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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 for Silicon Chip, 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 72 Silicon Chip Martins Augusto, who lives in Portugal, and it can be downloaded from his site: siliconchip.com.au/link/ab87 So I went ahead and ordered an LTDZ V5.0 from the Banggood website (siliconchip.com.au/link/ab88), paying US$46 plus US$4.87 for shipping, which came to a total of $75. 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. Australia's electronics magazine 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 recently (December 2021; siliconchip.com.au/ Article/15139), 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 2018 issue, specifically my review of the Digitally Controlled Oscillator module (May 2018; siliconchip.com. au/Article/11073). 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: siliconchip.com. au/link/aajc By the way, the LTDZ draws about 100mA from the PC in standby mode, siliconchip.com.au Fig.1: block diagram of the LTDZ 5.0 module. The most important sections are the two ADF4351 synthesisers and the STM32 ARM microcontroller. rising to about 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. 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 The internals of the Geekcreit LTDZ spectrum analyser. siliconchip.com.au Australia's electronics magazine January 2022  73 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. turn, goes to an analog-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 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, high-end 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 also 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 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. 74 Silicon Chip Australia's electronics magazine As mentioned earlier, Mr Augusto’s VMA SSA software can be downloaded for free (siliconchip.com.au/ link/ab87). 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 siliconchip.com.au 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. 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 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, 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. 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. 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. siliconchip.com.au 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. 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 is 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 Australia's electronics magazine 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 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 January 2022  75 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. 76 Silicon Chip Australia's electronics magazine 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 13th October 2017 (siliconchip.com.au/ link/ab8a), 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, as he admits; crunching the scanning data to truly remove the notching would be pretty complicated. siliconchip.com.au In another post dated 4th February this year (siliconchip.com.au/link/ ab89), Mr Augusto announced that a colleague of his named Domenico 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 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 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 IAM-81008 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 having 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 SC article. SMD Test Build it yourself Tweezers ● Resistance measurement: 10W to 1MW ● Capacitance measurements: 1nF to 10μF ● Diode measurements: polarity & forward voltage, up to about 3V ● Compact OLED display readout ● Runs from a single lithium coin cell, ~five years of standby life ● Can measure components in-circuit under some circumstances Complete Kit for $35 Includes everything pictured, except the lithium button cell and brass tips. October 2021 issue siliconchip.com.au/Article/15057 SC5934: $35 + postage siliconchip.com.au/Shop/20/5934