Devo ad un mio caro amico la riscoperta di un vecchio, interessante sistema di ricezione dei segnali. Non sarebbe altro che un eterodina, se non fosse che manca l'alimentazione anodica. Conosciamo già dei sistemi di supereterodina nei quali la valvola mescolatrice non vede l'anodica, ma il segnale dell'oscillatore rivelato. Qui vediamo un ondametro con una  valvola R con la sola alimentazione del filamento. Lo strumento, quando riceve un segnale, invece che far accendere una lampadina nel controllo di un trasmettitore o di ascoltare il segnale rivelato da un carborundum, riceve la risultante dell'onda in arrivo e quella dell'oscillazione che, quasi miracolosamente, appare nella valvola disalimentata .

da Bollettino Telegrafico del regio esercito, 1922 articolo del tenente Angelo Terranova delle officine rt ed e del genio militare
....osservando il circuito si nota che in questo ricevitore si è introdotta una semplificazione interessante dovuta allo scrivente e cioè che l'eterodina è priva di batteria anodica. L'unica sorgente è la batteria a 4 volt del filamento. Questo risultato si è potuto ottenere con l'attacco al punto di mezzo dell'induttanza alla batteria. L'energia così generata è più debole di quella della normale eterodina provvista di batteria anodica ma è più che sufficiente come eterodina in ricezione.

praticamente l'induttanza è tra placca e griglia con presa centrale al + del filamento, alla griglia arriva anche il quadro di ricezione.

L'articolo dopo le illustrazioni è del 1924


Continuous wave syntoniser

MSC., Royal Aircraft Establishment, Farnborough.
[MS.re cezved, 30th October, 1924.1
ABSTRACT. The syntoniser is a valve generator of feeble high-frequency alternating current,
the periodicity of which can be varied over nide limits. The instrument is used for the measurement
of the wave-length of high-frequencq oscillations and also as a heterod!ne for C.W. reception.
The instrument described was developed by the l i r Service during the war.
The various applications of this sy-ntoniser and also the method of its accurate calibration from
a tuning-fork of knon-n frequency are described in detail.
THEsy ntoniser is essentially an instrument designed for the accurate measurement of the
wave-length of continuous high-frequency oscillations, although it can also be used for other
The instrument which is here described was developed by the Air Service during the
war and was used considerably for both ground and air work.
It is essentially a small generator of high-frequency alternating current, ahich may, by
the adjustment of a variable condenser, be made to produce oscillations of any desired
frequency within its range.
It consists of a three-electrode valve x-ith its grid and anode circuits coupled inductively,
the anode circuit being tuned by a variable condenser. S o extra battery is required for the
supply of anode current, the effectke E.X.F. n hich maintains the high frequency current
being the difference of potential between the positive terminal of a 6-volt filament heating
battery to which the anode is connected and the average potential of the filament*. In
order to increase this difference of potential between the anode and the average potential
of the filament, one of the filament terminals of the valve is connected TO the positire
terminal of the 6-1-0lt battery through a resistance of two ohms. The mean potentia! of the
grid of the valve is set at tno volts positive by means of a tapping on a h e d potentiometer
connected across the 6-volt battery, the potentiometer being an internal component of the
The grid and anode inductance can both be varied by means of a three-nay switch in
order to provide sei-era! ranges of viave-lengths as required.
The following is a description of the general construction of a syntoniser designed to
cover a range of ivave-lengths from 1000 to goo0 metres. The scale of wave-lengths is
covered in three ranges:
Range (A) ... ... 1000 to ijoo metres,
Range (B) ... . . , z joo to jooo metres,
Range (C) ... . . . jooo to 9000 metres,
the change from one range to another being obtained by operating a rocker srritch mounted
on the top panel. Adequate oT-erlap of the ranges is pro\-ided.
* N. Lea, Brit. Pat. 128383 (i9iS).
Fig. I gives a general view of the instrument and Fig. 2 its internal arrangement. Fig. 3
is a diagram of the connections of the circuit.
(a) Anode and Grid Coils
Ran<qe (A): 1000 to 2500 metres. The grid and anode inductances are wound on an ebonite
tube the inside of which forms the valve chamber. The anode coil is wound in two sections
Fig. I Fia. 2
joined in series, the grid coil being wound in a groove cut in the valve chamber between the
two sections of the anode coil.
Range (B): 2500 to 5000 metres. The inductances for this range are made up of the
inductances used in Range (A) and extra inductances wound on a grooved former.
This former has eight grooves and the windings in
alternate grooves are connected in series. One set of coils
thus forms, with the inductance of Range (A), the anode
inductance, and the other the grid inductance.
Ranp (C): 5000 to 9000 metres. The inductances for this
range are made up of the inductances used in Range (A)
and Range (R), and extra inductances, all being joined in
series. 'l'he extra inductances are wound on another
grooved former, the windings in the alternate grooves being
connected in series. One set of coils forms, with the inductances
of Range (A) and Range (R), the anode inductance,
and the other the grid inductance.
From the diagram of connections shown in Fig. 3, it
will be seen that the rocker switch short-circuits the inductances
which are not being used.
(b) Variable Condenser
This condenser has a maximum capacity of 0~0007j2 p F and forms, with the anode coils,
the oscillating circuit of the instrument. The rotary dial is of polished ebonite and is engraved
with three scales. Scale (A) is engraved every 20 metres from 1000 to 2500 metres. Scales
(B) and (C) are engraved every IOO metres from 2jOo to 5000 metres and from 5000 to
goo0 metres respectively, sufficient overlap being provided on each range by adjustment of
the various inductances.
(c) Wace-length Switck
This switch is of the rocker type and is similar to the standard telephone listening and
ringing key. There are four movable and eight fixed blades. These are so interconnected
that the inductances not in use are short-circuited.
The switch changes both anode and grid inductances. It is mounted on the top panel of
the syntoniser and its three positions are engraved (il), (B), and (C).
(d) Telephones
TKO terminals are provided for connecting telephones when the syntoniser is used as a
wavemeter for spark or continuous x;ave reception.
The telephones are inserted in the anode circuit of the valve and a link is provided so that
the condenser which is connected across the telephone terminals may be short-circuited
when it is desired to use the instrument as an oscillator.
(e) Screen
A screen of copper foil strips is fitted to the inside of the case and is connected to the
negative battery terminal. The object of this screen is to prevent alteration to the calibration
by stray capacity.
(a) Beats
In the operation of a syntoniser use is made of the well-known phenomenon of beats.
If a receiver R is placed in the vicinity of two H.F. oscillating circuits A and B (for example,
two syntonisers), and their respective frequencies are so adjusted as to be slightly different,
a beat note of pitch corresponding to the difference between the two frequencies is heard
in the receiver. If the frequency of the circuit oscillating at the higher frequency is reduced,
the beat note is lowered until what is known as a “nul beat” occurs, at which point the two
frequencies should be exceedingly close together. If now the frequency of the same circuit
is gradually reduced, the pitch of the beat note rises again until it becomes inaudible. The
number of cycles over x-hich a “nu! beat” occurs map be of the order of a hundred or more
depending on the sensitiveness of the receiver to respond to low-frequency alternating
currents. That is, nothing may be heard in the telephones for a change of frequency of one
of the circuits from, say, jo,ooo to 50,100 if the second circuit is tuned to a frequency of
jo,ojo. This may not be sufficiently accilrate for many purposes, in which case use is made
of the double beat method of reception. Kith this method a change of frequency of less than
one cycle per second can be detected. In this method the receiver or a third circuit is made
to oscillate at a frequency adjusted to give a beat note of approximately j O O Tvith, say,
circuit A. If circuits A and B had been previously adjusted to approximately the same
frequent!. by the single beat method, then the pitch of the beat note produced by A and R,
and B and R might differ to the extent of. say, j o frequency, that is, the two notes heard
in the telephones may be joc and 450. These different notes give rise to a second beat in
the telephones vihich by adjustment of circuit B can be made to have a frequency of, say,
one in five seconds. Under these conditions the two circuits A and R are tuned to a freqriencv
not differing by more than one-fifth of a cycle per second. Alteration in the frequency of
the receiver or third oscillating circuit makes no difference to the frequency of this second
beat, as the frequency of the third oscillating circuit is not between the frequencies of
the first and second circuits. By this method two oscillating circuits can be tuned to
the same frequency with extreme accuracy.
(h) Ifarmonics
The oscillation of the syntoniscr, or in fact any C.W. valve circuit, is by no means :I pure
sine wave as most of the odd and even harmonics down to the twentieth or niore can easily
be detected. ‘I’his means that a syntoniser tuned to a fundamental wavc-lrngth of, say,
16,800 metres will produce I ‘ nul beats ” in a receiver oscillating at the follow in^ wavelengths
(a) Fundamental wave-length ... ... 16,800 metres
(h) 2nd Harmonic ... ... ... 8,400 .. (4 3rd ... ... ... 5,000 I ,
(4 4th 3 ) ... ... ... 4,200 I ,
( p ) 5th ... ... ... 3,360 .. etc.
, I
Since the receiver is also fairly rich in harmonics it is possible by fairly tight couplings
to obtain ‘ I nul beats” between harmonics of both receiver and syntoniser.
This production of harmonics by an oscillating valve circuit is extrrmelv tiseful for c;lIibration
work, but is liable to cause considerable trouble in high power valve transmission.
(c) Method of Calibration
In the calibration of a syntoniser reference must be made to some riltim;\te atantI;lr‘i,
the frequency of which is accrirately known. This standard may he an acct1r;ltely caIihr;~tctI
Fig. 4
tuning-fork or a high frequency alternator (say, 8000 -) the frequency of which can be
determined by mechanical means. The tuning-fork method is now usually employed for
this work and a rough outline of the method will be described below.
Various valve circuits are known which can be made to oscillate at audio-frequency, but
there is one which is particularly rich in all the odd and even harmonics. This circuit,
which is nade use of for wave-length calibration, was developed by Abraham and Bloch
and is known as the 1Iultivibrator. The connections of this circuit are shown in Fig. 4,
and an oscillogram of the oscillation after magnification is shown in Fig. j. If the standard
tuning-fork has a frequency of 1000,th e multivibrator is so adjusted as to give a note in the
telephones of exactly the same frequency, the two notes being adjusted to equality by the
usual beat method.
An oscillating circuit can novi be set to definite frequencies by beating with the various
harmonics of the multivibrator. The following table gives the frequencies at which ‘‘ double
beats ” occur, assuming that the multivibrator has a fundamental frequency of 1000.
Frequency of
oscillating circuit
at which double Frequency of harmonics of Frequency of
I ‘ nul beats ” occur multivibrator Beat note double beat
10,000 cycles
10600 ,,
11,000 ),
I 1 , j O O ~,
II,joI“ ,,
( 9,000 cycles- 9th harmonic
111,000 ,, -11th ,,
~10,000 ,, -10th ,,
Irz,ooo ,, -12th ~, IO00 ,,
;m; cycles] 0
(11,000 ,, -11th ,,
/io,ooo ,, -10th ,,
/II,OOO ,, -11th ,,
.{IZ,OOO ,, -12th ,,
~ I I , O O O ,, -11th ,,
(IZ,OOO ,, -12th ,,
500 >,
499 >, 2
* This shom the result of hcorrect adjustment.
It can be seen that these double beats occur at frequencies differing by every 500 cycles
per second.
The harmonics to which each beat note corresponds must now be determined. These can
be found without tracing back to the fundamental of the multivibrator by the following
Two points on the oscillating circuit giving two wave-lengths, one of which is twice the
other, the longest being so chosen as to give a double “nu! beat” with the multivibrator.
are first determined as follow: Let the short and long wave-lengths be represented b>-
A and ZA respectively. The frequency of 2A is thus accurately a multiple of joo cycles per
second. The wave-length A can now be fixed by another oscillating circuit tuned to the
second harmonic of zA, i.e. a wave-length of A.
The number of “nul beats” S occurring betmeen the points of the oscillating circuit A
and zA inclusive is now determined. As shown in the above table, these “nul beats”
correspond to frequencies differing by joo. Then
Frequency of 2.A = (S- I) joo.
In order to f i p~oi nts on a syntoniser with fixed differences of wave-length between them
instead of points which differ by a constant frequency, it is necessary accurately to tune an
oscillating circuit to a ware-length corresponding to the differences required. That is, if
a syntoniser is required to be calibrated at wave-lengths of every IOO metres, the standard
oscillating circuit must be tuned up to the latter. This can be done by the method described
above by using a suitable number of steps. It cannot be done in one step as the harrnonics
J.S.I. 9
of the multivibrator become exceedingly close together for wave-lengths below 20,ooo
Points on the syntoniser which produce “ nul beats ’’ with the Ioo-metre oscillating circuit
car, now be determined and these are accurately IOO metres apart. In spite of the multiplicity
of the “ nul beats ” under these conditions it is impossible to make a mistake so long
as one makes sure that there are a definite number of them between two wave-lengths
which have been determined independently, e.g. between wave-lengths of 1000 and 2000
metres there must be eleven “nul beats” inclusive of those occurring at 1000 and 2000
(d) Variation in the Calibration of a Syntonisei,
Kith the same valve and a constant 6-volt filament battery the calibration of a syntoniser
probably remains accurate to within one-tenth of I per cent. for a very considerable
period, assuming that no distortion of the variable condenser or variation in the value of
the inductances takes place. When telephones are used in conjunccion with the instrument
a small correction may be required.
The use of an extra anode battery may alter this calibration very considerably.
For accurate measilrements, therefore, neither telephones nor high tension battery should
Khen a valve is changed the calibration of the instrument should be checked.
be used.
(a) To Tune up a C. W. Transmitter
Khen extreme accuracy is not required, the sptoniser itself may be used as the receiver
by connecting a pair of telephones in the anode circuit of the valve. The instrument is
placed in the vicinity of the transmitter and the transmitter tuned until a “nul beat” is
heard at the required wave-1engTh. Care must be taken not to have the syntoniser too close
to the transmitter as the former is liable to cease oscillating if the coupling is too tight.
If the wave-length of the transmitter has been adjusted to the correct value, I ‘ nul beats ”
will be obtained for wave-length settings of the syntoniser 2, 3, 4, j, etc., times the fundamental
n-ave-!ength and for wave-lengths 8, 4, $, i, etc., of the fundamental wave-length.
If great accuracy is required, then the double beat method with a separate receiver must
be employed (see I11 (a)).
(bj To Tune in a C.W. Receiver
The syntoniser is set to the desired n-ave-length and placed near the receiver, and the
latter is nom tuned until the “nul beat” is found. If the tuning is correct, “nul beats”
will be heard for every multiple of the fundamental wave-length, e.g. if the receiver has
been tuned up to 1000 metres, “nul beats” will be obtained for syntoniser settings of
1000, 2000, 3030, poo, etc., metres.
(c) The Syntoniser used as a Separate Heterodyne
With high frequency amplifiers it has been found advisable to use a separate heterodyne
for C.W. reception. For this purpose the spntoniser is placed in the vicinity of the receiver
and tuned approximately to the wave-length of the incoming signal to give a suitable note
in the telephones. This note should not be affected by the tuning of the receiver. In this
case no extra anode potential need be applied to the syntoniser. Without high-frequency
amplification an anode battery is generally found necessary.