How much power should I use when running FDV?

Robert -- N5IKD

How much power should I use when running FDV?
Do I just need to keep an eye on the temperature of the transmitter?

K6OZY

Drive it at 100W. It operates much like MT63, low peak output.

Gerald-K5SDR

Read the section on configuring your radio at the link below. You do NOT want to run at 100w. http://freedv.org/tiki-index.php

K6OZY

That seems odd because metered output is normally around AVG 40W when at 100W.  So at 50W, it'd be super low.  Is the Flex amp not linear up to full power?  I thought the amp in the radio was rated well above 100W and it was tuned down to 100W?

Robert -- N5IKD

So 10 to 20 watts, per the instructions on the FreeDV web site :

"Drive your transmitter and amplifier so that it emits 10% to 20% of its rated power continuously. There is a 12 dB peak-to-average power ratio in our HF modem, and peak clipping in your amplifier will reduce the received S/N. Modern transmitters and amplifiers are only as linear, and only have as much headroom, as is necessary for voice SSB, thus we suggest you maintain amplifier headroom by operating well below your full power output. FreeDV is more efficient than SSB voice, and will achieve similar range to an SSB signal driven at higher levels, and better audio quality. We encourage you to ask manufacturers and reviewers to start rating transmitter and amplifier linearity and headroom for digital modes, not just SSB voice."

K6OZY

I'd rather clip the 12dB peak and use error correction on that clipped part than run it at 10% rated power all the time.   A 12dB peak-to-average is pretty ridiculous.  Not all of us have super antennas or 1.5KW amps.  I've been using FreeDV for years prior to the Flex FDV implementation without any issues this way - Drive it like MT63 and ignore the peaks.  Do what you want.

KY6LA_Howard

I only need 40W to drive my Amp to 1,500 W ... so my first FDV contact was with about 900W + 9dB Antenna Gain = about 8KW ERP... life is too short for QRP> 

Gerald-K5SDR

I would suggest that you talk to the guys on the FreeDV hang out frequency. They will know the technical details.

Eric-KE5DTO

We have actually handled the reduction in gain internally in SmartSDR with the FDV waveform, so you can run the slider all the way up to 100 and not get yourself in trouble.

Eric-KE5DTO

Correct.  While the freedv.org website says to drive it lower (for most transceivers), we handle the gain internally in SmartSDR's FDV waveform to correct for this to avoid excessive average power.

Robert -- N5IKD

Looks like the suggestions above are concerned with clipping the signal and not concerned about damaging the radio.

My concern is damaging the radio at 100W full duty cycle. Has this been studied?

DrTeeth

The Flex series 6x00 is rated at 100% power for 100% duty cycle modes - confirmed by FRS when I asked a similar question here.

DrTeeth

Oy. Who needs sunspots when you are squirting that much power into the atmos, LOL?

K6OZY

I've beat the **** out of my 6700 forever now on digital modes regularly doing 4-6 min solid transmissions on heavy duty cycle modes like Olivia and other MFSK modes that are worse than MT63/FDV multi-carrier modes.   The fans kick in as expected, stabilize the temp crawl, and then recover fast.   Your duty cycle on FDV will be more like SSB, which means I would not be concerned at all about duty cycle.   The 6xxx series has been the best digital platform I have ever used.

Rob Fissel

This was a concern when I was researching my 6300. I do a lot of digital mode stuff, and my IC7100 with the IT-100 tuner limited me to 30W of key-down RF output. 

FYI the Signature Series data sheet confirms:

1-100W SSB, CW, FM*, RTTY, DIGITAL;1-25 W nominal AM (carrier)

FreeDV's tutorial, while mentioning driving your rig at 10%-20% of its rated output, mentions the following:

We encourage you to ask manufacturers and reviewers to start rating transmitter and amplifier linearity and headroom for digital modes, not just SSB voice.

Fortunately, Flex has already done that for us. 

An ICOM FAE's response to me was that while there was no official rating for digital "key down" modes, it was reccomended to keep it below 50% rated output at all times for modes like FreeDV or ICOM's DV mode. My tuner limited me to even less than that. 

If Flex maintains that the rig can handle 100W of key down 100% duty cycle, then that's what I'm gonna do when I play with FreeDV tonight. 

Steve-N5AC

Graham (KE9H) is the expert here as he and Ed did the gain balancing.  But I'll throw in my two cents here and Graham can come back and correct any of my assertions when he has a minute (he's on a plane to Dayton at the moment).

In a single-carrier mode like RTTY, CW, JT65 or Olivia, only a single carrier is present on the band at once.  You can run full power as the peak power to average power are equal (peak-to-average = 0dB).  Other modes like PACTOR, MT63 and FreeDV are multi-carrier modes.  Think of them as a bunch of RTTY signals emitted by the same transmitter at the same time.  Sometimes, the carriers will add together and sometimes they will ALL add together (there are 9 carriers in FreeDV as I recall -- the center that is idle and stronger and 4 to either side.  You should check this for accuracy as I'm saying this from memory).  When they add together, the total power is a lot more than it is if the carriers all alternated.  This difference in peak and average power is the peak-to-average power and it is 12dB in FreeDV.  

Sometimes coding is used to ensure that all carriers are not on at the same time.  In FreeDV, there is some error correction so we can effectively run as if there were a lower peak-to-average and just clip above that level -- in this case the symbols will be received wrong, but the error correction will take care of the problem for us.  This is generally the recommendation in FreeDV.

So we have adjusted the gain from the codec so that you can set the power to 100 and it will do the right thing, which I recall is to run at about -6dBFS allowing for a 6dB peak-to-average.