constant level in an AGC circuit. In
the digital frequency/phase detector chip, phase and frequency of the IF signal are compared with
those of a reference signal from a second phase locked oscillator operating at 200 MHz. The
detectors output is an ±error signal proportional to the frequency and phase difference between
the 200 MHz IF and the 200 MHz reference signal. Frequency and phase lock acquisition is
achieved by applying this error signal to the Gunn bias tuning. The frequency locking range is ±
100 MHz. The output of the phase-locked Gunn oscillator is determined by:
Fgunn = (9220 MHz x 10) + 200 MHz Both the 9.22 GHz PLO an the 200 MHz PLO are locked to a common 10 MHz OCXO
which determines the long term stability of the output frequency of the phase-locked Gunn.
The OCXO stability specs are:
# ±5 x 1E-11/day
# ±5 x 1E-9 for -40B to +50B C
Short term stability:
100ms:
# ±1 x 1E-11

1s:
# ±1 x 1E-12

3.1.2 Transmitter Automatic Gain Control (AGC)
The transmitter output is leveled by an AGC circuit. A portion of the Gunn signal is
coupled to a power detector. The detector produces a voltage proportional to the incident power.
This voltage is connected to one of the inputs of a differential amplifier and the other input is
connected to a reference level that sets the desired output power of the transmitter. The output of
the differential amplifier is buffered and applied to a voltage controlled ferrite modulator that is
in the line before the detectors directional coupler. The ferrite modulator voltage drive is
adjusted by the loop until the detected power level matches the selected output level.
The transmitter output power is adjusted to 10mW measured at the feed horn input and
the maximum output power available is estimated at 20mW. Calibrated attenuation values of
3dB and 10dB can be inserted by the AGC circuit. This is an useful feature when troubleshooting
problems in the receiver system..

3.1.3 Auxiliary circuitry
Auxiliary circuitry built into the transmitter provides temperature control of the
transmitter enclosure and remote monitor and control of the Gunn oscillator output power and
phase-lock status. The transmitter telemetry is sent to the control room via an optical fiber link or
radio transceiver link. The choice of data link type between the tx tower and control room will be
made on basis of cost and other considerations (installation, RFI etc).
3.2 Dual Channel Receiver Electronics
A detailed block diagram of the receiver system is shown in Figure 4. The signals from
the main feed and the reference feed are down converted and amplified in identical IF chains.
The IF gain of both channels is adjusted to offset the large signal level difference between the
main and reference channels.
A phase locked Gunn oscillator operating at 92.06 GHz provides the common LO for a
pair of room temperature balanced mixers. Using a common oscillator makes the coherence
problem more tractable. Enough isolation is built in the LO path to decrease channel cross-talk
levels by more than 120 dB. 92.06 GHz
Gunn
Oscillator
Frequency/Phase
Lock Box
MHz
PLO
Ø Lock
Refer.
Refer.
Gunn
200MHz
9186
Ø-Lock
IF
Gunn
Ref.
200 MHz
9186 MHz
318.6 MHz
Reference
Channel
Main
Channel
FGunn= (9186 MHz x 10)
340 MHz
4 MHz BW
340 MHz
4 MHz BW
21.4MHz
400KHz BW
Harm
Mixer
PRIME FOCUS RECEIVER
92.4 GHz
Weather Proof/Temperature controlled enclosure
A Input
B Input
HP 8508A
Vector
Voltmeter
14 bit
A/D
A/D
14 bit
B Mag.
B-A Phase
ANTENNA RECEIVER CABIN
10 MHz
Standard
M&C BUS
Interface
Box
+200 MHz
21.4MHz
400KHz BW
-52dBm
-11dBm
CLK
-3dBm
-3dBm
60
20
10mW
10mW
0dBm
0dBm
10dB
10dB
20
20
0-10dB
0-10dB
S&H
S&H
-7db
-7dB
-2dB
-2dB
-7dB
-7db
T SYS = 1600K DSB
= 1600K DSB
SYS
T
Figure 4: Dual Channel Rece