Back to Basics - The Superhet

Back to Basics – The Superhet

by Terry Adams

(G4CHD)

Slide No: 1

Historical Timeline

1820:Hans Christian Ørsted demonstrated that a wire carrying a current wasable to deflect a magnetized compass needle

1831: Michael Faraday discovered electromagnetic induction. He proposedthat such forces extended into the empty space around the conductor

1838:Morse demonstrates telegraphy

1861:Transcontinental telegraph by wire

1873: James Clark Maxwell described the theoretical basis of thepropagation of electromagnetic waves in his paper 'ADynamical Theory of the Electromagnetic Field'

1874:Ferdinand Braun discovered rectifying abilities of crystals

1883:Edison invents the Edison effect

1886:Heinrich Rudolf Hertz produces and detects electric waves

1888: Hertz validated Maxwell's theory through experiment

1893: Nikola Tesla delivers a lecture "On Light and other High FrequencyPhenomena". Tesla used sensitive electromagnetic receiversthat were unlike the less responsive coherers used later by Marconi

Slide No: 2

Historical Timeline (2)

1894:Sir Oliver Lodge demonstrated the reception of Morse code signallingusing radio waves using a "coherer"

1894: Jagdish Chandra Bose, demonstrated use of radio waves in Calcutta.He ignited gunpowder and rang a distant bell using electromagneticwaves, proving that signals can be sent without using wires

1894:Alexander Popov built a coherer

1895: Guglielmo Marconi is the first to achieve successful radio transmission

1896: Marconi awarded patent for radio. This is the initial patent for radio

1895: Marconi is the first to achieve successful radio transmission

1896: Marconi awarded first patent for radio

1896: Bose went to London on a lecture tour and met Marconi

1897:J.J.Thompson discovers the electron

1897: Marconi established the radio station on the Isle of Wight

1898:Marconi opened the first radio factory on Hall Street, Chelmsford

1899: Bose announced his invention of the "iron-mercury-iron coherer”

1900: Reginald Fessenden made weak transmission of voice over airwaves

Slide No: 3

Historical Timeline (3)

1901: Marconi claims to have received in St. John's, Newfoundland a radiosignal transmitted from Poldhu in Cornwall

1901:Fessenden noticed beats were detected between two signals

1904:John Ambrose Fleming patented the thermionic diode

1906: Fessenden used an Alexanderson alternator and rotary spark-gaptransmitter to make the first radio audio broadcast, fromMassachusetts. Ships heard Fessenden playing O Holy Night on the violin

1906:Cat's whisker diodes developed made of mineral crystals eg galena

1906:Lee De Forest invented the triode by adding a grid to the Fleming valve

1907: Marconi established first permanent transatlantic wireless service fromClifden, Ireland to Glace Bay, Nova Scotia

1910:Edwin Armstrong invented regenerative receiver utilising a valve

1912: The RMS Titanic sank

1918:Armstrong invented superheterodyne (superhet) receiver

1919:Walter Schottky invented tetrode valve

1926:Bernhard D.H. Tellegen invented the pentode valve

Slide No: 4

Spark Gap Transmissions

Initially, receiver was a simple

untuned loop but later tuned loops

were used

Around 1890, the coherer,

a primitive form of radio

detector was invented by French

scientist Édouard Branly.

It consisted of a tube containing two electrodes spaced a small distance apart,with metal filings in the space between them.

When a rf signal is applied to the device,

the initial high resistance of the filings reduces,

allowing an electric current to flow through it.

The coherer was the first device used to detect

radio signals in practical spark gap transmitter.

Slide No: 5

Tuned Radio Frequency (TRF) RX

The triode valve's amplifying properties enabled the TRF RXto be developed in which

all amplification and selectivity

is done at rf

Circuit shown uses 6 triodes - two tuned rf,

one grid-leak detector/amp and three af stages.

Generally 2 or 3 RF amplifiers are needed to

filter and amplify the received signal to a level sufficient to drive the detectorstage with each stage tuned manually

Slide No: 6

Regenerative & Super Regenerative RX

The regenerative receiver was invented and

patented in 1914 by Edwin Armstrong.

It consisted of an amplifying valve with its output

connected to its input through a feedback loop,

(tickler coil) providing positive feedback.

Regenerative receivers achieve gain and

selectivity through positive feedback thus

reducing need

for many

expensive valves

required

by the TRF RX.

In 1922,

Armstrong

developed the

super-regenerative receiver which uses supersonic quenching to prevent

oscillation. Works best above 50MHz.

Slide No: 7

Basic Requirements of any Radio Receiver

All radio receivers must provide the following : -

Selectivity

Sensitivity

Amplification

Demodulation

Slide No: 8

Disadvantages of Early Receivers

Problems included :-

Lack of gain

Lack of selectivity

Instability

Inconsistent bandwidth

Resulted from trying to achieve all selectivity and

amplification at the tuned radio signal frequency

As the receiver frequency is tuned, changes in eg skin effect

and other losses will alter the Q Factor and Bandwidths achieved

Slide No: 9

The solution to these problems was first proposed during

WW1 by Edwin Armstrong which he finally implemented

in 1918 as the Superheterodyne (Superhet) receiver

In which the incoming rf signal was 'heterodyned' (mixed)

with a local oscillator signal to produce a constant

Intermediate Frequency (IF) at which most of the

amplification and selectivity would be achieved

Heterodyning uses a Mixer – where 2 frequencies

are combined in a nonlinear device to create 2 new signals,

the sum (f1 + f2) and the difference (f1 − f2)

One is chosen as the desired IF,

and the other is filtered out

The term IF was used since historically its value was

between that of the rf and af

The Superheterodyne (Superhet) Receiver

Slide No: 10

Consider :-

IF = 0.5 MHz

Input RF = 7 MHz

Local Oscillator (LO) frequency is

chosen using the Mixer difference

output, and is thus either 7.5 MHz (high side) or 6.5 MHz (low side)

High side value is often preferred (ie 7.5 – 7 = 0.5 MHz)

If a new RF of 7.2 MHz is now to be tuned, the Local Oscillator (VFO) ischanged to 7.7 MHz (7.7 – 7.2 = 0.5 MHz)

In this way, the Local Oscillator (LO) becomes the tuning control

Selection of the 0.5 MHz IF (and rejection of the sum output) is done in the IFstrip using tuned circuits, mechanical or crystal filters to achieve eg a 6 kHz(AM/FM), 2.7 kHz (SSB) or eg 250 Hz (CW) bandwidths

Superhet Receiver Theory

Slide No: 11

Consider tuning 7 MHz

RF signal (LO set to 7.5 MHz)

with 0.5 MHz IF :-

Unfortunately, an 8 MHz input

RF signal will also mix with the

7.5 MHz LO to produce a 0.5 MHz IF causing interference

This 8 MHz interfering signal is termed an Image Frequency and is 2 x IFaway from the required rf signal frequency.

To eliminate this interference, the Image Signal is prevented from reachingthe Mixer by a filter between the aerial and the Mixer. The filter in thisexample must accept 7 MHz and reject 8 MHz which is not impossible butwould be easier if a higher IF had been used.

Superhet Receiver Image Problem

Slide No: 12

Output from Aerial can be very small – eg few μV

Needs amplification in a RF Amp before feeding

the Mixer

The level of amplification must be chosen to not

overload the mixer when strong signals are present

The RF Amplifier must have a low noise design

since any noise introduced will be amplified later

Automatic Gain Control (AGC) may applied

Bandpass Filter is used block the Image Frequency which is

2 x IF from wanted signal. Hence use of a high IF makes

Image Rejection easier

RF Amp and Local Oscillator tuning is ganged to enable them to tracktogether

The RF Amp also helps prevent local oscillator signals radiate from the aerial

Superhet Receiver – RF Amp & Filter

Slide No: 13

Local Oscillator (VFO) generates single frequency that

must be pure, noise and distortion free as possible

The tuning is usually ganged with that of the RF Amp

Mixer is a non-linear (multiplier) device and can be either

Passive using one or more diodes (lower output) or

Active using valves or transistors to amplify output

Active Mixers improve isolation between ports but are less tolerant tooverload and consume more power and can generate more noise

Unbalanced Mixers output some of the RF and LO

signals besides the sum and difference products

Double Balanced Mixers (shown opposite) only

output the sum and difference products

Superhet Receiver – LO & Mixer

Slide No: 14

IF Amp provides majority of gain and selectivity

at single IF frequency that doesn't change as

the receiver is tuned

IF chosen from standard frequencies

eg 455 kHz, 1.6 MHz, 9 MHz, 10.7 MHz – the higher the IF, the easier tosuppress the Image Frequency

However, the lower the IF, the easier and cheaper to achieve the required gainand selectivity

Filters normally use coupled tuned circuits at low IF

or crystal filters at higher IF

Superhet Receiver – IF Amp & Filter

Slide No: 15

The output of the IF strip feeds the

Demodulator which for an AM signal can

be a simple diode Envelope Detector as

shown opposite, but for an FM signal,

a Discriminator is needed which

produces an output related to the

frequency changes in the carrier

Ideally, it is not sensitive to amplitude changes but usually the IF stripamplifies to the limit to give a constant maximum amplitude output

For SSB/CW a Carrier Insertion Oscillator (CIO) and the SSB IF feed aDouble Balanced Mixer. The CIO injects the missing Carrier and the AMoutput is then demodulated as per above

The example as shown in the next slide should help explain the principle

Superhet Receiver – Demodulator

Slide No: 16

The output of the IF strip feeds the Demodulator/Detector which in turn feedsthe AF Amp and ultimately the loudspeaker.

The resultant simplified Block Diagram for a Superhet becomes :-

Superhet Receiver Block Diagram

Slide No: 17

Superhet Receiver – Example

Slide No: 18

When choosing the IF for a superhet radio there is a trade-off to be madebetween the advantages of using either a low or a high frequency IF:

High frequency IF: The use of a high frequency IF means that thedifference between the wanted frequency and the unwanted image is muchgreater and it is easier to achieve high levels of performance because the frontend filtering is able to provide high levels of rejection

Low frequency IF: The advantage of choosing a lower frequency IF is thatthe filters that provide the adjacent channel rejection are lower in frequency.The use of a low frequency IF enables the performance to be high, whilekeeping the cost low

Accordingly there are two conflicting requirements which cannot be easilysatisfied using a single intermediate frequency. The solution is to use a doubleconversion superheterodyne topology to provide a means of satisfying bothrequirements

Double Superhet Receiver

Slide No: 19