1 Valve Fremodyne VHF Receiver
Written by Bryce Ringwood   


Introduction
 

During the 1950's, FM broadcasting was introduced in many countries as a means of producing high-quality radio transmission, and also to prevent further congestion of the AM medium wave band. “FM” stands for “Frequency Modulation”. In this mode of transmission, the frequency of transmission varies in sympathy with the sound waves being broadcast. (In an AM transmission, the strength of the signal varies.) Because an FM transmission requires a wider bandwidth than AM, FM broadcasting is done on VHF (Very High Frequency) in the 88 to 108 MHz waveband. (AM  is broadcast on low frequencies between 480 kHz and 1.6MHz). There are other benefits to FM broadcasting, such as the huge reduction in noise and atmospherics, also FM radios work inside steel and concrete buildings, whereas AM does not.

General View of Fremodyne

This circuit is almost the same as that developed by the Hazeltine Corporation in the late 1940's in the USA. The difference is that I haven't included a coupling capacitor between the two halves of the valve. Also, I used an ECC84 valve instead of a 12AT7, because the ECC84 was readily available from my supplier (RSE lectronics). In the 1940's, valves were expensive, so Hazeltine developed this “Fremodyne” circuit around a single valve. Fremodyne is derived from FREquency MODulation and DYNE. A conventional FM tuner uses at least 4 valves and uses a special FM detector, such as a ratio detector or Foster-Seeley discriminator. Nowadays, a complete stereo FM radio is available on a single chip. (e.g. Philips TEA 5712 integrated circuit.)

Before you begin...

This project is not recommended for beginners and the circuit requires a little bit of patience to get it to work correctly. It also requires a mains power supply capable of providing about 6.3 volts at 0.4 Amps and a high-tension (HT or B+) supply of between 80 and 100 volts. (It will work with a supply of 5.0 volts, without noticeable difference. The minimum HT supply is 40 volts, with a big drop in performance. Best HT supply seems to be between 85 and 90 volts.) Unlike the AM one-valve receiver, the output is very low and it will only drive a crystal ear piece. The original intention of the circuit was to drive a valve amplifier.

This set is for experimenters who want to learn something about electronics and who feel get a great sense of achievement when they meet the challenge. If you want a simpler, more practical kit for use with a HiFi, consider the solid-state FM tuner kit from RSE lectronics.

Also, bear in mind that complete stereo radios can be bought occasionally for less than R20-00, so this kit is certainly not for anyone trying to simply save money.

What to Expect

This radio is surprisingly sensitive. It will receive all the local stations on an outdoor antenna. My set even picked up stations with no aerial at all. The audio output level is not very great – comparable to a crystal set through a crystal ear piece. The reason for this is the de-emphasis circuit, which has a 100k resistor in series with the audio output from the cathode of the valve. This will not affect a crystal ear piece or high impedance input to a valve or FET amplifier, but it will not work, even with 4000 Ohm headphones. I was able to get acceptable volume from a computer sound card.

The sound quality is not as good as an FM receiver with a “proper” FM detector, but you may not notice the difference.

How it Works

fremodyne circuit

Figure 3 -Fremodyne Circuit Diagram

The receiver is s superheterodyne covering 88 to 108 MHz. The intermediate frequency has been chosen as 21.7 MHz. In a superheterodyne receiver, the received signal is mixed with the output from a local oscillator to produce a sum and difference frequency. One of these is chosen to be the intermediate frequency, which is amplified and detected to produce an audio output. To take a concrete example, in the above circuit, suppose we want to receive Radio Highveld on 94.7 MHz. Since we have an intermediate frequency of 21.7 MHz, then we need an oscillator frequency of 73MHz. (94.7 – 73 = 21.7). There will also be a frequency of 73 + 94.7 = 167.7 generated, which is filtered out by the tank circuit.

In an ordinary superhet, the intermediate frequency would be amplified and detected by 3 or more additional valves.

In this circuit, the intermediate frequency is amplified and detected by a super regenerative detector. In this type of circuit (as in a regenerative detector) the signal is fed back from the output to the input, resulting in a huge amplification. Unlike the regenerative detector, the regeneration is controlled by quenching the oscillator at a supersonic frequency – in this case roughly 30kHz. Although a super regenerative detector can function as a true FM detector by locking the nominal 30kHz quench frequency to the FM signal, this circuit does not work in this manner. Instead, the detector operates by tuning one side or other of the signal and using the slope of the selectivity curve to detect the signal.

In order to achieve higher quality, broadcasters boost the treble of an FM signal. This boosted treble has to be removed, and this is done using the de-emphasis circuit.

Some care is needed with the intermediate frequency. Since the set uses a super regenerative circuit, harmonics of the 21.7 MHz frequency will be generated. These will be at 43.4 MHz, 65.1 MHz, 86.8 MHz, 108.5MHz, and so on. You will see that none of the harmonics fall within the FM band. Alas, other frequencies are generated as sums and differences of the 21.7 and oscillator frequencies. These are referred to as “spurs”.

As a final note, super regenerative receivers do not work well in AM band and short-wave receivers because the quench frequency has to be low enough not to cause interference. This means that it is audible.

Construction

First of all, wind all the coils as follows:
 

L1 – 5 turns 20 gauge wire wound on 6mm diameter former. (Wind round a drill bit.)

L2 – 8 turns 20 gauge wire wound on 6 mm diameter former. Both air-cored.

Tank coil. 22 turns 32 gauge wire on 3 mm former with dust core.

FM RFC – 1 metre of fine wire wound in a single layer on a 22 MegOhm resistor.

HF RFC – 3 Metres fine wire, scramble wound on a 22 Meg resistor.

The tank coil could be a problem, since coil formers are no longer available. You might try 27 turns on a high value resistor. Note that the coil is shunted by a 15k resistor to lower the efficiency (or “Q factor”). This is to allow sufficient bandwidth for the FM signal.
 

Next step, place the 2p antenna capacitor on the board before placing the tuning capacitor.

Place the remaining components as follows:
 

Fremodyne Component Layout
Figure 2: Component placement

 

Losc is the 8 turn coil, Lant is the antenna coil, L1. Be sure you get the orientation of the valve base correct!!

Alignment

Connect a power supply, a long wire antenna and a crystal ear piece to the radio. Space the turns on the antenna coils as far as you can. If you have a signal generator, set it to 21.7 MHz and tune the tank coil. If you don't, there is a strong Islam station transmitting on 21.46 MHz during afternoons. You should receive this on the long wire. This will suffice for now.

Now remove the long wire and use an FM dipole. Squeeze or extend the oscillator coil so the receiver tunes the entire FM band. You should hear all the stations – unless you are living a long way from a transmitter.

Tune the tank coil a tiny amount so that you don't receive any harmonics in the FM band.

Finally squeeze the antenna coil for maximum strength on a weak signal (Radio 702 on 92.7 MHz in my case.) You might try putting a small value capacitor across the antenna coil. I put a 3pf capacitor across mine.

Results

I don't have a complicated spectrum analyser, so I used my PC and a communication receiver to scan all the frequencies from 18 to 86 MHz. This is the result:

Frequency Spectrum Fremodyne
 

Figure 3: Frequency Spectrum - The frequencies at 65 MHz are Junk from my PC

Printed Circuit Layout

Fremodyne PC Board

 
Joomla template by a4joomla