My working hypothesis was that the ALC meter value is relevant for digital modes, the input audio signal should be at a level where you have a small ALC meter value, but where you reach on an external power meter the commanded power of the FT991. The question is whether a higher audio input signal with a increased ALC meter value will cause more splatter in the WSJT-x transmit signal of the FT991.
Setup: FT991 and PC in the configuration the way it is used on the HF bands, both are connected via a microham keyer II. The commanded output power levels on the FT991 are 10, 25, 50 and 80 Watt. (See the discussion below under advisory note discussing the attenuators and the SDR.) Measured are: the peak level in dB of the WSJT-X generated tune signal and the signal level of highest parasitic frequency seen by the sdrplay duo, and also the noise floor in the waterfall graph produced by the SDR. The external power meter is that of a Palstar tuner put in bypass mode.
The following values show the dB difference between the signal peak and the parasitic frequency signal level for the lowest audio input level generated by the PC resulting in a near zero ALC indication and a power on an external power meter equal to the commanded level on the FT991:
- 80W 25.6 dB
- 50W 24.2 dB
- 25W 27.2 dB
- 10W 26.0 dB
There is an uncertainty of approximately 2dB in the reported values depending on the indicated ALC meter value in relation to the level of the audio input signal send by the PC to the FT991, whereby it should be remarked that the signal level of the parasitic signals relative to the central frequency deteriorate up to 2dB for larger ALC meter values.
Conclusion: there is no indication that a high or a low power level matters in the discussion. High audio levels will increase the level of parasitic frequencies by approximately 2dB, the parasitic frequencies are approximately 26dB below the central peak in the WSJT-x tune signal produced by the transmitter.
Example FT8 spectra made by FT991 measured with SDR console and an airspy HF+
What is causing the spectrum we see in figure 1?
In my opinion the spectrum is not caused by the SDR, it is also not caused by the PC generating the WSJT-x signal, instead this is what the FT991 produces. For comparison, in figure 4 you find the spectrum of the transmitter in CW mode, this signal has nothing to do with the WSJT-x program or the PC, it is directly generated by the FT991 in CW mode. We see the same features as in figure 1.
I can also rule out the airspy HF+ and the SDRconsole software because a signal generator (for details see this article) is able to create a 25 Hz wide signal with a carrier to noise ratio of at least 100 dB on the waterfall plot. The spectral width is related to the sampling rate of the used software, in reality the bandwidth of the test signal is less than 25 Hz. In the SDRplay directory you find an spectrum analyzer for a SDRplay duo and it offers some more flexibility for sampling the signal, see figure 5:
I also tested a FT990 transceiver, same set-up but with the SDRconsole software, the FT990 performs somewhat better than the FT991, it suggests that a TX IMD of -40 dB can be achieved.
Advisory note with regard to attenuators and SDR
Your SDR is already a fairly good spectral analyser, but it is way to sensitive because it lacks the ability to attenuate transmit signals from a FT991.
Do not try to feed anything more than about 1 mW into any SDR because you will kill it (it has protection diodes but I don’t insist on the challenge) For this experiment you need a high power rating attenuator, so the first one on the chain of attenuators should be able to handle at least the power you transmit, the big black body in the figure below can handle up to 100W with an attenuation of 20dB. Even that is not good enough, you need to go down to 130 dB attenuation (in total) to get a signal close to the noise floor of your SDR, so this is where the rest of the attenuators play a role. (The four BNC to BNC attenuators were not used, also the SMA attenuators were not used) Used were as said the 20dB big black block, the 10dB N-N attenuator, and the HP-335C/D stepper attenuators. The last one becomes useful to tune the signal level close to the noise floor of your SDR and of course within its dynamic range.
An n-bit SDR will have a dynamic range of 10log10(4^n) dB, so 8 bit is 48dB, 12 bit is 72 dB and 16 bit is 96 dB, the airspy HF+ has 18 bit ADC, so its theoretical dynamic range is 108 dB. With for instance 100 Watt of transmit power you want the signal to arrive at some 60 dB (well within the dynamic range of the SDR) above its noise floor which is approximately -140 dBm, so you need a signal level of -140 + 60 = -80 dBm. To reach -80dBm starting from 100W = 10*log10(100*1e3) = 50dBm you need 130 dB attenuation. The BBB plus the N-N connector give me 30dB; the extra required 100 dB comes from the HP335C/D.
There are quality differences between the transmitter intermodulation distortions (TX IMDs). ARRL test reports take a test range of -20 to -35dB, the lower the number the better it is, our FT991 TX IMD measurement is confirmed in the ARRL test report described in QST November 2015, page 45 to 51. The measurement itself is relatively easy to do with a set of attenuators and software, to validate the quality of the SDR spectrum analyzer I used various test signals of around 1dBm to see whether the bandwidth and the obtained carrier to noise ratio are acceptable. Very narrow signals of better than 5 Hz bandwidth and 100dB difference between carrier and surrounding noise can be obtained with the SDRplay frequency analyzer.
The consequence of TX IMD is that any transmitter will always cause some splatter in the spectrum, it will also happen in the FT8 output signal. The consequence is that a receiver can pick up the TX IMD, strong FT8 signals may therefore cause duplicates at nearby frequencies, duplicates of the FT8 signal can picked up by the decoder.
Examples of duplicate FT8 signals occur in my option rather often, at least, I see them every day. In figure 8 there is an example at 947 Hz offset with a nearly duplicate found by the decoder. I could not eliminate this signal in the receiver, reducing the audio signal did not help, what will help is to attenuate the overall signal, but then you will also lose the weaker FT8 signals. The station from Croatia was the only one that was causing this problem and everything suggests that it is caused by the station itself and not the receiver that I used, nor the ALC behavior, nor the audio level. The observed TX IMD may very well explain why we see this in the WSJT-x band activity window.
Figure 8: At 947 Hz offset you see the main signal with a SNR of 13 dB, and a duplicate at -23 dB at 797 Hz, the distance is 150 Hz.
Last update: 15-Jul-2019 6:56