DAX transmission with low freq signal

Sergey R5AU

Hi All,

I need advise with the DAX issues in case transmitting signals with low frequency (50-100-150)

in this case I can see on PAN and waterfall  signal on other side band, but ONLY in case transmission with low Freq., below 3 screenshots with BPSK63 (transmitting "1" continuously ) with central freq. 100Hz, 500Hz and 1500Hz. Digi apps -  Fldigi (but tested with any other with the same results(on the "Phone" inset on the right side of the SSDR), TX filters: Low Cut -50Hz, High Cut - 3500Hz). This effect independent with TX power slider on PAN and waterfall.

Is it really transmitting on air or FPGA algorithm issue ?

BPSK63 Central Freq=100Hz:

BPSK63 Central freq =500Hz:

BPSK63 Central freq =1500Hz:

BTW on last 2 screenshots visible always tone at the carrier freq. which level (on the PAN)depends with power level.

Tim - W4TME

I cannot tell what the signal strength is because the scale is cut off on the right hand side.  Can you show this in your screen shots?

Sergey R5AU

Sure Tim, all 3 screenshots repeated with the same conditions, the 4th with RF Power 100w

BPSK63 Central Freq=100Hz:

BPSK63 Central freq =500Hz:

BPSK63 Central freq =1500Hz:

BPSK63 Central freq =1500Hz (RF=100w)

Tim - W4TME

I doubt it is transmitting over the air as any signal outside of the TX filter is not transmitted.  Bit is is interesting and I'll have engineering take a look at it.

Sergey R5AU

Thank you Tim.

Bill W2PKY

The bottom trace is similar to JT-65 when DAX is corrupt.

Sergey R5AU

Well Bill, in the same time reception extremely well in any digi mode, On the other screenshots below I just change audio Freq in FlDigi. DAX restart make no sense , however in case DAX buffer is over this is visible as additional noise on waterfall with broken signal reception.

Tim - W4TME

Engineering believes that this is inter-mod since PSK is similar to a 2 tone test. The TX Filter in DIGU set at a 50Hz lowcut will have a slow roll-off which could let the inter-mode show more prominently.  As you noted, you need to move the PSK tone away from the filter edges to mitigate the issue.

Sergey R5AU

Well Tim, sounds a bit interesting due to the on hight cut of the filter I have no such effect. BTW in DIGI operation I am using 2 different methods of turning: - with cursor on waterfall of the digi apps(ex. Fldigi) with wide filter in SSDR - With slice on the station signal with narrow SSDR filter Last method not so operative and nice, to improve it I am turn On XIT+RIT with shift "-1500"Hz, then possible turn up on the station signal by click on the SSDR waterfall very precisely and operative with. Is it possible to make modification in SSDR to use second method ( 1500Hz as a turning cursor instead carrier frequency of the silice) without using RIT and XIT. I guess it will be god improvement for Digi.

Tim - W4TME

Have you set the "Sweetspot" in Fldigi to 1500 Hz and then use the Fldigi QSY feature to re-tune the slice so that the signal of interest is in the center of the filter?  Then you can use the narrow or wide RX filters easily without worrying about the filter edges.  I alos create a TX filter centered at 1500 Hz that is wide enough to accommodate the transmitted bandwidth and the sidebands.  Example I use a 200 Hz wide filter for PSK31

Sergey R5AU

Will try it, usually I am started with two instance of the FlDigi and using FlDigi-To-Dxkeeper apps to have full integration between DXkeeper and FlDigi, nevertheless I will try it, thank you.