Set 7. DX receiver with two tuned circuits
This receiver has been built in three parts, each part will be described on a separate page:
1- The antenna tuner unit
2- The detector unit
3- The transformer unit
In this receiver I tried to obtain the best possible
selectivity and sensitivity, by using a proper circuit and good quality
materials.
Before I started building this receiver, I performed many experiments
in order to obtain the best possible Q factor in the tuned
circuits.
This resulted in circuits with a (unloaded) Q above 1000 for the most part of
the mediumwave band.
Experiments en measurements carried out with the antenna tuner unit are described here.
The loudspeaker used is a driver unit
from a hornloudspeaker, for more information on the
sensitivity of this driver unit, see the loudspeakertest.
For a good performance, the coils of the antenna tuner unit and the detector
unit must be placed at a distance of 42 cm.
More information and measurements on this topic, you will find here.
The sensitivity of the receiver.
To measure the sensitivity of the receiver, the receiver
was connected to a signal generator.
The signal generator was AM modulated with 1000 Hz, this 1000 Hz was a square
wave with a modulation depth of 100 %.
Between signal generator and receiver, I connected a
variable attenuator and a dummy antenna.
The output voltage of the dummy antenna was 100 mVpp with the attenuator at 0
dB.
This 100 mVpp was measured without connecting the receiver.
When we connect the receiver to the dummy antenna, the output voltage of the
dummy antenna will increase, this happens because the coil and capacitor in the
dummy antenna then form a part of the resonant antenna unit circuit.
| Attenuator setting |
Voltage out of dummy antenna (unloaded) |
Audio |
| 0 dB | 100 mV pp | 1600 mV pp |
| -10 dB | 32 mV pp | 540 mV pp |
| -20 dB | 10 mV pp | 120 mV pp |
| -30 dB | 3.2 mV pp | 10 mV pp |
| -40 dB | 1.0 mV pp | < 1 mV pp |
| -43 dB | 0.7 mV pp | sound just audible. |
In the last column the amplitude of the detected
audiosignal is given, the audio signal is loaded with the transformer unit with
speaker.
With the attenuator at -43dB the 1000 Hz test tone was just audible.
The (unloaded) antenna voltage is then 0.7mVpp or 0.25mVeff.
For listening to a normal audio signal, the antenna signal must however be
stronger.
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With this receiver I took part in national and international receiving
contests.
In 2005 this receiver was the 1st price winner in the "Elmer Memorial Crystal Radio DX Contest". For loglists of the received stations, click here. |