11/12/2020 at 10:18 •
After getting the Wideband FM Demodulation to work and achieving much better results then expected, it was time to test the FM Modulation mode. The base configuration is the same as used for FM Demodulation.
In FM Modulation, the internal ADC is used to control the FM deviation/shift. For this the ADC was set-up to use the differential inputs, GPADC13. The Analog Input circuit transforms the single ended AC coupled input into a differential signal in the ADC input range of +/- 500 mV around a VCOM of 800mV.
Bellow is a figure of this signal conditioning, in yellow is the Analog Input signal, the audio signal injected into the module, and in green and blue the differential ADC inputs.
This figure also shows a problem with the differential amplifier in use, with the current circuit the amplifier is not very stable and introduces an oscillation on top of the differential output signal. In the future a different amplifier will be used and tested but for now, even with this oscillation, the results are good.
The sampled signal by the ADC is then used to control the FM deviation. The conversion gain from ADC count to frequency deviation is controlled by the FSKDEV register and here it is set to use the maximum frequency deviation possible, +/- 62.5kHz, which gives the desired Wideband FM Modulation.
Bellow is a figure showing the RF output spectrum of the Module. The spectrum of the FM Modulation with a full range audio signal as the input, 100% volume from my laptop, is shown in yellow and with a half range signal, 50% volume, in red.
To listen to the FM Modulation any conventional FM receiver can be used, I used the FM receiver of my smartphone and it worked great! More information on the set up used to achieved this as well as an audio sample file output by my smartphone FM receiver are available on the website.
With this, the Module can now both modulate and demodulate FM signals. The next step is to test Narrowband operation, using a microphone as the Audio Input signal and implementing push-to-talk functionality.
10/06/2020 at 11:57 •
As seen in the last project logs/updates, the module is capable of demodulating and outputting audio FM and AM signals. The audio quality is OK, not very good due to the way the DAC works and the not very good filtering of it. Over the last few days I researched different filter typologies to improve the audio quality and I tested a new one that uses the same circuitry as the previous filter, that is already present in the Module.
The previous filter used was a double RC Low Pass Filter, as can be seen in the figure bellow on the left. The new one tested is a double LC Low Pass Filter where the resistors of the RC filters are exchanged with inductors, as can be seen in the figure bellow on the right.
The double RC filter is a second order low pass and the new tested LC filter is a fourth order low pass, increasing the DAC filtering substantially. Bellow is a figure comparing the frequency response of both filters with ruffly the same cut-off frequencies:
Tests showed that the new LC filter improved the Audio quality as can be seen on the figure bellow where the Audio Output of the Module is shown, when in FM Demodulation mode. This can be compared with the previous project log, "FM Audio Demodulation", figure. The test conditions were the same.
09/02/2020 at 20:04 •
Analog FM demodulation, and modulation, is a unique and officially supported special feature of this transceiver. ON Semiconductor even provides an application note on how to set up and use the transceiver as an analog FM transceiver, "AX5043 Use as Analog FM Transceiver". Both the application note and the configuration used are for Wideband FM signals, the type used in FM Radio Broadcast. FM Radio Broadcasts were actually used to test the FM demodulation with good success!
As with the AM Demodulation, the basis of the FM Demodulation is the internal DAC. For FM Demodulation the DAC is setup to output the Frequency Tracking value. With the transceiver setup properly, the FM signal is first down-converted into the IF stage where the frequency change over time is tracked and output by the DAC. This tracking information is also accessible over the SPI interface of the transceiver.
To show the FM modulation in the IF stage, the DAC was configured to output the SAMPLE_ROT_I (or SAMPLE_ROT_Q) and a RF signal generator connected to the module is used to generate the FM signal, using as the modulating source a pure 500 Hz tone. In the figure bellow the FM signal in the IF stage is shown in blue and the pure 500 Hz tone is shown in yellow.
With the DAC setup to output the frequency tracking value, the FM demodulation, the signal observed is shown in the figure bellow. In blue is the FM demodulated signal and in yellow the original audio signal used as the modulation source.
As with the AM demodulation, a lot of noise is visible on the demodulated signal. This noise is also very clearly audible in the audio output of the module, through the audio jack. The main source of this noise is from the not well filtered DAC output.
Samples of the output audio can be listened to on my website, as well as all the configurations used to achieve the FM demodulation.
A little teaser on what I'm testing and working on for this project can be seen on my twitter, very early stages still.
07/26/2020 at 20:15 •
This very unconventional use of the AX5043 transceiver was triggered by a question posed to me by a fellow Hackaday member, if the transceiver could be, somehow, used as an Audio AM receiver. Over the past week I explored this idea and it turns out that it actually can!
The basis for this is the very versatile DAC output of the transceiver which, among others, can output the amplitude tracking information of the signal at the entrance of the demodulator block. With the DAC set up to do this, the transceiver has to be "tricked"/setup to keeping the amplitude modulated signal at the IF stage, where this tracking information is calculated. The signal at the IF stage is visible in the figure bellow in blue (DAC set to output either SAMPLE_ROT_I or SAMPLE_ROT_Q), in yellow is the original audio wave:
With all properly setup, the DAC outputs the demodulated AM signal, the desired Audio signal, as can be seen in the figure bellow. In blue is the demodulated AM signal as output by the DAC and in yellow the original audio wave:
It can be seen that the demodulated signal is very noise. The main source of this noise is the not well filtered DAC output, even after a second order low pass filter. The transceiver uses a Sigma-Delta DAC. Even with this noise the performance is much better then expected, and by plugging in a headphone to the audio jack of the Module one can listen to AM audio transmissions!
More information on the set up used to achieved this as well as an audio sample file output by the Module are available on my website.
07/15/2020 at 19:31 •
The newest hardware revision, revision 3, arrived and I assembled and tested it over the last weeks. This revision fixes all the hardware errors and problems discovered with the revision 2 like the AX5043 Oscillator Input, analog output amplifier/filtering and in the power supply/distribution circuits.
The RF circuit, matching networks, were kept the same and this is reflected by the RF performance and input matching (S11) which are the same as with revision 2.
Besides that, the PCB color was changed to blue which in combination with no bodges looks much better and clean then the previous revision:
With this, I now have two assembled and working modules. This, with the antennas that arrived and where tested a few weeks ago, enables now communication testing between modules as well as range testing/comparison with different antennas and parameters.
Files for the new revision are available on my website.
06/30/2020 at 21:16 •
Some months ago, I ordered a bunch of dual-band antennas to use in this project and they finally arrived! The antennas I bought are shown in the figure bellow, from left to right they are a Nagoya NA-771 Clone, a Nagoya NA-771 Original, a UV-106UT Clone and a UV-108UT Clone.
All four antennas where tested with a R&S VNA to get there S11 parameters and in the future, I will perform field tests with the VUHFRadio to compare their ranges and “gain”.
The figure bellow shows the comparative result of the two NA-771 antennas in the Sub-GHZ band:
And the results for the same band for the UV-106UT and UV-108UT antennas are shown in the figure below:
Overall the NA-771 antennas have better S11 characteristics for the desired bands of this project. In general a S11 of bellow -10dBm is desirable (VSWR better then 2:1) as this means that over 90% of the power is delivered to the antenna, less than 10% is reflected.
A bit more in-depth results are available on my website as well as all S11 results files for download. I will also, overtime, add more results for other antennas to that page.
06/21/2020 at 19:42 •
During this week I performed some BER testing on the transceiver for both AFSK-1200 and GMSK-9600 in UHF and VHF. In UHF the results were great with both modulations having a BER over 10-5 with input power above -105 dBm. For VHF, the results were not so good, with a BER over 10-5 only achieved with input power over -95 dBm, for both modulations. I am not sure why this is, and it requires further testing.
The effect of increasing the receiver bandwidth on the BER was also tested and it is very noticeable, as is expected. The BER decreases about 10-1 for each doubling of the receiver bandwidth, this with input power of -105 dBm.
Full results are available on my website at the bottom of the page.