Fig.5: when connecting the ADF4351 module to a Micromite no extra
components are needed, unlike with an Arduino. However, the newer
version of the module requires a 10kΩ resistor between CE and +3.3V.Note that there is also provision for
feeding in a different reference signal,
via the SMA socket labelled MCLK. In
order to do this, you’d need to remove
the 0Ω resistor connected to pin 3 of
the onboard XO. If you are using the
onboard XO, the MCLK socket can be
used to monitor its output via a scope
or frequency counter.
The resistors and capacitors con-
necting the CPOUT and SW pins of IC1
(pins 7 and 5) to the VTUNE pin (pin 20)
form the low-pass loop feedback filter.
The RF output signals from the
RFOA+ and RFOA– pins (12 and 13)
are taken to the RFOUT+ and RFOUT–
SMA sockets via matching/filtering
circuits using L2, L3, L5 and L6, plus
two 1nF capacitors.
Notice that the output pins are fed
with the +3.3V supply voltage via L2
and L3. In this module, the auxiliary
RF outputs RFOB+ and RFOB– (pins 14
and 15) are not wired up.
The whole module operates from a
3.3V supply, derived from the 5V in-
put at CON2 via REG1, an ASM1117
low-dropout regulator. This AVDD rail
powers all of the analogue/RF cir-
cuitry directly.
The digital supply rail, DVDD, is de-
rived from AVDD using LC filters com-
prising inductors L4 and L1, plus a
number of bypass capacitors.
There are two indicator LEDs. LED1
is connected between the DVDD line
and ground and indicates when the
module has power, while LED2 is con-
nected to the LD (lock detect) pin of
IC1 (pin 25) and indicates when the
PLL is in lock.
All the other components are for by-
passing and stability, apart from fuse F1
and diode D1, which prevent damage
in the event that the 5V power source
is connected with reversed polarity.Controlling it with an Arduino
I initially hooked the module up to an
Arduino Uno using the simple circuit
shown in Fig.4. The three main control
lines MOSI, SCK and LE are not taken
directly to the DAT, CLK and LE pins of
the module, but instead via 1.5kΩ/3kΩ
voltage dividers.
This is because the inputs of the
ADF4351 can only cope with 3.3V
signals, whereas the Arduino outputs
have a 5V output swing.
The LD signal fed back from the
module to the Arduino’s D2 pin does
not need a divider because it’s going
the other way and the Arduino inputs
function well with a signal having a
swing of 3.3V.
Note also that Fig.4 indicates
that the 5V supply for the module
can come from either a plugpack or
from the 5V output of the Arduino. I
adapted an Arduino sketch I found on
the internet, written by French radio
amateur Alain Fort F1CJN (http://bit.
ly/pe-may19-af).
Mr Fort’s sketch was written for an
Arduino with an LCD button shield,
but I decided to adapt it so that it
would work with the simple configu-
ration shown in Fig.4, relying on the
Arduino IDE’s Serial Monitor to send
commands to the ADF4351 and to in-
dicate the PLL’s output frequency and
whether it was locked or not.
I also connected one of the PLL mod-
ule’s RF outputs to my frequency coun-
ter, via a prescaler, so I could monitor it.
The results were quite impressive. I
could type in any frequency between
35MHz and 4.4GHz, with a resolution
of 0.01MHz (10kHz) and the module’s
output would lock to that frequency in
the blink of an eye.
I also monitored the current drawn
by the module and found that it variedbetween 110mA and 145mA, depend-
ing on the output frequency.
I also checked the accuracy of the
module’s 25MHz on-board reference
XO and found it to be 25.0000734MHz,
or only 73.4Hz high. Since this equates
to an error of just +2.936ppm, it seems
quite accurate.
So that’s one easy way to get the
ADF4351 module going with an
Arduino. The sketch (ADF4351_and_
Arduino_SC_version.ino) is available
for download from the Practical Elec-
tronics website.Driving it from a Micromite
I also hooked the module up to a Mi-
cromite LCD BackPack combination
and wrote some code so that it could
be controlled via the LCD touchscreen.
The circuit is shown in Fig.5 and it’s
about as simple as you can get. In this
case, no resistive dividers are needed
on the SCK, MOSI and LE lines be-
cause the Micromite’s logic pins have
a swing of 3.3V.
I used a ‘software’ SPI port rather
than the hardware one used by the
Micromite to communicate with the
LCD and touchscreen, to prevent pos-
sible interaction.
The embedded C code (CFUNC-
TION) needed to provide this added
port is included in the MMBasic pro-
gram I wrote for this approach. Soft-
ware SPI port performance is limited,
but that isn’t a problem as the amount
of data to transfer is small.
A USB charger was used to supply
5V to the ADF4351 module because its
current drain is a little too high for the
BackPack to provide.
The software uses just two screens,
as shown below. The initial screen (at
left) displays the current frequency
and gives you the option of touching
the button at the bottom if you want
to change it.
You will then get the second screen,
which allows you to key in a new fre-
quency, which is displayed below the
current frequency.