The basic Audion circuit
The diagram opposite illustrates the principle of direct grid detection. The two coils on the left, as well as the 500 cm variable capacitor, constitute the tuning block. The cell composed of the 2 MΩ resistor in parallel with a 250 cm capacitor is the heart of the direct detection. The triode detects the HF signal from the tuning block. The choke coil, made of 300 turns, filters the HF component so that it does not reach the 60V power supply.
The Audion circuit with capacitive feedback
The diagram opposite illustrates the principle of direct grid detection. The two coils on the left, as well as the 500 cm variable capacitor, constitute the tuning block. The cell composed of the 2 MΩ resistor in parallel with a 250 cm capacitor is the heart of the direct detection. The triode detects the HF signal from the tuning block. The choke coil, made of 300 turns, filters the HF component so that it does not reach the 60V power supply.
Experimentation with the basic Audion circuit
RLC calculator
It was after extracting the tuning capacitor and transformer from a faulty clock radio that can be seen in the photo below that the idea came to me to experiment with the principle of grid detection. Measurements on these 2 components give
Applying Thomson's formula \(\displaystyle f_0=\frac {1}{2\pi \sqrt{LC}}\) with \(\lambda_0=\frac {c}{f_0}\) shows that the LW band can be received.
The following experiment is based on a circuit described in Berché (Practice and Theory of Vintage Radio) shown here.
(Fig. 1)
I propose to put the principle diagram of the circuit to the test of SPICE.
Personally, I use a free tool from Linear Technology that I find simple, flexible, and above all without commercial limitations. It can be downloaded here LTspice/SwitcherCAD III (4MB)
| L1 | RL1 | L2 | RL2 | CVmin | CVmax |
| 73 µH | 2.66 Ω | 7 530 µH | 24 Ω | 18 pF | 155 pF |
Applying Thomson's formula \(\displaystyle f_0=\frac {1}{2\pi \sqrt{LC}}\) with \(\lambda_0=\frac {c}{f_0}\) shows that the LW band can be received.
| L2 | CV | \(f_0\) | \(l\) |
| 7 530 µH | 18 pF | 432 301 Hz | 694 m |
| 7 530 µH | 155 pF | 147 318 Hz | 2 036 m |
The following experiment is based on a circuit described in Berché (Practice and Theory of Vintage Radio) shown here.
(Fig. 1)
I propose to put the principle diagram of the circuit to the test of SPICE.
Personally, I use a free tool from Linear Technology that I find simple, flexible, and above all without commercial limitations. It can be downloaded here LTspice/SwitcherCAD III (4MB)
Berché recommends \(100pF < C1 <150pF\) and \(200K\Omega < R < 500K\Omega\). These limiting ranges come from considerations on low AF distortion if the condition below is met.
I provisionally choose C1=120pF and
R=330KΩ. The earpiece I have is rated
for 1KΩ. After measuring L=0.5H and Rsérie=1KΩ
. The capacitance C2 is roughly set to 2nF.
It remains to define the HF transformer. SPICE requires knowledge of (\(L_1\), \(R_1\)), (\(L_2\), \(R_2\)) and \(k\) the coupling coefficient between \(L_1\) and \(L_2\).
The circuit below allows measuring the inductances \(L_{AB+}\) and\( L_{AB-}>\) of the circuit in the opposite figure, then determining the mutual inductance coefficient \(M\) and therefore the coupling coefficient \(k\). $$\begin{cases}L_{AB}(+) = L_1+L_2+2M=8.70mH\\L_{AB}(-)= L_1+L_2-2M=6.48mH\end{cases}$$ $$M=\frac {L_{AB}(+) +L_{AB}(-) }{4}=\frac {2.22}{4}=0.555mH$$ $$M=k\sqrt{L_1L_2}$$ We get (\(L_1 = 73 \mu H ,R_{L1} = 2,66 \Omega\)) et (\(L_2 = 7,52 \mu H, R_{L2} = 24 \Omega\) ) et \(k = 0,748573\)
The circuit below allows measuring the inductances \(L_{AB+}\) and\( L_{AB-}>\) of the circuit in the opposite figure, then determining the mutual inductance coefficient \(M\) and therefore the coupling coefficient \(k\). $$\begin{cases}L_{AB}(+) = L_1+L_2+2M=8.70mH\\L_{AB}(-)= L_1+L_2-2M=6.48mH\end{cases}$$ $$M=\frac {L_{AB}(+) +L_{AB}(-) }{4}=\frac {2.22}{4}=0.555mH$$ $$M=k\sqrt{L_1L_2}$$ We get (\(L_1 = 73 \mu H ,R_{L1} = 2,66 \Omega\)) et (\(L_2 = 7,52 \mu H, R_{L2} = 24 \Omega\) ) et \(k = 0,748573\)
Let's summarize the parameters of the HF transformer that will be provided to SPICE:
(\(L_1 = 73 \mu H ,R_{L1} = 2.66 \Omega\)) and (\(L_2 = 7.52 \mu H, R_{L2} = 24 \Omega\) ) and \(k = 0.748573\)
(\(L_1 = 73 \mu H ,R_{L1} = 2.66 \Omega\)) and (\(L_2 = 7.52 \mu H, R_{L2} = 24 \Omega\) ) and \(k = 0.748573\)
Dummy antenna
To drive the primary of
this transformer, an HF generator in series with a dummy antenna consisting of a series RLC circuit well known to radio amateurs will be used. Another dummy antenna model is given elsewhere.
The simulation schematic will therefore be
The current in the secondary of the transformer varies with frequency as follows
At the resonance frequency (145.2KHz), the signal power in the primary of the transformer is of the order of 0.55µW
The Grid voltage of the triode is shown
To compare with the power absorbed by the grid
The power balance is illustrated by
The power gain is \(\displaystyle G(dB)=10\log\frac {113.5}{0.697}=22.12dB\)
Sources and references
[1] Paul BERCHE, "Pratique et théorie de la TSF", Librairie de la Radio, Paris, 1937, reviewed by Roger RAFFIN, 1958.
[2] Lucien CHRETIEN, "Théorie et Pratique de la Radioélectricité", Editions Chiron, Paris, 1933.
