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Review By Allan Linton-Smith
TOUCHSCREEN
54MHz to 13.6GHz
Signal Generator
Here's an excellent example of the march of technology. It wasn't that long
ago that low-cost ADF4351-based signal generator modules became available. These could generate signal frequencies up to an amazing 4.4GHz.
But in a little under two years, the new ADF5355-based modules go up to a
whopping 13.6GHz. All this for around and $250, with touchscreen control!
T
hese modules need no modifications or additional circuitry; just
plug in a 5V DC supply or USB cable,
attach your cables and go!
In this review, we'll describe the
ADF5355-based unit and show you some
practical applications.
This device is based around the Analog
Devices ADF5355 Microwave Wideband
Synthesizer (with Integrated VCO). It's
84
Silicon Chip
paired with an SM32 colour touchscreen
for control.
This chip has many similarities to the
ADF4351 which was reviewed by Jim Rowe
in the May 2018 issue, starting on page 82
(siliconchip.com.au/Series/306).
However, the module described back
then (which cost about $30) was 'bare
bones' and had no user interface; you had
to build one. That isn't the case here, as
Australia’s electronics magazine
this unit comes fully assembled and ready to
use, as a complete (if basic) test instrument.
The ADF5355 module we obtained has
four outputs. The middle SMA connector
on the right-hand side delivers signals
from 54MHz to 6.8GHz, while the bottom
connector provides identical signals but
180° out of phase.
The top right-hand output (connected to
the spectrum analyser in the accompanysiliconchip.com.au
ing photo) has a signal which is multiplied
by two compared to the other two. The
output from this terminal can sweep from
13.4GHz to 13.6GHz in one millisecond, in
100kHz steps (as shown on the screen; remember, the frequencies from this output
are doubled). I find that amazing!
The ADF5355 achieves this by having
an integrated VCO with a fundamental output frequency of 3400-6800MHz. Its output goes to divide by 1, 2, 4, 8, 16, 32, or
64 circuits that allow the user to generate
RF output frequencies as low as 54MHz
(ie, 3.4GHz ÷ 64).
For applications that require isolation,
the RF output stage can be muted, with
the mute controllable via the touchscreen.
Note that you can also purchase similar
touchscreen-controlled modules with the
lower-spec ADF4351 chip that we mentioned above. These are available from
various internet sellers for around $78 including delivery, and are a very convenient option for those who don't need to go
above 4.4GHz and just want to plug in a
power supply and go!
Specifications (as per manufacturer's data sheet)
•
•
•
•
•
•
•
•
Frequency range: 53.125MHz to 6.8GHz (outputs A) and
106.25MHz to 13.6GHz (output B)
Accurate from -40°C to +85°C
Low phase noise: typically -103dBc/Hz (13.6GHz, 100kHz offset)
RF output power: 8dBm at 1GHz, -3dBm at 6.8GHz
Programmable output power level: +5dBm to -4dBm
Power supply: 5V DC
Internal VCO frequency range: 3.4GHz to 6.8GHz
Harmonic content: -22dBc (2nd harmonic), -20dBc (3rd harmonic)
of phase), it also multiplies the VCO frequencies by two, to create signals from
106MHz to 13.8GHz. These go to RFoutB;
see the red box in Fig.1, the block diagram
from the device's data sheet.
Outside this red box, the block diagram
is very similar to that of the ADF4351.
We described its operation in detail in the
aforementioned May 2018 article, so if
you want a more complete description of
its operation, please refer to that article.
Note that there is no RFoutB- output,
possibly because the IC is already overcrowded and the designers were seeking
to achieve the maximum frequency for the
minimum price.
Interestingly, the transistor count for
this IC has increased from 36,955
(ADF4351) to 103,665 plus 3,214 bipolar
semiconductors. That's 2.8x more transistors – so perhaps there is not much
spare room!
Block diagram
Internally, the ADF5355 is almost identical to the ADF4351, except that the VCO
(voltage-controlled oscillator) core operates at higher frequencies, from 53MHz
to 6.8GHz.
As well as dividing the VCO output frequencies by up to 64 and then sending
them to RFoutA+ and RFoutA- (180° out
FUNCTIONAL BLOCK DIAGRAM
REFIN A
REFIN B
10-BIT R
COUNTER
×2
DOUBLER
CLK
DATA
LE
AV DD
AV DD
CE
DVDD
VP
RSET
We paid $248 including postage for our
module – but bearing in mind that even
old benchtop GHz generators can cost
thousands, that's peanuts! We spotted a
20-year-old 20GHz Anritsu generator on
eBay for $16,000. This module is well and
truly affordable by comparison!
We connected the module's output to
a spectrum analyser to check it out (see
Fig.2). Sweeping over the 13.6-13.8GHz
range, the output level was reasonably
flat. We only measured an output level of
-5.12dBm (124mV into 50Ω), but that is
nevertheless very useful.
Note that the spectrum analyser trace
is set on "max hold" during the sweep, to
give a graph without dips. Resolution at
this frequency is 1MHz, so you won't see
the 100kHz troughs.
The output power from the manufacturer's data sheet, shown in Fig.3, indicates how the output level decreases as
VVCO
VRF
MULTIPLEXER
÷2
DIVIDER
MUXOUT
CREG 1
LOCK
DETECT
DATA REGISTER
FUNCTION
LATCH
CREG 2
CHARGE
PUMP
CPOUT
PHASE
COMPARATOR
VTUNE
VREF
INTEGER
REG
FRACTION
REG
VCO
CORE
MODULUS
REG
VBIAS
×2
VREGVCO
OUTPUT
STAGE
THIRD-ORDER
FRACTIONAL INTERPOLATOR
RFOUTB
PDBRF
÷ 1/2/4/8/
16/32/64
N COUNTER
OUTPUT
STAGE
CPGND
AGNDRF
SDGND
RFOUTA–
ADF5355
MULTIPLEXER
AGND
RFOUTA+
AGNDVCO
Fig.1: the ADF5355 block diagram, taken from its data sheet. Aside from the frequency doubler and extra output in
the red box, it is similar to that of the ADF4351 chip described in the May 2018 issue.
siliconchip.com.au
Australia’s electronics magazine
May 2020 85
lead and four standoffs for mounting. We
highly recommended that you mount the
module in a Jiffy box or similar, because all
the soldered connections are left bare on
the PCB, and these can easily short against
a metal object, which may spell D-O-O-M
for your $250 signal generator.
We were very impressed with the output of this little unit; it very nearly achieves
the levels specified in the Analog Devices
data sheet. At these high frequencies, even
a simple SMA to BNC adaptor can change
the signal characteristics, either through
power loss or standing waves. This may
result in a sweep which is not flat and also
create analytical errors when testing devices using various adaptors and connectors.
The ADF5355 module we obtained has four outputs, two which go from
53.125MHz to 6.8GHz and are 180° out of phase from each other, and one which
goes from 106.25MHz to 13.6GHz in 100kHz steps. All outputs can be swept
across a user-defined frequency range over a period of 1ms or more.
Usability
OUTPUT POWER (dBm)
The touchscreen is easy to use but it,
and the buttons, are quite small so big fat
fingers may upset the settings. The solution is to use some sort of pointer – a pencil will work. It then performs very smartly.
No written instructions accompanied our
module, but we were able to extract them
from a link provided by the seller. However, because they were originally written in
Chinese, the English is a bit “lost in Google translation”. While it is quite intuitive to
use, the learning curve is still a bit steep.
We will give a few hints later.
If you purchase one of these, press your
seller to include a printed manual, because
a lot of the feedback online is complaints
about the lack of a manual.
It did, however, come with a USB power
Fig.2: a spectral analysis of the module's output level when
sweeping from 13.4GHz to 13.6GHz. The result is commendably flat, although the overall level is a little low at -5.12dBm
(or 124mV into 50Ω
Ω). The analysis resolution is 1MHz, so you
won't see the 100kHz troughs caused by the stepped sweep.
(The test setup used to capture this is shown on page 84).
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Silicon Chip
The manufacturer suggests that this chip
could be used for wireless infrastructure,
microwave links, satellite comms, clock
generation, test equipment and instrumentation.
As an example of the latter, we used
it to test for cable losses, by connecting
the cables between the generator's output
and the input of a spectrum analyser, and
sweeping up to 2.5GHz.
Two three-metre cables were tested, one
made of standard coax with BNC connectors and the second, a low-loss 3mm cable with SMA connectors. The results are
shown in Fig.5.
You can see that the coax cable with BNC
connectors loses 12dB more signal than the
low-loss cable at 2.5GHz, and this demonstrates that very good cables are required
at high frequencies.
This module would also be convenient
10
9
8
7
6
5
4
3
2
1
0
–1
–2
–3
–4
–5
–6
–7
–8
–9
–10
–40°C
+25°C
+85°C
1
2
3
4
5
6
7
FREQUENCY (GHz)
Figure 19. Output Power vs. Frequency, RFOUTA+/RFOUTA−
Fig.3:
graph10pF
(from
theCapacitors,
manufacturer's
data
sheet)
(7.5nH a
Inductors,
Bypass
Board Losses
De-Embedded)
showing the chip's output power. Note how the
output level decreases as the frequency is increased.
Australia’s electronics magazine
siliconchip.com.au
12714-016
frequency increases.
In Fig.3, we've again connected the
module to a spectrum analyser (this time
a more modern device) and set it to sweep
from 100MHz to 6.5GHz. The markers show
the amplitude at a few different frequencies
over this range; again, the trace was set on
"maximum hold" to give a usable graph.
Our module was pretty well in line with
the specifications, but there are some additional losses at multi-GHz frequencies due
to PCB layout, adaptors, connectors and cables. At these dizzy frequencies, you have
to be careful to keep signal paths short!
Practical applications
Fig.5: one practical use for this module (among many)
is testing cable losses in combination with a spectrum
analyser that lacks a tracking generator. Here you can see
that low-loss coax (in blue) loses, err, less signal compared
to the box standard coax (mauve).
Fig.4: we set up the module to sweep from 100MHz to
6.5GHz and plotted the resulting output level on a slightly
more modern spectrum analyser. It was set on "maximum
hold" to give a better result. These results are not far off
what Analog Devices specifies for the chip, although some
small losses in the board are apparent.
for testing frequency counters and similar
devices. Naturally, there are many other
ways to use this module, and we leave that
up to your imagination!
The future!
If progress continues at this pace, in two
years we should have an affordable 50GHz
signal generator IC. Already as we write
this, there is a Texas Instruments signal
generator IC available which has a higher
output power and better stability than the
ADF5355.
It is the LMX2594 15GHz Wideband PLLatinum™ RF Synthesizer with phase synchronisation and JESD204B support.
No doubt there will be bigger and better to come!
SC
Some useful tips
• Our supplier advised us
not to remove
the screen cover, as the
screen surface is easily scratched.
• Mount the module in a
box of some
sort to protect it.
• Always stop the sweep
before changing any values.
• Always set the start freque
ncy before
the stop frequency.
• After using the keyboard,
press Enter,
then Back, and keep pre
ssing Back
until the keyboard disapp
ears.
• You cannot set the resolu
tion to less
than 100kHz.
• You can increase the sw
eep time, but
the minimum is 1ms.
The ADF5355-based touchscreen module at right, and the cheaper ADF4351-based touchscreen module at left. You can
see that the module at right has more output connectors and a slightly different user interface.
siliconchip.com.au
Australia’s electronics magazine
May 2020 87
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