Using Cheap Asian Electronic Modules
By Jim Rowe
Geekcreit’s LTDZ V5.0
Spectrum Analyser
This compact unit is low in cost but can perform spectral analysis from
35MHz to 4.4GHz. It also includes a tracking generator for frequencydomain analysis of filters, RF amplifiers and similar items. It needs to be
controlled from a PC via a USB cable (which also provides its 5V DC power
supply), using a very impressive free application.
A
bout a year ago, I bought
an earlier version of the Geekcreit LTDZ spectrum analyser,
which came as a ‘naked PCB’ module.
The idea was to check it out and write
a review, but I wasn’t too impressed
when I tried it out.
The software needed to control it was
both difficult to find and rather flaky,
and the unit itself had poor sensitivity
combined with a relatively high noise
floor. There wasn’t much I could say
about it that was positive, so I decided
to give it a pass.
But earlier this year, I found that an
improved version of the analyser had
become available (the LTDZ V5.0), coming inside an extruded aluminium case
and not costing all that much more than
the original ‘naked’ version.
I also discovered that although Geekcreit was still recommending the same
control software that I had found so
problematic, a much better program
had appeared – one that you can download for free.
It’s called VMA Simple Spectrum
Analyser (VMA SSA), written by Vitor
Martins Augusto, who lives in Portugal,
and it can be downloaded from his site:
https://bit.ly/pe-feb23-vma
So I went ahead and ordered an
LTDZ V5.0 from the Banggood website (https://bit.ly/pe-feb23-ltdz), paying
40
£48 (free one-week shipping to UK). I
also downloaded Mr Augusto’s VMA
SSA software.
As you can see from the photos, the
LTDZ V5.0 is quite compact at 62 x 55
x 19mm, not counting the two SMA
connectors extending from the input/
output end.
It also weighs only 83 grams. It comes
complete with a 950mm-long USB2.0
cable, with a Type-A plug at one end
and a micro Type-B connector at the
other end, to connect it to a PC.
The LTDZ V5.0 is quite well made,
although the panels at each end of the
case in the unit I received had holes
for the countersink-head mounting
screws which were not countersunk.
This made it look unfinished until I
removed the panels and countersunk
their holes to complete the job.
This also gave me the opportunity
to examine the PCB inside and take
its photo. All of the components in
the LTDZ V5.0 are mounted directly
on this PCB.
Like the Geekcreit VHF-UHF signal generator module I reviewed last
month (PE, January 2023), the LTDZ
V5.0 uses the Analog Devices ADF4351
digital PLL synthesiser chip. In fact, it
uses two of them: one in the analyser
section, and one in the tracking generator (TG) section.
The ADF4351 is quite a complex
device, but we had a pretty detailed
description of how it works in the
May 2019 issue of PE, specifically my
review of the 35MHz-4.4GHz Digitally
Controlled Oscillator module.
So please read that article if you
want to know more about how this chip
works. You can also find the data sheet
for it at: https://bit.ly/pe-dec22-ad1
By the way, the LTDZ draws about
100mA from the PC in standby mode,
rising to roughly 350mA when it’s
scanning with the tracking generator
also running.
How the analyser works
I have prepared a block diagram
(Fig.1) that shows how the LTDZ 5.0
works. The ADF4351 chip at the bottom of this diagram forms the heart
of the analyser section, while the one
at upper right provides the tracking
generator function.
The STM32F103 MCU (microcontroller) handles the operation of both
sections, directed by the software
running in the PC. The two USB signal lines (D– & D+) from the LTDZ’s
micro-USB connector at upper left pass
through a CH340G USART chip before
reaching the MCU. The micro has an
8MHz clock crystal, while the CH340G
has a 12MHz crystal.
Practical Electronics | February | 2023
Fig.1: block diagram of the
LTDZ 5.0 module. The most
important sections are the two
ADF4351 synthesisers and the
STM32 ARM microcontroller.
(Block Diagram)
Geekcreit LTDZ Spectrum Analyser
Both ADF4351 synthesiser chips are
supplied with their master reference
clock from the 25MHz crystal oscillator at centre right. But they are controlled by the MCU via two separate
SPI (serial program interface) ports. The
analyser ADF4351 is controlled via the
MCU’s SPI1 port (SPI_SCK, SPI_MOSI
and SPI_NSS), while the tracking generator ADF4351 is controlled via the
SPI2 port.
The spectrum analyser section of the
LTDZ involves the devices and signal
paths shown at lower left in Fig.1.
This spectrum analyser operates
similarly to a ‘superheterodyne’
radio receiver, where incoming signals at a relatively high frequency are
shifted down to a much lower fixed
IF (intermediate frequency) before
being detected.
In this case, the ADF4351 at lower
centre corresponds to the local oscillator (LO). Its output is fed to one input
of the IAM-81008 double-balanced
mixer while the Analyser’s input signal goes to the other input. So its output will be the heterodyne products of
the two signals.
The mixer’s output signal then goes
through a low-pass filter to remove any
‘sum’ heterodyne components, leaving
only the difference, which is the IF signal we want.
This is then fed to an AD8307 logarithmic amplifier and detector, which
generates a DC output voltage proportional to the IF signal level. This, in
turn, goes to an analogue-to-digital
input (ADC123) of the MCU.
As a result of all this, the MCU can
measure the input signal level corresponding to the current frequency of
the ADF4351’ local oscillator’.
As the MCU changes the LO frequency over the selected range, it can
send measurements of the input signal
Practical Electronics | February | 2023
level at each point back to the software running in the PC. The software
can then take these measurements and
present them as a graph, plotted against
frequency. That’s how this type of spectrum analyser works.
This is the same basic system used in
many spectrum analysers (while some
instead use very fast sampling and a
digital Fourier transform). But in place
of the simple low-pass filter between
the mixer and the log detector, highend models have several selectable
bandpass filters which offer a choice of
resolution bandwidth (RBW) settings.
Most higher-end units also have a
wideband amplifier between the RF
input connector and the mixer’s input,
increasing the analyser’s input sensitivity. This is so that they can analyse
lower level signals, like those from
many antennas.
The tracking generator is really just
the second ADF4351 chip, which the
MCU can program to provide an output signal of the same frequency that is
currently being sensed by the analyser
section, at a relatively constant level of
approximately 0dBm (224mV).
The tracking generator can be
switched on or off using pushbutton
switch S1, so it can be turned on only
when needed.
There are four indicator LEDs shown
in Fig.1. LED1 indicates when the tracking generator is enabled, LED3 when
the LTDZ has power applied, LED4
when the analyser section is working,
and LED2 when both ADF4351s are
locked to the designated frequency.
The VMA SSA application
As mentioned earlier, Mr Augusto’s
VMA SSA software can be downloaded
for free (https://bit.ly/pe-feb23-vma).
You can also download a 54-page PDF
User Guide from the same page.
However, after downloading and
installing the app, you have to contact
him by email to obtain an activation
code before you can run it. This activation code will only function for up
to three months, after which you will
have to request another code. Or, if
you wish, you can make a small donation via PayPal of around US$10, after
which you will be sent a ‘permanent’
activation code.
The internals of the Geekcreit LTDZ spectrum analyser.
41
but when you unzip it, you will get
the main EXE file plus several auxiliary files.
All you have to do is copy it to a
suitable folder and then launch the
executable. But don’t install it to ‘C:\
Program Files’ or ‘C:\Program Files
(X86)’ because Windows 10 limits
access to files in those folders, which
can cause problems.
Screen 1: the VMA SSA software output when the LTDZ input is terminated
with a 50W resistor over its frequency range of 35-4400MHz.
Screen 2: the LTDZ input was now connected to an external VHF/UHF discone
antenna with a plot over 200-208MHz. The average signal level was –49dBm
over that range.
Screen 3: a Gratten GA1484B VHF-UHF signal generator was used to provide
the LTDZ with an unmodulated 2.5GHz output at 0dBm. The software was then
set to scan over 2.4-2.6GHz.
After using VMA SSA for a short
time, I was so impressed that I sent Mr
Augusto a donation of $25 and received
a permanent activation code. There is
no doubt in my mind that it’s massively
42
better and much easier to use than the
NWT4.11.09 software that Geekcreit
still recommends.
Incidentally, the file you download
from Mr Augusto’s site is zipped,
Trying it out
All I had to do initially was plug the
LTDZ into my computer using the supplied cable and launch the VMA SSA
software. Next, I clicked on its Setup
menu, to tell it the virtual COM port
number which the LTDZ has been
assigned (in my case, COM3) and the
particular Analyser model.
The VMA SSA application can work
with five different units, with the LTDZ
V5.0 listed as ‘SMA Simple Spectrum
Analyser Version 2 – 35MHz-4.4GHz
– ADF4351’.
You then need to select the ‘Spectrum’ option at the top left of the
screen. This gives you the main screen
for spectrum analysis, as shown in the
screen grabs.
Most of the screen is occupied by the
centre plotting graticule, with a narrower graticule below it that can show
a ‘waterfall’ display (although the two
can be swapped, if you wish). On the
right are most of the control setting
controls, with a large START/STOP
button at the top.
Click on any of the small Frequency
setting boxes on the right opens a ‘keyboard’ dialog box that makes it easy
to enter a new frequency. This also
applies if you click on any of the other
small boxes; for example, the ‘Samples’
box, the ‘Wait (us)’ box or the ‘Marker1’
or ‘Marker2’ boxes.
Screen 1 shows what was displayed
when I fitted a 50W termination to the
LTDZ input, set VMA SSA for the full
span of 35-4400MHz and clicked the
START button. This is the ‘noise floor’
of the LTDZ, which I found to be almost
constant at –76.9dBm over the whole
frequency range.
Screen 2 shows what was displayed
when I connected the input of the
LTDZ to an external VHF/UHF discone
antenna, and set the VMA SSA software to scan from 200MHz to 208MHz
(the frequency range used by Sydney’s
DAB+ transponders). The full range of
transponder signals is shown, with an
average level of about –49dBm. Note
those five sharp ‘notches’ though; more
about this shortly.
The next step was to power up my
Gratten GA1484B VHF-UHF signal generator and set it to produce an unmodulated output of 2500MHz (2.5GHz) at
Practical Electronics | February | 2023
0dBm. I then connected its output to the
LTDZ input via a 2m-long SMA-SMA
cable, and set the VMA SSA software
to scan from 2400MHz to 2600MHz (a
span of 200MHz).
This resulted in the display shown in
Screen 3, where you can see the main
signal spike at 2500.00MHz accompanied by a pair of smaller spikes (about
–66dBm) about 25MHz on either side.
There are also a couple of much smaller
spikes of –73/-74dBm, about 75MHz
on either side.
I’m sure those extra spikes are not
coming from my signal generator,
because they don’t show up when
I check it with my Signal Hound
USB-SA44 spectrum analyser.
They are probably the result of the
LTDZ’s fixed and relatively wideband
RBW. The other thing to note about
this display is that the amplitude of
the main signal in the centre is about
–13dBm, quite a bit lower than the generator’s 0dBm output.
This is considerably lower than
you’d expect, even allowing for losses
in the 2m long SMA-SMA cable (about
2.5-3.0dB).
Notch artefact
The next step was to leave the signal
generator set to 2500MHz with 0dBm
output and connected to the LTDZ
input, but to change the VMA SSA
app’s frequency settings to give a much
smaller spectrum span of 10MHz (ie,
5MHz either side of 2.5GHz). This gave
the display shown in Screen 4.
The spike at 2500MHz has now
expanded into a pair of ‘twin peaks’,
with a fairly deep notch between them.
The twin peaks reach an amplitude of
about –2.5dBm, much closer to the correct value. But the notch in the centre
reaches down to about –31dBm, which
is a bit disconcerting.
It turns out that this kind of notch is
basically due to the fixed and relatively
wide RBW of the LTDZ and similar
low-cost analysers. As Vitor Augusto
explains in his blog post dated 13 October 2017 (https://bit.ly/pe-feb23-vma2),
the fixed and wide RBW causes them to
have a ‘blind spot’ in the centre of their
‘scanning slot’ as the Analyser moves
the input signals past it.
It’s this blind spot that causes a notch
in the centre of signals with a narrow
bandwidth. That’s why professional
(and much higher-cost) spectrum analysers give you a choice of RBW settings,
as low as 10kHz
Mr Augusto has included a notch
function into his VMA SSA app,
which, when selected, can fill in this
kind of notch by replacing it with a
straight line between the twin peaks.
But this is just a cosmetic workaround,
Practical Electronics | February | 2023
The ‘front’ of the LTDZ module houses the SMA sockets for the RF input
and output connections. There are two status LEDs which show the current
operating mode.
The ‘rear’ of the module houses a micro Type-B USB socket for connecting to a
computer, plus two more status LEDs to indicate STM32 operation and power,
and a pushbutton labelled ‘KEY’ which controls the tracking generator.
as he admits; crunching the scanning
data to truly remove the notching
would be pretty complicated.
In another post dated 4 February
2022 year (https://bit.ly/pe-feb23vma3), Mr Augusto announced that
a colleague of his named Dominico
had put much work into improving
the performance of LTDZ analysers.
This is both in terms of improving the
hardware (presumably concentrated
around the low-pass filter) and revising
the firmware in the STM32F108 MCU.
In his February post, Mr Augusto
provided a link to a beta version of
Dominico’s revised firmware. However,
he didn’t give any details of Dominico’s
changes to the LTDZ’s hardware.
More details on the current product
Getting back to my review of the product as it stands today, I decided to try
using the LTDZ’s tracking generator to
perform a couple of spectrum scans of
circuitry connected between the tracking generator output and the Spectrum
Analyser input.
The first item I scanned was a FlightAware ADSB bandpass filter. This was
connected via a 150mm-long SMASMA cable. Then after pressing the
‘Key’ button (S1) on the rear of the
LTDZ’s case to turn on the tracking generator, it was simply a matter of setting
VMA SSA to scan between 800MHz
and 1300MHz, and clicking on the
START button.
The filter’s bandpass curve was then
displayed, as shown in Screen 5. The
filter has a flat response from 1000MHz
to 1150MHz, with an insertion loss of
about 4dB, falling away quite steeply
at either end. Just the shot for receiving ADSB signals centred on 1090MHz!
Finally, I ran a series of tests using
SMA-SMA fixed attenuators, again
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Screen 4: a ‘close-up’ of the output from Screen 3, this time with a range of
2495-2505MHz, which shows the singular peak from before was actually a pair.
Screen 5: the bandpass curve over 800-1300MHz of a FlightAware ADSB filter.
Note the flat response between 1000-1150MHz that falls away at both ends.
Screen 6: the plot of a Mini-Circuits –30dB attenuator over the full 35-4400MHz
range is fairly smooth until it starts dipping past 3.7GHz.
connected between the TG output
and the analyser’s RF input using
a 150mm-long SMA-SMA cable.
For these tests, the VMA SSA app
was set for a full scan from 35MHz
to 4400MHz, to show how the
44
attenuators behaved over the entire
range. I also checked the span with
the 150mm long cable by itself, for
reference.
Screen 6 shows the result for a
Mini-Circuits –30dB attenuator. As
you can see, it’s reasonably smooth
over the full range, apart from a small
bump in the centre and a couple of
dips at about 3700MHz and 4100MHz.
Overall, it just curves slowly upward
from –30dBm at 35MHz to –25dBm at
2400MHz, then slowly downward to
–30dBm at about 3400MHz and further
down to about –40dBm at 4400MHz.
The result when checking the
150mm cable by itself was somewhat
flatter, varying from about –5dBm at
35MHz to –3dBm at 470MHz and then
curving down and up by less than
2dB right up to 4400MHz. But it also
had dips at 3700MHz and 4100MHz,
which might be due to reflections in
the cable.
My verdict
The Geekcreit LTDZ V5.0 spectrum
analyser is a low-cost unit that must
be used in conjunction with a PC,
and operates over a wide frequency
range, from 35MHz to 4400MHz. It
also boasts a tracking generator covering the same frequency range, with
an output level of around 0dBm.
Used together with Mr Augusto’s
VMA SSA application, it’s capable
of performing a surprising number of
spectrum analysis jobs.
But it does have a few shortcomings, of which the most irritating
is probably those ‘notches’ which
appear in the centre of narrow-band
signal peaks. These are caused by
the fixed and wide bandwidth of
the low-pass filter between the IAM81008 double-balanced mixer and the
AD8307 log amplifier/detector.
The LTDZ does have another shortcoming: its relatively low sensitivity.
Its noise floor is about –76dBm, which
corresponds to 35μV. That means it
will be effectively ‘blind’ for signals
below 50μV or so.
Presumably, this low sensitivity is
because there is no amplifier between
the LTDZ’s RF input connector and
the input of the IAM-81008 mixer.
So it might be possible to improve
the sensitivity by connecting a lownoise wideband amplifier ahead of
its RF input.
There are a few of these currently
available, some even have the amplifier circuitry inside a shield – either
on the PCB, or by fitting the complete
amplifier inside a small metal case.
I have ordered a couple of these
amplifier modules to try them out
with the LTDZ, and if the results are
satisfactory, I will cover them in a
future article.
Reproduced by arrangement with
SILICON CHIP magazine 2023.
www.siliconchip.com.au
Practical Electronics | February | 2023
