April 1969 Radio-Electronics
[Table of Contents]
Wax nostalgic about and learn from the history of early electronics.
See articles from Radio-Electronics,
published 1930-1988. All copyrights hereby acknowledged.
Have you ever used any of
these voltage-variable capacitor (VVC) types: varicaps, epicaps, minicaps, voltacaps,
varactrons? If you answered "no, but I have used varactor diodes," then the more
correct answer would have "yes, I have, but by a different name." Construction was
similar for all variations. This article from a 1969 issue of Radio-Electronics
magazine reports on some of the earliest forms of diodes specifically designed to use
a reverse bias on the PN junction to control the effective capacitance of the device
for use in frequency tuning circuits. The first uses were for electronically tuning
local oscillators in mixing stages, and then for making tunable filters. Capacitance
ratios greater than 10:1 with some VVCs allowed tuning over a very wide range. At the
time the article was written, there was not universally agreed upon schematic symbol
for the VVC, as illustrated in Figure 2. Varactor-tuned television channel selectors
made for much smaller tuner assemblies with fewer adjustments for alignment across
the entire VHF and UHF bands. Here are some really good photos of the vintage
channel selectors (see parts
and 2, also).
Variable-Voltage Tuning: How It Works
A new kind of tuning "capacitor" for the new sets
By Robert F. Scott
Senior Technical Editor
Fig. 1 - Capacitance vs reverse voltage curve for voltage-variable
capacitor (VVC). Nominal capacitance is the value obtained with 4 volts of reverse bias.
Fig. 2 - There is no common voltage-variable capacitor (VVC) symbol.
Here are the most common ones in use. R-E uses the one shown at right in bottom row.
During the revolution of radio communications, the galena crystal and spark gap gave
way to vacuum tubes and now the vacuum tube is rapidly being replaced by semiconductors.
The variable tuning capacitor - which, except for permeability tuning in auto radios,
seemed to be forever with us - is now bowing out in favor of a solid-state equivalent.
These semiconductors that are replacing the tuning capacitor are generally known as voltage
variable capacitance (VVC) diodes, varactors, voltage variable diodes or capacitance
diodes and by such trade names as Varicaps, Epicaps, Minicaps, Voltacaps, Capistors and
The VVC is a special type of silicon diode that acts like a capacitor when its pn
junction is back-biased. The effective capacitance of a diode varies as the formula 1 √V
as the voltage across its terminals is varied. The capacitance versus bias voltage curve
for a typical VVC diode is shown in Fig. 1.
There doesn't seem to be a standard symbol for the VVC. Some of the more common ones
are shown in Fig. 2.
The earlier VVC's had a low maximum capacitance and a maximum capacitance ratio of
around 5.5:1 which limited their use to modulation and afc circuits. Recently, high-Q
VVC's have been developed with capacitance ratios greater than 26:1 for a voltage range
of 0 to 10 volts. This makes it possible to use them as replacements for the conventional
mechanical variable capacitor tuning over as much as a 3 to 1 frequency range of any
band on a receiver or signal generator. The VVC offers many advantages over its conventional
mechanical equivalent. Among them are:
• Small size - they average around 0.1 inch in diameter and 0.3 inch long.
• No need for mechanical coupling to dial or drive mechanism.
• High-speed tuning.
• Greater electrical stability because of immunity to shock and vibration.
• No moving parts to wear out or come loose.
• Temperature coefficients are known and easily compensated.
• Tuning controls (a potentiometer or pushbutton switch to adjust the de bias)
can be remote from the tuner.
VVC Front-End for AM Radio
VVC diodes have been used for tuning and bandswitching in some European TV sets and
in some Japanese and European AM/FM radios since around 1964. The front-end (converter)
of a pushbutton AM tuner is shown in Fig. 3. It was described in an application note
for the BA 163 made by ITT.
The BA 163 is a silicon epitaxial diode with a capacitance ratio of greater than 26:1
over a voltage range of 0 to 10 volts. Its Q ranges from 200 (minimum) to 500 (typical)
from 150 to 500 kHz at 1 volt and from 300 to 1500 kHz at 10 volts.
In the diagram, one BA 163 is connected in series with a 0.047-μF capacitor across
the high-impedance winding of a ferrite-type antenna coil. The series capacitor blocks
the dc control voltage and prevents it from shorting to ground. Its value is high enough
so it does not affect the capacitance range of the VVC. C1-L1 form a trap to reject signals
in the set's i.f. range.
The oscillator circuit works in the common-base mode with feedback from collector
to emitter. The oscillator voltage in the collector circuit is limited (by design) to
1 volt p-p to limit distortion.
Fig. 3 - Front-end of AM tuner designed by ITT to illustrate the use
of the BA163 capacitance diode.
Fig. 4 - Basic circuit for adjusting tracking to two VVC diodes.
Fig. 5 - Basic circuit of all-channel TV tuner where D2, D4, D5 and
D6 tune stations. The others switch bands.
Fig. 6 - Front-end and voltage-regulator circuit in the Panasonic
R-J500 battery-powered transistor portable radio.
Antenna-tuning diode D1 is fed from the 10-volt bias line through an adjustable voltage
divider consisting of R3 and R4, and the oscillator diode is fed from a fixed voltage
divider consisting of R1, R2 and R4. R3 is adjusted for proper tracking between the oscillator
and antenna circuits.
The bias voltage for the tuning diodes is applied through interlocking pushbutton
switches S1-S6. When S1 is closed, R5 is used for continuous tuning. Switches S2-S6 and
pots R6- R10 are for preset pushbutton tuning. Potentiometer R11 sets the maximum bias
voltage developed across the control pots.
The BA 163 is supplied in matched sets of 2, 3, 4 or as is required. For any two,
the maximum ratio of voltages at 30 pF is 1:1. The basic tracking circuit for any two
diodes is shown in Fig. 4. Potentiometers R1 and R2 are set so the capacitance of D1
and D2 is 30 pF. The capacitance ranges that can be obtained by varying voltage Vab
(with R1) are 120-260 pF, 30-120 pF and 10-30 pF. When the ratio of the voltages across
D1 and D2 is constant, the capacitance differential is less than 5% from 120 to 260 pF,
less than 2% in the 30-120-pF range and less than 1 pF between 10 and 30 pF.
VVC All-Channel TV Tuner
With the coming of varactor-tuned TV tuners, the channel selector can be mounted on
any convenient part of the cabinet - or even in a remote location - instead of being
mounted directly on the tuner shaft as in capacitor-tuned conventional models. Standard
Kollsman Industries has announced a new solid-state all-channel TV tuner whose circuit
will probably resemble Fig. 5, which is a basic diagram prepared from US patent No. 3,354,397,
issued to Karl H. Wittig and assigned to SKI.
In this circuit, VVC's are used for channel selection and for switching between vhf
and uhf TV bands. On the vhf channels, antenna coil L1 is tuned by varactor D2. The collector
tank circuit of the RF amplifier is composed of L4 tuned by varactor D4.
The oscillator voltage is tapped off, amplified, rectified and then added to supply
voltage to compensate for low batteries.
Oscillator coil L6 is shunted by varactors D5 and D6 connected back-to-back and tied
to the tuning bus. (In the oscillator circuit, the RF voltage across the tank coil is
high compared to the tuning voltage applied to the varactors. If a single diode were
used as in the antenna and mixer tuned circuits, this high RF voltage would lead to frequency
instability and high harmonic output. With two varactors back-to-back, the dc tuning
bias varies the capacitance of both by the same amount and in the same direction - as
with a split-stator capacitor-while the RF voltage causes equal and opposite capacitance
The channel selector consists of a special two-section potentiometer (R2-R3) ganged
to band switch S1. As the arm of the channel selector is moved to the right (toward the
junction of R2 and R4) the voltage applied to D2, D4, D5 and D6 goes more positive. This
decreases their effective capacitance and tunes the antenna, mixer and oscillator circuits
to successively higher channels in the vhf band.
Coils L2, L3 and L5 are isolated from the circuit during vhf operation by the very
low effective capacitance of D1, D3 and D7 which is produced by the negative bias on
the switching bus.
As the arm of the channel selector passes the mid-point of its travel, it moves to
the junction of R3 and R5 and S1 switches to the high position. D1, D3 and D7 are now
forward-biased so they act as short circuits or closed switches which connect L2, L3
and L5 in parallel with the vhf coils. This reduces the effective inductance in the antenna,
mixer and oscillator circuits so the uhf channels can be covered.
Battery Portables Too
When VVC tuning was applied to the Panasonic R-1500 battery-powered transistor portable,
special circuits were added to insure that a stable 10 volts was available for the Capistors
(D1 and D2) in the antenna and mixer circuits so as to compensate for the normal decrease
in battery voltage with time and use. The front end and compensating circuits in the
R-1500 are in Fig. 6. The set has five pushbuttons: one to select manual tuning and four
for pre-set stations.
When the set is first turned on, battery current from the 9-volt line flows through
D4 and D5 and turns on converter Q1. As Q1 starts oscillating, a part of the oscillator
voltage is tapped off the emitter circuit and fed through C4 to the base of Q9. Transistors
Q9 and Q10 amplify the oscillator signal and develop a fairly high RF voltage across
RF choke L3.
This voltage is rectified by D4 and D5 and added to the converter supply voltage and
stabilized by Zener diode D6. This stabilized voltage is also fed through R38 and R39
to the manual tuning or preset pots as bias for the Capistors in the antenna and mixer
The afc circuit (not shown) consists of a separate i.f. amplifier stage - fed from
the output of the second i.f. amplifier-feeding a discriminator that develops a correction
voltage. This voltage is applied to the anode of the oscillator Capistor (D2) to aid
or oppose the tuning bias and tune the station in right on the nose.
Posted January 3, 2019