China Gadget: 35MHz – 4.4 GHz LTDZ Spectrum Analyzer

In VHF contests, the question sometimes arises of how far I can drive my power amplifier without causing too much interference. In Ilmenau, we have many stations in a relatively small area, and we don’t want to interfere with each other unnecessarily. The other recurring question when setting up for the contest: Does my transverter on the mast transmit anything at all?

For both problems a small spectrum analyzer is the ideal instrument. A spectrum analyzer is a measuring instrument that scans an adjustable frequency range and plots the measurable field strengths as a graph. Possible peaks can then be measured in their size and the exact frequency. Spectrum analyzers were expensive professional equipment in the 10k€++ price segment years ago. Now there are several types between 30 and 500 € on AliExpress. Yes, our Chinese…

At my friend Jens, DH1AKY, I saw the Tiny Spectrum Analyzer, which goes from 100 kHz to 350 MHz and has a 2.5″ display. A nice thing, but I wanted a bigger display and if possible also cover our transverters in the 13cm and 9cm band, so see something up to about 3.5 GHz.

So I spontaneously decided to order a no-name 35 MHz – 4.4 GHz spectrum analyzer at AliExpress, which was offered there by many dealers in the price range of 40 – 150 €. So for a total of 50,-€ such a copy landed with me.

 

In the package was the hand-sized spectrum analyzer and surprisingly a small rubber  antenna. Nothing else, also no manual.

So I connected it to a USB charger (USB-C) and charged the internal battery.

In the meantime, you can take a look at what kind of antenna this is. So this plugged into my nano VNA. All matching SMA.

Note the yellow curve, which makes a clear dip at about 148 MHz.

Aha, so this is obviously an antenna for PMR-446 walki-talkies. More suitable would have been a wideband antenna, which I have to order for a few Eur when I get the chance. But for the first use of the part, the PMR antenna is a nice gimick.

Technics

The missing manual is not a big problem at first. Men seldom read manuals anyway 😉 and you quickly get to know how things work when playing with the device.

The Spektrum Analyzer does not have a touchscreen, everything is operated via the rotary/push button on the side. Sometimes this is a bit awkward, but it works. The functionality is also not really big:

• Start/Stop scanning
• Start frequency
• Bandwidth
• Offset and gain

The first unpleasant thing to notice is that the span (swept frequency range) or the stop frequency results from the set bandwidth. The smaller the bandwidth, the smaller the span. I can understand the principle behind it, there are exactly 700 points on the display and the step size is just so that each display point is a measuring point. This keeps the refresh rate constant at about 800ms. But I still think this is much too short thought, with a little more programming effort, this could have been made nicer and more useful. This is so typical Chinaware. A German engineer would have invested more, before such a thing is let loose on mankind (I say now, as a German engineer 😉 ).

It’s not immediately obvious, but the manual reveals that the “Magnification” and Offset settings directly change the calibration of the device. The calibration is hardly credible anyway with the mini hardware and the giant frequency range. Who needs it, however, has the possibility to provide defined ratios here even for the interested frequency range via a known attenuator (and depending on the requirement of a known calibrated signal source). Practical consequence is however that one cannot simply magnify a weak signal out (see below), without adjusting the calibration. In my opinion, an instrument calibration also does not belong in the first menu level (but there is only one here).

So, as in the X-axis, a zoom option has been forgotten in the Y-axis as well. As a software engineer, who has to deal with similar things every day, I grab my head. The uninitiated hobbyist (I can’t call this anything else) has saved the entire abstraction layer for the representation of the data. Eh, this is not a hobby project (or is it?), the part is obviously mass produced.

First Tests

As a test device I took my Baofeng UV-5 dual-band walkie-talkie.

Great, you can see the transmission frequency as a clear spike (see first picture). The part works in principle. But the width of the spike seems visually too large. The set bandwidth of 100kHz is not plausible. More about this below. Probably the power of the PMR antenna on the 70cm band is too high due to the walkie-talkie next to it. However, this is all less bad in principle.

 

Much more unpleasant: When my Baofeng transmits in the 70cm band, I see needles on the spectrum analyzer even at about 150MHz. Should the Baofeng produce such spurious emissions there? That would be pretty cheeky.

So let’s compare it with a real spectrum analyzer of the 20k€ class, a Rhode&Schwarz FSL. There is nothing to see at 150 MHz. So the culprit is the small spectrum analyzer, which obviously sees some kind of mirror frequencies here. That is now really unpleasantly, although with the cheap hardware now not so unexpectedly. I was allowed to spend a beer evening with the developer of the FSL input part at the beer table at the Ham-Radio fair 🙂 In any case, I can say that the effort here is on a several orders of magnitude different level. In this respect, the expectations of the Chinese miracle should be kept very low, but it is still unattractive.

 

Next try, I want to see my IC-9700 with my Yagi on the roof transmitting on 144MHz. For this I first take an antenna resonant on 144 MHz for the spectrum analyzer. Nevertheless I see only a very timid peak. So the part is quite insensitive, which is almost clear.

To see something nevertheless, one can increase the “Magnification” in the menu (see above). In other words, turn up the digital gain. Unfortunately, this also moves the noise level. I.e. the signal disappears completely from the screen. With the offset settings you can get the signal back. With a little bit of persistence you can get the transmit signal of the IC-9700.

Again annoying at this point is that the software makes it so hard for you by complete lack of any inherent intelligence. It would be quite possible to adjust the offset automatically and only change the scale accordingly. Then you could easily adjust the sensitivity of the device to the conditions via the magnification.

Deeper internal inspection

This spectrum analyzer is also sold as a bare PCB without case, display and battery. There is also a reference to a Windows software for use, but I have not yet found and thus not yet tested. The PCB and the manual (Google search: “ltdz spectrum analyzer manual”) give some insight into the internals.

The core of the spectrum analyzer is an ST microprocessor with ARM core. This controls the tracking generator, an ADF4351, which specifies the usable frequency range of 35 – 4400 MHz. Apparently there is no input amplifier, the input signal and the tracking generator meet directly at a mixer. Behind the mixer sits a detector and after that an AD-converter. The specified 90dB dynamic range fits well with a 16-bit ADC. According to the manual, the detector should have a fixed bandwidth of 500 kHz.

Due to the price, one has to live with the following disadvantages:

– The missing input amplifier makes the spectrum analyzer quite deaf. The manual points out to connect an external amplifier if necessary. These are also available for little money, but in portable field use this might be rather uncomfortable. But for fixed applications you could build something together with a suitable antenna.

– Of course there is no switchable attenuator, which would be useful for strong signals.

– On the hardware side, there is only a fixed bandwidth of the detector of 500kHz. The software adjustable lower levels are not supported in the hardware. Possibly the above described backlash frequency problem is caused by this. In any case the peak of the Baofeng which appears too wide. In expensive commercial spectrum analyzers one drives an unimaginably larger expenditure at this point.

– Of course all measures to compensate the frequency response of the analog circuit are missing. A little bit is compensated by software, the parameters available for this in the menu are described very rudimentary.

Summary

These were the impressions of a first quite short test. With time, you will probably warm up to it even better and what it brings for the actually intended application, you have to see first.

I haven’t searched and tried the PC software yet. That will surely come.

For 50,-€ you should not ask too much and first the spectrum analyzer is also a great toy. The hardware is really minimalistic. That is first for the price OK, but for the double money one could have gotten the tenfold use out. The software, however, is on a dilettante level and disappoints at several points. That is really annoying. Maybe it will improve. Theoretically, the ST processor should be able to be updated, but it is completely unclear whether anything will follow.

Therefore, I don’t recommend buying it, but I don’t know if there is anything better in the cheap class that would cover my use case.

Should someone still decide to buy it: This spectrum analyzer is offered by many Chinese dealers in many versions (bare PCB, device without and with battery, possibly with different accessories). You have to be very careful. The same version as described here but WITHOUT battery I have also seen for twice the price.