How to Adjust RF Power Output for Best IMD when Operating Digital Modes

How to Adjust RF Power Output for Best IMD when Operating Digital Modes

  System Dependencies
  Minimum PowerSDR Version:     2.0.x 
  Applicable Hardware:     FLEX-5000/3000 

Content provided by: Tim, W4TME & Brian, WB6RQN

When using a traditional analog transceiver to adjust the power output when running digital modes, it is customary adjust the radio's RF output until the ALC begins to register and then back it off a little bit.  With a FlexRadio SDR running PowerSDR and using VAC for the digital audio transfer between PowerSDR and the sound card digital mode program, the following technique can lead to better control and cleaner signals.

In the olden days, the mic gain was how you controlled power output from your radio so to those of us who grew up with older radios, this approach seems natural. You would just advance the mic gain until just below the level where you actuated the ALC (onset of clipping) and you were all done. If you wanted less power you just turned down the PA RF gain (power).

What typically happens is that people set the AF input gain control a bit too high and then compensate by turning down the RF drive level. The result is clipping at an early stage within the radio. Most analog radios can tolerate a bit of over-driving at an early stage. The result is minor compression of the peaks and a minor increase in distortion. In fact, for SSB operation this might even provide just a bit more "punch" to the signal.

In the case of an SDR, the clipping is hard flat-topping, usually from exceeding the range of the A/D converter at the audio input. The rise in distortion is very rapid. So in order to ensure that all the stages are operating linearly, one must be sure to keep the input level below that which produces any clipping at all. Fortunately for us the FlexRadio Systems software defined radios have an indicator of AF input level at different stages in the TX audio chain. If ALC, which is at the end of the TX audio chain is indicating less than 0dB, you are not clipping the signal in the radio. With the VAC TX Gain (input signal) at 0 and the drive level set for 100, the RF output is 100 watts if using a pure tone. If you want to reduce the output and still make sure you have some headroom to prevent clipping, it is easier to turn down the VAC TX Gain (input level). Setting the VAC TX Gain in order to produce a -5dB ALC level should produce a RF output that is close to 50% of the PAs rated power.

Procedure for Adjust RF Power Output for Best IMD when Operating Digital Modes

Step 1. Set the PA Gain to 100.
Set your PA drive at 100. When you keep the audio input levels below -1dB, this will produce a signal that has the most optimal IMD figure with the maximum RF output.

Step 2. Set the TX Meter to Forward Power (FWD PWR)
You are going adjust your RF output by changing the VAC TX Gain (AF input) and not by turning down the drive on the PA. So you need the TX meter set for Forward Power rather than ALC.

Step 3. Adjust the VAC TX Gain (audio input) Until you Reach the Desired RF Power Level Out.
For PSK31 or other low wattage digital modes, adjust the VAC TX gain until you hit 35 watts output. On my setup that is about a value of -5.  For other higher power digital modes, adjust the VAC TX gain up until you reach the desired RF output level that does not exceed 100 watts PEP.

Step 4. Check to See if you are Over Driving the Radio's AF Input
Now change your TX meter to ALC and you should be well below 0 dB. If you run the VAC TX gain up to 0, you should be very close to 0 db on the ALC TX meter reading and consequently putting out about 100 watts PEP. You never want to exceed 0 dB on the TX ALC meter or your signal will be distorting due to DSP clipping.  Peaking at -1 dB is the recommended maximum audio input for digital mode operation.

BTW, if you are operating PSK you need to realize that PSK31 is not pure PSK but has envelope shaping in order to minimize the sidebands.  This means that there is amplitude variation along with the phase shift in the carrier. It is a good idea to include some headroom so that the amplitude peaks are not clipping. Setting the VAC TX Gain to produce a ALC level output of something less than -3dB is probably safe.

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Posted Thursday, October 30, 2008 by Administrator