TX Gain - Thetis vs power meter RF output

[Robin Matthews]

Hi

Has anyone else noticed a difference between the RF output reported in Thetis and an external watt meter for different TX gain settings?

This experiment was performed using Thetis v2.10.6 HL2 Beta 3 into a dummy load. The meter is a Nissei DG-503 and not calibrated. However, even for an uncalibrated meter, I would not anticipate seeing a jump in this way.

Thanks

Robin, GW0NTM

[Steve Haynal]

Hi Robin,

I suspect this is a software issue. We have measured power output on the HL2 in the past and also spot check different levels during factory testing and don't see this behavior. It is possible the Thetis software is trying to create a larger power output range as some like to see power output go to zero. That requires reducing the amplitude of the DAC samples which decreases the bit resolution which I think is undesirable. The HL2 has a native range of about 7.5 db which the TX output can be adjusted based on what the AD9866 DAC can do.

To make sure there isn't a problem with your particular HL2, I'd try the same experiment again with SparkSDR or Quisk.

73,

Steve

kf7o

[Robin Matthews]

Thanks for the info Steve. I'll try this when I am back from vacation on the 2nd.

Best

Robin, GW0NTM

[Robin Matthews]

Hi Steve

I've finally managed to run some further experiments.

SparkSDR (on Mac)

From 0 - 195 on the TX slider the power increases smoothly from 0 to 3.02w. At 196 on the TX slider, the power decreases to 1.01w and then increases smoothly to about 6.75w at 255 on the TX slider. So essentially the same behaviour I saw with Thetis (although I'm not sure how the slider values are related).

I was going to try PiHPSDR, but at the moment, I don't seem to be able to be able to generate a carrier in tune. I will read the manual to see where I may have gone wrong.

Do you think this is a hardware issue?

Best

Robin

[Robin Matthews]

Hi

RTFM and have now set the PA calibration levels correctly in PiHPSDR.

From 0 - 23 on the TX Drv slider the power increases smoothly from 0 to 3.03w. At 24 on the TX Drv slider, the power decreases to 1.07w and then increases smoothly to about 6.75w at 100 on the TX Drv slider. So essentially, similar experience with SparkSDR and Thetis.

Best

Robin

[Steve Haynal]

Hi Robin,

It is hard to determine yet if this is a hardware problem. I can't think of any HL2 fault mode which would result in the behavior you see. 

You mentioned your meter was not calibrated. It feels like the meter may be switching from a lower power mode to a higher power mode right in the range you want to measure. Do you have an attenuator? You could measure the output of that and it should change the range over which your meter is operating. Maybe there are some settings in your meter you can try changing.

This drop in power is big enough that you should see it with the HL2 if it is real. If you enable full duplex and transmit into a dummy load, the receiver will still pick up enough power to register a signal. Do you also see this drop in power when monitoring with the HL2's receiver? 

73,

Steve

kf7o

[Robin Matthews]

Hi Steve

Hmmm.... good idea.

When I zoom the panadapter to the -60 to -40 range, and use duplex, I can see a gradual and continual rise in signal as I increase the TX Drv slider. There is no downward jump, so I think that means there is no issue with the TX attenuator, but it's an issue with my Nissei DG-503 measuring at these low levels.

Thanks for your help Steve.

Best

Robin, GW0NTM