before digital communications was widely adopted, there was a great
need for stable frequency-determining devices / systems. That is
to say, low bit error rates (BER) for digital communications are
not the sole motivation for oscillators with low short-term and
long-term stability and low levels of jitter. One obvious need for
precise frequency control is radar, in order for accurate ranging
(the second 'R' in radar) and in the case of Doppler systems, for
accurate radial velocity reporting and clutter cancellation. Those
capabilities existed long before digital systems came online. Aside
from radar, precise frequency was needed in order to reduce guard
band width between assigned channel assignments, thereby enabling
more broadcast stations (commercial and military) to coexist in
an allotted frequency band. This article presents various automatic
frequency control (AFC) topologies used for accomplishing frequency
July 1966 QST
Wax nostalgic about and learn from the history of early electronics. See articles
QST, published December 1915 - present. All copyrights hereby acknowledged.
Stable Microwave Oscillators
By Wilfred Jensby, WA6BQO
The author of this article discusses, in general terms, the
evolution of microwave stabilizing devices and offers some interesting
advice on circuits for improving the effectiveness of the amateur
Glenn Tomlin, WA6KKK, with his C-band setup during an attempt
to break the existing DX record. This installation was atop
Lama Prieta Mountain near San Jose, California. The attempt
failed because line-of-sight conditions were not established
between the site shown and opposite end of the circuit -
Yosemite Park, 135 miles distant.
The tools of a microwave enthusiast. Shown here: a typical
i.f. strip, an old-style klystron at the left, and a modern
klystron tube at the right. The newer unit is an Eimac 1K20XE.
Fig. 1 - Block diagram of the cavity-stabilized automatic-frequency-control
Fig. 2 - An example of a crystal-stabilized i.f.-discriminator
a.f.c. system. This method offers better stability than
is possible with the arrangement shown in Fig. 1.
Fig. 3 - A crystal-stabilized i.f. phase comparator system
of the a.p.c. variety. This has become a popular system
among amateur microwave groups.
Being interested in the microwave
frequencies for a number of years, and desiring to extend one of
the microwave-band distance records, I chose the 5600-Mc. band for
my experiments. In 1959, using 2K26 klystrons and the polaplexer
system,1 a 3-mile contact was made between Jack Taylor,
W7VCM, and myself. Since then, equipment problems and lack of time
have prevented me from establishing any DX records.
I had the opportunity to accompany Len Garrett, W7JIP, while he
and W7LHL extended the X-band distance record to 265 miles. Because
of transportation problems to the tops of the mountains, exotic
equipment was not used. In the case of W7JIP and W7LHL, 3- and 4-foot
dishes were used, 1-watt klystrons provided the signal, and receiving
i.f. strips with a 1-Mc. bandwidth were used. Waveguide balanced
mixers were used ahead of the i.f. strips and cavity wavemeters
were used to monitor the frequency of the klystrons.
not an easy matter to set up a portable microwave link because considerable
effort is required in aiming the parabolic antennas. Then, too,
the transmitter frequency and power must be monitored, to say nothing
of actually establishing the contact. It would appear, nevertheless,
that a 250-mile distance record should be attainable on any of the
microwave bands below X-band.
Improving Present Techniques
Most microwave enthusiasts have been working with klystrons
that deliver 100 milliwatts of r.f. power output.2 Other
operators have purchased 1-watt klystrons from war-surplus dealers.
This power limitation suggests the need for improved techniques
in the transmission and reception of microwave signals if greater
coverage is desired.
A significant improvement can be realized
by switching from the more common wide-band i.f. system to narrow-band
receiving techniques. Many i.f. strips being used have bandwidths
on the order of 1 Mc., or more. By using a communications receiver
with an i.f. bandwidth of 10 kc., a gain in sensitivity of approximately
20 decibels is possible - a gain which is not easily achieved by
working with transmitters or antennas. (In microwave work, the antennas
are usually limited in size to dishes that are between 4 and 6 feet
in diameter because of transportation problems to mountain tops.)
With increased i.f. selectivity, the microwave transmitter will
require a frequency stability that is common to crystal-controlled
transmitters designed for low-frequency use. If the polaplexer approach
is not used, the receiver's local oscillator must also be quite
There are several ways to achieve signal stability
at microwave frequencies. For future experiments, W7JIP plans to
use a u.h.f. crystal-controlled transmitter which drives a klystron
multiplier for X-band use.3 I have tried replacing the
klystron multiplier with a varactor diode multiplier at 5600 Mc.,
but have had limited success to date. Perhaps if I were to mount
the varactor diode directly in the waveguide the efficiency of the
multiplier would be improved because of the excellent bandpass filter
characteristics of the waveguide.
Automatic Frequency Control
By applying automatic frequency control (a.f.c.) to a klystron
oscillator, a stable microwave signal can be secured. Methods for
applying a.f.c. date back to World War II with information describing
typical circuits and techniques being available in the M.I.T. Radiation
Lab Series, Vol. 11, p. 58. One method developed then was to compare
the klystron frequency with that of a high-Q microwave cavity which
had a high short-term stability, and used a servo loop to control
frequency drift. A frequency-sensitive device or discriminator was
used to obtain the error voltage. One type of microwave discriminator
developed was the 'Pound' discriminator4 which is effective
but sensitive, and is difficult to adjust because of possible drift
in the diodes and in the d.c. amplifier used in the system. Fig.
1 shows a simple block diagram of this scheme. The amount of stabilization
obtained is dependent upon the voltage sensitivity of the klystron
and the gain of the d.c. amplifier. The typical performance of a
system such as this (at X-band) would be a center-frequency drift
of about 10 kc. per degree C of the reference cavity because of
temperature. A frequency drift as much as four times greater than
this would result from d.c. drift in the a.f.c. system. To obtain
better performance requires the use of expensive and bulky equipment.
For this reason this system was not often used outside of microwave
Crystal Stabilized A.F.C.
system shown in Fig. 2 provides increased stability at a fixed frequency
by mixing a sample of the klystron's output with the harmonic of
a crystal oscillator-multiplier. This produces a low-frequency signal
which might be 60, 30 or 10 Mc. The resultant signal is amplified
by an i.f. strip and fed into an f.m. discriminator. The d.c. output
from the discriminator is amplified and used to stabilize the oscillator
frequency. The heterodyne method is capable of higher accuracy and
stability than the direct comparison method provided the crystal
oscillator is well designed and is temperature controlled. Generally,
crystal-controlled v.h.f. oscillators provide a short-term stability
of better than 1 part in 108 and aging rates of less
than 1 part per million per week. Commercially available crystal
and oven combinations provide temperature coefficients as low as
1 part in 108 per degree C. The stability of the i.f.
discriminator is often the limiting factor in this system. As low
an i.f. frequency as possible should be used in combination with
good electrical and mechanical design to minimize drift of the discriminator
center frequency. An f.m. superhetrodyne receiver can be used with
the added advantage, if tunable, of being able to move the klystron
frequency while 'locked'. Fig. 4 contains a block diagram of a complete
microwave transmitter-receiver using automatic frequency control.
The system was developed by the San Bernardino Microwave Society,
Automatic Phase Control
The a.f.c. systems just
described have never become too popular because they were never
available as complete units, ready to connect to a klystron and
power supply. Several years ago the Dymec Division of Hewlett-Packard
Co. introduced an oscillator synchronizer which was designed for
the purpose of phase-locking a klystron oscillator to a crystal-controlled
reference signal and achieving a short-term stability of 1 part
in 108/second. This unit eliminates all long-term drift
with klystrons and minimizes the incidental f.m. caused by klystron
noise, power-supply ripple, and mechanical shock. It can also be
used for frequency modulation of the klystron by replacing the i.f.
crystal-oscillator reference with a v.f.o.
The automatic-phase-control system (a.p.c), shown in Fig. 3, is
similar to the heterodyne a.f.c. technique, shown in Fig. 2, except
that the discriminator and d.c. amplifier are replaced by a phase
comparator and i.f. reference oscillator. The a.p.c. system is an
electronic servo system which does not permit a steady-state frequency
error to be developed between the controlled oscillator and the
reference frequency. This improvement results from the integration
of frequency change by the phase comparator, which produces an error
proportional to phase, rather than frequency difference. The comparator
output is passed through an RC stabilizing network and added in
series with the normal reflector supply voltage to the klystron.
The stabilizing network is essentially a simple low-pass filter
which stabilizes the loop gain of the phase-lock system and prevents
an oscillatory condition from developing.
Fig. 4 - The 3300-Mc. "Rock Lock" arrangement shown in this
block diagram is being used at W61FE-W6OYJ, the San Bernardino
Microwave Society, Inc.
Fig. 5 - Some phase detection and stabilization circuits
for microwave systems. The circuit at A is a balanced-input,
diode phase detector and stabilizer (DYMEC Oscillator Synchronizer
At B, the circuit of an unbalanced-input, diode phase detector
and stabilizing network (Microwave Journal, Sept. 1964).
At C, a gated-beam tube phase detector and stabilizer (Electrical
Design News, March 1962).
Shown at D, a switchable discriminator or phase detector
circuit. In all circuits, resistors are in ohms, K = 1000.
Decimal value capacitors are in u.f., others are pf.
The phase comparator
has a higher gain and larger voltage swing than the i.f. discriminator,
which eliminates the need for d.c. amplification. This simplifies
the circuit and also makes it easier to couple the control signal
into the reflector circuit of the klystron, which may have a potential
as high as -2000 v.d.c. However, klystrons used by hams, such as
the 723A/B-2K25 series, use a reflector voltage between -300 and
-700 volts which is not difficult to handle. The voltage swing available
from a phase comparator can reach ±20 volts, providing as wide an
electronic locking range as most klystrons can handle. For a typical
klystron reflector tuning sensitivity of 1 Mc. per volt, this means
a locking range of ±20 Mc. Once an oscillator is captured, or locked,
the klystron reflector voltage can actually vary through a range
of ±20 volts without changing klystron frequency or losing phase
If phase lock is lost, because the klystron shifted
phase beyond the control range of the phase-comparator error voltage,
the klystron jumps to the frequency to which it would normally have
drifted had it not been locked. To return the klystron to a phase-lock
condition it is necessary to retune it to a point within the locking
range where the difference frequency at the phase comparator falls
within a smaller range, known as the capture, or pull-in range.
This capture range is determined by the locking range, but because
the loop stabilization network controls the bandwidth of the system,
the capture range is smaller than the locking range. For a locking
range of ±20 Mc., the capture range is typically ±2 Mc. Most commercial
equipment incorporates an automatic search oscillator which sweeps
the klystron frequency near its natural frequency until it is captured
by the phase-lock loop. This can be simply a 1-kc. sine wave or
sawtooth signal which is added to the reflector voltage.
Most of the parts of a microwave system are familiar
to a v.h.f. amateur. Such circuits include i.f. amplifiers, discriminators,
crystal oscillators and multiplier chains. The output frequency
of the multiplier chain should be 200 Mc, or above, with a power
output of about 1 milliwatt. This means that the design should be
similar to that of a 432-Mc. converter's local oscillator. The multiplier
energy drives a harmonic diode, or in some designs may drive the
mixer diode directly. The r.f. power to the mixer diode from the
klystron can be from 0.1 to 1 milliwatt. The output of the microwave
mixer drives an i.f. amplifier which should have 50- to 7O-db. gain
and a bandwidth of 10 to 20 per cent of center frequency. Surplus
i.f. strips should be entirely satisfactory and can be modified,
if necessary, to include one or two stages of limiting. Conventional
discriminator and f.m. detector circuits are used in a.f.c. circuits.
Typical phase-comparator detector circuits are shown in Fig.
5. The circuit of Fig. 5A can be used either as a discriminator
or phase comparator and is found in some v.h.f. telemetry receivers.
The circuit shown in Fig. 5C was used in a National Bureau of Standards
X-Band receiver which had an i.f. bandwitdh of 45 c.p.s. - illustrating
the capability of the system, and the stability requirements of
the reference oscillators. Additional information can frequently
be obtained by writing to the manufacturers of microwave equipment
and requesting instruction books with schematics. These can usually
be purchased at a reasonable price.
A possible future article
will describe a complete C-Band microwave link with more details
concerning construction. In the meantime, I hope that others will
be making use of our amateur microwave bands.
1 Prechtel, "Experimental Transceivers
for 5650 Mc.," QST, August, 1960.
2 Peterson, "Practical
Gear for Amateur Microwave Communication," QST, June, 1963.
Garrett and Manly, "Crystal Control on 10,000 Megacycles," QST,
4 Pound, "Frequency Stabilization of Microwave
Oscillators," Proc. IRE, December, 1947.
"Phase Locking Microwave Oscillators To Improve Stability and Frequency
Modulation," The Microwave Journal, January, 1963.
and Strandberg, "Phase Stabilization of Microwave Oscillators,"
Proc. IRE, July, 1955.
Posted October 30, 2013