December 1942 Radio-Craft
[Table
of Contents]
Wax nostalgic about and learn from the history of early electronics.
See articles from Radio-Craft,
published 1929 - 1953. All copyrights are hereby acknowledged.
|
Homodyne reception, although we don't often refer to it today using
that term, involves mixing the modulated signal with a local oscillator
that is tuned to the same frequency so that the demodulated signal
is at baseband. In other words, the result of a homodyne nonlinear
mixing process is a sum frequency of 2x the signal input and the
difference frequency is DC (at the low end
of the modulation). That is a simplistic explanation, and
this article goes into a little more detail about methods, advantages,
and disadvantages. Why not just make things simple and make every
receiver a homodyne circuit? The answer is that with homodyne operation
every theoretically possible mixer spurious product will fall inband
without any means of filtering them out. Sometimes it doesn't matter,
but especially in today's crowded radio spectrum it just is not
workable because the interference level would be too intolerable.
Homodyne Reception
Possibilities of the System as an Aid to Selectivity
The "homodyne" system of reception is a little-known member of
the family of radio "dynes," so let us first see how it is related
to its cousins heterodyne, super (sonic) - heterodyne and autodyne.
The word "dyne" is derived from the Greek for power, so that heterodyne
merely means putting in energy at a different frequency, and becomes
"supersonic-heterodyne" if the frequency difference is greater than
audible (e.g. 465 kc/s), while autodyne means putting in its own
power, i.e. a self-oscillating detector. Similarly, homodyne means
that energy is put in at the same frequency, i.e. in synchronism
with the carrier of the signal which it is desired to receive, and
this is the system which may be able to help us with the selectivity
problem.
Interference

Interference and effect of bias in diode detection. The
diode conducts during the parts of the cycle are shown shaded.

Homodyne reception compared with other methods of obtaining
selectivity. Note the similarity of homodyne reception to
the bandpass curve.
|
Interference can be divided into two categories, the type which
involves the carrier of the wanted signal, and the type which does
not. In the first category we have the direct heterodyne between
the wanted carrier and a neighboring carrier, "side-band splash"
which consists of heterodynes between the wanted carrier and the
side-bands of the interfering signal, and cross-modulation; in all
of these the output of interference is merely proportional to the
weaker of the two frequencies which are beating together so that
increasing the strength of the wanted carrier makes no difference
to the interference. Before, we can benefit from the homodyne principle,
therefore, adjacent carriers must be spaced far enough apart for
the heterodyne note to be outside the audio-frequency band, or alternatively
the heterodyne must be eliminated by means of a "whistle filter"
of some sort. The latter alternative is not the ideal solution,
since it involves eliminating the same frequency (or rather a band
of frequencies) from the program; but if the filter has a narrow
enough attenuation band, it may be a tolerable method. It seems
likely to take a very tong time to produce sufficient public demand
for high-fidelity broadcasting on the medium-wave band to secure
the sacrifice of a number of stations to adequate spacing of channels;
in fact, it is a debatable point whether the introduction of wide-band
U-H-F broadcasting would render superfluous high fidelity on the
medium-wave transmissions, or whether the experience of really good
quality would lead to a demand for it on all transmissions. Assuming,
however, that we have by some means eliminated the adjacent-channel
heterodyne, and taken the necessary precautions against cross-modulation
(which means practically building a receiver with RF stages that
never overload), the residual interference will consist of the whole
modulated signal (carrier plus side-bands) of a transmitter on a
neighboring frequency.
Selectivity Limitations
There is an essential distinction between the wanted and unwanted
signals, by reason of the fact that they have different carrier
frequencies, and so it may be possible to eliminate the interference
which consists solely of the independent signal more effectively
than heterodynes, etc., which involve the carrier of the desired
signal. But first one must answer the natural question, why selective
circuits? Now, reasonable program enjoyment requires a signal/interference
ratio of 40 db., and for high-fidelity reception the ratio should
be 60 db., i.e. a voltage ratio of 1000: I; add to this the condition
that ideally one should be able to receive the weaker of two adjacent
stations, say with a field-strength ratio of 10: I, and if the reader
then thinks it is easy to design a receiver with adjacent-channel
selectivity of 10,000: I, he need not worry about homodyne receivers.
Linear Detectors
The phenomena underlying homodyne reception actually occur to
some extent in every receiver using a linear rectifier; (that is
to say almost every modern receiver that has a reasonably strong
signal tuned-in); one of the phenomena is that a linear rectifier
is most sensitive to signals that are in the same phase as the strongest
signal out of several applied to it. In the ordinary-diode rectifier,
the diode is automatically biased by the signal so that it is only
conducting for a small part of the cycle, say the extreme positive
values of the voltage wave, as shown in Fig. 1. If now the amplitude
of the signal is varied by modulation, there will be a change in
the height of the voltage-peaks, therefore an increase or decrease
of diode conduction, and this in turn will change the bias voltage
so that conduction occupies the same proportion of the whole cycle
as it did for the original amplitude. But the bias voltage on the
diode is in fact the rectified output, so that variation of this
voltage with the input represents an output signal proportional
to the amplitude modulation of the input signal.
Detector Discrimination
Now suppose there is added to the input a smaller signal, at
a different frequency, as suggested by the dotted curve in Fig.
1. The first positive peak of this second signal falls fairly well
on the conduction period (determined mainly by the strong signal)
and therefore increases the rectified current; but, the second positive
peak falls in a non-conducting period and therefore cannot affect
the output, while the second conduction period is accompanied by
a negative peak of the smaller signal, which reduces the rectified
output and so tends to oppose the effect produced in the first conduction
period. It is obvious that the weaker signal has relatively little
effect if of different frequency from the stronger one, since it
is the latter which decides when the diode is conducting: as often
as not the weaker signal comes up positive when the diode is thoroughly
cut off by the. stronger signal, and on those occasions when the
diode is conducting, the weaker signal is as likely to be negative
as positive. This is only a very rough picture of the action, because
the frequency-difference is greatly exaggerated in Fig. 1, and no
allowance is made for changes in duration of the conduction periods
when the weak signal reaches a maximum or minimum near the edge
of a conduction period; when it has been properly worked out mathematically,
the ratio of the AF outputs due to modulation on the strong signal
S and on the weak signal W is approximately 2S2/W2,
and the phenomenon is known as "the apparent demodulation of a weak
signal by a strong one" (or, more briefly, "rectifier discrimination").
To see how useful this is, suppose that by means of selective circuits
we have made the wanted station supply a carrier voltage 10 times
greater than that of the unwanted station at the input to the detector:
this represents a signal/interference ratio of 20 db., which would
not be very good. But if S/W = 10, the ratio of the audio-frequency
output voltages is 2S2/W2 = 200, or 46 db.,
which is tolerably satisfactory.
Selectivity and Tone Correction
In early receivers this gain from linear detection was not always
obtained, because the signal level at the detector was so small
that the detector did not function as an on/off device, as described
in connection with, Fig. 1, but as an approximately square-law device
which conducted rather better in one direction than the other; since
the stronger signal was thus not sufficient to stop conduction for
part of the cycle, the weaker signal could always produce some effect,
regardless of its phase relation to the stronger signal, and no
rectifier discrimination was obtained. One of the first specialized
systems to obtain this advantage (though the mechanism was not at
first understood) was the "tone-correction" type of receiver. The
RF circuits (including the RF, if any) were made. of maximum Q,
so that a very high gain was obtained at carrier frequency and low
modulation frequencies, though the higher sidebands were relatively
cut by a very large amount, and after detection the severe top cut
was corrected by AF tone-correction circuits.
Rectifier Discrimination
Owing to the strong carrier, this gave good "rectifier discrimination,"
but the top boost in the AF circuits exaggerated any harmonics produced
in the process of rectification or by asymmetry of the RF circuits:
2 per cent to 5 per cent of harmonics in the output of the detector
could become something like 50 per cent harmonics in the loudspeaker,
and the popularity of this system was short-lived. In fact, it died
a natural death with the development of the super-heterodyne and
AVC; the latter required a large enough amplitude at the detector
to insure linear rectification, while the former provided the means
of getting sufficient gain, and at the same time made it technically
feasible to use selective band-pass circuits with a square-topped
response, giving good adjacent-channel selectivity without requiring
tone-correction.
But good tuned circuits are expensive and critical in adjustment,
even when they work at a fixed intermediate frequency, and of recent
years the number of high-powered transmitters has been greatly increased,
so that once again selectivity, is a problem. The tone-correction
system was on the right track, but the top boost in the AF circuits
was an intolerable nuisance; the solution then appears to be to
increase the amplification of the carrier only, while retaining
a uniform amplification for all the sidebands from lowest to highest,
and this is the homodyne system. The three systems are represented
diagrammatically in Fig. 2: diagram (a), normal receiver with square-topped
response curve; (b), sharp circuits requiring subsequent tone-correction;
and (c), homodyne receiver with carrier only accentuated. If wanted
and unwanted signal reach the detector with equal amplitudes, the
result will be a hopeless jam; but if we can add to the desired
signal an artificial carrier of just over 30 times the existing
carrier strength of either, we immediately obtain a rectifier discrimination
2S2/W2 equivalent to 66db., and reception
is perfect, without any disturbance of the audio-frequency response
characteristic. In fact, the audio-frequency performance is improved,
because an incidental advantage of the homodyne system is the elimination
of one source of distortion in the detector. With a normal diode
detector feeding a load circuit whose AC impedance is less than
its DC resistance, distortion occurs when the depth of modulation
exceeds some value such as 75 per cent (depending upon the ratio
of AC to DC load); but when the carrier has been artificially increased
for homodyne reception, the depth of modulation will always be small,
so that the ratio of AC to DC detector loads is no longer critical.
Artificial Carrier
The problem, of course, is how to produce this artificial carrier,
which must be exactly in phase with the original carrier of the
wanted signal, and there are two main lines of attack. According
to one method, the carrier is selected from the input by some form
of filter, and amplified more than the sidebands. There are various
methods of inserting the filter in the circuit, and a method of
selective negative feedback has been suggested as suitable (Patent
No. 533784, abstract published in Wireless World, Jan., 1942); but
this does not go far towards solving the problem, for the filter
still has to have a very narrow response, even if it is connected
in the negative-feedback line instead of in a straight-forward coupling
between two stages of amplification. It can be assumed that the
receiver is a superhet., and probably the IF will be 465 k.c. while
the lowest audio-frequency can be put at 50 cycles per second. (Any
rise in the response to frequencies below 50 cycles per second can
be easily offset by a falling-off in the characteristics of loudspeaker
and AF amplifier.) The carrier-selecting filter must therefore have
a band-width of not more than 50 cycles per second in 465 k.c./s
which is a fairly difficult proposition even for a crystal filter.
In addition, the intermediate frequency must then be correct to
something like 20 cycles per second, which means that both the accuracy
of tuning and the stability of the local oscillator must be as good
as 20 parts in a million for the higher-frequency end of the medium-wave
band, and proportionately better for short-wave working.
The other line of attack is to use a local oscillator, somewhat
similar to the IF beat oscillator used for CW reception, to generate
the extra carrier voltage, and synchronize this oscillator with
the signal carrier. Probably most experimenters have done this at
some time or another with a receiver using a reacting detector:
if the reaction control is smooth enough, reception free from beat
note can be obtained although the set is gently oscillating. But
this is not really a fair example of homodyne reception, since it
involves also a great increase of Q of the tuned circuit, and hence
loss of high audio frequencies, which would not be present with
a separate oscillator. In any case, this is hardly a method of reception
to let loose on the general public. But granted the use of a superhet
circuit and a separate oscillator tube for generating the carrier,
which is then a practically constant frequency, there are possibilities
in the way of designing the oscillator specially so as to hold synchronism
over as wide a range of frequency as possible, though even so, tuning
would need to be exceptionally accurate, and oscillator drift small.
One of the troubles is that on 100 per cent modulation the carrier
of the signal to be received falls to zero, and the homodyne oscillator
would then be almost certain to drop out of synchronism. (Some data
on the effect of modulation on the synchronization of an oscillator
were published by Eccles and Byard in an article in Wireless Engineer,
Jan., 1941, Vol. 18, p. 2.) Another snag is that the artificial
carrier from the local oscillator would predominate in the output
from the detector, so the DC component could not be used for AVC,
which would have to be derived from an independent IF circuit free
from carrier injection.
Possibilities of Development
It is clear that a good deal of development would have to be
done before a commercial broadcast receiver could be built on the
homodyne principle. (Perhaps the problem might appeal to some of
the amateurs whose transmitters are close down "for the duration.")
But the whole history of radio is the development of tricky laboratory
apparatus into something approaching a foolproof piece of household
equipment. For example, think back to the days of the earliest receivers
and compare them with the present-day superheterodyne. Instead of
single-knob tuning and a dial engraved in k.c., meters, and station
names, one used to have two dials, marked only in degrees, which
had to be simultaneously at the correct setting before any but the
local station could be received. Instead of AVC to keep a constant
output level, there used to be a reaction control which usually
needed progressive adjustment as one tuned round the waveband, in
order to keep a high level of sensitivity. Instead of independent
volume and tone controls, there would probably be a reaction control
supplemented by a rheostat in the filament of tile RF tube to control
gain, and the expert would balance reaction and gain adjustments
to secure the desired volume and band-width. Looking at this transformation
of the radio receiver, and the parallel transformation of the television
receiver from a 3D-hole scanning disc in front of a neon lamp into
the cathode-ray type of receiver, it does not seem unduly optimistic
to say that the difficulties inherent in the homodyne system of
reception could be overcome in a commercial design.
-Wireless World (London).
Posted
|