On the left side of the front panel are the
connectors for antenna and ground.
The socket for the headphone is on the right-hand side.
The dimensions of the front panel are 12 x 5 cm.
Receiver with ceramic resonator
This receiver is using a ceramic resonator for tuning,
instead of a LC circuit.
Ceramic resonators behave a little bit like a LC circuit, so they can be used
for tuning to a certain frequency.
The resonator only works on one frequency, so the receiver can receive only one
station.
On the left side of the front panel are the
connectors for antenna and ground. |
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Ceramic resonators are available in several frequencies between 375 and 503 kHz, they are low cost components often used in, for instance, tv remote controls.
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The resonators are not available in frequencies
corresponding to station frequencies on the medium wave band, but we can do
something about this.
When the casing of the ceramic resonator is opened, we find a thin ceramic disc
inside, mounted between two flexible contact plates.
The contact plates are connected to the soldering pins of the ceramic resonator.
The dimensions of the ceramic disc determine the frequency of the resonator.
If we make the disc smaller, the resonance frequency will go up, and we can
reach a certain medium wave frequency.
In this receiver I used a 485 kHz resonator, the disc
having dimensions 4.7 x 4.7 mm and a thickness of 0.5 mm.
I then reduced the dimensions of the resonator disc from 4.7 x 4.7 mm to 3.4 x
3.4 mm, this causes a frequency change from 485 to 675 kHz, which is the
frequency of my local radio station.
The thickness (0.5 mm) must not be changed, because then the conductive surface
of the disc will be removed.
To make the disc smaller, I used fine sandpaper (#320),
holding the disc between my fingers and moving it across the sandpaper.
Remove the same amount from both length en width, thus preserving the square
shape of the disc.
Every time I removed some material, I measured the frequency, because when you
remove too much, there is no way back.
I don't know if frequencies in the top of the
medium wave band can be attained with this method, because the disc must then be
very small.
Circuit diagram of the receiver.
The ceramic resonator determines the frequency of reception, in this case it is
675 kHz.
Two germanium diodes are used, connected as a voltage doubler.
Detection with one diode is not possible in this case, because a ceramic
resonator does not pass DC current (unlike a parallel LC circuit).
With trimmer capacitor C2 we can adjust the resonance
frequency slightly.
With C2 at 0 pF the resonance frequency is 675.9 kHz
With C2 at 40 pF the resonance frequency is 674.5 kHz.
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Backside of the receiver. The resonance disc is mounted between the original
contact plates (not visible in the picture). |
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Response curve of the resonator. The horizontal scale is 5 kHz /cm. The curve is measured with the diodes connected,
and loaded with 82 kΩ. |
The results:
Compared to a receiver with a LC circuit, this receiver gives less sound output,
this is because the resonator has a lower impedance then a LC circuit.
This results in a lower voltage at the diodes, which will therefore have higher
losses.
However, with a 5 meter antenna wire, reception of my local station is
satisfactory.