NEW METHOD OF DETECTING FM SIGNALS

• Document Type: CREST
• Collection: General CIA Records
• Document Number (FOIA) /ESDN (CREST): CIA-RDP80-00809A000700160396-4
• Release Decision: RIPPUB
• Original Classification: U
• Document Page Count: 5
• Document Creation Date: December 22, 2016
• Document Release Date: September 8, 2011
• Sequence Number: 396
• Publication Date: March 8, 1954
• Content Type: REPORT

Sanitized Copy Approved for Release 2011/09/08: CIA-RDP80-00809A000700160396-4
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Sanitized Copy Approved for Release 2011/09/08: CIA-RDP80-00809A000700160396-4
 Sanitized Copy Approved for Release 2011/09/08: CIA-RDP80-00809A000700160396-4
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NEW MH1'HOD OF DETECTING FM SIGNALS
Radio amateurs building FM receivers often run into difficulties in tuning
the frequency detector.
Recently, a new phase detector circuit suitable for detection of FM signals
has been suggested. This circuit, giving good-quality operation, is simple to
set up and tune.
Numerous tests have shown that a 6L7 tube operates very well in this cir-
cuit. We suggest that the 6A10 (6SA7) and 6A8 tubes will operate as well.
The circuit can be employed in both superheterodyne and straight receivers
where the use of a conventional discriminator as an FM signal detector is very
difficult.
The suggested circuit in simplified form is shown in Figure 1 figures re-
ferred to are appende7. Here the FM signal voltage is applied to the third
grid of the heptode. The coil Lc and capacitor Cc form the load circuit for
the last stage of rf or 1-f amplification.
If a small positive voltage (30-50 v) is applied to the screen grids, the
tube will have a characteristic with a short left portion (-1.5 to 3 v) and at
higher signal levels there will be a natural limit to plate current.
The first grid of the heptode is connected to the circuit LkCk which is
tuned to the average band-pass frequency of the rf amplifier (or i-f amplifier):
i.e., to the frequency of the unmodulated signal.
Hereinafter we shall refer to this circuit as the "square-law detector."
There must be no parasitic coupling between the first and third grid cir-
cuits. It is better to place them on opposite sides of the chassis. Oscilla-
tions in the square-law detector are excited due to the effect of the electron
stream controlled by the third (signal) grid on the first grid.
Let us examine the operating principles of this circuit.
As we know, the first and third grids have practically the same effect on
the plate current of a mixer tube in the left portion of the characteristic.
Let us assume that a voltage of the same frequency (synchronized excitation) is
applied to both grids. Let us see what changes take place in the form of the
oscillations and the coverage value of the plate current when the relative phase
of the voltages in both grids varies. For the sake of simplicity, let us assume
that a voltage is applied to the grids which is rectangular in form and of such
an amplitude that, in the negative half cycle on one of the control grids, the
plate current cuts off independently of the sign of the voltage on the second
grid.
We can readily obtain such conditions with a 6L7-type heptode. The plate-
current pulse form (Figure 2) will strongly depend on the relative phase shift
between the grid voltages. It is obvious that the plate current can flow only
during that portion of the cycle when the voltages on both are positive in sign.
The wave form of the plate current will resemble the exciting voltage only when
both voltages are completely in phase. With a 900 negative shift of voltages,
the plate current pulses will be twice as narrow as when the voltages are in
phase.
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current generally stops flowing. Hence the mean valueJofctheyplateJcurrentlcan
be controlled through shifting the relative phase of voltages on the grids.
An analogous condition is obtained if the control grids are fed by voltages
which are not rectangular but sinusoidal in form. If the voltages on the grids
are insufficient to stop the plate current entirely, its value will still be
changed within certain limits.
Returning to the circuit shown in Figure 1, if the third grid is supplied
an alternating voltage of the frequency to which the square-law detector is
tuned, the voltage induced in this circuit will have a phase lag of 900 behind
the excitation voltage, and some average value will be established for the plate
current Io (Figure 3).
If the excitation frequency is increased, the relative phase shift will
also increase, resulting in a reduction of the plate current. At an excitation
frequency lower than the resonance frequency of the square-law detector, the
relative voltage phase shift will be less than 900, and the plate current will
be increased. It is obvious that if FM oscillations are applied to the third
grid, variations in the average value of the plate current will correspond to
a modulating signal, and we can take of oscillations from the plate load of the
tube.
The characteristic of this detector is quite linear within the limits of
the pass band of the square-law detector. Hence it follows that the circuit
-at have enough attenuation so that, in detuning by the deviation value, sup-
pression of the resonance characteristic will not be more than 0.7 of the res-
onance value.
An increase in attenuation of the square-law detector tends to decrease the
transconductance of the detector characteristic. In practice, a pass band of
approximately 300 kc is adequate for our assumed 75-kc deviation.
A straight receiver with a 300-kc pass band requires a circuit with a Q of
about 200. On ultrashort waves this circuit is not feasible when coupling with
the tube is direct because of the shunting action of the tube's input resistance.
But even in some extra bands the transconductance characteristic will be suffi-
cient.
In certain cases, to obtain the required pass band it is necessary to shunt
the circuit through a resistance of 20,000-25,000 ohms.
To improve the operation of the circuit, the input capacitance of the
square-law detector must be selected as a minimum.
The permissible Q for the circuit may be calculated by the formula
fi,.ZV. > (1)
0.3
where fN.2V, is the carrier frequency of the audio channel in Mc.
The equivalent resonance resistance for the circuit (needed for selecting
the shunt) can be calculated by formula 2, if the circuit capacitance is even
approximately known:
where Roe is the equivalent resistance of the circuit in thousands of ohms; f
is calculated in Mc; and Ck is the capacitance of the circuit in fcft,fd. 11.ZV.
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Figure 4 shows the actual circuit for this detector. The capacitor Ca,
which shunts out the plate load, not only blocks the high frequencies, but also
suppresses higher modulation frequencies. This is necessary because of the
rise of the frequency response of an FM transmitter modulator in this region.
The magnitude of the of voltage taken from the detector plate load depends
on the operating conditions of the tube, primarily on the screen-grid voltage,
which, in its turn, must be selected in relation to the excitation voltage on
the receiving circuit. The higher this voltage, the higher will be the screen-
grid voltage at which we shall obtain a limiting effect.
It must not be forgotten that cathode, screen-grid, and plate currents con-
tain both of and rf components and that, therefore, power-supply and automatic
bias circuits must be blocked by high capacitances, which are shunted by nonin-
ductive capacitors.
The phase detector is easily tuned by ear from the operation of a trans-
mitter. If there is an rf oscillator, the square-law detector can by tuned by
a milliammeter connected to the plate circuit of the detector. The signal from
a 2- to 3-v oscillator is fed to the third detector grid. By changing the
square-law detector tuning, from milliammeter readings one can obtain a curve
like that in Figure 3 (in this case the resonance frequency, not the excitation
frequency, varies), and adjustment can be made to correspond to the average (be-
tween greatest and least) deviation of the milliammeter.
An rf pentode can be used in this circuit. In this case, the receiving
circuit is connected to the first grid, and the square-law detector to the pent-
ode grid. To increase the square-law excitation, a 500- to 1,000-ohuu resistance
is connected between the plate and the load which is shunted by a capacitor.
The square-law detector is excited through the plate-suppressor grid interelec-
trode capacitance. The coupling can be improved by connecting a 3- to 5- Ne fd
capacitor between the plate and the pentode grid.
It should be noted that, if a pentode is used, the additional resistance
in the plate will attenuate the square-lav detector to some extent, and a shunt
may not be required.
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 Sanitized Copy Approved for Release 2011/09/08: CIA-RDP80-00809A000700160396-4
Sanitized Copy Approved for Release 2011/09/08: CIA-RDP80-00809A000700160396-4
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