August 1965 Electronics World
Wax nostalgic about and learn from the history of early electronics. See articles
Electronics World, published May 1959
- December 1971. All copyrights hereby acknowledged.
Here from a 1965 issue
of Electronics World magazine is a really nice write-up
on electrical noise, both how it originates and how it affects receiver systems.
Although vacuum tubes were still the predominant active amplification components
in 1965 (the date of this article), semiconductors were already solidly ensconced
in the signal detector role. I have to confess to learning a new term that I probably
should be familiar with: Equivalent-Noise-Sideband-Input, or ENSI. It appears also
Reference Data for Engineers: Radio, Electronics, Computer, and
Communications. Interestingly, this is the first time in a long time I
have seen noise referred to as "grass;" the drawings make it clear why the moniker
was created. We were taught to use "grass" in USAF radar tech school and used it
in common parlance once on duty as radar technicians.
Receiver Noise from Antenna to Detector
By Joseph Tartas
The sensitivity of a radio receiver is governed by the amount of internally generated
noise, how this noise originates, how it is measured, and how it can be reduced
are covered in this article.
The most common byproduct of our civilized world is noise in one form or another.
We rise in the morning to noise; we ride to and from work with noise; and we eat
and relax with noise. In its most common form, noise is the effect of vibration
upon our eardrums, and subsequently through the message centers of our nervous system,
upon our brain.
In electronics, however, noise is a different matter. More than being just annoying,
it is the limiting factor in the reception of intelligence through the medium we
The ultimate goal is the reception of intelligence and the reproduction of the
originally transmitted material in its truest fidelity, whether the receiver be
a simple table radio or an extremely complicated and sensitive piece of electronic
laboratory equipment. The weak signal received at the antenna must be amplified,
sorted out of the myriad of other signals and assorted electrical impulses, amplified
to a greater degree than the undesired signals and noises, converted to a varying
d.c. voltage by a detector, and then applied to functional circuits to actuate a
speaker, CRT, relay, or any of a multitude of recording devices.
Kinds of Noise
Fig. 1. - (A) Passband of receiver showing both desired
and interfering signal. (B) Detector output with and without a.g.c. Bias change
due to a.g.c. can reduce or eliminate desired signal when the interfering signal
Fig. 2. - If the interfering signals shown on the right
occur between T1 and T2, their relatively short duration and low amplitude have
no effect. If situation is reversed, receiver would then be blocked for that period
The various types of noise can be broken down into two basic categories: natural
and man-made. There are various methods of dealing with noise, but no matter what
the source, there is a limit to the amount of reduction we can achieve.
Man-made noise includes both intentional and unintentional signals within the
passband of the receiver; interference from sparking, such as automotive ignition,
arcing of generator or electric motor brushes, and defective wiring; and a host
of other causes, most of which result from an electric spark. Such a spark produces
interference over an extremely wide frequency range and is impossible to filter
out if it is near the level of the signal to be received.
Intentional signals might include those that are produced within the passband
of the receiver, either due to drift of the transmitter or poor design of the receiver.
Such a signal, if strong enough in intensity, will cause the first few stages of
the receiver to draw grid current and hence block (in the case of tubes) or saturate
(in the case of transistors). If automatic gain control (a.v.c. or a.g.c.) is used,
the strong undesired signal will cause drastic reduction in the gain of the stage
or stages, and the desired signal will then be amplified to such a small degree
that it will be unintelligible or unusable. (Fig. 1.)
This effect is known as cross-modulation, and its effects are determined by the
type of tube used in the r.f. or i.f. stages, the bandpass of the receiver, the
shape factor of the selective circuits (relation of peak to skirt bandwidth), and
the type of interference. If the interference has a different time relation or duration
than the desired signal, it may have little or no effect on the intelligibility
of the received signal. (Fig. 2.)
Another effect of undesirable signals within the passband is known as intermodulation
distortion. Intermodulation distortion results when an interfering signal is applied
to the mixer along with the desired signal. The non-linearity of the mixer produces
both sums and differences of the two, in addition to the modulation of one by the
amplitude variations (modulation) of the other. In addition to this, if the interfering
signal produces cross-modulation sufficient to bias the r.f. stage back to a non-linear
point, then the r.f. stage itself will act like a mixer and produce its own sums
and differences, which will then be passed on to the mixer stage to further produce
sums and differences of those that get through the passband of the r.f.-output,
Natural interference causes noise in a form we call static. Such interference
results from lightning, aurora, whistlers, and, to some extent, so-called galactic
noise. Lightning, or other static electricity, produces a spark during discharge,
and hence yields the same result as man-made static. The only difference here is
that the intensity of a lightning discharge is tremendous and in most cases completely
blanks out the receiver, whether or not the signals coming through are desired or
undesired. Aurora, on the other hand, produces a wide variety of effects, from sizzling
static to rapid fade or flutter, not to mention the complete blanketing of electromagnetic
circuits such as telephone lines. Whistlers are usually of short duration and, because
of Doppler effect, change frequency to some degree as they progress. Depending upon
the selectivity of the receiver, they may pass completely out of the passband before
the a.g.c. can react. Galactic noises, except for u.h.f. and microwave receivers,
and some v.l.f. equipment, with antennas designed specifically for the reception
of these fixed noises, are not of any significance to the average reception medium,
since the inverse-square law of r.f. signal strength vs distance shows them to be
too minute. It is the natural law that makes the intentional reception of these
stages difficult in radio astronomy. Galactic noises are so weak that other noises
tend to obscure them, and the search for better reception methods is an unending
Receiver noise, other than that due to antenna or cable defects, is the most
important factor to consider. Since the antenna itself is essentially a passive
device, there is no significant noise contribution from it other than that received
by it in the form of the electrical radio signals.
In most cases, the sensitivity of a receiver is set solely by the front-end of
that receiver itself; that is, between the antenna terminals and the first i.f,
stage. Because the r.f. stage, mixer, or first i.f. stage can contribute to this
sensitivity, they must be considered separately and together.
Fig. 3. - Germanium diode will start to conduct at a lower
signal than will silicon.
The relation among sensitivity, gain, detector levels, and selectivity seems
to be a confusing one to many, yet each is a separate entity. Further confusion
is often added by the concept of noise, signal-to-noise ratio, noise figure, and
noise voltage. It is one aim of this article to clarify the meanings and relationship
among these terms.
Sensitivity means just what it says. It is the degree to which a receiver is
sensitive to a signal. In other words, a more sensitive receiver will be able to
reproduce the same intelligence from a smaller amplitude signal, all other things
being equal. A less sensitive receiver will require a larger signal, all other conditions
being the same. However, the sensitivity at the input end will also depend upon
the amount of gain, or amplification, that follows it and the level desired or obtained
at the detector. By a detector, we mean the point at which r.f. is converted to
d.c., AM, or any other type of intelligence. For practical purposes in this discussion,
let us consider the receiver as being a high-frequency communications or radar receiver
with a diode detector, the intelligence being in the form of positive square pulses.
This is something that could be observed on an oscilloscope and thus serves well
to aid in the explanation.
Because of the characteristics of the usual r.f.-type silicon or germanium diode
detector, the last i.f. stage must have sufficient output to drive the detector
well into the linear region. For most of these diodes, the output must be at least
0.5 volt to produce a minimum of distortion and good linearity of the modulation.
If we put a pulsed signal into the receiver and increase its level until we can
see a small output from the detector but no random noise, we have no way of knowing
the actual sensitivity of the receiver. Additional amplification between the r.f.
stage and the detector might show either of two things: the increase of output signal
level resulting might also be accompanied by the appearance of noise such that any
decrease in input signal would cause the output signal to disappear into the noise;
and a decrease in input signal would still produce an output with no visible noise.
If additional gain produces noise at the output, no further gain would help and,
for the given conditions, the maximum sensitivity of the receiver has been attained.
This is also known as the threshold sensitivity.
If, however, no noise appears, the receiver still has inadequate gain between
the r.f. stage and the detector, and the sensitivity can be further improved by
additional stages of gain. This should be increased to the point where noise does
begin to appear in the output, and we are then at the same point discussed previously.
The point of adequate amplification has been reached when sufficient noise is seen
at the detector output. This noise level should be about 0.5 volt or half the desired
The selectivity of the front-end is the ability of the receiver to receive or
reject certain signals close to each other, Thus selectivity means to select, to
reduce or remove interference, and reduce cross-modulation and intermodulation interference.
Traps and bandpass transformers eliminate specific signals that are sufficiently
displaced from the desired signals and limit those that are not sufficiently out
of the passband to be rejected completely.
The greatest amount of receiver noise is due to the action of electrons within
the first tube or transistor. If the first stage is a converter rather than an r.f.
stage, then the relative noise out of the receiver will be greater for a given input
signal level. If a mixer (converter) stage follows an r.f. stage but the r.f. stage
has little gain, then the relative noise level out of the receiver may be somewhere
between the two levels. This will be explained subsequently.
Origin of Noise
Even in the most perfect component, some noise is always developed due to the
random motion of free electrons within the material. Since the velocity of the electrons
represents current flow, then a varying a.c.-voltage component is produced due to
the natural resistance of the material. Because electron motion is always random,
so is its velocity, and hence the frequency and amplitude of the noise voltages
are continuously changing. The net result is called white noise, and its energy
is evenly distributed throughout the entire radio spectrum.
Because the r.m.s. voltage taken over a relatively long period of time is measurable,
we know that for a given bandwidth the voltage would be the same at 100 kc. as.
it would at 100 mc. because of this even distribution. This noise voltage, when
due to the antenna and transmission line, is usually negligible compared to the
tube or transistor noise and is further dependent upon the temperature, the resistance
across which the noise voltage is developed, and the bandwidth of the component,
circuit, or receiver.
Although the average noise voltage is zero, the a.c. component exists in cycles
varying in a perfectly random manner and has a constant r.m.s. voltage. The mean-square
noise voltage associated with the input circuit is found by: en2 = 4kTRΔƒ,
showing that the noise is dependent upon the temperature (T), bandwidth (Δƒ),
and the equivalent input resistance (or noise resistance) for the first stage (R)
k is a natural constant, a relation of energy-per-degree of temperature discovered
In a vacuum tube, random emission and random velocities of the electrons as they
leave the cathode or arrive at the plate produce a random modulation of the d.c.
plate current component known as "shot noise," If a tube is relatively noisy, then
the equivalent resistance in the grid circuit is quite high. Hence, the merit of
a tube in relation to its noise contribution is dependent upon how low an equivalent
resistance can be assumed to be present in the grid circuit. This equivalent resistance
for a triode is simply 2.5/Gm, but for a pentode it is much higher because the electron
stream is divided into two paths - plate and screen - and the random modulation
of the d.c. plate component is considerably greater.
For any given tube type, there is a value of source resistance for which the
best noise figure of that tube can be achieved. This information has been calculated
for various types and is normally found in the manufacturer's data sheet for that
tube. For transistors, the information is usually given as a typical noise figure
at some test frequency, with a fixed-source impedance. In addition, curves are often
shown that give the range of frequencies over which the transistor or tube is usable,
and these show both the variation in noise figure with frequency and the difference
in noise figure with different source impedances.
Except for the filaments, a transistor presents a similar problem in relation
to noise. Since the flow of electrons (or holes) in both cases is from a lower to
a higher energy level, the same sort of thermal agitation takes place, and the modulation
of collector current produces a noise voltage. The transistor, however, because
of its base-spreading resistance, methods of biasing, and impedances, has until
recently resulted in a relatively high noise level. For the sake of discussion,
however, the terms tube and transistor may be used interchangeably.
Fig. 4. - Typical input circuits. (Top) Vacuum-tube cascode.
(Bottom) Grounded (common) emitter circuit. Except for the bias and input circuits,
r.f. circuit is similar in both.
As the frequency is increased, there are further effects of transit-time loading
with a reduction in gain that appears to increase the noise of the stage, and the
equivalent input resistance seems to be as much as five times greater.
If the signal and the noise in the first stage are amplified sufficiently, then
all the noise added by the next stage will be hardly significant. For example, if
the noise voltage at the input of the first stage is one microvolt, and the signal
is two microvolts (representing a 2:1, or a 6 db signal/noise ratio), and the gain
of the stage is 20 (or 26 dbv ), then the noise . voltage and signal voltage at
the input of the next stage will be 20 and 40 microvolts respectively. If the next
stage then adds one microvolt of noise to this, then it represents an increase
of 1/20 or 5%. The third stage, if the second stage is similar to the first, will
then add 1/400 or 0.25%.
The formula for this is usually given as: NFtotal = NF1
+ (NF2/Gain1) where NF represents the noise figure of that
stage in db.
It is therefore essential that the first stage have the lowest possible noise
contribution with the highest possible gain. Because the lowest noise figure in
a tube is obtained with a triode, this is normally used as the input stage, or more
often, two triodes in a cascade circuit are employed (Fig. 4). This circuit
comprises a low-noise triode in a grounded-cathode stage with little or no gain,
to eliminate Miller effect (change in tube capacitance with gain change from a.g.c.),
followed by a grounded-grid stage with the gain of a pentode, to eliminate the grid-plate
capacitance feedback problem. The cathode impedance of the second stage, usually
a few hundred ohms, serves as a low-impedance output load for the first stage, thus
making this section extremely stable.
Transistors, until recently, were notorious for their noise and could not be
used as input stages where good sensitivity was desired. As a result, hybrid receivers
were constructed, with input stages using low-noise vacuum-tube circuits (usually
the cascode) followed by transistor circuits for the mixer and i.f, amplifiers.
Recent improvements in semiconductor materials have resulted in germanium transistors
with noise figures of 2 db up to several hundred megacycles and silicon transistors
with noise figures near 3 db for the same frequency ranges. The nearest devices
until then were the low-noise triode tubes, such as the W-E 417B, and the more recent
RCA nuvistors and G-E ceramic subminiature triodes, but these required considerably
more power for the active elements.
Fig. 5. - Detector outputs. Top row, left to right: output,
no visible noise; increased gain, still no noise; same condition, noise visible,
shows adequate gain; signal reduced to original 1-volt level, noise disappears,
signal good; noise level approaching that of signal. Bottom row, left to right:
adequate gain and noise figure; gain not improved but noise reduced; same as last
case but with additional gain after first stage, over-all signal still good; last
condition shows maximum sensitivity (tangential sensitivity) when bottom of noise
in pulse is equal to top of noise outside.
At reasonably low frequencies (up to 500 mc.), there is a definite advantage
to the use of one or more r.f. stages to give adequate amplification with minimum
noise contribution. Because the conversion gain of a triode used as a mixer (again
the less noisy tube) is less than its gain as an amplifier, noise contribution can
be minimized if the gain of the preceding stage or stages is adequate.
The input equivalent resistance as a mixer is 4/Gm, instead of the 2.5/Gm as
an amplifier, and the conversion noise figure is higher than that of the amplifier
At some high frequency, known as the transition frequency, the use of an r.f.
amplifier ahead of the mixer becomes impractical because of the transit-time loading
effect, excessive feedback, insufficient gain, and other problems.
In this case, the signals are fed directly to a converter or mixer. This may
consist of a special tube circuit or, at the higher frequencies where the reactances
within the tubes or transistors cannot be tuned out, the use of mixer crystals or
diodes is preferred. These are usually preceded by tuned circuits or cavities to
provide the necessary selectivity to eliminate images or other interfering signals
within the passband of the following circuits.
Because there is no amplification in this type of converter, and even a loss
in signal during the conversion process, the first i.f. stage then becomes an important
factor in the over-all noise of the receiver. In this case, the total noise figure
becomes: NFtotal + L (NFi.f. + t - 1) where L is the diode
conversion loss and t is the diode noise temperature. The total noise then obviously
rests heavily upon the noise contribution of the i.f. amplifier. Just as in the
case of the r.f. stages, the first i.f. stage must have highest gain and lowest
noise in order to have a minimum noise contribution from the i.f. stages as a whole
when this system is operating.
Of equal importance in the consideration of low-noise circuitry is the matching
of impedances between antenna and input stage, or input stage and mixer. Aside from
the consideration of maximum power transfer, it is essential to provide the match
from the source to the correct input noise impedance. It is only under these conditions
that the minimum noise conditions can be achieved. In transistor circuits, some
designers intentionally load down the input circuits with excessive damping resistance
in order to make the stage more stable. This is often done at the sacrifice of gain,
since this additional power is dissipated uselessly in the loading resistor. If
such excessive loading is attempted, it should not only provide the correct noise-match
impedance, but it should also be done by using the low input impedance of the transistor
itself. In this way the excess power is not dissipated in a resistor but instead
is dissipated as useful power within the active elements of the transistor, providing
a stable amplifier but with more gain (and hence better sensitivity for the same
noise voltage). This is the ideal solution.
Fig. 6. - A simple noise generator. The output is fed to
receiver antenna terminals while the input circuits are adjusted for maximum noise
for fixed settings of noise potentiometer.
We have discussed the requirements of receivers with respect to noise, and in
order to obtain the best possible noise figure for a receiver, it is necessary to
make some adjustments. A noise generator as an outside noise source is quite often
used in laboratories, but a calibrated signal generator may also be used.
A noise generator uses a temperature-limited diode, also known as a noise diode,
with a variable anode current source. Because the noise output has a wide spectrum
and its amplitude is directly related to the anode current, the generator can be
calibrated directly in noise figure. It is connected through suitable d.c. isolation
to the receiver input. The input is shunted by a resistive termination equal to
the antenna impedance, and the noise power at the receiver output is measured. The
anode current of the noise generator is then increased until the noise power out
of the receiver has been doubled. The noise figure (usually in decibels) is then
read directly off the generator. Theoretically, this represents the number of db
of noise greater than a perfect receiver where all of the noise was due to the antenna.
This perfect receiver would then have a noise figure of zero db.
The indicator for output power may be an oscilloscope, a peak-reading v.t.v.m.,
or a thermocouple indicator. When reading voltage, the power is doubled when the
voltage is increased by 1.414 times the original value.
The noise factor is then F = 20IbRant, where Ib
is the anode current of the noise diode and Rant is the equivalent antenna,
or receiver input impedance.
When trying to optimize the performance of a given receiver for best noise figure,
it is not necessary to have an actual noise figure. A noise generator may be made
from a high-frequency diode, and the receiver circuit is then adjusted for minimum
relative noise. Such a generator is shown in Fig. 6.
The signal-generator method depends largely upon the bandwidth of the receiver
and the linearity of the detector. If the detector level is adequate, as discussed
earlier, the measurement may be considered quite reliable.
The standard of receiver measurement is that of the equivalent-noise-sideband-input,
or ensi. In this method, the unmodulated carrier is applied to the input of the
receiver (always through its proper antenna impedance) and the noise power is read
on an r.m.s.-reading instrument. This level should be three to ten times as great
as the expected noise voltage, or about 2 to 5 volts. The carrier, still at the
same level, should now be modulated at 400 cps and 30%. The signal-power output
is then measured through a filter that will pass only 400 cycles (this eliminates
the problem of different bandwidths in the receivers). The ensi is then equal to
where Es is
the unmodulated carrier level, and Pn and Ps are unmodulated
and modulated output power, respectively.
Initially, it is necessary to select a tube or transistor with a good noise figure.
This information, along with the best noise-match impedance, is usually provided
in the manufacturer's data sheet. Sometimes it is necessary to use some other impedance,
or because of supply voltages available, it is not possible to duplicate the manufacturer's
test conditions exactly. In this case, the best conditions must be arrived at.
A triode tube usually produces the best noise figure when it is biased for class
A operation where the gain is maximized, consistent with linearity. In a transistor,
it is most often found that the best noise figure is obtained when the collector
current is quite small, usually around 1 to 2 ma.
Depending upon the circuitry, it is usually advantageous to adjust the matching
transformer and its loading for best noise figure. By observing a signal (either
on an oscilloscope, or modulation through a 400-cycle filter) , adjust for the greatest
signal-to-noise ratio for a given generator output. When the noise match has been
optimized, the smallest input signal will be seen above the noise.
The noise figure may be further improved in a grounded-cathode tube circuit,
or in a common-emitter transistor circuit, by neutralization. This reduces the effects
of the grid-plate or base-collector capacitance upon the input impedance. The usual
practice is to place a small inductance between the grid and plate through a series-blocking
capacitor. When the reactance of the inductor is equivalent to the reactance of
the grid-plate capacitor, the two will be in parallel resonance, and the effect
is the same as replacing the capacitive reactance with a very high series reactance
of the value of QX, the "Q" of the inductance, and the reactance of either the coil
It will be found that the maximum noise voltage, maximum gain, and maximum sensitivity
will result simultaneously with the best noise figure. Do not look at the noise
voltage alone when adjusting for best noise figure, as this is deceiving. A signal
must be visible with the noise in order to determine whether or not the ratio of
signal to noise has increased or decreased with an increase in noise voltage. See
Posted March 30, 2020(original