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NATIONAL RADIO ASTRONOMY OBSERVATORYGreen Bank, West Virginia 24944
Electronics Division Internal Report No. 126
MICROWAVE LINK FOR DATA TRANSMISSION
N. V. G. Sarma
JANUARY 1973
NUMBER OF COPIES: 150
MICROWAVE LINK FOR DATA TRANSMISSION
N. V. G. Sarma
Introduction
The success of any long baseline interferometer observations in radio astronomy
depends upon the solution of two technical aspects, namely, provision of coherent local
oscillator signals at the observing stations and the method of bringing the IF signals,
without affecting their phase relationship, to a central location for real time correlation.
By the very nature of baseline lengths involved (10 km to 50 km), cable transmission
is not a very attractive solution because of the signal losses and dispersion in the cable.
Microwave link is a feasible solution to achieve the above objectives and the present
report describes such a system for transmission of data from a remote station to the
control station in which the phase fluctuations due to atmospheric variations are largely
eliminated.
1. 0 Design Considerations
The NRAO 3-element interferometer is a double sideband interferometer and has
a maximum baseline of 2. 7 km and it was felt necessary to extend the baseline to 35 km
by locating a smaller dish (45 ft. dia. ) at the remote station so that it can be made very
effective for the study of small diameter sources such as quasars and nuclei of some
external galaxies. The interferometer operates either in dual frequency mode (11 cm
and 3. 7 cm) or in dual polarization mode at one of the above wavelengths. It can also
be operated as a line interferometer at 21 cm wavelength. Hence, the microwave link
should be capable of transmitting the above data to the control station and the design
specifications are as follows:
1. The carrier frequency of the link should be high so that it (including its
harmonics) will not interfere with the existing radio astronomy frequencies that are
being used on the site at Green Bank.
2. Two IF channels, each 30 MHz wide (5-35 MHz) should be brought back to the
control station and the isolation between the two bands should be at least 40 dB.
2
3. Digital data giving the information of telescope position and other param-
eters such as temperature inside the front-end box and total power outputs from the
front-end box, etc., should be carried on the link.
4. A 100 kHz signal should be sent back to the control station for use in the
phase lock loop of the phase coherent local oscillator system.
5. Voice channel should be provided for communication between the two sta-
tions.
Based on the above criteria, it is decided that:
1. The main carrier frequency of the link will be 17. 5 GHz.
2. The two IF bands will be carried as double sideband (AM) signals on the
main carrier.
3. Digital data will be carried as FM on a sub-carrier at 87 MHz which, in
turn, is transmitted as DSB signal on the main carrier.
4. The 100 kHz and voice signals will be carried as FM on another sub-carrier
at 90 MHz which, in turn, is carried as DSB signal on the main carrier.
As the NRAO 3-element interferometer is a double sideband type, the require-
ments on the stability of IF phases and delay time are not so stringent when compared
to a SSB type. However, these variations should be much less than the minimum delay
steps introduced in the system for delay tracking. Measurements of path length varia-
tions at 9. 6 and 34. 5 GHz over a path length of 64. 25 km (ref. 1) shows that one could
expect short-term (20 min. duration) variations of about 0. 2 ns, whereas the long-term
(24 hr. duration) variations could be about 1 ns. These variations are quite acceptable
in the case of NRAO interfereometer.
1. 1 Transmitter Power Requirements
Maximum baseline ..................................... 32 km
Minimum signal/noise at the receiver • • 20 dB
Transmitting and receiving antennae ....... 6 ft. paraboloids
Receiver system noise temperature ....... 1400 °K
3
Receiver bandwidth ..................................... 200 MHz
Free space path loss ................................... 149 dB
Loss due to multipath fading ....... ............. 20 dB
(ref. 2) 99% reliability
Loss due to rainfall (ref. 2) ...................... 30 dB
(rate 10 mm/hr)
Total (maximum) path loss ................ 199 dB
Net path loss (after subtracting ................. 199 - 96 = 103 dB
transmitting and receiving antenna
gains)
Receiver input noise power ........................ k T...... AF = -84 dBmsysSignal power required (20 dB S/N) ........... -64 dBm
Signal power required with syn-
chronous detection ............................... -64 - 3 = -67 dBm
• . Transmitted power ............................. -67 + 103 = + 36 dBm = 4 watts
A traveling wave tube amplifier operating at 17. 5 GHz can be used immediately follow-
ing the double sideband modulator and will be able to supply the required power.
2. 0 General Descri tion of the System
A block diagram of the microwave link system is shown in Figure 1. CH 2 IF
is converted up by means of 42 MHz local oscillator signal to occupy the band 47-77
MHz. In addition, part of the 42 MHz signal is also added to the spectrum so that it
can be used to convert down the CH 2 IF to its original spectrum (5-35 MHz) at the
receiving end. FM sub-carriers carrying the other data are also added to the combined
IF spectrum and then used to modulate the 17. 5 GHz main carrier in a double balanced
mixer. The resulting spectrum is shown in Figure 2a. The output from the DSB modu-
lator goes through a current-controlled ferrite attenuator and then amplified in a TWT
amplifier to the required power level. On the receiving end, the received power is
first amplified in a low-noise tunnel diode amplifier and then mixed with a 17. 4 GHz
4
local oscillator signal to obtain the spectrum as shown in Figure 2b. The resulting
spectrum is symmetrical with respect to the 100 MHz signal which is the difference
between the link carrier frequency and the local oscillator signal of the first converter.
Assuming that these two oscillators (17. 5 GHz and 17. 4 GHz) are stable oscillators,
one can see that the resulting 100 MHz signal contains the information regarding phase
and amplitude variations caused by the atmospheric variations in the link path. Hence,
this 100 MHz signal is used to control the amplitude of the signal spectrum (AGC action)
and also generate a second local oscillator (VCX0) signal whose phase is the same as
that of the 100 MHz signal in a phase lock loop. After mixing the IF spectrum with the
phase locked oscillator (100 MHz) signal, we get back the spectrum with which we
originally modulated the 17. 5 GHz carrier. The received spectrum, after second
conversion, is shown in Figure 2c. CH 1 IF is separated out by means of a low pass
filter and CH 2 IF is converted down to its original spectrum by means of the 42 MHz
carrier. FM sub-carriers are separated out and detected in the FM receiver.
One technique which is employed and found very useful in obtaining good isola-
tion between the two IF bands to introduce a certain amount of delay (60 ns in the
present case) in one of the IF bands before modulation on the main carrier and then
introduce the same amount of delay in the other IF band after they are separated in the
receiver.
2. 1 Double Sideband Modulator and Transmitter
Block diagram of the DSB modulator and transmitter is shown in Figure 3. The
carrier oscillator (17. 5 GHz) is a phase locked Gunn oscillator manufactured by Micro
mega. The primary reference is a crystal controlled oscillator around 100 MHz. The
quoted stability of the phase locked oscillator is 1 p. p. m. over a 24-hour period and
puts out 25 mW of power. The Gunn oscillator can also be locked to an external oscillator
of better stability and about the same frequency as that of the crystal oscillator. A
current-controlled ferrite attenuator is introduced between the DSB modulator and TWT
amplifier so that the transmitter power could be controlled (remote operation) in case of
severe fading due to bad atmospheric conditions. The TWT amplifier is capable of
putting out 10 watts (saturated) power and has a maximum gain of 48 dB. Output power
5
from the TWT amplifier is monitored by means of a 20 dB directional coupler and dis-
played on the front panel meter. A photograph of the front panel and the component
layout inside the chassis of the DSB modulator is shown in Figure 4.
2. 2 FM Modulator
A block diagram of the FM modulator is shown in Figure 5a. Detailed circuit
diagrams are given in Figure 5b. A photograph of the front panel view and the com-
ponent layout is shown in Figure 6. Voltage-controlled transistor oscillators are used
as FM modulators at both the frequencies, 87 and 90 MHz. In order to improve the cen-
ter frequency stability of the VCO, a portion of the output is fed to an accurafely tuned
frequency discriminator which is included in a feedbacksloop containing the FM driver
and VCO. FSK modulation is employed in the case of the digital data which is at a rate
of 2. 5 kHz. Modulation index can be adjusted by means of a front panel control and is
usually set at 3 or more. It is also displayed on the front panel meter. In the case of
100 kHz signal, modulation index cannot be increased beyond 1 because of limitations
on the maximum rate of VCO tune. Audio modulation index is generally set by listening
to the voice quality coming down the link.
3. 0 Receiver — Front-End Box
This box is mounted at the back of the 6-ft dish and is temperature controlled.
The power received by the antenna is first amplified in a low noise (NF = 5. 5 dB) tunnel
diode amplifier of medium gain (G = 15. 5 dB) operating at 17. 5 GHz and then mixed
with a local oscillator signal at 17. 4 GHz in a waveguide balanced mixer. The local
oscillator signal is obtained from a phase locked Gunn oscillator, similar to the one used
in the transmitter. The converted spectrum (0-200 MHz) is then amplified in a low noise
(2. 8 dB) wideband IF amplifier and sent down the tower to the control building. A block
diagram of the electronics contained in the front-end box is shown in Figure 7.
3. 1 Receiver
Block diagrams of the main receiver are shown in Figures 8a and 8b and the
individual circuits are shown in Figure 8c. A photograph of the front panel, top and
bottom views of the receiver chassis, is shown in Figure 9, The incoming signal
6
spectrum (0-200 MHz) is first amplified and then passed through a MIC amp -attenuator
whose AGC terminal is controlled by the amplitude of the 100 MHz signal. The bias to
the control terminal is also displayed on the front panel meter marked receiver gain
and the mean level can be set at any desired gain level by means of a 10-turn poten-
tiometer (marked gain) accessible on the front panel. When set at the mid-position on
the meter, AGC action is effective tn . approximately 25 dB variation of RF input signal
to the attenuator. If the input signal varies beyond the above range (due to increased
path losses because of heavy rain, etc. ), transmitted power may be controlled by means
of the current-controlled ferrite attenuator inserted before the TWT amplifier. Table 1
gives the total RF power (as measured by means of a power meter) at the 100 MHz
mixer for different settings of gain potentiometer, assuming AGC action is effective.
TABLE 1
Gain Power Input toPotentiometer 100 MHz Mixer
Setting
0 16.75 -14.4
10 -12. 515 -11. 020 -9.925 9.030 -8.135 -7.440 -6.750 5.6
Apart from generating the AGC voltage, the 100 MHz signal is also used to
generate a phase-coherent local oscillator signal (100 MHz) in a phase lock loop as
shown in the block diagram. Phase of the 100 MHz RF input to the mixer can be made
exactly equal to the local oscillator signal derived from PLL by means of the phase
trimmer provided in the PLL branch. When this happens, the amplitude of the con-
verted signals will be maximum. The loop filter in the phase lock system is a passive
second order loop followed by a DC amplifier of gain of 100. The loop bandwidth is
7
5 kHz. The phase lock loop has a pull-in range of ± 30 kHz and a lock-in range of
± 300 kHz. VCXO has harmonics separated every 2 MHz and so we have to use a
band pass filter in order to reduce the harmonics. They are about 50 dB down after
the filter. Cosine output from the phase comparator is used for display on the front
panel meter to indicate the condition of lock. 100 MHz PLL MONITOR output on the
front panel can be connected to a scope while obtaining lock by means of bias knob on
the front panel. This bias effectively increases the pull-in range to ± 80 kHz.
After the second conversion in the 100 MHz mixer, the whole input spec-
trum gets folded and occupies from 0-90 MHz. FM side bands are routed to the FM
receiver while IF sidebands are separated in the IF Processor. CH 1 IF is sepa-
rated by means of a low pass filter and amplified to the required level. CH 2 IF is
separated by means of a high pass filter and down conversion with the help of the
42 MHz sub-carrier present in the spectrum. Total power present in the two IF bands
can be monitored by means of the front panel meter. They are also present at the back
connectors for computer sensing. LOCK indicator voltage (approximately 7 V) is also
available at the back panel connector for display at the console.
3.2 FM Receiver
87 MHz and 90 MHz FM sub-carriers are first separated by means of band
pass filters and then passed through limiters before they are fed to frequency discrimi-
nators. Block diagram of the FM receiver is shown in Figure 10a while Figure 10b
shows the individual circuits. A photograph of the front panel and component layout
inside the chassis is shown in Figure 11. Digital data and 100 kHz signals coming out
of the FM receiver are monitored by means of the front panel meter.
4.0 Operation and Maintenance
Dual frequency or dual polarization operation: Connect the two IF outputs
from the 45-ft front-end box to CH 1 and CH 2 IF inputs at the back panel of the DSB
modulator chassis. All the signal levels in the modulator are adjusted for -3 dBm in-
puts of IF bands. Connect digital data, 100 kHz and voice signals to the input connec-
tors provided at the back panel of the FM modulator chassis. With the front panel switch
8
in DIG-DATA position, adjust the modulation index (screw driver adjustment) to ap-
proximately mid-scale (10 V scale) on the front panel meter. Next switch to 100 kHz
and adjust the modulation index to full scale (10 V scale). Remove the 100 kHz input,
keep the switch in AUDIO and adjust the AUDIO ADJ. so that the meter reads approxi-
mately half scale (1 V scale). Connect back the 100 kHz input. For switching on the
TWT amplifier, follow the instructions given in the instruction manual. The front
panel meter on the DSB modulator is calibrated such that it reads mid-scale for 1 watt
power at the TWT amplifier output.
At the receiver end of the control station, one may have to turn the PLL
bias knob to obtain the lock. Turn the knob to mid-position after obtaining the lock.
Lock indicator reading should be around 0. 7 on the front panel meter. If the gain
meter reads full scale, front panel attenuator may have to be decreased because the
input signal level is low. The gain meter reading will also be affected by the gain pot
setting. Adjust the gain pot so that the total power outputs of CH 1 and CH 2 read ap-
proximately 0. 3 on the front panel meter. This will correspond to -3 dBm IF outputs.
Whenever the gain pot is adjusted, care has to be taken to retune the phase trimmer
for maximum IF outputs. The front panel attenuator may be adjusted so that the gain
meter reads approximately 0. 4. In this position, AGC action is effective for 25 dB
variation of signal input.
For line frequency (21 cm) operation, terminate CH 2 at the power combiner
as shown in Figure 3. Feed the line IF to CH 1. On the receiver end, by-pass the
low pass filter in CH 1 as shown in Figure 8b so that the whole spectrum (22 MHz
82 MHz) is available at CH 1 IF out. One may have to introduce a sharp cut-off low
pass filter (0-82 MHz) if the FM sub-carriers (87 and 90 MHz) give trouble.
Maintenance of the equipment is relatively simple. On the transmitter side,
one may have to retune the frequency discriminators located in the FM modulator
chassis if the center frequency drift is excessive. Zero crossing frequency of the fre-
quency discriminator determines the center frequency of that particular VCO. It can
be adjusted either by retuning the detectors or by adjusting zero adj. potentiometers.
The latter adjustment may introduce a slight non-linearity. It can also be adjusted by
the CENTER FREQ. adjust potentiometer in the FM driver and feedback control box in
the FM modulator chassis. TWT amplifier tube may have to be replaced after 10, 000
9
hours of operation. The instruction manual may be referred to for that information.
On the receiver end, we do not expect major maintenance. Frequency dis-
criminators in the FM receiver chassis may have to be retuned if any similar changes
are made in the transmitter side. Most of the performance , checks are monitored on
the front panel meters and hence trouble-shooting is relatively easy. If it becomes
difficult to obtain lock, check the input (IF input) to the receiver on a spectrum ana-
lyzer. 100 MHz signal should be at least -50 dBm for satisfactory operation.
The microwave link system has been field tested on a shorter baseline
(approximately 2. 5 km) with a horn as the transmitting antenna and the 6-ft dish as
the receiving antenna. With a transmitting power of 1 watt, it gave reasonably good
performance even though signal strengths are about 16 dB less than the predicted
values. It is believed that this is attributable to the obstruction caused by trees along
the transmission path.
Acknowledgements
I am very grateful to Dr. S. Weinreb for his continued encouragement and
advice during the progress of this project and also for making this visit to NRAO pos-
sible by offering me a visiting appointment. I have had many helpful discussions with
J. Coe during the course of this project and received a great deal of help during the
testing stage. R. Ervine assisted in construction of this equipment.
References
1 H. B. Janes, M. C. Thompson, Jr. , D. Smith and A. W. Kirkpatrick,
"Comparison of Simultaneous Line-of-Sight Signals at 9. 6 and 34. 5 GHz",
LE_LE_Ztamae_Antennas and Propagation, AP-18, pp. 447-451, July 1970.
2. Reference Data for Radio Engineers, 5th Edition, ITT.
- 10 -
List of Figures
Figure 1 Block Diagram of the Microwave Link
Figure 2 Sideband Locations in the Spectrum
Figure 3 Double Sideband Modulator and Transmitter — Block Diagram
Figure 4 Front Panel and Top View of the DSB Modulator
Figure 5a FM Modulator — Block Diagram
Figure 5b FM Modulator — Individual Circuits
Figure 6 Front Panel and Top View of the FM Modulator
Figure 7 Link Receiver Front-End Box — Block Diagram
Figure 8a Link Receiver — Block Diagram
Figure 8b Link Receiver — IF Processor — Block Diagram
Figure 8c Link Receiver — Individual Circuits
Figure 9 Front Panel and Top and Bottom Views of the Receiver
Figure 10a FM Receiver — Block Diagram
Figure 10b FM Receiver — Individual Circuits
Figure 11 Front Panel and Top View of the FM Receiver
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lock
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Figure 9 — Front Panel and Top and Bottom Views of the Receiver
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10
a —
FM
Receiv
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— B
lock
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m
IN 1
51
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Fig
ure
10b —
FM
Rec
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— I
ndiv
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Figure 11 — Front Panel and Top View of the FM Receiver