Apache Labs Community Forum

I often run into ANAN users running AM and at times see them set up to transmit with a considerable amount of asymmetry. Since SDR architecture first generates a DSB signal and then adds the carrier at the end, you are not actually modulating the carrier as you do with plate modulation or Class E rigs. Regardless of what value you choose for the AM Carrier Level setting, the DSB signal with all of your voice content will be the same.

So, is there any advantage to setting the software to run with an asymmetrical looking signal?

Apparently not, since the DSB signal is unchanged regardless of the carrier level that is set in the software. In the test that I posted at YouTube, the ANAN-200D is transmitting AM with a CFC version of OpenHPSDR using an AM Carrier Level setting of 100. With Pure Signal toggled off, my bench scope shows 125% positive peaks and 95% negative peaks. With Pure Signal toggled on, the scope shows 100% positive peaks and 99% negative peaks, which is almost perfectly symmetrical. (Note that with OpenHPSDR, when the AM Carrier Level is set to 100 it is impossible to modulate beyond 100% negative while running AM. You will absolutely not pass through the baseline and no "wavelets" will be generated.)

The video capture shown below was made using an SDRPlay receiver which has a bit of latency due to its USB interface so the VU meter that is tracking audio loudness doesn't quite synch up with what is seen in the pandadapter. What you hear in the video is the actual transmitted audio of the ANAN-200D as it was received by the SDRPlay.

The VU meter does not show a significant difference in the audio level whether asymmetry (no Pure Signal) or symmetry (Pure Signal toggled on) is present in the transmitted signal. Since the symmetrical signal is as loud as the asymmetrical signal, and since Pure Signal cleans up the IMD and as a bonus, improves the THD of the transmitted audio from ~3% to 0.2% it seems to make sense to use Pure Signal when running AM to attain symmetry with no sacrifice in loudness.

At any rate, that's my opinion!

73,

Rob W1AEX

https://www.youtube.com/watch?v=0_92HNhpfnU

There is a suspicion out there that Pure Signal removes or limits the positive peaks when it is used with AM. Hopefully, the video below will put that rumor to rest.



In the first scenario, the ANAN 200D is modulated with a 1000 cps tone right to the threshold where the ALC look-ahead algorithm soft limits the signal to prevent clipping at the DAC. The AM Carrier Level in OpenHPSDR (v3.4.1) is set to a value of 100 as Pure Signal is toggled ON and OFF several times. Keep an eye on the amplitude of the positive peaks as Pure Signal is toggled.

In the second scenario, the ANAN 200D is modulated with a 1000 cps tone right to the threshold where the ALC look-ahead algorithm soft limits the signal to prevent clipping at the DAC. The AM Carrier Level starts out with a value of 100 and is varied to values less than 100 and then greater than 100. Keep an eye on the amplitude of the positive peaks as the AM Carrier Level is adjusted.

The way AM is generated with OpenHPSDR is about as good as it gets!

73,

Rob W1AEX

I know this is an older post but has openhpsdr changed since this was posted?
I notice my carrier setting goes up to 125 and when it is set at 100 according to my rea monitor it breaks 100% negative easily I need to push up to a setting of at least 108 then I see 120 positive and can not break past 99 negative this is with pure signal on or off