6400 sensitivity discussion

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  • Updated 5 months ago
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I've always felt the 6400 reciever was lacking when it came to RX sensitivity, particularly when it comes to pulling out weak signals. Tests I've done with signal generators and the recent Sherwood report confirms this. It's the worst on the list!Using a preamp at +16 on bands above 40m is a must if you're trying to hear a weak signal. The method listed in the article below is irrelevant when it comes to the 6400. https://helpdesk.flexradio.com/hc/en-...
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A.J. AJ2I

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Posted 5 months ago

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KF3F

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For what it's worth, my 6600 with an RF Gain of 16db receives the same as my Kenwood TS-850.
Signal strength is the same as well as weak signal copy capability.
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W9HH - Steve

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I always wondered where the 6400 would end up on Sherwood's charts and suspected the results wouldn't be as good as everyone originally thought. So when the tests were finally published on the website a few weeks back, I was a bit disappointed. Still love the radio though, and it does just about everything I need it to do for my style of operating. No plans to sell it because of the test results, It's not going anywhere. Maybe this will put things in perspective:

https://www.youtube.com/watch?v=up6QB3qyc6M

73 -
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Gayle Lawson

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I have looked at the receiver block diagrams of several of the radios listed in 
Sherwood's table.  Most of the radios have a dedicated "RX RF amplifier" stage in front
of their mixers to help obtain  sensitivity values. 

Flex 6000 radios have a dedicated variable -10 dB to +32 dB amplifier stage in front of their 
16 bit A to D converter, however, Flex calls this amplifier a "PRE-AMP" not a "RX amplifier". 
The Flex PRE-AMP is the same as other manufacturers RX amplifier.  The beauty of Flex radios is that you can set the gain that you need per band.  I think that we may have gotten hung up over naming conventions.

It would be nice to know what the Rx sensitivity of other radios is with out the fixed gain 
RX amplifiers.  I know that the FT-5000 can be set to 0, 10 and 17 dB gain.  The FT is speced at 
.2 uV with 17 dB gain RX amp enabled.  Flex 6400 was measured at .6 uV with 16 dB of gain.  
Yeah that's down from the FT but we got +24 dB or 32 when needed.

I think that Flex is doing very good with ~-111 sensitivity, and  no gain in front of the 
16 bit A to D converter.  Remember Sherwood measured both sensitivity and close in 3rd order IMD 
with  16 dB gain in front of the A to D.  (100 dB IMD isn't to shabby)

I'm happy with the sensitivity of my Flex radios.
  .

Gayle K0FLY   
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Paul Bradbeer

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Good post, Gayle. The important thing is that we're discussing different RX architectures. Flex has enough gain in the RX system to hear very low down (about -140dBm) ; the fact that you may have to use 32dB of gain to get there is irrelevant!
I use my 6400M extensively on 6m weak signal, and at my quiet QTH I am using the +32dB preamp. So what? ...Nothing wrong! Paul M0CVX
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A.J. AJ2I

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My difficulty is running out of AGC-T when you run the preamp that high. I usually run mine about 25 when I'm at +16. But when you get to +24 or +32 you can goto 10 on the AGC-T
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Ken - NM9P

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Are you saying that your "sweet spot" where the noise just begins to drop on a quiet frequency is not reached until you move from the right all the way down to 25?  (Or 10, with +24 or +32)  On which band, and with what antenna?   
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Manuel Maseda

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Great post Gayle. For those that don’t know him Gayle is a highly respected retired Rockwell Collins employee.
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Gayle Lawson

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Hi Manuel:

Thanks for the shout out, "Highly Respected" maybe a stretch. HI  HI

73 Gayle
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A.J. AJ2I

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Ken, what happens is since I find myself using +16 of preamp on 20m and above, I find myself using the AGC-T more like an RF gain which I know is wrong way to use it, to quiet the background noise., even with the EQ 200-2300hz wide. With this setup my AGC-T is usually around 25-30. I know NR is probably more a useful tool.for this, but found the NR is pretty useless feature. So the only choice isto move AGC lower and lower and usually with +24 or +32 at times I have to goto 10.
(Edited)
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Bill -VA3WTB

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A.J I wonder what the difference is. On my 6500 on 20m, I have +10 gain,  the AGC-T is set to 10. I turned on the NR and set it to 15 and I can hear stations down in my noise floor just fine. The NR cuts about 75% of the noise, it does a nice job.
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Ken - NM9P

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A.J. If you are running the AGC-T down that far below the sweet spot in order to cut down on noise, then you are also cutting lots of the weaker signals out. Which is ok if you are working strong signals and want a quiet background. But if you are listening for weaker signals on a quiet band, you have just cut off everything you gained with the preamp.

But these rigs are flexible with lots of controls, so people can set them up any way they want if it fits their needs.
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A.J. AJ2I

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Gotcha,thanks ken. Will experiment more
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K5ROX

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I have many radios, many pretty old and my flex 6600 will not receive a signal that my others wont, my 6600 isn't any better.  I had a 6300 and it was same.
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James Whiteway

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Poor antennas can make a great receiver preform poorly. Good antennas can make an average receiver work well. Even to the point of overloading the radio.
The better the antenna the better, to a point, any radio will receive.( and transmit)
James
WD5GWY
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Steve

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Terrible title for this thread.. it would make one think that the radio sucks and it does not.  usable sensitivity
is dependent on many variables.  Just my opinion.
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Steve - KC5F

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Agreed. I'm about to jump in and buy a 6400 or 6400M, but from this and other threads some folks seem to think I should just save money and buy an IC7300 and get better sensitivity, noise blanking or whatever. They haven't quite convinced me, though!
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Winston VK7WH

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Steve, I think the major take out from this is that some people don’t understand that the Flex architecture is different from the radios that the 6400 is being compared with. The Flex RF Amp/Attenuator can be switched in or out on different bands, as required, in order to achieve the best balance between sensitivity and dynamic range. In addition the settings of each band can be recalled each time you return to that band.

When I first received my 6700 five years ago, my immediate reaction was that “The receiver is sensational”. I still feel the same way 5 years later.

Good luck, whatever you decide to buy

Winston VK7WH
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James Whiteway

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I wouldn't let this or any thread, completely influence me in regards to purchasing a new radio.
I had an Icom 7300 for a backup radio and the ONLY thing it had better than my Flex 6600M was the Automatic Notch Filter.
Sensitivity is much better on the Flex. Plus, the Flex receiver is much quieter than the 7300.
If you are looking for an inexpensive radio that has a good bandscope( but, no external monitor capability) the 7300 is hard to beat for the price.
But, a 6400M is a good choice for not a lot more money.( new) It really boils down to what YOU want in a radio. And what you feel comfortable paying.
James
WD5GWY
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Steve - KC5F

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Thanks, both. I'm a cw operator, and not seriously considering the 7300. And an automatic notch filter wouldn't help much on cw! But from reading everything I can find on the Flex, I have no doubt one would be great for me. Just trying to decide between a 6400M now or a 6400 now and a Maestro later.
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Michael Walker, Employee

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Steve, give me a call at 512 535 4713 and just ring the Sales extension as it rings my phone.  If we don't answer, leave a message as we return all calls.  This is best discussed in a conversation.

Mike 
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Michael Wheatley

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What are the recommended preamp settings for 6400 on 160-20? 17-6?  I saw the post in which Tim Ellison recommended +16 but I'm not sure he meant all bands.

Michael -- KM6LHD
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George Molnar, KF2T, Elmer

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One thing to remember, gang. Per ITU-R P.372.7, radio noise vs frequency data indicates that it pretty much doesn't matter outside of a lab environment. Most every modern receiver has an MDS at or below typical noise environments in the real world. A few dB that you could not possibly discern shouldn't be an issue.

As others have said, other factors, including IMD, filtering, dynamic range, etc., will quickly overwhelm the putative noise level differences. Radio manufacturers often "trade off" one value for another for marketing or engineering ease. It's not indicative of a "good" radio vs. a "bad" one.

Most of all, your environment affects your receive capacity. If you are experiencing noise in excess of the ITU table, don't be surprised. Noise levels have been increasing steadily in the developed world as more noise sources (switching supplies, a million kinds of devices, energy-saving lamps, etc.) that the real curves probably ought to be revised upward.

Rob Sherwood is a careful professional who provides excellent information. It is our job to read it in context and understand the very real differences between lab testing and the real world. 
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N5LB - Lionel B

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Well said!
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Robert Lonn

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Tonight I was on 20 meters listening to the Maritime Mobile net.. Here in Ca. it was not that great, the signal was only about a S-3.. So after making all the adjustments I could on my 6600M I decided to switch to Diversity Reception.. One antenna was a Carolina Windom, the other was a 65 foot Longwire.. On went the earphones, and it is such a cool feature, and what was a hard to understand conversation became very clear and readable.. NR was set to about 50%, Bandwidth was 2.3..

So after I heard the part and called the interested party, I switched on my Icom IC-9100 radio to see how it would here the station.. This is not an apples to apples comparison, but the Icom was basically useless.. I am not familiar with what other radios can do diversity like the Flex Can, you need two built in independent receivers, but I presume that when you switch to Diversity you need to Phase Lock the two receivers together, and having a 10 MHz reference input probably helps as well.. You hit the A<>B button to assure both are on the same frequency.

I was using the 6600M stand alone, not using SmartSDR external software, that should not matter much.. I think the 6600M can do this, I dont think the 6400M can do the same function as it does not have 2 SCU???

OH!!! I am still on 2.xx software, do we know if 3.xx has any enhancements to diversity  reception, I know a lot of folks wanted more flexibility on how it operated..

Robert
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HCampbell WB4IVF

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In another thread Chuck/W9WLX provided a link to an update of the Flex article on setting preamp gain.  Apologies if it’s already been discussed and is old news.  In any case it's interesting and explains reasons for / differences in preamp gain settings between the Flex radios:

Why is the FLEX-6600(M) / FLEX-6400(M) noise levels +10dB higher vs. the FLEX-6500 or FLEX-6700?

Now to the difference between the FLEX-6500/FLEX-6700 and the FLEX-6600. The FLEX-6600 incorporates 7th order bandpass filters on all of the HF contest bands. The 6500/6700 incorporate 3rd order filters on those bands. The 7th order filters provide greater than 50 dB of band to band isolation between the contest bands at the cost of higher insertion loss. This is excellent for Field Day, SO2R, and other contesting configurations. This additional loss is easily compensated with additional preamp gain if required.

Let's take a look at the default preamp gain strategy for the 6400 and 6600 radios. We used the "Rural" noise floor numbers for our calculations to set the default gain for each band. Note that the 6400 filters have a lower loss on the contest bands because it uses 3rd order preselector filters on all bands just as the 6500 and 6700 do. Below you can see the ITU typical noise power in dBm in a 500 Hz bandwidth with S unit call outs in red.



Given the Rural noise floor for each band and the receiver noise for each band we calculated a the target sensitivity to be 10 dB below the ITU typical. We then picked the preamp or attenuation setting. The table below shows the gain setting and the expected MDS with no antenna attached for that setting on the respective radio. If you need more or less gain for your location, antenna and operating conditions use the procedure above to adjust accordingly.


(Edited)
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Robert Lonn

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TOP SECRET!!!!
FLEX SDR Radio and SmartSDR!! WOW!!! We have come a long way baby!!!

https://www.youtube.com/watch?v=V9xUQWo4vN0

Robert

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Clay N9IO

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Sending this link to my FB "friends."
Thanks Robert.