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How to Adjust RF Power Output for Best IMD when Operating Digital...
How to Adjust RF Power Output for Best IMD when Operating Digital...
How to Adjust RF Power Output for Best IMD when Operating Digital...
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.
This KB article may reference additional files that are available on the FlexRadio Systems web site Downloads page. Please use the URL(s) below to download the referenced materials.
An Adobe Acrobat Reader may be required to open the file. You can download Adobe Acrobat from here.
KB Source Document(s):
None Referenced
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Tags:
Best Practices for the FLEX-3000
,
Best Practices for the FLEX-5000
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Details
Last Modified:
Friday, July 1, 2011
Last Modified By:
Administrator
Type:
HOWTO
Rated 4 stars based on 3 votes.
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