It’s fun to build electronics circuits.
It has been a long time since the SSB mode became the mainstream of the Amplitude Modulation communication in the HAM world. Still, however, there are a certain number of hams who are interested in the AM mode and they lively communicate in a roll call meeting using the AM mode on weekends.
Meanwhile, the AM radio is also firmly favored. Recently, listening to the AM radio using a smartphone application or on the Internet instead of using a radio has become popular. You can listen to the AM radio with a very simple germanium radio but I found a radio that was designed with one chip AM 3-terminal IC installed. I was very impressed.
Then I decided to make a 50 MHz AM receiver using the 3-terminal IC. I found that the 3-terminal IC is quite cheap---it only costs 25 JPY (Approximately 0.25 USD)! I felt a little guilty and finally bought five ICs online.
The construction of the 50 MHz AM receiver is as illustrated below. The circuit down-converts the AM radio wave and the audio is output from an AM radio that the 3-terminal IC is used. This is what was called “a Crystal Converter method” in the past.
The frequency range of the AM radio is Middle Frequency (MF) and is between 500 kHz and 1600 kHz. Because of this low frequency range, the single conversion type superheterodyne receiver is rather difficult to gain enough image characteristics against the 50 MHz frequency. The challenge for this circuit structure is that how to acquire enough image frequency immunity.
When the local oscillator frequency is set to lower than the receive frequency, the circuit is affected by the image frequency that is lower than the 50 MHz band. With regard to the 50 MHz band, it is difficult to produce a filter that has good frequency selectivity to reduce image frequency interference because of its high frequency. The problem can be solved if I make a double superheterodyne receiver, but the circuit construction will be complex. After all, I chose to make a superheterodyne receiver, fortunately, because there are no amateur radio stations with a strong signal in my neighborhood.
(Japanese band plan)
To make a circuit of each section as a unit, I divided the whole circuit into some blocks so that I can reuse them later. The circuit consists of the RF Amplifier section, the Double Balanced Mixer (DBM) section, the Local Oscillator section, and the AM Radio section. The AM Radio section does not include the AF Amplifier section to reuse the radio section later.
The construction of the AM radio section is quite simple---it’s finished by just connecting a 3-terminal IC to a tuning circuit!
I didn’t have any coils that can be used for the resonance circuit so I decided to make it by myself. For the coil bobbin, I considered many things and found that a coupler (outside diameter: 24 mm, 0.9 in) for a water pipe vinyl chloride (diameter: 13 mm, 0.5 in) should be suitable. I winded a Polyurethane wire (0.3 mm, 1.18 in) on the bobbin for 94 times for the tuning circuit and for 10 times for an antenna coil.
I reused a stopper that was attached to a coil I already had. Luckily a screw core was attached together. I hunted for a variable capacitor in my junk box and used for this circuit.
A coil and a receiver circuit
The power voltage of the 3-terminal IC would be 1.5 V so I used a transistor as an emitter follower circuit to produce 1.5 V. I connected a green LED between the base and the ground to produce constant voltage. A forward direction voltage of the LED was measured as 1.97 V and the base-emitter voltage is about 0.5 V. As a result, 1.5 V can be output.
I made a storing case for the radio using acrylic plates and coated with a spray paint. This activates as a radio by connecting a crystal earphone (or a ceramic earphone) to the output section.
A simple structured IC NJM368 is used for the AF Amplifier section. The output power of this IC is approximately 1 W and its gain is approximately 26 dB. When connecting it to the 3-terminal IC radio, a practical amount of audio level can be acquired. I also made a case for this section and stored all together to use as another unit.
The following figure describes the RF Amplifier section that down-converts from the 50 MHz band to the AM radio band, and the Double Balanced Mixer (DBM) section.
I used a GG (Grounded Gate) amplifier, FET 2SK125, for the RF Amplifier section and its gain is not so high, approximately 10 dB I suppose. This circuit has been used a lot since old days. I have had many dice typed DBM manufactured by TDK. A simple low pass filter is incorporated in the output section and its characteristics are illustrated below.
I constructed a down converter on a universal PCB.
I found a 24.75 MHz crystal unit, which is very appropriate for this circuit, on the Internet and put it into the Local Oscillator section. By oscillating the crystal oscillator, the doubled frequency 49.5 MHz can be taken out. Normally the oscillator can only provide a one third fundamental frequency so an overtone oscillation circuit is required. I made the logic circuit I have been used for several times as a multiplying circuit to double the frequency.
When putting two signals---an input signal oscillated by a crystal unit and another signal that is delayed and inversed the input signal---into an EX-OR circuit, the leading and the trading edges are detected and the frequency is doubled. After that, the signal whose spurious is suppressed through a resonance circuit consists of an analog buffer and a collector is taken out.
The signal level of the local oscillator is specified as + 7 dBm in the specification sheet so I designed the circuit to match the level. I also constructed the DBM section on a universal PCB as well as the down converter.
Connecting these blocks all together and now the AM receiver is completed! I don’t think there will not be many problems because the construction is relatively easy but further consideration probably required to obtain the enough sensitivity and gain level.