April 1958 RadioElectronics
[Table
of Contents]
Wax nostalgic about and learn from the history of early electronics. See articles
from RadioElectronics,
published 1929  1948. All copyrights hereby acknowledged.

Declaring any kind of straight LC tank circuit
to be high stability is a bit of a stretch when compared to the Q available simply by adding a crystal,
even in 1958. Tone modulation was an early method for achieving remote control of model airplanes, boats,
and cars. The number of channels with these tone modulation systems is two times the number of modern
proportional systems in that moving the rudder left took one channel and moving it right took another.
Up and down elevator likewise took two channels. Therefore, this four channel system is only two channels
by today's terminology. Technology evolved into fully proportional systems on crystalcontrolled 27 MHz
using pulse position modulation (PPM), then to 72 MHz, and today nearly every every R/C uses a
combination of frequencyhopping spread spectrum (FHSS) and directsequence spread spectrum (DSSS) in
the 2.4 GHz ISM band.
Tone Modulator for RC
Simple circuit has stable oscillator for toneoperated radiocontrol
systems
By E. L. Safford, Jr.
One of the most perplexing problems facing the RC enthusiast when he turns to a toneoperated system
is the construction of an oscillator that will provide the various frequencies accurately and with long
time stability. The tone modulator described in this article is an answer to the problem. It not only
provides the tones but also a method of modulating the radiofrequency oscillator.
Fig. 1  Fourchannel twotube 6meter RC rig.
The circuit is a modified Hartley oscillator using a standard (pushpull plates to voice coil) audio
output transformer as its tapped inductor. The size or type of this transformer is not critical and
its wattage may vary from 2 for a batteryoperated system to 10 for a system using a 6V6 as the oscillatormodulator
tube. The latter will modulate a 5watt carrier adequately. One end of the primary winding is connected
to the plate, the center tap to Bplus and the other end to the stabilizing element which is a seriesresonant
circuit. The screen is connected to Bplus and the cathode is grounded.
The theory of operation is simple. The transformer provides feedback necessary for oscillation, but
the seriesresonant circuit prevents all frequencies from being impressed on the grid except the resonant
frequency of the tuned circuit. In practical operation, it is possible to vary the frequency slightly
by changing the value of the grid resistor, but this should be adjusted for the best, purest and cleanest
note. Changes in frequency should be obtained by changing the value of the coil or capacitor or by using
entirely different tuned circuits.
Fig. 1 shows the circuit as actually used. A fourchannel tone section which modulates an oscillator
generating a radiofrequency carrier of 5054 mc was required. The unit was to be batteryoperated,
using not more than 135 volts for Bplus and has a power output of approximately 1 watt.
To meet these requirements, two 3Q4's were used. The audio oscillatormodulator section uses one
as a pentode while the rf section uses the other triodeconnected. The value of the grid resistance
was varied from 5,000 to 50,000 ohms and the frequencies generated range from 600 cycles to 3,200 cycles.
A separate tuned circuit is used for each tone. The keying switches are placed between the grid and
the ends of the coils. A 47,000ohm grid resistor gave the best operation over the whole range with
voltage variations from 75 to 135.
The rf section is modulated by connecting the plate of the rf oscillator through a radiofrequency
choke and small bypass to the plate of the modulator tube. Bplus connects only to the center tap of
the transformer and the screen of the audio oscillatormodulator tube.
Fig. 2  Another way of arranging tuned circuits in oscillatormodulator.
TunedCircuit Calculations
The tuned circuits are built around Chicago NSI1 0.8h and NSI2 2.4h coils. It is possible to
use one inductance and switch various series capacitances into the circuit to obtain the required tones.
The sharpness of the resonance, however, is a function of the choke's reactance. Thus, the use of one
choke for the entire range means that the Q varies and the accuracy of the tone along with it.
The Chicago chokes are listed as they have a minimum Q of 20. However, less expensive units, such
as the Stancor C2328 (0.8 h) and the Stancor C2334 (2.8 h) may be used.
Using either a single choke or the less costly chokes will result in some degradation in the frequency
stability. This will be small when applied to tunedcircuit receivers, but important with tunedreed
units.
The best stability and ease of adjustment would be with variableinductance chokes, like UTC's VIC
types, using a separate unit for each frequency. A simple adjustment of the core would set up the proper
tone for transmission. Of course, this is the most expensive method.
The values of the series capacitors can be calculated by the standard formula:
where C is capacitance in farads, L is inductance in henries and the frequency is in cycles per second.
This can be rearranged to solve for the capacitor as:
where C is capacitance in microfarads, L is inductance and frequency is in cycles per second.
For example, if you wanted a 1,000cycle tone using the 0.8h coil:
Shunt this capacitor with a small variable padder, say 100 to 750 μμf, to allow some adjustment
of the frequency. If you come up with odd values for the capacitor, remember if you parallel values,
say .01 and .001 μμf, the total capacitance will be .011 μμf. If you use two series capacitors,
the resultant value can be found from:
Therefore, you might use a single coil and several values of capacitors to obtain the different frequencies
or use two coils, one 2.4 and one 0.8 h, the smaller for frequencies above 1,000 cycles and the larger
for frequencies below that. Fig. 2 illustrates this arrangement.
Posted April 25, 2017
