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 source stabilization.
July 1966 QST
of Contents]These articles are scanned and OCRed from old editions of the
ARRL's QST magazine. Here is a list of the
QST articles I have already posted. All copyrights (if any) are hereby acknowledged.
See all available
vintage QST articles.
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 microwave
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
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.
In 1960 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.
It is 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
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.
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 stable.
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.
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 laboratories.
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, Inc.
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 lock.
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.
and Manly, "Crystal Control on 10,000 Megacycles," QST, November, 1963.
4 Pound, "Frequency Stabilization of Microwave Oscillators," Proc.
IRE, December, 1947.
- Benjaminson, "Phase Locking Microwave
Oscillators To Improve Stability and Frequency Modulation," The Microwave
Journal, January, 1963.
- Peter and Strandberg, "Phase Stabilization
of Microwave Oscillators," Proc. IRE, July, 1955.
Posted October 30, 2013