The "Monode" Noise Generator
April 1967 QST Article
noise diodes are fairly inexpensive these days and are widely used
for measuring noise figure of systems and for generating specific
signal-to-noise ratios when testing receiver performance. This article
from a 1967 edition of QST describes a method for using a 'hot resistor,'
aka 'monode,' as a noise reference source. When the temperature
(T) and the resistance (R) is known, a noise power can be calculated
with a precision limited by the precision of the T and R measurements.
The tungsten filament of a pilot lamp is used as the resistor.
April 1967 QST
Wax nostalgic about and learn from the history of early electronics. See articles
QST, published December 1915 - present. All copyrights hereby acknowledged.
The "Monode" Noise Generator
Hot-Resistor Noise-Figure Measurement
By Ronald E. Guentzler,
This article describes a noise generator that should
find use in amateur work either as a noise source for noise-figure
measurements or as a reference source for comparison with the output
from some other noise source. It is inexpensive and simple to construct.
The "Monode" noise generator is essentially a hot resistor whose
noise output is known when the temperature and resistance are known.1
The hot resistor is the tungsten filament of a No. 12 radio pilot
lamp heated from a d.c. source. The term "Monode" is derived from
vacuum-tube terminology, a monode being a one-element vacuum tube.
The Monode noise generator was constructed to obtain a known
source of random noise to check the performance at 147 Mc. of a
5722 temperature-limited diode generator similar to the one in the
Handbook.2 The reason for desiring a means of checking
the 5722 generator arose from comments by J. A. Huie3
and A. van der Ziel4 regarding the effects of stray capacitance
and inductance on the noise output of the 5722 generator at high
frequencies. (The output of a 5722 generator was found to be 12
percent or 0.5 db. high at 147 Mc. before compensation!)
Two other Monode noise generators were built to prove that the
principle of the Monode noise generator was indeed practical at
lower frequencies. These generators are for use in the 6- and 40-meter
The Resistor as a Noise Generator
A resistor at any temperature above absolute zero
generates a noise power
P = KTB watts, where
1.38 X 10-23 Joules/Kelvin degree,
T is the temperature
of the resistor in degrees Kelvin, and
B is the bandwidth in
cycles per second.
When the temperature of a resistor is
other than some reference temperature, T0 (usually taken
as 290°K), it may be convenient to use the terms "excess noise
temperature" or "excess temperature," which are defined as the temperature
of the resistor minus reference temperature; i.e.,
= T - T0.
The term "excess noise" is commonly
used; the excess noise is the excess noise temperature divided by
the reference temperature; i.e.,
E.N. = (T - T0)
The excess noise may be given in db., where
E.N.db. = 10 log10 (T - T0) /T0.
In order to obtain enough noise for convenience of measurement,
the resistor may be raised to many times room temperature. The filament
of an incandescent lamp makes a good hot resistor because tungsten
is a well-behaved material and has a high melting point. The temperature
can be raised by passing a direct current through it.
The Monode Generator
The Monode has the
advantages of simplicity, low cost, and being an absolute standard.
The disadvantages are fixed output and the necessity for tuning
each amateur band (but not within the band). The complete generator
is composed of three basic parts: a regulated variable-voltage power
supply, a room-temperature "quiet" termination (R1),
and the noise generator with its r.f. filtering and coupling network
The variable d.c. voltage from the power supply is used to heat
the filament of the lamp. The d.c. is filtered by means of RFC1
and a 0.001-µ.f. capacitor, C3, to eliminate any
r.f. noise component that might be present in the power supply.
RFC2 is used to conduct into the lamp the d.c. required
to heat the filament while preventing the thermal noise generated
in the hot lamp filament from being lost in the supply. The noise
generated in the filament is coupled to the output connector, J2,
by means of C1; this capacitor also serves the function
of resonating the lead and lamp-filament inductances so that the
output impedance is purely resistive.
|A resistor is about as basic a noise generator as you
can get. The filament temperature of a No. 12 dial lamp
can be adjusted to the desired resistance with sufficiently
high noise output, and the corresponding noise temperature
is available from the calibration curve given in this article.
With these data, measurement of receiver noise figure becomes
Fig. 1 - Circuit of the Monode
noise generator. Except as indicated, capacitances are in µf.;
capacitor with polarity marked is electrolytic, other fixed capacitors
are disk ceramic. Resistances are in ohms.
trimmer (Centralab 822-EN or equivalent). See text for values for
frequencies other than 144 Mc.
C3-Disk ceramic (see text).
inc.-Silicon, 500 ma., 100 volts p.i.v. or higher.
Zener, 18 volts (1N1819 or equivalent).
-Neon pilot-light assembly.
J2-Chassis-mounting coaxial connectors.
carbon adjusted to 50 ohms.
RFC1, RFC2-1.8-µh., 1000-ma. choke
rectifier transformer, 30 volts, 2 amp. A 24-volt, 1-amp. filament
transformer may be used if a suitable value of resistance is substituted
in power-supply filter.
The major portion of the noise generator can be
built using any mechanical construction desired. The one described
was built on a 3 1/2 X 19-inch relay-rack panel. The power supply
is mounted in a 3 X 4 X 6-inch aluminum chassis fastened to the
rear of the panel. The r.f. filter network and the quiet termination,
R1, and its connector, J1, are mounted in
a small Minibox. The No. 12 lamp, C1, RFC2
and J2 are mounted in an identical Minibox. The two Miniboxes
are fastened together and to the panel; the connectors J1
and J2 protrude through holes in the panel.
obvious innovation would be to have the Minibox containing the lamp
physically separate from the power supply and connected to it by
means of a flexible cord. In this event, the coaxial socket J2
would be replaced by a plug and R1 would be mounted in
a separate plug.
The power supply is a conventional bridge-rectified,
RC-filtered supply with a shunt Zener regulator. The supply was
made electrically larger than necessary because it was not known
at the time of construction what lamp type would be used in the
final version. The 1/2-ampere, 18-volt capability gives a range
of voltages and currents large enough for experimental purposes.
The actual maximum output required for the No. 12 lamp is 10 volts
at 200 ma. The regulation is probably not necessary.
noise-generator portion of the unit requires more than usual care,
considering the frequency for which the unit is designed. Stray
inductance and capacitance are not particularly important, although
they should be kept low; this is the opposite of the 5722 generator
where stray inductance and capacitance result in improper amounts
of noise output. However, losses cannot be tolerated; i.e., any
resistance appearing in the noise-generating circuit other than
the hot lamp filament must be eliminated. This is again the opposite
situation from the 5722 generator where losses will, in general,
have no deleterious effects and can be beneficial.
2 is a photograph of the noise-generating portion of the unit. The
components are mounted in such a way that the lead lengths are as
short as possible in order to keep their losses low. The lamp is
mounted in a "socket" constructed from two of the metal inserts
taken from a miniature-tube socket. An entire tube socket cannot
be used because the pin spacing is improper. Also, sockets introduce
the possibility of losses.
The Monode described here is
usable at frequencies below 144 Mc. with slight modification. Two
separate noise-generating portions were built, one for use on 6
meters and one for use on 40 meters. For 6 meters, C1
is a 50-380-pf. mica trimmer, RFC1 and RFC2
are Ohmite Z-50 inductors, and C3 is the same as listed
for 2 meters. For 40 meters, C1 is two 0.001-µf.
fixed mica capacitors in parallel, RFC2 is an Ohmite
Z-7 inductor, RFC1 is omitted, and C3 is a
For the other high-frequency bands,
use the appropriate Ohmite inductor for RFC2, omitting
RFC1; use a 0.1-µf. disk ceramic for C3.
C1 should be the size required to resonate the lamp filament
and lead inductance in order to present a pure 50 ohms at the output
Although the temperature of the lamp filament can be varied by varying
the applied .c. voltage, only one temperature of operation is usable
because the resistance of the filament is also a function of the
applied voltage, and this resistance must be set to give the proper
output impedance. Some means of impedance measurement in the band
in which the unit is to be used should be available; this can be
either an impedance bridge or meter or an s.w.r. bridge known to
be properly calibrated. The impedance-measuring device must be sensitive
enough to operate on small amounts of r.f. This is necessary to
insure that the r.f. getting into the lamp does not heat the filament
to a temperature greater than that resulting from the applied d.c.
A good check can be made by applying the r.f. while the d.c. is
off. The lamp should not glow.
Fig. 2 - The noise- generating head of the Monode.
Leads between the lamp, C1 and the coax connector
are kept to the shortest possible length. The quiet termination
and r.f. filter are in a similar box bolted to the bottom
of the one shown.
Fig. 3 - Filament temperature in degrees Kelvin vs.
applied d.c. voltage, G.E. No. 12 pilot lamp.
With the impedance-measuring
device connected and operating, the d.c. lamp voltage is applied
and the voltage and C1 are adjusted until the output
impedance at connector J2 is 50 ohms, purely resistive.
The value of the lamp voltage is noted, and whenever the unit is
to be used the voltage is set at this value. If the Monode noise
generator is to be used as a reference for comparison with other
noise generators, it is important that the output impedances of
all the generators be the same. The best way to make sure that they
are the same is to measure all of them at the same time, with the
same measuring device, and at the same frequency.
temperature of the filament can be found from Fig. 3. This curve
applies to General Electric No. 12 lamps, and may not be applicable
to lamps of other than G.E. manufacture.5 The excess
temperature or excess noise can be calculated by means of previously
given formulas. For example, assume that as a result of the impedance
adjustment step it was found that the lamp must operate at 8.4 volts
in order to give an output impedance of 50 + j0. With the aid of
Fig. 3 the lamp temperature is found to be 2430°K when operated
at 8.4 volts. This is the noise temperature. If room, or reference,
temperature is 290°K, then the excess temperature is 2430 -
290 = 2140°K, and the excess noise in db. is 10 log10
(2140/290) = 8.7 db.
R1 should be adjusted to
give an impedance of 50 + j0 when viewed through J2 This
resistor should be an inherently nonreactive composition type such
as the Ohmite "Little Devil."
Using the Monode Generator
In using the Monode as a source of noise for noise-figure
measurements of a receiver, the quiet termination of the Monode
is first connected to the input of the receiver under test by means
of a coaxial cable having x db. loss. (If the separate noise-head
construction is used, x is taken as zero.) The output noise power
from the receiver is noted; call this reading A. The Monode is then
set to its operating voltage, its output is connected to the input
of the receiver through the same cable, and the output power of
the receiver is noted; call this reading B. The noise figure of
the receiver in db. is then found from the formula
= E.N.db. - x - 10 log10 (A-1),
where E.N.db. is the excess noise of the Monode noise
generator in db. Note that B and A must be in units of power and
not in db.
For example, assume that a coaxial cable with
0.9 db. loss is being used between the Monode noise generator and
the receiver; this makes x = 0.9 db. Assume that the excess noise
of the Monode is 8.7 db. (from the previous example). Further, assume
that the receiver noise output was 1 milliwatt with the quiet termination
and 4 milliwatts with the Monode connected; this makes B/A = 4.
Therefore, the receiver noise figure is 8.7 - 0.9 - 4.8 = 3.0 db.
comments are intended to provide a basis for further experimentation,
especially at frequencies above 150 Mc.
Skin effect does
not significantly increase the a.c. resistance of the lamp filament
from the d.c. value, at frequencies up to 150 Mc., because the filament
diameter is small (approximately 0.001 inch) and the resistivity
of tungsten is relatively high. Therefore, it would be expected
that the resistive component of the output impedance as seen from
J2 would be the same as the d.c. filament resistance
as calculated by taking the ratio of the lamp voltage and current.
However, there is about 1 pf. shunt capacitance across the lamp
resulting from the lamp leads, and there is a significant amount
of inductance effectively in series with the filament resistance.
This inductance is principally a result of the coiled filament and
the filament support leads; the total inductance is approximately
0.04 microHenry. As a result of the inductance and capacitance,
the resistive portion of the filament impedance, as viewed from
J2, is increased in magnitude. This increase is significant
only at 50 Mc. and above. For the unit pictured in Fig. 2, the d.c.
lamp resistance is 45.6 ohms when the output impedance is 50 + j0
at 147 Mc.
The impedance step-up effect was not observed
in an earlier version of this unit. The d.c. filament resistance
and the output impedance were both 50 ohms. This was considered
a bit of good luck until the noise output was found to be too low.
It was discovered that the inductor RFC2 was lossy and,
since it was effectively in parallel with the lamp, its associated
losses lowered the apparent filament resistance, the output impedance,
and the noise temperature. This is why the Ohmite inductors are
A different lamp type might eliminate the impedance
transformation problem and the necessity for retuning or rebuilding
the Monode noise generator for each different amateur band. An ideal
lamp type for noise generation would be one with a straight (not
coiled) filament mounted in a small-diameter tubular bulb having
the lead-in wires appearing at the opposite ends of the lamp. If
the leads are brought out from opposite ends of the bulb, the lamp
could be coaxially mounted. This mounting scheme offers the possibility
of low stray inductance and capacitance.
I wish to express
my appreciation to Mr. Donn R. Hobbs of the Miniature Lamp Department,
General Electric Company, Nela Park for many pieces of information
regarding miniature lamps.
1 A. van der Ziel, Noise, Prentice-Hall, 1954, pp. 60-61.
2 The Radio Amateur's Handbook, 42nd ed., 1965, pp.
3 Huie, "A
V.H.F. Noise Generator," QST, Feb. 1964,
4 A. van
der Ziel, op. cit., pp. 63-69.
5 Smithsonian Physical
Tables, 9th ed., rev. 1956, Table 85 and Test No. 7824, Miniature
Lamp Dept., General Electric Co., Nela Park, 1966.