Comparison of cheap RF modules with ASK modulation. Part 1. Methods.

Disclaimer. Anything in this article is a private opinion guaranteed by Universal Declaration of Human Rights (UDHR). The author doesn’t impose it to anybody. Parameters are shown in this article are relative, and don’t reflect standard parameters of this type of the devices. If one would use different methods of measuring (or different types of measuring devices), the result different. The author doesn’t bear any responsibility for any consequences related to devices described in this article, including a lost profit. Decisions made by a reader are implemented at his own risk. Manufactures who think they are defamed can present their own refutations and the author will publish them with (or without) changing his own opinion.

Some time ago I was designing a 2-band (433 and 315 MHz) device. It should have been receiving and transmitting digital signals in ASK modulation in both bands simultaneously. So, I ordered at Aliexpress 2 sets (TX & RX) of modules for 315Mhz and 433MHz. But there was a problem in the process of debugging – 315Mhz channel was operating from the 433MHz source. I thought there was a mistake in schematic or code, but there was a simple reason – the 315MHz receiver was decoding the 433MHz signal. I had to invent some methods to test these modules and identify at least some parameters such as working frequency, sensitivity or signal strength. Unfortunately, I have no serious equipment to test modules by all required parameters. But I don’t pretend for scientific character of this investigation, though it was able to compare different types of modules. But I did my best to minimize most of errors. My first sets of modules had terrible parameters, so I decided to order almost all types of cheap RF modules with ASK modulation that present at Aliexpress. After the all work was done I decided to publish its results because I never found similar information before.

Some modules have the similar design, even they have different marks or different operation frequencies. So, if it has the similar schematic or the chip used you can suppose module characteristics just look at them.

But one should remember if you order modules, and you’ll get it, it doesn’t mean that you get exactly the modules you ordered. In my case, some modules were 433MHz though they were marked as 315MHz (as I've described above). So, try to check them before you insert them into your device. One can use the signal-generator or the transmitter with the known operating frequency and see what is at the receiver output. Besides, some modules I got didn’t operate at all. So, it’s strongly recommended to test everything you get from sellers.

Part 1. Methods of measuring.

Transmitters.

I have no specific antenna for 315MHz, so I used 433MHz pin antenna for both bands:

The antenna was connected directly to the transmitter output via SMA-connector without any matching. The second “ground” pin of SMA-connector wasn't connected at all.

I got some spiral antennas when this investigation was almost done. So, the measuring with the spiral antenna is mentioned in the table below. But these results can be applied to other modules too – in the most cases, spiral antenna operates better than the pin one.

All measures for transmitters were carried out by RF-explorer. It’s the cheapest RF spectrum analyzer. It looks like a DIY device, and it's available for buying in many places. It has a range from 15 up to 2400MHz and can measure the signal strength and the type of spectrum. I don’t think that this device is precise, but it can measure different parameters of different devices comparatively. I think that if I can measure different devices at the same conditions, I can get relatively accurate results.

Rf-explorer view with the spectrum of one of 433MHz TX-module. One can see a frequency and a level of the main band

The distance between the transmitter antenna and RF-explorer antenna was 1,5 m for all modules. Antennas were parallel to each other.

Supplying voltage was 5V (+/- 0,05.

Measuring parameters:

• Current consumption;
• Main signal frequency in MHz and its level;
• 2^nd harmonic and its level;
• 3^rd harmonic and its level.

Receivers

The same antennas were used for receivers. The antenna was connected directly to a receiver input without any matching.

I used an old Soviet device named X1-50. It’s a RF sweep generator with frequency response measuring. The best feature of this device is an ability to get module response while the frequency of its input signal is changing. So, it is able to measure the width of the range of receiving signal. Sweep frequency is about 50Hz, what is much less than any module bandwidth so we can ignore any restrictions of the speed of transmission and module built-in noise reduction. As a result, one can see different bands of receivers (many modules have more than 1 band), and make conclusions about jam-protection of the module if it has parasitic bands.

Main disadvantage of X1-50 is a frequency counting. It is able to count frequencies multiple to 1 MHz only. So, precision below 1MHz is provided only by an eye. So, there could be an error in measuring about 0,3-0,5MHz.

Examples of X1-50 measuring:

Here is a band for 433MHz RX-module (but it’s not 433MHz as you can see). The distance between marks (splashes) on the screen is 1 MHz. So here is the band from appr. 435,8 to 436MHz. So, do you still use cheap modules without testing? 

Here are 2 receiving bands of 315MHz module. There are 2 bands – from appr. 316,0 to 317,5 MHz and from 318,2 to 319,5 MHz. So, what about selectivity?

X1-50 has an output RF level adjustment. It’s used to determine module sensitivity – if module can receive a signal without any noises with maximum attenuation, this module is more sensitive than others. So, the maximum level of attenuation when module is still receiving a signal was written in the table below. I have to repeat: this level is NOT an absolute sensitivity level, it’s a relative one in comparison to other modules measured with this method.

RF output of X1-50 has 75 Ohm impedance, so antenna was connected to X1-50 through self-made 50-75 Ohm matching circuit:

The distance between the transmitting antenna connected to X1-50 and the receiver antenna was 0,5 m for all modules. Antennas were parallel to each other.
Supplying voltage was 5V (+/- 0,05V).

Measuring parameters:

• Current consumption;
• Main band width;
• Secondary bands width (if the module has);
• Sensitivity – attenuation of output RF signal when module is still receiving the signal without any noises.

Unlike transmitters using of pin antenna instead of spiral one gave more sensitivity. Second “ground” pin of SMA-connector wasn't connected at all, cause’ when it was connected to GND, sensitivity was falling down.

I'm sorry about many words above, but I had to explain the logic of my investigations.

So, let’s see the first results.

    The 2nd part with transmitters results.

    The 3rd part with receivers results.

  The last part with conclusions.