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Items relevant to "Simple VHF FM/AM Radio":
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Explore the mysteries of “slope
detection” with this:
Simple VHF FM/AM
Regenerative Receiver
If you want to build an AM/FM radio it takes
at least one special IC or quite a raft of discrete
components, doesn’t it? Wrong. As shown in this
article, you can build a simple VHF receiver
quite simply, particularly if it uses “slope
detection” for the FM stations.
By ANDREW WOODFIELD
M
ANY READERS will remember building their very first
radio. The projects that you
built after this are probably a forgotten
memory now but that first receiver,
well, is often quite special. Perhaps,
like many, it was a simple crystal set.
In my case, I built a one-transistor
86 Silicon Chip
reflex receiver, the parts bought with
money earned from helping to paint
a holiday beach house.
Tuning in those first sounds after
hours of careful soldering, with the
help of a long-suffering ham-radio-operator uncle, was nothing short
of amazing for me. Then came an
endless round of careful testing and
adjusting of wire antennas and the
earth connections to get the best performance out of that simple receiver.
(I don’t recall anything that made a
real difference!)
It all made for a memorable summer,
listening to music, news and cricket
broadcasts on the AM band.
These days, most youngsters prefer
listening to music and DJ chatter on the
VHF FM band. Of course, you can buy
an IC to build a radio but that hardly
falls into the ‘simple’ category despite
the modest number of parts required.
Also, special ICs can be hard to find
and expensive.
Making a simple FM radio receiver,
at least at first glance, therefore appears far more appealing for someone
www.siliconchip.com.au
Parts List
1 PC board, code 06212021, 37
x 31mm
1 battery holder for two AAA
cells
2 x 1.5V AAA cells
1 crystal high impedance earphone
1 miniature slide switch
1 BF199 or equivalent RF small
signal transistor (Q1)
1 BC549 or equivalent high gain
small signal transistor (Q2)
1 10µH RF choke (L2)
L1: see text
Fig.1: the circuit uses just two transistors. Q1 and its surrounding parts
form a regenerative detector stage, with the receiver’s frequency set by
tuned circuit L1 and VC1. The output from this stage is fed to audio
amplifier stage Q2.
just starting out in the hobby. However, designing a simple radio for FM
which is truly repeatable is more of
a challenge.
After all, FM cannot be received
on a simple crystal detector, can it?
(Actually, it can, but it’s a complex
and challenging construction project.)
And there’s no point in a design that
won’t go first time.
This article describes a simple FM
radio that’s inexpensive to build. It
uses only a few more parts than a
basic one-transistor AM radio or a
crystal set, yet it can receive speech
and music with reasonable quality
from FM stations.
Based around a proven super-regenerative receiver design, it’s also
easy to build, and all of the parts
are readily available. Finally, with
a little adjustment, local VHF AM
airport radio services can be received
equally well.
Regenerative receivers
Regenerative receivers, of which
this design is an example, are tuned
amplifiers which are held right on the
edge of oscillation. Any amplifier with
too much feedback will oscillate - the
loud squeal and howl of an audio
amplifier with too much feedback is a
www.siliconchip.com.au
memory we don’t enjoy! In this case,
however, the tuned amplifier’s gain is
allowed to rise until it just begins to
start to oscillate.
The difference with a regenerative
receiver is that as soon as it begins to
oscillate, the circuit instantly reduces
the amplifier’s gain so that it drops
out of oscillation again. With careful
design, this type of tuned amplifier/
oscillator can be made to fluctuate
continuously in and out of oscillation,
rapidly, right at this very high gain
operating point.
There is considerable debate about
the exact manner in which a regenerative receiver operates. Perhaps because
this time-shared amplifier-oscillator
action is inherently non-linear, such
high-gain amplifiers readily detect
amplitude modulation speech and
music on received radio signals. The
typical amplifi
er/oscillator quench
frequency (as this on-off switching
effect is called) varies between 10kHz
and 100kHz.
In simple regenerative receivers,
like this design, the quench frequency
is not fixed precisely. It changes with
component characteristics, temperature, supply voltage, as well as with
external effects such as the presence
of metal objects, or even as your hands
Capacitors
1 47µF 16V PC electrolytic
1 4.7µF 16V PC electrolytic
1 33nF (.033µF) MKT polyester
2 22nF (.022µF) MKT polyester
1 4.7nF (.0047µF) MKT polyester or ceramic
1 6-60pF plastic trimmer capacitor
1 6.8pF 50V disc ceramic
2 1 nF 50V disc ceramic
Resistors (0.25W, 1%)
1 330kΩ
1 3.3kΩ
1 22kΩ
1 2.2kΩ
1 10kΩ
1 100Ω
1 4.7kΩ
Miscellaneous
Hookup wire, solder, case to
hold PC board, etc.
get closer to the circuit. Regardless,
the tuned amplifier is still kept on the
edge of oscillation.
The quench rate is often controlled
by a resistor and capacitor in simple
regenerative receiver designs. Such
components cannot reliably control
all of the dynamics of a high-gain
amplifier when large changes occur,
of course. If the receiver is tuned over
large ranges, for example, the amplifier
may stop oscillating at some point,
or it may begin to oscillate and never
properly quench.
This is one reason why many simple
regenerative receivers have a ‘reaction’
or ‘feedback’ control.
This allows precise adjustment of
quench to keep the receiver as close
as possible to the optimum setting.
This design avoids this problem by
selecting a compromise value for the
resistor-capacitor pair and by limiting
December 2002 87
2.2k
1nF
10k
22nF
6.8pF
10H
+
4.7F
22nF
Q1
4.7k
22k
12021260
L1
VC1
33nF
330k
100
4.7nF
frequency is offset from the centre frequency of the receiver’s tuned circuits.
Since the receiver is rapidly turning
on and off at the quench frequency,
this gives rise to considerable hiss in
the received audio, especially when
not receiving a signal. This is a very
characteristic sound in regenerative
receivers. Since the quench frequency
is so audible, R5 and C8 are used to
reduce the level of this hiss.
C6 isolates the bias voltage on the
audio stage around Q2 from the signal
and bias voltages around Q1. Q2’s
bias is set using a very simple bias
chain using two resistors; R6 and R7.
This requires that Q2 be a high gain
transistor but these are no more expensive than similar types and readily
obtainable.
A crystal earphone in used to listen to the final detected sound. This
minimises loading on the circuit, increasing the sound level considerably.
It also saves a further amplification
stage with another transistor, as well
as the cost of a speaker and matching
transformer.
The audio received with this arrangement is amazing. One of the
prototypes produced music and sound
that could be clearly heard more than
a metre away from the receiver. For
simplicity and to save considerable
cost, there is no volume control on the
receiver. We did say this receiver was
simple, didn’t we? If the audio level is
too loud, R6 can be reduced to 220kΩ.
A further major advantage of this
receiver design is its miserly battery
drain. Prototypes averaged well under
1mA with a pair of AAA batteries,
allowing for many hours of use. This
is probably one of the most important
considerations for younger builders
(and parents!) keen to avoid the continual cost of battery replacement or
expensive rechargeable cells.
Because the receiver oscillates mo-
2 x AA CELL HOLDER
3.3k
3.3k
+
47F
Q2
TO CRYSTAL
EARPIECE
S1
OFF
ON
the frequency range to the FM broadcast
and nearby VHF aviation bands.
Circuit description
The receiver has two basic sections:
(1) The regenerative detector, which
amplifies and detects the signal; and
(2) A simple one-transistor audio
amplifier.
The full circuit is shown in Fig.1.
Q1 and surrounding components
form the regenerative detector stage.
The receiver’s frequency is set by the
tuned circuit L1 and VC1. Capacitor
C4 provides a path for RF feedback
to encourage oscillation. The quench
frequency is primarily set by R4 and
C5 and as oscillation begins to rise, the
increasing current through R4 ensures
that Q1 is eventually limited, in turn
halting oscillation.
As the regenerative receiver is tuned
across a signal, the current through
R3 varies with the modulation on
the received signal. With amplitude
modulated signals, such as those used
by airports, the strength of the signal
changes in sympathy with the audio
signal. If the receiver is tuned to this
Fig.2: most of the parts fit on a small
PC board which can be assembled
in about 10 minutes. The receiver is
tuned by adjusting trimmer capacitor
VC1 with a plastic alignment tool (eg,
a discarded knitting needle sharpened
to fit the slot).
signal, this variation in received signal level is detected and converted to
small variations in collector current
in Q1. In turn, this small signal is
amplified by Q2.
The process by which this receiver
detects FM is a little more complex.
FM signals are generated when the
audio signal changes the frequency of
the transmitter rather than it’s ampli
tude. When a very selective tuned
circuit is adjusted closer and closer
to the frequency of an FM signal,
the amplitude of the received signal
will increase. If the tuned circuit is
suffi
ciently selective, the changing
frequency caused by the modulation
on the FM signal will cause an amplitude change in the signal across the
tuned circuit.
This same effect was used in the
earliest receivers to detect FM signals.
The ‘slope’ of the tuned circuit’s selectivity allowed this change in signal
amplitude with changing frequency.
This was called “slope detection”. If
you tune an FM signal using an AM
receiver, the best sounding audio
will be received when the FM centre
Table 1: Resistor Colour Codes
No.
1
1
1
1
1
1
1
88 Silicon Chip
Value
330kΩ
22kΩ
10kΩ
4.7kΩ
3.3kΩ
2.2kΩ
100Ω
4-Band Code (1%)
orange orange yellow brown
red red orange brown
brown black orange brown
yellow violet red brown
orange orange red brown
red red red brown
brown black brown brown
5-Band Code (1%)
orange orange black orange brown
red red black red brown
brown black black red brown
yellow violet black brown brown
orange orange black brown brown
red red black brown brown
brown black black black brown
www.siliconchip.com.au
mentarily on each quench cycle, it is
possible for the ultra-low microwatt
oscillator signal to be detected in
nearby conventional receivers. This
was a major problem with 1930’s
valve versions of these receivers.
These operated at many times greater
power levels and the loud levels of
radiated hash at the quench rate of
the receiver could be heard in every
nearby receiver - hardly a desirable
characteristic!
Modern transistor equivalents,
including this design, seldom encounter the same problem. In part, this is
because they operate at much lower
power levels. Instead of 90 to 150V DC
power supplies required for a valve,
this receiver makes do with just a sniff
of current from a pair of AAA cells;
ie, just 3V.
To further prevent this problem arising with this design, we’ve not made
any provision for an external antenna.
The receiver is highly sensitive and
good reception can be achieved without any extra antenna. Also, attaching
a short 1m long wire to the circuit is
possible, say via a 4.7pF ceramic capacitor to the collector of Q1, it will
shift the received frequency a little
since it will partially load the tuned
circuit, reducing the effectiveness of
the detector.
Construction
The receiver can be built either using the PC board and housed in any
convenient case. One of the prototypes
was built into a small peppermint tin.
(It’s actually something of a little joke.
The tin was a marketing giveaway
from a manufacturer of one of the
most advanced digital mobile radio
systems currently produced. It somehow seemed appropriate to recycle
it to house one of the oldest types of
analog radio circuits.)
Begin construction by carefully
inspecting the PC board for any unwanted short circuits between tracks
or other manufacturing defects. Check
for undrilled holes, too.
Mount all of the resistors and capacitors first. Then make and install the
two coils, L1 and L2. L1 can be made
by winding four turns of enamelled
copper wire around a convenient 6mm
diameter former. A drill bit or a pencil
work well.
L2 is a small RF choke. If one cannot
be found, you can make it by winding
30 turns of 36 gauge enamelled copper
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Fig.3: this diagram shows the extra parts that are required in order to
use low-impedance stereo headphones (eg, from a portable CD player or
a Walkman). This involves adding an extra audio stage based on
transistor Q3 and a small audio transformer.
wire on a 1MΩ 0.25W resistor.
VC1 is miniature plastic trimmer
capacitor which is used to tune the receiver to your favourite station. Insert
it into the PC board carefully before
soldering and don’t use too much
heat when soldering this into place.
The plastic can melt if the trimmer
gets too hot.
If you wish to only receive one station or you only want to tune a small
range of frequencies, you could replace
VC1 with a fixed capacitor. A 22pF
ceramic capacitor works to cover the
aviation band and 33pF can be used for
the FM band. This may require some
adjustment of coil L1, depending on
the actual capacitor used to precisely
tune into the signal you want. You
may need to add or subtract a turn or
two to L1 to allow the fixed capacitor
to be used.
Install the two transistors next,
making sure that the audio transistor
(BC549) is used for Q2 and the RF
transistor (BF199) for Q1. The receiver
won’t work if they are reversed.
Then add the wiring for the switch,
the battery and the earphone.
Earphone options
There are several earphone options.
If possible, use a high impedance
crystal earphone, although they can be
hard to find in some locations. Most
large parts suppliers almost always
stock them.
An alternative is to use a piezo
speaker recovered from an old toy or
from one of those greeting cards that
plays a tune or a few pre-recorded
words.
However, while these little speakers deliver lots of volume at high
audio frequencies, they don’t do so
well at mid-to-low audio frequen-
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A small peppermint tin was used to
house one of the author's early
prototypes (and it wasn’t even built
on a PC board).
cies. However, you can add a simple
horn to improve the sound quality
somewhat. It can be made by cutting
the top 100mm section from a plastic
soft drink bottle. A hole around 8mm
diameter was drilled into the cap and
the piezo speaker was then hot-glued
to the cap.
The resulting sound is not loud but
it is clearly audible from a metre or
more in a quiet room. Painted, it gave
A piezo speaker scrounged from an
old toy or from a greeting card can
be used directly with the circuit
shown in Fig.1.
A simple horn made by cutting the
top 100mm section from a plastic soft
drink bottle can be used to improve
the sound quality of a piezo speaker
(see text for details).
90 Silicon Chip
a 1930’s look to one of the prototype
receivers.
By the way, don’t try to use a pair
of stereo earphones from a Walkman.
Their 32Ω impedance is much too low
for this receiver. However, if you still
would like to use these, then you’ll
need to add a small amplifier stage to
the receiver and the current will rise
substantially. The required components and changes to the receiver are
shown in Fig.3.
Suitable transformers include
Dick Smith or Altronics Part Number M-0216. If possible, connect the
earphones so that the left and right
earphones are in series. This helps increase the volume further. Fig.4 shows
the earphone connections required.
Fig.4: this diagram shows how the
connections to a stereo headphone
plug are made. Only the ring and
tip terminals are used – there is
no connection to the sleeve.
06212021
Suitable transistors
RF transistors should be used for
Q1, while audio transistors are suitable for Q2 and Q3, the latter being
required if the stereo headphone
modification is added. Suitable transistors include:
Q1: BF 115, BF184, BF199, BF494,
MPSH11, 2N2222, etc (ie, RF transistors with hfe>100 and fT>250 MHz).
Q2: BC109, BC549, 2N3904 (ie, almost any high-gain small signal audio NPN transistor is likely to prove
suitable).
A variety of these transistors were
used on the three prototypes built, all
working almost identically. The main
difference was the current drain, with
this varying between 0.6 and 1mA,
depending on the RF transistor being
used.
Testing and operation
Before proceeding further, carefully
check the PC board again and the location and orientation of all parts. Check,
especially carefully, the orientation
of the two transistors. Are they in the
correct location? Are all resistors in
the correct location too? Check the
underside of the PC board for poorly
soldered joints or shorts caused by
solder bridges where connec
tions
have been accidentally soldered too
closely together.
Once you’ve checked the layout
again, and with so few parts, testing
is as simple as connecting the battery
and switching on the power to the receiver. You should hear a loud hiss in
the earphone. If that’s the case, adjust
the trimmer capacitor until you hear
an FM station.
Fig.5: this is the full-size etching
pattern for the PC board. Check
your board for defects before
installing any of the parts.
If you don’t hear a signal, it’s likely
that the coil you’ve wound for L1 is a
little too large. The simple solution is
to, firstly, turn off the power to the receiver, then spread out the coil’s turns.
Spread the turns of the coil apart so
that it occupies a length of, say, 12 or
15mm. Then, turn on the power again
and try tuning again.
If you don’t hear any hiss at all, turn
off the power and recheck all your
connections, especially those going
to the earphone. You can check that
the audio amplifier stage is working
by pressing your finger lightly on the
underside of the PC board with the
power on (Don’t worry - The battery
voltage is high enough to be dangerous) and press on the base of Q2. You
should hear a low hum if it’s operating
correctly. If you cannot hear anything,
check the battery and the battery holder’s connections carefully.
Tuning
Once you have the receiver operating, the receiver can be carefully
tuned into your favourite station.
This should be done with a plastic
or insulated adjustment tool. An old
plastic knitting needle or discarded
piece of plastic rod from a kitset model
plane works well. This minimises any
frequency shift caused by the body as
SC
it gets close to the circuit.
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