Clean TX at Low Power?

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  • Updated 3 years ago
I am thinking of removing the attenuator form my Expert 1.3K.  My Flex 6500 would drive the amp with about 5 watts.

Is the Flex 6500 transmit output just as clean at 5 watts as it is at 20 watts?   I am aware that some radios lose transmit quality when power is very low.  If that's the case with the Flex, I will leave the attenuator in the amp.

73 de K1ESE
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Posted 3 years ago

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Martin Ewing AA6E

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Interesting question.  Somebody should measure this.  You can think of reasons why low power settings might have higher percentage distortion, but it's not clear if it would be significant in the real world.  Operating your amp with higher gain could also (theoretically) lead to worse RF feedback or other amp linearity issues.

Personally, I wouldn't make a mod like this if there's no big benefit.  You only save a few watt-hours.

73 Martin AA6E
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Any 6x00 can make the measurement by using the Full-Duplex option and an audio two tone test generator such as:

On the (windows) sound menu set the playback default device to Dax Audio TX, enable DAX and VOX on the 6x00. Set volume levels etc.

In Full Duplex be ultra-careful!  Connect a dummy load and transmitter port to the load. I set the receiver antenna to an unused port (xcvr). There is plenty of isolation but best to test with 1 watt at first.

results 3rd order IMD test for 2 tones(550 and 1800):
5 W :   -49dB

100 W : -60dB   !!!!!!

Hmmm.  Can't be..  60dB down on 10 meters at 100 watts. I've measured this before and found -43dB at 100 watts. Below are the graphs. Perhaps someone else can try the measurements.


100 Watts:

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If those readings are accurate, it would appear to be much cleaner at higher power.  Not sure if that difference also shows up at 5 watts vs 20 watts.
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Ken - NM9P

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Be careful about believing what you see on the receiver panadapter when transmitting with FDX.  

While what you see within the TX Slice's filter flag is fairly accurate, I have found that the rest of the display can exhibit some spurious artifacts due to overloading and leakage within the rig when transmitting.  This often shows up during digital transmissions, causing people to freak out when they see phantom spurs on their rtty or psk31 signals.

The 6000 receiver is almost as good as an expensive spectrum analyzer, but there are a few caveats...this is one of them.

Perhaps recent software updates have changed this.  If so, I will be glad for someone from FRS to correct me on this.
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Tim - W4TME, Customer Experience Manager

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Perhaps recent software updates have changed this.  If so, I will be glad for someone from FRS to correct me on this. 

No deltas
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John - K3MA

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I have been running my 6500 and SPE 1.3K with 4-6 watts of drive for over a year and never a issue reported with 3rd order products not being quite acceptable.  I think the larger problem will come from trying being conservative in your expectation of the max amp output.  I keep mine down around 1K watts and do not try to drive it to 1.3K or higher.  If you do the same you should expect a clean output.
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David Livingston

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My SPE 2K has no issues running as low as 2 watts with this new release.
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Gerald - K5SDR, Employee

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I am not sure I can add much without doing measurements myself at low power.  Monitoring the internal RF leakage on the panadapter is not a good method for the reasons Ken stated above.  You would need to put the transmitter through a power attenuator (e.g. 40 dB/100W) to get the signal level down to receive levels and then do the measurements.  In that case, you could "carefully" make measurements with a FLEX-6000 or a commercial spectrum analyzer.  With a power attenuator in line, you can't get a much better spectrum analyzer in terms of dynamic range than a 6000.
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Doug Hall

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I measured the Flex third order TX IMD just now using my 6300, and the results are attached. I used Audacity to generate a 700/1900 Hz two-tone signal and fed it to the DAX TX input. The 6300 was running on 14 VDC and feeding a dummy load with a 60 dB attenuator on the output feeding the spectrum analyzer. There are two graphs, the first is at 100w output, and the second is at 5w output. The marker values shown on the screen can be read as the third order IMD value. Keep in mind that this is how far down the third order product is from either tone. Subtract 6 dB (or add -6 dB) if you want ARRL-style readings, as I believe they measure from PEP in their tests. This is just for 80m where the rig happened to be sitting. Sometimes TX IMD suffers at frequency extremes. For example my Kenwood TS-2000 is significantly more stinky on 160m than on the other bands. But a quick check of the other bands on the 6300 indicates that they are all decent in terms of TX IMD. In any case I believe it answers the question about whether the IMD suffers at lower power levels. It does not appear to.
Doug K4DSP

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Tim - W4TME, Customer Experience Manager

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Thanks for sharing the plots, Doug.