6400 sensitivity discussion

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  • Updated 11 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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Posted 11 months ago

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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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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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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.
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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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Gotcha,thanks ken. Will experiment more
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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.
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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.

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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!