July 1966 QST
Wax nostalgic about and learn from the history of early electronics. See articles
QST, published December 1915 - present (visit ARRL
for info). All copyrights hereby acknowledged.
Long 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 1966 QST magazine article
presents various automatic frequency control (AFC) topologies used for accomplishing
frequency source stabilization.
Stable Microwave Oscillators
By Wilfred Jensby, WA6BQO
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
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 system.
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
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
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 stable.
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.
The 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. superheterodyne 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.
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.
Fig. 4 - The 3300-Mc. "Rock Lock" arrangement shown in this
block diagram is being used at W61FE-W6OYJ, the San Bernardino Microwave Society,
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 Handbook).
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 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
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. bandwidth
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,
3 Garrett and Manly, "Crystal Control on 10,000 Megacycles," QST,
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,
- Peter and Strandberg, "Phase Stabilization of Microwave Oscillators,"
Proc. IRE, July, 1955.
Posted August 10, 2021
(updated from original post on 10/30/2013)