Part Two: Superhetrodyne Receivers
Regenerative receivers were simple, easy
to build and worked well enough to serve the needs of early hams.
Tubes were scarce, expensive and short lived in the early days so this
was a good thing. The broadcast boom had fueled the development of
good, affordable tubes, including power tubes. The latter paved the
way from spark to cw. As more and more broadcast stations got on
the air the need for receivers with better selectivity became pressing.
Broadcast sets of the day used two, three, or more RF stages ahead of a
triode detector. At first neutralized triodes were used in the RF
stages, latter tetrodes such as the type 224 (aka 24A). More RF stages
added selectivity IF THE STAGES TRACKED DURING TUNING! Selectivity
was better at the low end of the band, poorer at the higher because the
Q of the stages varied as the frequency changed.
The American inventor, Edwin H. Armstrong,
who had previously developed the regenerative receiver, came up with the
next innovation, the superhetrodyne circuit. In the superhetrodyne,
also referred to as the superhet, the incoming signal was converted (usually
down in frequency) to another frequency, referred to as the Intermediate
Frequency. The Intermediate frequency, or IF stages operated at a
constant high Q to provide improved selectivity. The receiver was
tuned primarily by the action of the local oscillator which was the heart
of the frequency converter stage. This stage operated by generating
a beat note between the incoming signal and the local oscillator.
The difference between these two signals was the intermediate frequency.
This frequency conversion occurred
in a new type of circuit known as a mixer or frequency converter.
The latter included the oscillator, while the former used a separate oscillator.
At first triode or tetrode tubes were used as mixers with a separate triode
oscillator. Later combined mixer - oscillator tubes known as converters
were introduced. The first such tube was the 2A7 pentigrid tube.
Other converter tubes were the 6K8 triode-hexode; 6F7 triode - pentode;
6J8 and 7J7 triode - heptodes; 7A8 octode; and the 6SA7 and 6BE6
pentigrid types. These were later joined by battery operated types.
The IF frequency used was usually
somewhere between 400 to 500 khz, although some receivers used lower frequency
IF's (100-300 khz). A higher Q could be obtained with lower frequency
IF transformers with a resultantly sharper selectivity using the same number
of stages than a higher frequency IF. The use of a low frequency
IF had it's problems however. Since the mixer stage produced both
sum and difference results, there would be TWO input frequencies for the
same oscillator input that could produce the desired IF output. If
the IF frequency was low compared to the input frequency, then the two
input frequencies would be close together and the wrong one could be heard
along with the desired one. The undesired input was known as the
'image' response. Adding selectivity to the input of the receiver
would help reduce this. This meant using one or more ganged RF stages
ahead of the frequency conversion stage. This brought back many of
the same problems that existed with the old TRF receivers.
Most of the better early superhet short wave receivers had one or more
RF stages. Some even had three! The famous National Radio HRO
was the best known receiver of the era.
Another way of getting around the
'image' problem. This was to use a higher IF frequency and then convert
down again to a lower one to obtain selectivity. The result was a
double conversion circuit. Some sets used triple conversion, spreading
the first and last IF's even further apart and using very low frequency
last IF's to get very narrow selectivity curves (ideal for CW!).
Some examples were:
IF #1 = 1500khz, IF #2 = 455khz; IF#1 = 4500khz,
IF#2 = 455khz, IF#3 = 50khz.
By this time the diode detector had
come back and replaced the triode grid leak type. Diode detectors
gave better linear response, and could handle strong signals without overloading.
They did not have any gain, but this was no longer a problem! To
copy CW signals (or SSB!) without the use of the oscillating regenerative
detector, another oscillator was required to re-inject the missing carrier.
This was known as the Beat Frequency Oscillator or BFO. The BFO operated
at the IF frequency, and was adjustable to set the desired beat note.
In early receivers it was free running, modern sets use a crystal oscillator
here. Sometimes a mixer or converter tube was used as the detector
for CW. The BFO-Detector stage was not unlike the receiver's converter
stage, but with the output at audio frequency. This type of detector
is known as a product detector. Product detectors did not become
popular until SSB, early sets stuck with the diode.
Broadcast sets introduced another
development; Automatic Gain Control or AGC. AGC tried to keep the
gain of the receiver constant by reducing the gain of the RF and IF stages
for strong signals. A portion of the output of the detector was applied
as negative bios to the grids of the amplifier stages to reduce their gain.
Unless a separate AGC detector was provided from the signal detector the
AGC circuit had to be disabled while receiving CW or the BFO would swamp
out the AGC. Most sets had a manual gain control to be used when
the AGC was shut off. Another new wrinkle was a signal strength meter.
Known first as the R meter, later called the S meter, it monitored the
AGC voltage to give an indication of the incoming signal strength.
The signal strength meter could also be used as a tuning indicator.
Broadcast sets usually replace the meter with a 'magic eye' tube.
These cathode ray types gave a shadow indicator on a phosphor coated target.
Example tubes were the 6E5, 6U5, 6AF6, and 1629 (6E5 with 12v heater and
octal base used in AN-ARC5 transmitters).
With time the manufacture of electronic
parts became more precise and mass produced receivers could be designed
for accurate dial calibration. The vernier dial and calibration book
became a thing of the past. Receivers now had calibrated, direct
reading dials. There were three kinds of these. The Airplane
Dial, so called because it resembled an airplanes flight instruments, used
a round fixed scale with a moving pointer. A variation of this used
a moving round dial behind a meter face with a fixed cursor line.
National Radio made various styles of both kinds of these dials.
The final kind was the linear 'slide rule' dial. The first two kinds
of dials were used on the more expensive receivers, the slide rule dial
with it's pulley and cord construction was mostly used on cheaper sets.
There were a few slide rule dials with anti-backlash rack and pinion gearing
(an example was made by James Millen) that were used on professional sets.
Another popular slide rule dial was made by Edystone. The vernier
scale dial did remain on the HRO up till the last model (HRO 500), although
later versions of the receiver also had a slide rule dial. The very
smooth HRO dial was still preferred by many, it was copied by receiver
home brewers up to the present day. (With a linear tuning vfo the
HRO dial can be read directly.)
