Superhet & straight receivers:
The main difference between a superheterodyne (superhet) receiver and a straight (TRF) receiver is how they receive and process radio signals. Both are designed to pick up radio transmissions, amplify them and turn them into audible sound, but they use quite different methods. This difference was particularly important in the development of military and vintage valve radio equipment.
Straight receiver
A straight receiver, often called a TRF (Tuned Radio Frequency) receiver, works by amplifying the incoming radio signal directly at its received frequency.
The signal passes through:
- An aerial input circuit.
- Tuned radio-frequency amplifier stages.
- A detector, which extracts the audio.
- An audio amplifier.
- A loudspeaker or headphones.
Advantages:
- Relatively simple circuit design.
- Can be straightforward to construct and maintain.
- No frequency-conversion oscillator required.
Disadvantages:
- Selectivity can be difficult to maintain across a wide tuning range.
- Tuning several RF stages together can be awkward.
- Performance and sensitivity can vary considerably with frequency.
Superheterodyne receiver (Superhet)
The superhet takes a different approach. Instead of amplifying the incoming signal directly throughout the receiver, it converts it to a fixed intermediate frequency (IF).
The process works as follows:
- The aerial receives the incoming radio signal.
- An RF stage selects and amplifies the signal.
- A local oscillator produces another frequency.
- A mixer combines the two frequencies.
- The resulting intermediate frequency is amplified.
- A detector extracts the audio.
- Audio stages amplify the sound.
For example, a receiver tuned to 1,000 kHz might convert the signal to an IF of 455 kHz.
The advantage is that the receiver can use carefully designed, fixed-frequency IF amplifiers to provide good sensitivity and selectivity.
Direct-conversion receiver
There is another type called a direct-conversion receiver, sometimes called a zero-IF receiver. This is different from a traditional straight TRF receiver. A direct-conversion receiver mixes the incoming signal with a locally generated signal at, or very close to, the received frequency. This converts the signal directly down to audio frequencies.
It does not use the conventional fixed intermediate-frequency amplifier stages found in a superhet. It is particularly useful for certain amateur radio applications, including CW and SSB reception.
Comparison of the three types
| Feature | Straight TRF | Direct conversion | Superhet |
|---|---|---|---|
| RF amplification | Yes | Sometimes | Yes |
| Local oscillator | No | Yes | Yes |
| Mixer | No | Yes | Yes |
| Fixed IF stages | No | No | Yes |
| Selectivity | Variable | Audio filtering | Generally excellent |
| Circuit complexity | Simple | Moderate | More complex |
Why the superhet became popular
The superheterodyne receiver offered several important advantages:
- Much better adjacent-channel rejection.
- More consistent sensitivity across its tuning range.
- Easier control of receiver bandwidth.
- Greater amplification without requiring numerous tuned RF stages.
- Better performance for communications over a wide range of frequencies.
These advantages made it particularly attractive for military communications equipment during the Second World War.
An interesting point about wartime military radios
When looking at British military equipment from the 1930s and 1940s, you will encounter many superheterodyne receivers. The Wireless Sets used by the British Army often employed valve-based superhet designs because reliable reception, good selectivity and sensitivity were important in the field.
The receiver design could make a considerable difference when trying to separate a weak military transmission from other signals. One useful distinction: a straight TRF receiver and a direct-conversion receiver are not the same thing. The former amplifies the incoming radio frequency directly; the latter mixes it down to audio.
Direct-conversion radio receivers explained
A direct-conversion receiver, sometimes called a homodyne receiver or zero-IF receiver, is an interesting type of radio receiver. It is particularly associated with amateur radio, especially the reception of Morse code (CW) and single-sideband (SSB) signals.
Unlike a conventional superheterodyne receiver, it does not convert the incoming signal to a fixed intermediate frequency such as 455 kHz. Instead, it converts the radio signal directly into an audio signal.
How does it work?
A basic direct-conversion receiver has five main sections:
- Aerial input: Picks up the incoming radio signal.
- RF amplifier: Provides initial amplification and some selectivity.
- Local oscillator (VFO): Generates a frequency very close to the incoming signal.
- Mixer: Combines the incoming signal with the oscillator signal.
- Audio amplifier: Amplifies the resulting audio so it can be heard through headphones or a loudspeaker.
The clever part is the mixer. It takes two radio frequencies and produces their sum and difference. The difference frequency is what we want.
An example using actual frequencies
Imagine you are receiving a Morse code transmission on 7,030 kHz. Your local oscillator is adjusted to 7,030 kHz, or very slightly above or below it. The mixer produces a difference frequency of approximately zero. For a CW transmission, you deliberately offset the oscillator slightly to produce an audible tone.
For example:
| Incoming signal | Oscillator | Audio output |
|---|---|---|
| 7,030 kHz | 7,030.7 kHz | 700 Hz |
You would hear the Morse transmission as a tone around 700 Hz, switched on and off by the transmitted Morse code. This is why direct-conversion receivers are so useful for listening to CW.
Receiving different types of transmission
Direct conversion is especially interesting because different modes behave differently.
CW
This is probably the simplest application. The oscillator is offset slightly from the incoming carrier, producing an audible tone. The Morse signal is heard as a series of beeps.
SSB
The receiver uses its local oscillator to reinsert the missing carrier. This allows the original speech to be recovered. For example, an SSB transmission on the 7 MHz amateur band can be converted directly into audible speech.
AM
A basic direct-conversion receiver can receive AM, but it is generally less convenient than a conventional AM receiver. Oscillator stability and tuning are important, and the receiver can suffer from distortion and interference from strong signals.
Advantages and disadvantages of direct conversion
| Advantages | Disadvantages |
|---|---|
| Relatively few components | Can suffer from oscillator drift |
| No conventional IF transformers | Can pick up strong unwanted signals |
| Good for CW reception | Sideband rejection can be poor |
| Excellent for simple amateur receivers | Strong signals can overload the mixer |
| Audio filtering can be very effective | Requires careful oscillator adjustment |
One problem is called audio image response. A simple direct-conversion receiver can receive signals on either side of its oscillator frequency and convert both into audio. This means two different radio signals can appear at the same audio frequency. More sophisticated designs use phase-shift techniques to overcome this problem.
What would a valve direct-conversion receiver look like?
A traditional valve version could be built using:
- One valve as an RF amplifier.
- One valve as a variable-frequency oscillator.
- A mixer or product detector valve.
- One or two audio amplifier valves.
- A variable tuning capacitor.
- An audio filter and headphone output.
The local oscillator would need to be particularly stable. Even a small frequency drift could make an SSB conversation difficult to understand. One interesting historical term is synchrodyne. This was an early name for a receiver using an oscillator synchronised with the incoming carrier. It is closely related to the modern direct-conversion principle.
Direct-conversion technology was known in the 1930s, although practical difficulties limited its popularity at the time.
Direct conversion for FM reception
In a basic direct-conversion receiver, the incoming signal is mixed with a local oscillator at approximately the same frequency. With FM, the information is carried by changes in the instantaneous frequency of the signal, rather than changes in its amplitude.
The mixer produces an audio-frequency signal whose frequency varies with the original FM modulation. However, simply amplifying this signal does not properly recover the audio. A frequency discriminator or another suitable FM demodulator is needed.
There are several approaches:
- Quadrature detection: Uses two signals 90 degrees apart to recover the frequency variations.
- PLL detection: A phase-locked loop tracks the incoming signal's frequency changes and recovers the audio.
- Frequency discriminator: Converts frequency changes into corresponding voltage changes.