- Very compact
- Battery or plugpack operation
- Stereo transmission
- Standard FM tuner required to receive transmission
- Crystal locked operation
- 14 selectable transmission frequencies
This 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 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
broadcasts 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 106.7-107.9MHz in
No tuning coils
Fig.1: block diagram of the Rohm BH1417F
stereo FM transmitter IC. The text explains how it works.
We first published an FM stereo transmitter in SILICON CHIP
in October 1988 and followed this up with a new version in April
2001. Dubbed the Minimitter, these earlier versions were based
on the popular Rohm BA1404 IC which is not being produced any
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
After that, it's just a matter of adjusting a single coil
when setting up the transmitter, to set for correct RF operation.
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
Fig.2: this frequency versus output
level plot shows the composite level (pin 5). The 50ms pre-emphasis
at around 3kHz causes the rise in response, while the 15kHz
low pass roll off produces the fall in response above 10kHz.
At the heart of the new design is the BH1417F FM stereo transmitter
IC made by the Rhom Corporation. As already mentioned, it replaces
the now hard to find BA1404 that has been used in the previous
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
As shown, the BH1417F includes two separate audio processing
sections, for the left and right channels. The left-channel
audio signal is applied 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 50ms 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 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.
Fig.3: the frequency spectrum of the
composite stereo FM signal. Note the spike of the pilot
tone at 19kHz.
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 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
(L-R) 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 D0-D3 (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).
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.
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
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.
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.
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
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 the filter is very difficult
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.
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
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
The left and right audio input signals are fed in via 1μF
bipolar capacitors and then applied to attenuator circuits
consisting of 10kΩ fixed 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 low-pass
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.
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)
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 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
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.
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
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 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.
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 56W 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.
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.
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
transients, 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.
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 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 soldering 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.
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
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 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
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
The capacitors can go in next, taking care to install the
electrolytic types with the correct polarity. The NP (non-polarised)
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 installed at this stage.
Table 2 shows their marking codes, to make it easy for you
to identify the values.
Fig.7 shows the winding details for coil L1. It comprises
2.5 turns of 0.5 - 1mm enamelled copper wire (ECW) wound onto
a tapped coil former fitted with an F29 ferrite slug. Alternatively,
you may also use any commercially made 2.5 turns variable
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
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).
This photo shows how the case is drilled
to take the RCA sockets, the power socket and the antenna
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.
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
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
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 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
In addition, a hole must be drilled in the lid for the power
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).
It's also necessary to remove the internal side mouldings
along the walls of the case to a depth of 15mm 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 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.
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 10mW 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.aca.gov.au
The class licence information for LIPDs can be downloaded
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
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 battery holder sits in the bottom
of the case, beneath the PC board.
The oscillator is now correctly tuned.
No further adjustments to L1 should be required if you subsequently
switch to another frequency within the 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
Fig.8: the full-size front-panel artwork.
All that remains now is to adjust trimpots VR1-VR3 to set the
signal level and modulation depth. The step-by-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 μ sockets, while VR3 can be adjusted
by moving the μF 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.
Fig.9: full-size etching pattern for
the PC board.
(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 Micromitter is ready for action.
2: Capacitor Codes
3: Resistor Colour Codes
||4-Band Code (1%)
||5-Band Code (1%)
||red red orange brown
||red red black red brown
|| brown black orange brown
|| brown black black red brown
|| green brown red brown
|| green brown black brown brown
|| orange orange red brown
|| orange orange black brown brown
|| brown black brown brown
|| brown black black black brown
|| green blue black brown
|| green blue black gold brown
|| orange white black brown
|| orange white black gold brown
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 or 7.68MHz crystal
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 2.5 turns variable coil (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 (VR1-VR3)
1 BH1417F Rohm surface-mount FM stereo transmitter
1 78L05 low-power regulator (REG1)
1 MPSA13 Darlington transistor (Q1)
1 ZMV833ATA or MV2109 (VC1)
1 24V 1W zener diode (ZD1)
1 1N914, 1N4148 diode (D1)
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
||87.7MHz to 88.9MHz in 0.2MHz steps
106.7MHz to 107.9MHz in 0.2MHz steps (14 total)
|Total Harmonic Distortion (THD)
|Low Pass Filter
||within ± 2dB (can be adjusted with trimpots)
|RF Output power (EIRP)
||typically 10μW when using inbuilt attenuator
|| 28mA at 5V
|Audio input level
||220mV RMS maximum at 400Hz and 1dB compression