Using Cheap Asian Electronic Modules
By Jim Rowe
Geekcreit’s
35MHz-4.4GHz
Signal Generator
This self-contained module is based on the Analog Devices ADF4351
wideband digital synthesiser chip. It has an onboard microcontroller unit
(MCU), OLED display and pushbuttons to set the desired frequency and
adjust the output level. All it needs is a 5V DC power supply.
I
f the ADF4351 sounds familiar,
that’s because it was also used in
the digitally-controlled oscillator
we reviewed (May 2019). But whereas
the earlier unit needed to be controlled via a separate microcontroller
such as an Arduino or a Micromite,
this one is a self-contained instrument, delivered ready to use.
It is larger than the earlier one, measuring 88 x 67mm compared to 48 x
36.5mm. But the price isn’t all that
much higher, currently setting you
back £30 including shipping to the
UK. It can be purchased from Banggood (https://bit.ly/pe-dec22-bg).
As shown in the photos, it comes
with two cables: a USB Type-A to
mini-B cable and a 240mm-long DC
cable with a plug on one end to match
the module’s DC input socket.
It also comes fitted with four
5mm-long nylon mounting spacers
and matching screws. But no case is
supplied, so you’ll either need to use
it as a ‘bare’ module, or come up with
your own arrangement.
On the PCB, there’s an STM32F103
MCU (lower left), a small OLED
(organic light-emitting diode) display
with a 128 x 64 pixel 25mm (1-inch)
Practical Electronics | January | 2023
diagonal screen, and seven pushbutton
switches. The five at lower right control the module, while the one in the
centre resets the MCU. The one near
the upper left with a square body and
blue actuator is the ON/OFF switch.
The ADF4351 synthesiser chip and
its surrounding components are all
in the upper right-hand corner of the
PCB. The two nearby edge-mounted
SMA sockets are the RF outputs, while
the vertical SMA socket near the centre
of the PCB is an input for an optional
external master clock, an alternative to
the onboard 100MHz crystal oscillator.
The ADF4351 chip at the heart of
the module is a digital ‘phase-locked
loop’ or PLL device, and a pretty
fancy one at that. But there isn’t space
here to give you a full explanation
of PLLs and how the ADF4351 itself
works. So if you want to know more
about these aspects, refer to the May
2019 article: 35MHz-4.4GHz digitally
controlled oscillator, which has a
comprehensive explanation.
A close-up of
the 1-inch OLED
screen when
using the ‘Point’
command from the
main screen.
27
Reproduced by arrangement with
SILICON CHIP magazine 2022.
www.siliconchip.com.au
Geekcreit 35MHz – 4.4GHz Signal
Generator Module
Fig.2: plot of the output level vs frequency when terminated by 50W.
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A brief rundown
The ADF4351 is a wideband digital
synthesiser IC with a ‘fractional-N’
PLL, allowing it to be programmed to
produce any desired output frequency
between 35MHz and 4.4GHz. It is
locked to a ‘master clock’ crystal oscillator of typically 25MHz or 100MHz.
It can be programmed to change the
output frequency in steps as small
as 10kHz, and can also provide an
output sweeping over a range of frequencies in steps of the same minimum size.
The whole chip is controlled/
programmed via a simple threewire serial peripheral interface
(SPI), in this case, via the onboard
STM32F103 MCU.
The data sheet for the ADF4351
can be found at: https://bit.ly/
pe-dec22-ad1
Lack of instructions
The Geekcreit 35-4400MHz signal generator module comes with very little
user information, so you have to work a
lot out for yourself. All you get is a brief
summary of its main specs and features,
Practical Electronics | January | 2023
Fig.1: the circuit diagram for the
Geekcreit signal generator module.
and you can download a circuit diagram that is not easy to decipher.
So before I began testing the module, I spent a couple of hours redrawing the circuit so that we can all see
how it works – see Fig.1.
Like the earlier module, this one
is fairly closely based on Analog
Devices’ evaluation board for the
ADF4351. That is described in their
User Guide UG-435, which you
can download from their website:
https://bit.ly/pe-dec22-ad2
How it works
In Fig.1, the ADF4351 (IC2) is on the
right, with its onboard 100MHz master clock oscillator to its left. These
form the actual VHF-UHF RF synthesiser ‘heart’ of the module. The two
complementary RF outputs emerge
from pins 12 and 13 of IC2, and are
fed via 1nF capacitors to the two SMA
output sockets at far right. The 3.3V
DC supply to pins 12 and 13 flows
via inductors L2 and L3.
Only the RF output from pin 12 of
IC2 (RFout+) has an onboard 51W terminating resistor.
The other components on the righthand side of Fig.1 are to provide IC2
with power, set its operating mode,
or feed it control signals. For example, the components between pins
7 and 20 at upper right form the
ADF4351’s low-pass loop feedback
Practical Electronics | January | 2023
filter (to optimise its performance),
while the capacitors at pins 19, 23
and 24 bypass key reference points
in its internal circuitry.
The digital control signals from
IC1 that direct IC2’s operation are
fed to pins 1, 2, 3 and 4 at centre left,
labelled CLK, DATA, LE and CE. The
only other signal that passes back
from IC2 is the LD (lock detect) signal
from pin 25, which is high when IC2
is locked to the requested frequency.
As well as being fed back to the
MCU, this signal is also used to illuminate LED2, the blue lock indicator.
The power supply section is at
upper left in Fig.1. This accepts
either 5V DC from mini USB socket
CON2, or 5-15V DC from concentric DC socket CON1. This flows via
on-off switch S7 to power indicator
LED1 and the rest of the circuit. The
incoming supply powers REG1 and
REG2, both of which are LT1763 LDO
(low drop-out) 3.3V linear regulators.
REG1 provides 3.3V to the control
circuitry, while REG2 generates a
separate 3.3V supply for synthesiser
IC2. The incoming supply to REG2
is via T1-T2, a balanced decoupling
transformer wound on a small ferrite
balun core.
As mentioned earlier, the control
circuitry is based around IC1, an
STM32F103C8T6 microcontroller,
and the 128 x 64-pixel OLED display
On starting the module, the OLED display lists the five available functions.
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Fig.3: a graph of the
module’s output at
2.5GHz with a 60MHz
span provides a
reasonably clean plot
with just two small
spurs at the edges.
Fig.4: the span is now
set over the range
50MHz to 4.4GHz with
the same 2.5GHz output.
Note the additional
spurs approximately
600MHz apart.
Fig.5: setting the output
frequency higher to
3GHz also provides a
clean plot.
Fig.6: the output
frequency now set
to 4GHz.
Fig.7: setting the module
to an output frequency
of 100MHz produces
a large number of
spurs at the harmonic
frequencies (ie,
multiples of 100MHz)
with varying amplitude.
module to its right. The four digital
signals to control synthesiser IC2 connect to pins 25-28 of IC1, while the LD
signal from IC2 is fed back to pin 29.
Pushbuttons S1-S5 at lower left
select the operating mode of the synthesiser, its operating frequency, output level and so on. The MCU provides
a series of menus and indications on
the OLED display to make this reasonably straightforward. The OLED display is driven via an SPI serial control
link from pins 14-17 of IC1.
The instruction and master clock
for IC1 is generated by an internal
oscillator using 8MHz crystal X2,
connected to pins 5 and 6.
Pushbutton S6 manually resets IC1
if necessary. The D– and D+ data lines
from mini USB socket CON2 are connected to pins 32 and 33 of IC1, so
its firmware can be updated from a
PC if needed.
It’s also possible to communicate
with IC1 via a second serial link connected to pins 34 and 37, brought
out to the pins of CON5. This is not
a physical connector, but provision
on the module’s PCB for fitting a fourpin SIL header.
Trying it out
When I received the unit and tried
powering it up, there were a couple
of problems. The first of these was
that the DC supply cable provided
with it turned out to have an open
circuit in its red (positive) lead. So
I had to discard it and substitute a
known good cable.
Then when I powered it up, I found
that the module was on regardless
of whether power switch S7 was
pressed or not. The cause turned
out to be a solder bridge under the
PCB joining its two active pins permanently. Luckily, I fixed that easily
with a soldering iron.
I also tried powering the unit from
a 5V USB plug pack, using the USB
Type A-to-mini Type B cable provided, which worked fine.
When the module is first powered
up, the OLED screen shows its function menu, or more accurately, the top
of it – listing the first three functions:
1. Point: used to set the module’s frequency to a particular figure, for
example 4375.05MHz
2. Sweep: used to set the start and
stop frequencies for sweeping over
a range
3. Step Fre: not clearly explained,
but seems to be used to set the frequency steps used during sweeping
Then if you continue pressing the
down (DWN) button, S5, you find the
remaining two options:
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Practical Electronics | January | 2023
4. Step Time: not clearly explained, but it appears to
be for setting the time between steps when sweeping
5. dB Set: see below
When you press the OK button (S4) to select this last
option, you get a screen giving a choice of four RF Power
settings: +5dB, +2dB, –1dB or –4dB. These appear to be
provided to allow ‘fine adjustment’ of the module’s RF
output level.
When I tried checking these output level options with
the module’s frequency set to 1GHz using my Agilent
V3500A power meter, I obtained the following results:
• With the +5dB setting, the meter registered +3.60dBm
• With the +2dB setting, it registered –0.24dBm
• With the –1dB setting, it registered –1.99dBm
• With the –4dB setting, it registered –4.52dBm
These were all measured with the meter connected to
the RFout+ connector, using a very short (<20mm) SMASMA coupler. So the reference (0dB) level appears to be
around –1dBm, and while these settings are not particularly accurate, they give you the ability to adjust the
unit’s output level somewhat.
The next step was to measure its RF output over the
whole frequency range, again using the V3500A power
meter. I did these measurements again using the SMASMA coupler, connected between the power meter and
first the module’s RFout+ connector, and then the RFoutconnector. In each case, the output not being measured
was terminated in 50W, to hopefully prevent any standing wave disturbances.
As you can see from Fig.2, the level from the RFout+
connector is about 4dBm lower than that from the RFoutconnector, probably due to the loading from the onboard
51W terminating resistor across RFout+. But apart from
that, both plots are relatively flat, rising slowly by about
2-3dB between 40MHz and 1GHz, and then wobbling a
bit to return very close to the 1GHz level at 4.4GHz.
So overall, both outputs were within the range of –4dBm to
+4dBm over the entire frequency range. Next, I checked the
module’s RF output signal purity at several different frequencies, using my Signal Hound USB-SA44B spectrum analyser
with the latest version of Signal Hound’s ‘Spike’ software.
The results were reasonably acceptable, bearing in mind
that the module’s outputs are essentially square waves
with significant harmonic content, along with the inevitable spurs you tend to get from any PLL-type synthesiser.
To illustrate this, Fig.3 shows the module’s output at
2.5GHz, with the analyser set for a 60MHz span (ie, 30MHz
either side of 2.5GHz). The main output is a reasonably
clean peak reaching about +1.5dBm in the centre, with
two small spurs at about –55dBm around 25MHz either
side. So far, it looks reasonably clean.
But now look at Fig.4, which shows what the analyser
displays when set to span over the total frequency range
from 50MHz to 4.4GHz, with the module still set to 2.5GHz.
Several additional spurs are visible, spaced at about
620MHz apart on either side of the main output, with
amplitudes varying between about –6dBm and –18dBm.
So the output is not nearly so clean as Fig.3 suggests.
The full-scan plots don’t look so bad with the module set to higher frequencies, though. For example, Fig.5
shows the result when the frequency is set to 3.0GHz,
while Fig.6 shows a similarly clean plot when it is set
to 4.0GHz.
On the other hand, Fig.7 shows the result with a full
scan showing what happens when the module is set to
produce a 100MHz signal. There’s now a virtual ‘forest’
of spurs, varying in amplitude from –10dBm down to
about –49dBm in alternating steps. Not a pretty picture!
Because of the lack of information regarding how to
get the module sweeping or stepping from one frequency
to another, I gave up trying to test those functions.
Summary
So although the Geekcreit 35MHz-4.4GHz signal generator module is a low-cost, self-contained unit that can
generate output signals of around 0dBm (1mW) over
that wide frequency range, it does have a few drawbacks and limitations.
One of these is the lack of much information on operating the module, especially with regard to getting it to
perform sweeping. Another is the large number of ‘spur’
components in the outputs, especially when it’s generating a frequency below about 1GHz.
That is because its outputs are essentially square
waves, rather than the sinewaves that are needed for
many signal generator applications. Filtering these to
produce a smoother signal is virtually impossible due
to the wide range of possible output frequencies; however, external filters could be used if you need cleaner
signals at specific frequencies.
And finally, because of its lack of any shielding
(especially for the RF generation circuitry around the
ADF4351), it would be tough to achieve accurate control over its output level.
But overall, the module would still be useful, for
example, if you want to generate digital clock signals
over a very wide range of frequencies. Just bear in mind
that to use it as the basis of a practical VHF/UHF signal
generator, you’d have to add shielding, output filtering
and a wide-range output attenuator system.
Terrington
Components
• Project boxes designed and manufactured in the UK.
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• Unique designs and sizes, including square, long and deep
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IoT devices.
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www.terrington-components.co.uk | sales<at>terrington-components.co.uk | Tel: 01553 636999
Practical Electronics | January | 2023
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