Flex 6K ADC overload prevention?

Barry N1EU

There was a posting to the Elecraft list describing a Field Day operation that included K3, Orion and Flex 6Ks.  It said that the Flex 6K receivers (ssb) were impaired when the K3 or Orion transmitted cw on the same band.  I know many things could possibly explain this but I just wanted to ask: does the SmartSDR software or Flex 6K firmware automatically dial in some front end hardware attenuation as necessary to prevent ADC overload?  I know that the K3 and Orion do just that ("hardware AGC" on s9++ signals).

73, Barry N1EU

Tim - W4TME

Yep, I suspect the 6000 was adversely affected by the phase noise from those transmitters.  I'd need to see a picture of it to be certain.  We have this issue when we demo the radio at some hamfests where there is a special event station.  The radio can easily handle the signal level, but the phase noise is incredibly wide and that wipes out a portion of the band. Signal is signal, no matter how it is produced. 

The FLEX-6000 ADC can handle a very large amount of aggregate signal and will overload somewhere in the neighborhood of +9 dBm.   When this happens you will see spurs or "images" on the panadapter.  There isn't an ADC attenuator that automatically kicks in to reduce the when you approach the ADC saturation level.  The AGC only operates on the recovered audio inside the RX passband filter.

Barry N1EU

Thanks Tim!  If ADC Overload was occurring, is there a visual indicator on the SmartSDR screen like there is in PowerSDR?

Tim - W4TME

There is not.  We have a pending feature request to add this indicator, but it has not been scheduled.

One thing I failed to mention is that if the aggregate signal level rises to the point of possibly damaging the ADC, there is a protection relay that will disconnect the RF signal path until the signal level drops.  I have actually heard this relay actuate when I accidentally transmitted 100W in the near field of the 6700s RX antenna.  It gets your attention rather quickly ;-) 

Steve N4LQ

Barry
Did the Orion and K3 impair each other? 

Bill -VA3WTB

Of coarse they would say no.... I wonder too if the Flex caused the other radios trouble, I would think not

Steve N4LQ

Of coarse they would say yes. 

Ross - K9COX

Tim, Is that feature present is the 6500 and 6300?

Barry N1EU

No they didn't.  

To me it sounds like lack of antenna isolation plus lack of operator smarts to dial in attenuation exacerbated the Flex 6K filter-less wide-open DDC front end feeding the ADC.

Barry N1EU

Barry N1EU

They said they didn't.

Steve N4LQ

So are we saying the 6000 needs hardware agc? I always wondered why it didn't have it. 

Bill -VA3WTB

Berry there were several Flex used this year in Field day, Maybe we will have some reports as to how they worked out for them.

Tim - W4TME

Yes, absolutely.  The ADC is one of the most expensive parts of the 6000 and we take extra measures to protect it from damage.

Tim - W4TME

The 6000 has a very clean signal when it comes to phase noise.

Barry N1EU

I'm starting to share your concern Steve.  An important piece of info is the maximum signal level that the ADC can handle and under what conditions that level is likely to be exceeded (e.g. on the lowbands, in contests from the east coast, from local stations, etc)

Tim - W4TME

With the exception of a few installations that are within sight of several major AM broadcast stations,  we do not have people with overload issues.  And I seriously doubt that the issue reported here at FD is an overload issue.  I have operated the 6700 within 200' if a 1500W special event station and the radio did not overload. 

Ross - K9COX

Which 6000 do you have Barry?

JoeMoffatt

I am the person who reported this on the Elecraft list.  I do not lack "operator smarts".   We dialed the attenuation WAY back to almost no RF Gain at all.  The K3 still affected it.  These were on two beams about 150 feet apart, 100 watts, beams pointed parallel to each other.    Now, according to Elecraft, the K3 has little phase noise.  According to Flex, it is has very little phase noise.  What is true?   I don't know.  I do know that K3s are famous for operating multi multi with little trouble inband.

JoeMoffatt

Specifically, this was a 6500 I am referring to on 14.250 ish and the CW was on 14.030 ish

JoeMoffatt

On top of that, this was never meant to be a bash Flex and totally praise Elecraft discussion.  I have both and both have their place.  It was meant to be a why did this happen, and how can we mitigate this in the future?    I had a lot of ops that came over to operate that are very interested in Flex purchases, and I want to be able to explain what our issue was with reasonable intelligence.

KY6LA_Howard

Joe Still trying to diagnose your comments k3 has lots of phase noise which is why they brought out K3S with the low phase noise board Dialed back RF gain.? Does not normally happen in 6000 as you can only insert -10DB attenuation

Barry N1EU

Hopefully, nobody is perceived as bashing anybody and we're all about understanding why you guys were seeing what you were seeing.  Joe, if you do try the field exercise again, perhaps you should focus on whichever band had the worst interaction between the ssb and cw rigs and try all combinations of the 6500, 6300, K3, and Orion as cw and ssb rigs on that band and see how each rig affects the other.

Bill -VA3WTB

Ya sounds like the K3 is a little dirty

KY6LA_Howard

@Barry Tim has stated the the maximum input to the ADC before it goes non linear is 9dBm which is the equivalent of about S9+82dB. Or an incredibly huge RF signal. even in the most extreme uses you are unlikely to ever encounter such a strong signal. It is unlikely to be exceeded in any conditions such as contests, field day, local stations, etc, etc

Bill -VA3WTB

It is very interesting, The flex has proven very good in close in transmitters, many use them in field day with no problems, a matter of fact out standing. So It is hard to know what their problems were.

KY6LA_Howard

Sounding more and more like a RFI issue. I suspect that the Flex has more cables around eg Ethernet. My best guess RF splatter from the K3 was getting into the cables going to the Flex which is why it was cutting out and the RF attenuators had no effect.

JoeMoffatt

This is what I suspect too Howard!   I am going to retest this in the upcoming weeks when we can to see what is going on.  I don't necessarily think it is either radio.   This K3 has the new synth board in it as well.  

Bill -VA3WTB

Joe it would be interesting, considering we already know how well the Flex works with close in signals, as they have been used in many field days so far. RF would explain what was going on.

Barry N1EU

There are a lot of variables involved, but it sure looks to me like "within 200ft of 1500w" is going to generate a lot higher signal than 9dBm on the receive antenna terminals.  I must assume that your receiver was on a different band than the 1500W transmitter Tim.

Tim - W4TME

Well, you don't point your RX antenna directly at the transmitting antenna, but they were S9+70ish. Being a direct sampling radio it receives on all bands at the same time.  But to your question, we were able to enabled a slice receiver on the same band and demodulate signals.  We have to tune up beyond the phase noise which was several hundred kHz.

Barry N1EU

So there's no bandpass filter between the rx antenna terminals and ADC?  I guess that was a bad assumption on my part.