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Build the . . .
Here’s an FM stereo transmitter that’s really
easy to build. It’s based on the new BH1417F
chip from Rohm and is crystal-locked to the
selected frequency so there’s no drift. Best of
all, there are no messy tuning coils to wind
and adjust.
T
By JOHN CLARKE
HIS NEW STEREO FM Micromitter is capable of broadcasting good quality signals over a
range of about 20 metres. It’s ideal for
broadcasting music from a CD player
or from any other source so that it can
be picked up in another location.
For example, if you don’t have a
18 Silicon Chip
CD player in you car, you can use the
Micromitter to broadcast signals from a
portable CD player to your car’s radio.
Alternatively, you might want to use
the Micromitter to broadcast signals
from your lounge-room CD player to
an FM receiver located in another part
of the house or by the pool.
Because it’s based on a single IC,
this unit is a snack to build and fits
easily into a small plastic utility box.
It broad
casts on the FM band (ie,
88-108MHz) so that its signal can be
received on any standard FM tuner or
portable radio.
However, unlike previous FM transmitters published in SILICON CHIP,
this new design is not continuously
variable over the FM broadcast band.
Instead, a 4-way DIP switch is used
to select one of 14 preset frequencies.
These are available in two ranges covering from 87.7-88.9MHz and 106.7107.9MHz in 0.2MHz steps.
No tuning coils
We first published an FM stereo
transmitter in SILICON CHIP in Octowww.siliconchip.com.au
Main Features
•
•
•
•
•
•
Fig.1: block diagram of the Rohm
BH1417F stereo FM transmitter IC.
The text explains how it works.
Very compact
Battery or plugpack operation
Stereo transmission
Standard FM tuner required
to receive transmission
Crystal locked operation
14 selectable transmission
frequencies
ber 1988 and followed this up with
a new version in April 2001. Dubbed
the Minimitter, these earlier versions
were based on the Rohm BA1404 IC
which is now obsolete.
On both these earlier units, the
alignment procedure requires careful
adjustment of the ferrite tuning slugs
within two coils (an oscillator coil
and a filter coil), so that the RF output matched the frequency selected
on the FM receiver. However, some
constructors had difficulty with this
because the adjustment was quite
sensitive.
In particular, if you had a digital
(ie, synthesised) FM receiver, you
had to set the receiver to a particular
frequency and then carefully tune the
transmitter frequency “through” it. In
addition, there was some interaction
between the oscillator and filter coil
adjustments and this confused some
people.
That problem doesn’t exist on this
new design, since there is no frequency alignment procedure. Instead, all
you have to do is set the transmitter
frequency using the 4-way DIP switch
and then dial-up the programmed
frequency on your FM tuner.
After that, it’s just a matter of adjusting a single coil when setting up
the transmitter, to set for correct RF
operation.
Improved specifications
The new FM Stereo Micromitter is
now crystal-locked which means that
the unit does not drift off frequency
over time. In addition, the distortion,
stereo separation, signal-to-noise ratio
and stereo locking are much improved
on this new unit compared to the earlier designs. The specifications panel
has further details.
BH1417F transmitter IC
At the heart of the new design is
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the BH1417F FM stereo transmitter
IC made by the Rhom Corporation. As
already mentioned, it replaces the now
obsolete BA1404 used in the previous
designs.
Fig.1 shows the internal features of
the BH1417F. It includes all the processing circuitry required for stereo
FM transmission and also the crystal
control section which provides precise
frequency locking.
As shown, the BH1417F includes
two separate audio process
ing sections, for the left and right channels.
The left-channel audio signal is ap-
plied to pin 22 of the chip, while the
right channel signal is applied to pin
1. These audio signals are then applied
to a pre-emphasis circuit which boosts
those frequencies above a 50µs time
constant (ie, those frequencies above
3.183kHz) prior to transmission.
Basically, pre-emphasis is used to
improve the signal-to-noise ratio of
the received FM signal. It works by
using a complementary de-emphasis
circuit in the receiver to attenuate
the boosted treble frequencies after
demodulation, so that the frequency
response is restored to normal. At
December 2002 19
Fig.2: this frequency versus output level plot shows the composite level (pin 5).
The 50µs pre-emphasis at around 3kHz causes the rise in response, while the
15kHz low pass roll off produces the fall in response above 10kHz.
the same time, this also significantly
reduces the hiss that would otherwise
be evident in the signal.
The amount of pre-emphasis is set
by the value of the capacitors connected to pins 2 & 21 (note: the value
of the time constant = 22.7kΩ x the
capacitance value). In our case, we
use 2.2nF capacitors to set the pre-emphasis to 50µs which is the Australian
FM standard.
Signal limiting is also provided
within the pre-emphasis section. This
involves attenuating signals above a
certain threshold, to prevent overloading the following stages. That in turn
prevents over-modulation and reduces
distortion.
The pre-emphasised signals for
the left and right channels are then
processed through two low-pass
filter (LPF) stages, which roll off the
response above 15kHz. This rolloff is
necessary to restrict the bandwidth of
the FM signal and is the same frequency limit used by commercial broadcast
FM transmitters.
The outputs from the left and right
LPFs are in turn applied to a multiplex
(MPX) block. This is used to effectively
produce sum (left plus right) and difference (left - right) signals which are
then modulated onto a 38kHz carrier.
The carrier is then suppressed (or removed) to provide a double-sideband
suppressed carrier signal. It is then
mixed in a summing (+) block with a
19kHz pilot tone to give a composite
Fig.3: the frequency
spectrum of the
composite stereo
FM signal. Note the
spike of the pilot
tone at 19kHz.
20 Silicon Chip
signal output (with full stereo encoding) at pin 5.
The phase and level of the 19kHz
pilot tone are set using a capacitor at
pin 19.
Fig.3 shows the spectrum of the
composite stereo signal. The (L+R)
signal occupies the frequency range
from 0-15kHz. By contrast, the double
sideband suppressed carrier signal (LR) has a lower sideband which extends
from 23-38kHz and an upper sideband
from 38-53kHz. As noted, the 38kHz
carrier is not present.
The 19kHz pilot tone is present,
however, and this is used in the FM
receiver to reconstruct the 38kHz
subcarrier so that the stereo signal can
be decoded.
The 38kHz multiplex signal and
19kHz pilot tone are derived by
dividing down the 7.6MHz crystal
oscillator located at pins 13 & 14. The
frequency is first divided by four to
obtain 1.9MHz and then divided by 50
to obtain 38kHz. This is then divided
by two to derive the 19kHz pilot tone.
In addition, the 1.9MHz signal is
divided by 19 to give a 100kHz signal.
This signal is then applied to the phase
detector which also monitors the program counter output. This program
counter is actually a programmable
divider which outputs a divided down
value of the RF signal.
The division ratio of this counter is
set by the voltage levels at inputs D0D3 (pins 15-18). For example, when
D0-D3 are all low, the programmable
counter divides by 877. Thus, if the
RF oscillator is running at 87.7MHz,
the divided output from the counter
will be 100kHz and this matches the
frequency divided down from the
7.6MHz crystal oscillator (ie, 7.6MHz
divided by 4 divided by 19).
In practice, the phase detector
output at pin 7 produces an error
signal to control the voltage applied
to a varicap diode. This varicap diode
(VC1) is shown on the main circuit
diagram (Fig.4) and forms part of the
RF oscillator at pin 9. Its frequency of
oscillation is determined by the value
of the inductance and the total parallel
capacitance.
Since the varicap diode forms part
of this capacitance, we can alter the
RF oscillator frequency by varying its
value. In operation, the varicap diode’s
capacitance varies in proportion to the
DC voltage applied to it by the output
of the PLL phase detector.
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In practice, the phase detector adjusts the varicap voltage so that the
divided RF oscillator frequency is
100kHz at the program counter output.
If the RF frequency drifts high, the frequency output from the programmable
divider rises and the phase detector
will “see” an error between this and
the 100kHz provided by the crystal
division.
As a result, the phase detector reduces the DC voltage applied to the
varicap diode, thereby increasing its
capacitance. And this in turn decreases the oscillator frequency to bring it
back into “lock”.
Conversely, if the RF frequency
drifts low, the programmable divider
output will be lower than 100kHz.
This means that the phase detector
now increases the applied DC voltage
to the varicap to decrease its capacitance and raise the RF frequency. As a
result, this PLL feedback arrangement
ensures that the programmable divider output remains fixed at 100kHz
and thus ensures stability of the RF
oscillator.
By changing the programmable divider we can change the RF frequency.
So, for example, if we set the divider
to 1079, the RF oscillator must operate
at 107.9MHz for the programmable
divider output to remain at 100kHz.
Frequency modulation
Of course, in order to transmit audio
information, we need to frequency
modulate the RF oscillator. We do that
by modulating the voltage applied to
the varicap diode using the composite
signal output at pin 5.
Note, however, that the average
frequency of the RF oscillator (ie, the
carrier frequency) remains fixed, as
set by the programmable divider (or
program counter). As a result, the
transmitted FM signal varies either
side of the carrier frequency according
to the composite signal level – ie, it is
frequency modulated.
Circuit details
Refer now to Fig.4 for the full circuit of the Stereo FM Micromitter. As
expected, IC1 forms the main part of
the circuitry with a handful of other
components added to complete the
FM stereo transmitter.
The left and right audio input
signals are fed in via 1µF bipolar
capacitors and then applied to attenuator circuits consisting of 10kΩ fixed
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Parts List
1 PC board, code 06112021, 78 x
50mm.
1 plastic utility box, 83 x 54 x
31mm
1 front panel label, 79 x 49mm
1 7.6MHz crystal (Hi-Q International (Australia) Pty Ltd.
GB02E QC49/A 7600.000) (X1)
1 SPDT subminiature switch
(Jaycar ST-0300, Altronics S
1415 or equiv.) (S5)
2 PC-mount RCA sockets
(switched) (Altronics P 0209,
Jaycar PS 0279)
1 2.5mm PC-mount DC power
socket
1 4-way DIP switch
1 4mm tapped coil former (L1)
1 4mm F29 ferrite slug
1 680nH (0.68µH) surface mount
inductor (1210A case) (Farnell
608-282 or similar)
1 68nH surface mount inductor
(0603 case) (Farnell 323-7886
or similar)
1 100mm length of 1mm
enamelled copper wire
1 50mm length of 0.8mm tinned
copper wire
1 1.6m length of hookup wire
3 PC stakes
1 4 x AAA cell holder (required for
battery operation)
4 AAA cells (required for battery
operation)
3 10kΩ vertical trimpots (VR1VR3)
Semiconductors
1 BH1417F Rohm surface-mount
FM stereo transmitter (Fairmont
Marketing) (IC1)
1 78L05 low-power regulator
(REG1)
1 MPSA13 Darlington transistor
(Q1)
1 ZMV833ATA (AE version
SOD323 package) surface
mount varicap diode (Fairmont
Marketing) (VC1)
1 24V 1W zener diode (ZD1)
1 1N914, 1N4148 diode (D1)
resistors and 10kΩ trimpots (VR1 &
VR2). From there, the signals are coupled into pins 1 & 22 of IC1 via 1µF
electrolytic capacitors.
Note that the 1µF bipolar capacitors
are included to prevent DC current
flow due to any DC offsets at the signal source outputs. Similarly, the 1µF
capacitors on pins 1 & 22 are necessary
to prevent DC current in the trimpots,
since these two input pins are biased
at half-supply. This half-supply rail
is decoupled using a 10µF capacitor
at pin 4 of IC1.
The 2.2nF pre-emphasis capacitors
are at pins 2 & 21, while the 150pF
capacitors at pins 3 & 20 set the lowpass filter rolloff point. The pilot level
can be set with a capacitor at pin 19 –
however, this is not usually necessary
as the level is generally quite suitable
without adding the capacitor.
In fact, adding a capacitor here only
reduces the stereo separation because
the pilot tone phase is altered compared to the 38kHz multiplex rate.
The 7.6MHz oscillator is formed by
connecting a 7.6MHz crystal between
pins 13 & 14. In practice, this crystal
is connected in parallel with an internal inverter stage. The crystal sets
the frequency of oscillation, while the
27pF capacitors provide the correct
loading.
The programmable divider (or
program counter) is set using switches at pins 15, 16, 17 & 18 (D0-D3).
These inputs are normally held high
via 10kΩ resistors and pulled low
when the switches are closed. Table
Capacitors
2 100µF 16VW PC electrolytic
5 10µF 25VW PC electrolytic
2 1µF bipolar electrolytic
2 1µF 16VW electrolytic
1 47nF (.047µF) MKT polyester
2 10nF (.01µF) ceramic
3 2.2nF (.0022µF) MKT polyester
1 330pF ceramic
2 150pF ceramic
1 39pF ceramic
1 33pF ceramic
2 27pF ceramic
1 22pF ceramic
1 10pF ceramic
1 3.3pF ceramic
Resistors (0.25W, 1%)
1 22kΩ
1 100Ω
8 10kΩ
1 56Ω
1 5.1kΩ
2 39Ω
2 3.3kΩ
December 2002 21
SPECIFICATIONS
Transmission frequencies .............................. 87.7MHz to 88.9MHz in 0.2MHz steps
....................................................106.7MHz to 107.9MHz in 0.2MHz steps (14 total)
Total Harmonic Distortion (THD) .......................................................... typically 0.1%
Pre-emphasis ....................................................................................... typically 50µs
Low Pass Filter ............................................................................15kHz/20dB/decade
Channel separation................................................................................ typically 40dB
Channel balance ................................... within ± 2dB (can be adjusted with trimpots)
Pilot modulation ..................................................................................................15%
RF output power (EIRP) .......................typically 10µW when using inbuilt attenuator
Supply voltage .....................................................................................................4-6V
Supply current ..........................................................................................28mA at 5V
Audio input level ..... 220mV RMS maximum at 400Hz and 1dB compression limiting
1 shows how the switches are set to
select one of 14 different transmission
frequencies.
The RF oscillator output is at pin
9. This is a Colpitts oscillator and is
tuned using inductor L1, the 33pF
& 22pF fixed capacitors and varicap
diode VC1.
The 33pF fixed capacitor performs
two functions. First, it blocks the DC
voltage applied to VC1 to prevent
current from flowing into L1. And
second, because it is in series with
VC1, it reduces the effect of changes
in the varicap capacitance, as “seen”
by pin 9.
This, in turn, reduces the overall
frequency range of the RF oscillator
due to changes in the varicap control
voltage and allows better phase lock
loop control.
Similarly, the 10pF capacitor prevents DC current flow into L1 from
pin 9. Its low value also means that the
tuned circuit is only loosely coupled
and this allows a higher Q factor for
the tuned circuit and easier starting of
the oscillator.
Modulating the oscillator
The composite output signal appears at pin 5 and is fed via a 10µF
capacitor to trimpot VR3. This trimpot
sets the modulation depth. From there,
the attenuated signal is fed via another
10µF capacitor and two 10kΩ resistors
to varicap diode VC1.
As mentioned previously, the phase
lock loop control (PLL) output at pin
7 is used to control the carrier frequency. This output drives high-gain
Darlington transistor Q1 and this, in
turn, applies a control voltage to VC1
via two 3.3kΩ series resistors and the
10kΩ isolating resistor.
The 2.2nF capacitor at the junction
of the two 3.3kΩ resistors provides
high-frequency filtering.
Additional filtering is provided by
the 100µF capacitor and 100Ω resistor
connected in series between Q1’s base
and collector. The 100Ω resistor allows
BANDPASS FILTER OPTION
We’ve designed the PC board so that it can accept a different bandpass
filter at the pin 11 RF output of IC1. This filter is made by Soshin Electronics
Co. and is labelled GFWB3. It is a small 3-terminal printed bandpass filter
and operates in the 76-108MHz frequency band.
The advantage of using this filter is that it has much steeper rolloff above
and below the FM band. This results in less sideband interference at other
frequencies. The drawback is that this filter is very difficult to obtain.
In practice, the filter replaces the 39pF capacitor, with the central earth
terminal of the filter connecting to the PC board earth. That is why there is a
hole between the 39pF capacitor leads. The 39pF and 3.3pF capacitors and
the 68nH and 680nH inductors are then not required, while the 68nH inductor
is replaced with a wire link.
22 Silicon Chip
the transistor to respond to transient
changes, while the 100µF capacitor
provides low-frequency filtering.
Further high-frequency filtering is
provided by the 47nF capacitor connected directly between Q1’s base and
collector.
The 5.1kΩ resistor connected to the
5V rail provides the collector load.
This resistor pulls Q1’s collector high
when the transistor is off.
FM output
The modulated RF output appears
at pin 11 and is fed to a passive LC
bandpass filter. Its job is to remove
any harmonics produced by the
modulation and in the RF oscillator
output. Basically, the filter passes
frequencies in the 88-108MHz band
but rolls off signal frequencies above
and below this.
The filter has a nominal impedance
of 75Ω and this matches both IC1’s pin
11 output and the following attenuator
circuit.
Two 39Ω series resistors and a 56Ω
shunt resistor form the attenuator and
this reduces the signal level into the
antenna. This attenuator is necessary
to ensure that the transmitter operates
at the legal allowable limit of 10µW.
Power supply
Power for the circuit is derived
from either a 9-16V DC plugpack or
a 6V battery.
In the case of a plugpack supply,
the power is fed in via on/off switch
S5 and diode D1 which provides
reverse polarity protection. ZD1 protects the circuit against high-voltage
tran
s ients, while regulator REG1
provides a steady +5V rail to power
the circuit.
Alternatively, for battery operation,
ZD1, D1 and REG1 are not used and the
through connections for D1 and REG1
are shorted. The absolute maximum
supply for IC1 is 7V, so 6V battery
operation is suitable; eg 4 x AAA cells
in a 4 x AAA holder.
Construction
A single PC board coded 06112021
and measuring just 78 x 50mm holds
all the parts for the Micromitter. This
is housed into a plastic case measuring
83 x 54 x 30mm.
First, check that the PC board fits
neatly into the case. The corners may
need to be shaped to fit over the corner
pillars on the box. That done, check
www.siliconchip.com.au
Fig.4: the complete circuit of the Stereo FM Micromitter. DIP switches S1-S4 set the RF oscillator frequency and this is
controlled by the PLL output at pin 7 of IC1. This output drives Q1 which in turn applies a control voltage to VC1 to
vary its capacitance. The composite audio output at pin 5 provides the frequency modulation.
that the holes for the DC socket and
RCA socket pins are the correct size.
If L1’s former doesn’t have a base (see
below), it is mounted by pushing it
into a hole that is just sufficiently tight
to hold it in place. Check that this hole
has the correct diameter.
Fig.5(a) & Fig.5(b) show how the
parts are mounted on the PC board.
The first job is to install several surface-mount components on the copper side of the PC board. These parts
include IC1, VC1 and two inductors.
You will need a fine-tipped solwww.siliconchip.com.au
dering iron, tweezers, a strong light
and a magnifying glass for this job. In
particular, the soldering iron tip will
have to be modified by filing it to a
narrow screwdriver shape.
IC1 and the varicap diode (VC1) are
polarised devices, so be sure to orient
them as shown on the overlay. Each
part is installed by holding it in place
with the tweezers and then soldering
one lead (or pin) first. That done,
check that the component is correctly
positioned before carefully soldering
the remaining lead(s).
In the case of the IC, it’s best to first
lightly tin the underside of each of
its pins before placing it onto the PC
board. It’s then just a matter of heating
each lead with the soldering iron tip
to solder it in place.
Be sure to use a strong light and a
magnifying glass for this work. This
will not only make the job easier but
will also allow you to check each
connection as it is made. In particular,
make sure that there are no shorts
between adjacent tracks or IC pins.
Finally, use your multimeter to
December 2002 23
Fig.5(a): this diagram
shows how the four
surface-mount parts
are installed on the
copper side of the
PC board. Make sure
that IC1 & VC1 are
correctly oriented.
Fig.6: here’s how to modify the
board for the battery-powered
version. It’s just a matter of
leaving out D1, ZD1 & REG1 and
installing a couple of wire links.
Fig.5(b): here’s how to install the parts on the top of the PC board to
build the plugpack-powered version. Note that IC1, VC1 and the 68nH
& 680nH inductors are surface mount devices and are mounted on the
copper side of the board as shown in Fig.5(a)
check that each pin is indeed connected to its respective track on the
PC board.
The remaining parts are all mounted
on the top side of the PC board in the
usual manner. If you are building the
plugpack-powered version, follow
the overlay diagram shown in Fig.5.
Alternatively, for the battery powered
version, leave out ZD1 and the DC
socket and replace D1 & REG1 with
wire links as shown in Fig.6.
Top assembly
Begin the top assembly by installing
the resistors and wire links. Table 3
shows the resistor colour codes but we
also recommend that you use a digital
multimeter to check the values. Note
that most of the resistors are mounted
end-on to save space.
Once the resistors are in, install PC
stakes at the antenna output and the
TP GND and TP1 test points. This will
make it much easier to connect to these
points later on.
Next, install trimpots VR1-VR3 and
the PC-mount RCA sockets. The DC
socket, diode D1 and ZD1 can then
be inserted for the plugpack-powered
version.
The capacitors can go in next, taking
24 Silicon Chip
care to install the electrolytic types
with the correct polarity. The NP (nonpolarised) or bipolar (BP) electrolytic
types can be installed either way. Push
them all the way down into their
mounting holes, so that they sit no
more than 13mm above the PC board
(this is to allow the lid to fit correctly
when the AAA batteries are mounted
under the PC board inside the box).
The ceramic capacitors can also be
Fig.7: this diagram shows the
winding details for coil L1. The
former will have to be trimmed
so that it sits no more than
13mm above the board surface.
Use silicone sealant to holder the
former in place, if necessary.
installed at this stage. Table 2 shows
their marking codes, to make it easy
for you to identify the values.
Winding coil L1
Fig.7 shows the winding details for
coil L1. It comprises 2.5 turns of 1mm
enamelled copper wire (ECW) wound
onto a tapped coil former fitted with
an F29 ferrite slug.
Two types of formers are available
– one with a 2-pin base (which can be
soldered directly to the PC board) and
one that comes without a base. If the
former has a base, it will first have to
be shortened by about 2mm, so that
its overall height (including the base)
is 13mm. This can be done using a
fine-toothed hacksaw.
That done, wind the coil, terminate
the ends directly on the pins and solder the coil into position. Note that the
turns are adjacent to each other (ie, the
coil is close wound).
Alternatively, if the former doesn’t
have a base, cut off the collar at one
end, then drill a hole in the PC board
at the L1 position so that the former is
a tight fit. That done, push the former
into its hole, then wind the coil so
that the lowest winding sits on the top
surface of board.
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Table 2: Capacitor Codes
Value
IEC Code EIA Code
47nF 47n 473
10nF 10n 103
2.2nF 2n2 222
330pF 330p 331
150pF 150p 151
39pF 39p 39
33pF 33p 33
27pF 27p 27
22pF 22p 22
10pF 10p 10
3.3pF 3p3 3.3
Be sure to strip away the insulation
from the wire ends before soldering
the leads to the PC board. A few dabs
of silicone sealant can then be used
to ensure that the coil former stays
in place.
Finally, the ferrite slug can be inserted into the former and screwed in
so that its top is about flush with the
top of the former. Use a suitable plastic
or brass alignment tool to screw in the
slug – an ordinary screwdriver may
crack the ferrite.
Crystal X1 can now be installed.
This is mounted by first bending its
leads by 90 degrees, so that it sits
horizontally across the two adjacent
10kΩ resistors (see photo). The board
assembly can now be completed by
installing the DIP switch, transistor
Q1, regulator (REG1) and the antenna
lead.
The antenna is simply a half-wave
dipole type. It consists of a 1.5m length
of insulated hookup wire, with one
end soldered to the antenna terminal.
This should give good results as far as
transmission range is concerned.
Preparing the case
Attention can now be turned to
680nH
IC1
68nH
VC1
It’s best to install the four surface-mount parts first (including the IC), before
installing the remaining parts on the top of the PC board. Note how the body of
the crystal lies across the two adjacent 10kΩ resistors (top photo).
the plastic case. This requires holes
at one end to accommodate the RCA
sockets, plus holes at the other end for
the antenna lead and the DC power
socket (if used).
In addition, a hole must be drilled
in the lid for the power switch.
It’s also necessary to remove the
internal side mouldings along the
walls of the case to a depth of 15mm
Table 3: Resistor Colour Codes
No.
1
8
1
2
1
1
2
www.siliconchip.com.au
Value
22kΩ
10kΩ
5.1kΩ
3.3kΩ
100Ω
56Ω
39Ω
4-Band Code (1%)
red red orange brown
brown black orange brown
green brown red brown
orange orange red brown
brown black brown brown
green blue black brown
orange white black brown
5-Band Code (1%)
red red black red brown
brown black black red brown
green brown black brown brown
orange orange black brown brown
brown black black black brown
green blue black gold brown
orange white black gold brown
December 2002 25
Above: the circuit can be powered from 4 x 1.5V AAA
cells if you wish to make the unit portable. Note that the
battery holder requires some modification in order to fit
everything inside the case (see text).
Left: this photo shows how the case is drilled to take the
RCA sockets, the power socket and the antenna lead.
below the top edge of the box, in order
to fit the PC board. We used a sharp
chisel to remove these but a small
grinder could be used instead. That
done, you also need to remove the end
ribs under the lid in order to clear the
tops of the RCA and DC sockets. The
front-panel label can then be attached
to the lid.
The battery-powered version has
a AAA cell-holder mounted upside
down in the box, with the base of the
holder in contact with the copper side
of the PC board. There is just sufficient
room for this holder and the PC board
to mount inside the case with the following provisos:
(1). All parts except for power
switch S5 must not protrude above the
surface of the PC board by more than
13mm. This means that the electrolytic
Fig.8: the full-size front-panel artwork.
26 Silicon Chip
capacitors must sit close to the PC
board and that L1’s former must be
cut to the correct length.
(2). The AAA cell holder is about
1mm too thick and should be filed
down at each end, so that the cells
protrude slightly over the top of the
holder.
(3). The tops of the RCA sockets
may also require shaving slightly, so
that there is no gap between the box
and the lid after assembly.
Test & adjustment
This part is a real snack. The first job
is to tune L1 so that the RF oscillator
operates over the correct range. To
do that, follow this the step-by-step
procedure:
(1). Set the transmission frequency
using the DIP switches, as shown in
Table 1. Note that you need to select
a frequency that is not used as a commercial station in your area, otherwise
interference will be a problem.
(2). Connect your multimeter’s common lead to TP GND and its positive
lead of to pin 8 of IC1. Select a DC
volts range on the meter, apply power
to the Micromitter and check that you
get a reading that’s close to 5V if you’re
using a DC plugpack.
Alternatively, the meter should
show the battery voltage if you’re using
AAA cells.
(3). Move the positive multimeter
lead to TP1 and adjust the slug in L1
for a reading of about 2V.
The oscillator is now correctly
tuned. No further adjustments to L1
should be required if you subsequently
switch to another frequency within the
Fig.9: full-size etching pattern for the PC board.
www.siliconchip.com.au
Silicon Chip
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$12.95
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80mm internal width
SILICON CHIP logo printed on
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PO Box 139
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Or fax (02) 9979 6503; or ring (02)
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The battery holder sits in the bottom of the case, beneath the PC board.
selected band. However, if you change
to a frequency that’s in the other band,
L1 will have to be readjusted for a
reading of 2V at TP1.
Setting the trimpots
All that remains now is to adjust
trimpots VR1-VR3 to set the signal
level and modulation depth. The stepby-step procedure is as follows:
(1). Set VR1, VR2 & VR3 to their
centre positions. VR1 and VR2 can
be adjusted by passing a screwdriver
through the centres of the RCA sockets,
ACA COMPLIANCE
This FM broadcast band stereo transmitter is required to comply with the
Radiocommunications Low Interference Potential Devices (LIPD) Class Licence 2000, as issued by the Australian Communications Authority.
In particular, the frequency of transmission must be within the 88-108MHz
band at a EIRP (Equivalent Isotropically Radiated Power) of 10µW and with
FM modulation no greater than 180kHz bandwidth. The transmission must
not be on the same frequency as a radio broadcasting station (or repeater
or translator station) operating within the licence area.
Further information can be found on the www.acma.gov.au web site.
The class licence information for LIPDs can be downloaded from:
www.aca.gov.au\legal\licence\class\lipd.rtf
www.siliconchip.com.au
while VR3 can be adjusted by moving
the 1µF capacitor in front of it to one
side.
(2). Tune a stereo FM tuner or radio
to the transmitter frequency. The FM
tuner and transmitter should initially
be placed about two metres apart.
(3). Connect a stereo signal source
(eg, a CD player) to the RCA socket
inputs and check that this is received
by the tuner or radio.
(4). Adjust VR3 anticlockwise until
the stereo indicator goes out on the
receiver, then adjust VR3 clockwise
from this position by 1/8th of a turn.
(5). Adjust VR1 and VR2 for best
sound from the tuner – you will have
to temporarily disconnect the signal
source to make each adjustment.
There should be sufficient signal to
“eliminate” any background noise
but without any noticeable distortion.
Note particularly that VR1 and VR2
must each be set to the same position,
to maintain the left and right channel
balance.
That’s it – your new Stereo FM MiSC
cromitter is ready for action.
December 2002 27
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