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| Figure 1 |
Recently I discovered a pile of old issues of Elektuur magazine in our local university library. In one of these (July/August 1973) I spotted a highly unusual circuit: a receiver without coils! 1. Elektuur is the name of the Dutch electronics magazine which in other countries is known as Elektor. The header of the article said: “Gevoelige spoelloze MW en LW synchrodyne ontvanger”, i.e. “Sensitive synchrodyne receiver for MW and LW without any coils”. When I saw the receiver, I knew I had to try it – I am fond of strange and simple circuits.
The radio consists of two phase-shifting transistor stages (T1 and T2) and a RF amplifier (T3). The gain of T1 is almost one (R4/R5). Phase shift of this stage is 90 degrees at a frequency determined by P2 and C4. The second stage (T2) is identical to the first; the frequency of 90 degrees phase shift is here determined by P3 and C5. The RF amplifier (T3) has 180 degrees phase shift. Therefore the output of T3 has the same phase (90+90+180 degrees) as the input signal. Positive feedback occurs via P4 and C6 to the base of T1. If the overall gain within the feedback loop is greater than 1, the circuit will oscillate at the frequency determined by P2/P3 (the tuning potentiometer) and C4/C5. The amount of feedback can be controlled by P4 (regeneration control). The circuit can be tuned from 100 kHz to 2 MHz with the stereo potentiometer P2/P3 (linear taper).
An antenna signal (order of magnitude 10-5…10-4 Volts) can modulate the oscillator signal. The antenna signal is brought to the emitter of T1 via the coupling caps C1 and C3, R1 and P1 (RF gain control). P4 should be adjusted to ensure that the circuit just (barely) oscillates and is synchronized with the antenna signal. By careful adjustment of P2/P3 and P4 the desired station can be received.
The modulated oscillator signal enters a detector stage (D1,D2) which is followed by an AF amplifier (T4). Capacitor C10 shunts the RF signal to ground. A printed-circuit board for the circuit was designed by Elektuur. The copper side is shown in Figure 2
An antenna signal (order of magnitude 10-5…10-4 Volts) can modulate the oscillator signal. The antenna signal is brought to the emitter of T1 via the coupling caps C1 and C3, R1 and P1 (RF gain control). P4 should be adjusted to ensure that the circuit just (barely) oscillates and is synchronized with the antenna signal. By careful adjustment of P2/P3 and P4 the desired station can be received.
The modulated oscillator signal enters a detector stage (D1,D2) which is followed by an AF amplifier (T4). Capacitor C10 shunts the RF signal to ground. A printed-circuit board for the circuit was designed by Elektuur. The copper side is shown in Figure 2
and the component side in Figure 3. Actual size of the board is 170 x 43 mm.
Does it work?
Can such a simple receiver without any coils actually work? The answer is yes, but don’t expect the performance of a megabuck superhet. There are (of course) a number of weak points:
1) Tuning is very tricky, one must very carefully adjust the tuning and feedback pots to receive a station. As Elektuur suggested, this drawback can be cured by adding a control for fine tuning (1k lin stereo, or 500 ohms lin stereo pot in series with P2 and P3). It is a good idea to replace P4 with a 10-turn pot or a variable resistor with reduction gear, although this may be overkill. Hand effects can be reduced by grounding the case of P2,P3,P4 and P5 (see the photograph above). I added a 100nF decoupling capacitor (ceramic) in parallel to C2 to clean the supply line.
2) Selectivity is only fair; this is not noticeable during daytime but it may become more problematic during the night.
3) Although the receiver tunes to longwave frequencies, I could acquire good reception only in the mediumwave band. And in that band I could only receive the strongest stations (Radio 1, Radio 10 Gold, the national broadcasting system of Norway, etc). This was on a short wire antenna (150 centimeters).
4) With long antennas, the frontend may be overloaded (especially at night). RF gain should then be reduced (using P1).
5) Since there is no ferrite (bar) antenna, but just a wire antenna the receiver is susceptible to interference. Fluorescent lights in the home, TVs, computers etc. should be switched off for optimal reception.
To end on a more positive note:
6) After the optimal setting of P2, P3 and P4 has been found, the tuning is remarkably stable when good quality ceramic capacitors are used for C4 and C5.
7) Under such conditions, the audio quality can be quite good. Use an AF amplifier with some bandwith limitation (e.g. a commercial CLC filter or a T-filter at the input). In my own case, a TBA810 amplifier immediately worked well but a TDA2003 amplifier worked only after such a filter had been added. It is great fun listening to classic rock on Radio 10 Gold with this minimalist receiver.
Can such a simple receiver without any coils actually work? The answer is yes, but don’t expect the performance of a megabuck superhet. There are (of course) a number of weak points:
1) Tuning is very tricky, one must very carefully adjust the tuning and feedback pots to receive a station. As Elektuur suggested, this drawback can be cured by adding a control for fine tuning (1k lin stereo, or 500 ohms lin stereo pot in series with P2 and P3). It is a good idea to replace P4 with a 10-turn pot or a variable resistor with reduction gear, although this may be overkill. Hand effects can be reduced by grounding the case of P2,P3,P4 and P5 (see the photograph above). I added a 100nF decoupling capacitor (ceramic) in parallel to C2 to clean the supply line.
2) Selectivity is only fair; this is not noticeable during daytime but it may become more problematic during the night.
3) Although the receiver tunes to longwave frequencies, I could acquire good reception only in the mediumwave band. And in that band I could only receive the strongest stations (Radio 1, Radio 10 Gold, the national broadcasting system of Norway, etc). This was on a short wire antenna (150 centimeters).
4) With long antennas, the frontend may be overloaded (especially at night). RF gain should then be reduced (using P1).
5) Since there is no ferrite (bar) antenna, but just a wire antenna the receiver is susceptible to interference. Fluorescent lights in the home, TVs, computers etc. should be switched off for optimal reception.
To end on a more positive note:
6) After the optimal setting of P2, P3 and P4 has been found, the tuning is remarkably stable when good quality ceramic capacitors are used for C4 and C5.
7) Under such conditions, the audio quality can be quite good. Use an AF amplifier with some bandwith limitation (e.g. a commercial CLC filter or a T-filter at the input). In my own case, a TBA810 amplifier immediately worked well but a TDA2003 amplifier worked only after such a filter had been added. It is great fun listening to classic rock on Radio 10 Gold with this minimalist receiver.
A Japanese radio amateur has even built a superheterodyne receiver without any coils ! 2.
References :
1 “Gevoelige spoelloze MG en LG synchrodyne ontvanger”, Elektuur, July/August 1973, p.760 (circuit no.63) See also the additional reference: “Variabel filter”, Elektuur, July/August 1973, p.773 (circuit no.91).
2 Kazuhiro Sunamura, “Radio made by op amp“, http://bit.ly/TVEFma
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