May 1974 Popular Electronics
Table of Contents
Wax nostalgic about and learn from the history of early electronics. See articles
published October 1954 - April 1985. All copyrights are hereby acknowledged.
Radio control (R/C) systems
operating in the 2.4 GHz ISM band, using one of or a combination of frequency
hopping and direct sequence spread spectrum scheme, have been in widespread use
since the early 2000s. As with any new technology, there was a lot of reluctance
to adoption of the systems based on a few reports (valid or not) of performance
issues - primarily lack of control range where communications between the transmitter
(Tx) and receiver (Rx) with a pilot and aircraft was lost and a crash ensued. Tx
power was already at the FCC-mandated maximum, so manufacturers quickly improved
receivers by adding diversity with a second Rx antenna. The receiver microprocessor
continuously monitors signal integrity from both antennas and uses the best one.
It is the same scheme that was already being used by WiFi routers also operating
at 2.4 GHz. By 2010 or so no R/C system companies were making 72 MHz (airborne)
or 75 MHz (ground) AM and FM systems. Technology for those former AM and FM systems
had improved significantly between the time this article appeared and when the 2.4 GHz
systems finally dominated. The physical size and weight of receivers has been reduced
to 1/2 or 1/4 of 1990s era versions, but not just because of the frequency and modulation
scheme change; it was due to an electronics industry-wide advance in component size
decrease and performance increase. Reliability improved as well for all kinds of
circuitry and components. IMHO, the greatest benefit of the 2.4 MHz spread
spectrum R/C systems was the ability of many aircraft to operate simultaneously
without worrying about being "stepped on" by someone on a directly adjacent channel
frequency (with an out-of-spec transmitter oscillator) or even by some idiot in
the pit area turning on his transmitter on your frequency to adjust his airplane
while yours is in the air (often causing a crash from interference). The 72 MHz
systems are assigned
frequencies at a 200 kHz spacing.
Spektrum R/C, one of the earliest 2.4 GHz R/C system pioneers, did a demonstration
sometime around 2006 with 100 airplanes in the air at the same time to prove interference
immunity. Entire books can be (and have been) written on the evolution of R/C systems.
See also "How to Get Started
in Radio Control Modeling" in the February 1974 Popular Electronics
How Radio-Control Systems Work - Types of Remote R/C Systems Used
to Operate Model Planes, Cars, and Boats
By Fred Marks
A growing number of hobbyists has been attracted to radio-control modeling due
to new solid-state designs and exam-free CB licensing. The electronic gear in today's
modeling provides wide response flexibility and high reliability in extremely compact,
lightweight packages. (See also "Radio Control For Hobbyists," February 1974. )
There are two commonly used basic schemes in R/C equipment. The simplest is a
refined version of single-channel, pulse-proportional control. The other, a digital
system, is a sophisticated feedback proportional control that utilizes pulse-position
Fig. 1A - Block diagram of contemporary five-channel digital-proportional
transmit and receive system is complete as shown except for servos.
Fig. 1A - The decoder system uses typical IC's.
The only tone-modulated, pulse-proportional system we know of on the market today
is made by Ace R/C. Its major advantages are minimal weight and low cost. It is
used primarily in situations where multi-channel flexibility is not required.
In the Ace R/C system, a 600-Hz tone is pulsed on and off at the transmitter
to drive an actuator in the receiver. The receiver demodulates the received pulsed
tone and produces a square-wave output that reproduces the pulses originally sent
to the transmitter's tone modulator.
The switching outputs of the receiver connect one end of the actuator or the
other (depending on the command initiated at the transmitter) to the positive voltage
supply. This type of system is limited to one proportional function. The repetition
rate is from 6 to 8 pulses per second. The model under control does not respond
to individual pulses. Instead, it follows the average position.
Digital systems all use the same pulse-position modulation scheme, with the r-f
envelope amplitude-modulated by a series of pulses at a specific interval (frame
length) of from 10 to 15 milliseconds. Each pulse is separated from the others by
a nominal 1.5 milliseconds. This separation is independently and continuously variable
by ±0.5 millisecond (maximum) for control. The decoded output to a digital
feed back servo is a pulse that duplicates the original control input to an accuracy
of 99.75 percent.
The block diagram of a typical digital-proportional transmit/receive system is
shown in Fig. 1. The wavetrain illustrated is for a five-channel system.
The objective of digital encoding and decoding is to obtain a specific width-control
pulse for each servo in the system. The length of the pulse must be variable over
the desired control range. Timing and synchronization of the scheme is shown in
Fig. 2. The times indicated are approximate and are representative of those used
in current systems.
A typical encode/decode process takes place as follows. The clock oscillator
in Fig. 1A establishes the repetition rate of the system, which is usually 10 to
16 milliseconds or 60 to 100 Hz. The trailing edge of the frame pulse triggers the
first stage. Upon being triggered, the first stage changes state for a period of
time determined by the control potentiometer.
The output of each stage is gated by a diode to an RC differentiator, and a small
spike at the input to the squaring block occurs at the trailing edge of the stage's
output pulse. This spike is also coupled to the next stage for triggering. Stage
two stays on for its commanded period (t2 in Fig. 2). Stages three through five
follow in like manner. Then 3 to 8 milliseconds are permitted to elapse to allow
the decoder to reset. The clock, having completed its 15-millisecond period, changes
state to restart the cycle.
Fig. 2 - Waveforms show timing and synchronization of digital
signals. Those at (A) are for one frame of encoder; (B) is for decoder; and (C)
shows control variations.
The gated output spikes appear at the input to the squaring amplifier in the
first frame of pulses. The squaring stage converts each spike to a well-defined
pulse with a 0.3-millisecond nominal width for control of the modulator.
The position of each pulse relative to the preceding pulse is the transmitted
information - hence, pulse-position modulation. The rise and fall times of the pulsed
r-f envelope must be carefully controlled to obtain an acceptable sideband spectrum.
The receiver attempts to reproduce the encoded envelope with the best possible
fidelity for presentation to the decoder.
The decoder, shown in block diagram form in Fig. 1B and schematically in Fig
3, is typical of modern IC decoders. The output of the receiver is a train of two
(for one channel) to nine (for eight channels) pulses per frame. The inverters switch
between a 4.8-volt high level to almost zero for the low level. Transistor Q1 serves
primarily as an inverter for the incoming signal, although it also provides a slight
amount of amplification.
The pulse train that comes off the collector of Q1 proceeds to two stages. The
stage consisting of inverters 4 and 5 generates the "set" pulse, while the one made
up of inverters 2 and 3 shapes the pulse train to form the clock or "shift" pulses.
The shift-pulse shaper accepts the pulse train, squares it, and slightly stretches
it by feedback through C3. The low output from inverter 2 is shifted high by inverter
6 to provide the proper output. The shaped clock pulses are then passed to an eight-bit
Upon receipt of the sync pulse, the diode and C4 act as a "sample-and-hold" circuit
or pulse stretcher. Discharging through inverter 4, C4 places the inverter in a
high state. At this point, the pulse is stretched across the entire pulse train.
The output from inverter 5 is square and low during the period when the pulse train
The shift register is set by having the output of the inverter high at the instant
the first clock pulse is received. The set is immediately driven low until after
the last pulse is received. Flip-flop FF1 in the register is inhibited from shifting
to high at its Q output until the next frame of information is received. (see Fig.
The two control functions are entered at the set (S) input of FF1 and at the
clock inputs. When there is no information present during the sync (set) pause period,
the FF1 through FF8 Q outputs are low. As soon as the first clock pulse is received,
FF1's Q output goes high. Unless it "sees" a high input at SA and SB, which it does
at the instant the first clock pulse is received, FF1 (at its Q output) cannot shift.
It remains high until the second clock pulse is received, at which time it goes
Fig. 3 - Schematic diagram of the decoder shows how the 8-bit
shift register in IC is used to simplify the circuit. Transistor Q1 is inverter
and amplifier for input.
Fig. 4 - Most of the functions in digital servo are included
in a single IC device.
Fig. 5 - Diode-resistor comparator determines relative width
of pulses in digital servo.
If the S input of FF1 were to remain high at all times, FF1 would simply shift
high alternately at its Q and not-Q outputs every time a clock pulse is received.
Now, FF1 cannot shift again as long as the output of inverter 5 is low. This is
why the pause between frames is caned the sync or "set" pause.
Bear in mind that the Q output of FF2 cannot shift high until the S input is
high. It sees a high level only when the Q output of FF1 is high. As soon as FF1's
Q output is driven high, FF2 is set to shift high at its Q output when the second
pulse, which sets the Q output of FF1 to low, is received. The Q output of FF2 remains
high until the third pulse is received to drive it low and the Q output of FF3 high,
and so on down the line through FF5. As soon as C.5 and C6 have discharged after
t-he last pulse, the output of inverter 5 returns to high, setting FF1 for the next
Figure 4 is a block diagram of a digital feedback servo. Almost all servo amplifiers
now consist of a specially designed IC that contains all the control functions shown
in the diagram. The only external components needed are used for dead-band, travel,
and feedback sensitivity trimming. The servo functions as follows.
First, a reference pulse is generated, its width determined by the feedback element
as positioned by the servo output arm. The incoming signal may be wider or narrower
than the reference pulse. The comparator, usually a diode-resistor network, determines
the relative width of the pulses (see Fig. 5).
The pulse stretcher and trigger convert the error pulse, measured in microseconds,
to a longer pulse suitable for turning on the servo driver. The driver then applies
B+ power to the servo motor, providing full power at any position.
As the motor and servo arm are driven, the feedback element is repositioning
to yield a pulse that is exactly equal in width to the input pulse. The repositioning
can take several frames to occur because full servo travel requires 0.5 to 1.0 second,
depending on such factors as gearing and battery voltage.
Radio control for modeling has come a long way since the 1950's. With today's
equipment, containing the sophisticated electronics described here, the modeler
has at his command a control system that responds in a manner very similar to the
systems used in full-size aircraft, racing and standard cars, and boats.
Posted April 14, 2020