Generally when we think about FM radio receiver, what will be first to usually pop into our mind will be a complex kind of radio receiver, more than a diode and coil as usually utilised in AM radio receiver. In actuality its not the case when we take into consideration the original FM radio receiver invented by the FM radio transmitting pioneer Edwin Armstrong. He was credited with the invention of the first practical FM radio receiver, which use ONLY one non-linear electronic component ( the vacuum tube).
The first FM radio receiver, using vacuum tube in regenerative topology.
With the invention of junction field-effect transistor, the vacuum tube was subsequently omitted from the circuit and being substituted by JFET. From my analysis on various FM receiver circuits found on internet I concluded a final generalised version of single transistor FM regenerative receiver as shown below:
Most of single transistor FM radio receiver circuits I found on the internet dont deviate much from the above generalised design,except few which are completely different from it. But like I said, most of it follow the above configuration.
Now I am going to explain each part of the circuit, each numbered point below refers to corresponding red circled number in the diagram:
1) Group of capacitors: Some of the designs use capacitors arranged only parallely, while others just in series. Some omit the variable capacitors. So this part depends on the designer's choice. The value of the capacitors much be carefully chosen cuz it will determine the bandwidth within where the circuit will resonate. For FM receiver, the corresponding resonant frequency must be within 80 to 108 MHz.
2) The JFET transistor: From analysis I found out collections of transistor types suitable for the operation. Here are the examples: MPF102 (frequently used), BF245, J310 ( is now obsolete), 2N4416, RS2003 (Radio Shack JFET), 2N3819, BF256.
3) RF choke: The purpose of RF choke is to block high frequency signal from reaching the audio amplifier. It can be home-made, by just winding copper wire around a cylindrical air core or ferrite torroid. Various kinds of choke are made depending on the designers' choices.
4) Resistor: The value of the resistor can be adjusted to obtain the most clear audio output.
5) Capacitor: Same with resistor at (4)
6)Feedback capacitor: Its value is also designer's choice. This capacitor taps the coil at (7) to form a regenerative topology.
7) Capacitor's tap points: Part of the coil where the feedback capacitor touches. The position where the feedback capacitor taps is designer's choice.
8) Antenna and antenna's tap points: antenna touches the coil at the antenna's tap points. The length of the antenna, and the position of tap are the designer's choices.
9) Coil: the number of turns, the length and the diameter are all designer's choices.
10), 11) and 12) are capacitor and resistors to regulate the power supply. Their values are again, designers' choices.
By using this generalised circuit, anyone can play with various choice of values of any of the involved components, coils and transistors without being restricted to the designs proposed on the internet. It enables everyone to even create their own variants of FM single transistor receiver. To be honest, I ve tried various FM single transistor designs but none of them worked. And I m still trying hoping one day I will found a working one and this general design helps me alot in constructing various new variants.
Below I will show u several circuit designs made by several hobbyists, which I redrawn and omitted the audio amplifier part to enable easy matching with the general circuit proposed before.
Design 1: Philip Crane's design
2) JFET transistor: MPF102
3) RF choke: 26 turns, 30 AWG wire, 8mm diameter coil
4) resistor: 10kOhm
5) capacitor: 4.7nF
6) capacitor: 4.7-5pF
7) Tap: close to JFET's Drain
8) antenna's tap: 2 turns from power supply
9) coil: 22AWG wire 7 turns 2 inches long coil around 5/16" former
10) capacitor: 1nF
11) resistor: 1kOhm
12) 9V power supply
Design 2: Andrew Mitz's design
2) JFET transistor: 2N4416
3) RF choke: 22uHenry choke
4) resistor: 10kOhm
5) capacitor: 5nF
6) capacitor: 24pF
7) Tap: centre tap, soldered
8) antenna's tap: no antenna
9) coil: gauge unknown (assume 18AWG), 6 turns around 0.5inches diamter former, length 0.75 inches
10) capacitor: 1nF
11) resistor: 1kOhm
12) 9V power supply
Design 3: Andrew Mitz's design
2) JFET transistor: 2N4416
3) RF choke: 100uHenry choke
4) resistor: none
5) capacitor: 5nF
6) capacitor: 24pF
7) Tap: centre tap, soldered
8) antenna's tap: 3/4 of a turn from 10nF capacitor, antenna has 10pF capacitor on it as shown
9) coil: 18AWG wire, 12 turns around 3/8 inches diamter former, close winding (close wrap)
10) capacitor: 10nF
11) resistor: 1kOhm
12) 9V power supply
Design 4: Patrick Cambre's design
2) JFET transistor: MPF102
3) RF choke: 20 wraps of 24AWG around 5/16 inches former
4) resistor: 10kOhm
5) capacitor: 4.7nF
6) capacitor: each 5.6pF
7) Tap: tap closest to Drain of JFET
8) antenna's tap: Tap closest to power supply
9) coil: 22AWG wire, 8-10 turns around 5/6 inches diamter former
10)1nF capacitor 11) and 12) as shown
Design 5: Alan Yates's design
2) JFET transistor: J310 or MPF102
3) RF choke: 25uHenry choke
4) resistor: 10kOhm
5) capacitor: 6.8nF
6) capacitor: 10pF
7) Tap: centre tap
8) antenna's tap: no antenna
9) coil: 5 turns 7mm diameter and length (inductance about 120nHenries)
10) capacitor: 1nF
11) resistor: none
12) 9V power supply
Design 6: mikroElektronika's design
2) JFET transistor: BF256
3) RF choke: 22uHenry choke
4) resistor: 10kOhm to 22kOhm
5) capacitor: 6-100nF
6) capacitor: 22pF
7) Tap: centre tap, soldered
8) antenna's tap: closest to the Drain of JFET
9) coil: 5 turns 0f 0.9mm diameter wire (close wrap) 9mm diameter coil
10) capacitor: 1nF
11) resistor: 1kOhm
12) 9V power supply
Thats all from me. Hope this will help.
