August 1965 Electronics World
People old and young
enjoy waxing nostalgic about and learning some of the history of early electronics. Electronics World
was published from May 1959 through December 1971. See all
Electronics World articles.
Here 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 in
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 technicians.
Receiver Noise from Antenna to Detector
By Joseph Tartas
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 call communications.
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
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.
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 is present.
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.
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.
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, mixer-input
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 of time.
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.
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.
Fig. 3. - Germanium diode will start to conduct
at a lower signal than will silicon.
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
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 1.0-volt signal.
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
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,
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.
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.
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
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
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
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.
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.
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 noise figure.
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.
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
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.
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
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
where Es is the unmodulated carrier level, and Pn
and Ps are unmodulated and modulated output power,
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 or capacitor.
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 Fig. 5.
Posted June 4, 2015