Why does the Flex 6600 need band preselectors?

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I’m loving my 6600 - an incredible instrument! As I dig deeper and deeper into the documentation and try to learn the art of SDR signal processing, I recall seeing that the 6600 has band preselectors. I’m guessing that these are some sort of high quality analog filters that sit ahead of the ADC. But I thought the whole point of an SDR was to move this sort of thing entirely into the DSP. For instance, Flex Radio’s VP of Engineering has a brilliant explanation of why an SDR architecture can handle saturation due to strong signals (https://helpdesk.flexradio.com/hc/en-...). If the ADC is fast enough and selective enough to use direct sampling, what is the advantage of using any preselectors at all?

Thanks to anyone who can shed light on this trade off.
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Jeremy Gilbert KC1JZE

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Posted 7 months ago

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Paul

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I guess it gives extra isolation if you want to run full duplex. The internal receive filters (on the RX SCU) would supplement the isolation provided by external TX filters on the TX SCU. Other than that, the preselector could offer some attenuation to big signals that might exist just outside the band edges. These could otherwise cause in band images.
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Bill -VA3WTB

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Jeremy, if there was an award for asking the best question, this would win. A very interesting one at that. I hope maybe Steve H or Tim can comment on this to get the technical answers.

But what I do know, is that the analog preselectors filters are about the best in ham radio witch makes the 6600 a contest killer. Even better than the claim to fame 6700.
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Neil D Friedman N3DF

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At a Dayton dinner last year, Gerald said the filters represented a value of about $1,000.  
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Paul

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I think thats about right. I recently bought an automatic 200W, 5 band filter bank from DX Engineering. It cost around £725. Works well with a custom interface from a USB bit cable. Cheapest way (but not quickest in my case) is to home brew.
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Greg - N8GD

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My understanding of the band preselect filters is that it provides isolation of the two receive SCUs in multi-transmitter contest situations (think Field Day) and isolation for those stations running SO2R and/or full duplex.  I have an older Flex-6300 and when I bought it, I recall seeing that the 6500 was recommended for those folks that were sited near strong broadcast stations, since the 6500 has some band preselection, whereas the 6300 does not.  The preselectors in the 6600 are far better.
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SteveM

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I think receive-only filters are not very useful for situations like field day. You would benefit much more by using high power band filters after each transmitter to prevent the cross-band spatter. Of course, the transmit filters are also active during receive, but the real benefit occurs during transmission.
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Varistor

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This is (partially) what good looks like for full legal power multi-radio station.



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SteveM

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A relay box for them, too? Nice!
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Bill -VA3WTB

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Visitor, wow thats way over the top for me...lol

The filters are not so bad in our radios though.
At field day a couple years ago I was working phone on 40m and another person working CW on 40m. our antennas were a bout 60 feet apart.
When I transmitted he could not hear his contacts, and at one point just gave up. When he sent CW I could never know he was transmitting. At one point we switched radios so he could use CW,,but for me  while using  his radio he knock me out every time he sent,,but I had lots of contacts anyways so I didn't mind.

But that shows how good my filters worked on my 6500.

With what some of you guys are doing you need much more...
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Geoff - W8GNM

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Those analog filters are there to protect the A/D converters from strong out of band signals, but the insertion loss of the 7th order filters is about 8dB which degrades the noise figure of the RF pre-amplifiers.  The filters can be bypassed by opening another slice that is outside the passband of the preselector filter normally used on the same A/D converter.   This can improve the noise figure on the VHF bands if necessary.

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Geoff - W8GNM

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Those analog preselector filters are there to protect the A/D converters from strong out of band signals, but the insertion loss of the 7th order filters is about 8dB which degrades the noise figure of the RF pre-amplifiers.  The filters can be bypassed by opening another slice that is outside the passband of the preselector filter normally used on the same A/D converter.   This can improve the noise figure on the VHF bands if necessary.

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Robert Lonn

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This is why Flex and their products are well engineered... Reminds me why 2 Fuel Injected V6 can differ as much as 100 Plus Horsepower... Engineering. Just because something claims it is a SDR, does not mean you will automatically get instant Performance from it... I am sure Flex has to make engineering decisions based on What the Ham really needs, What they expect, and how much will it cost to make it happen...

Most of us have been around this hobby for many years, and many different Brands!!! As the WORLD waited for the Icom 7610 and Flex 6400/6600 to hit the market place,,, The same folks are now waiting for the new Yaesu to hit the marketplace.. Now those same folks are hoping and somewhat Demanding that Yaesu fully support their remote software capabilities??? I just laugh at these comments!! I am not talking about Flex Owners!!

Yaesu, Icom and Kenwood build a basic SDR software package and just make minor Bug Fixes.. That is why All OF Us for the most part own Flex.. Yes we get FAST bug revisions, but we get Feature Enhancements with SSDR!! Some call this Future Proofing the radio... If the new Yaesu has 4 or 5 features that the Icom 7610 does not have, I see some, not all, of the Icom Owners selling their radio and changing brands...

Price will dictate this, they say the new radio will be in the $5000 range... But we already know that the Yaesu will NOT have a HDMI output!!! That was one of 5 Top Features that convinced me to go with the 6600M radio... But Hay!!!! You already know what I am saying....

Robert
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Neal Pollack, N6YFM

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What are preselectors for in an SDR Radio?

The direct sampling radio ADC chip will digitize EVERY signal it sees on the input pin.
That will include out of band signals, and strong adjacent out of band signals.
[Strong signals can desensitize the receiver too]
Every signal there needs to be digitized and fit into digital stream coming out.

If you use a filter in front of an SDR ADC to limit the input to the ADC chip to be ONLY your desired
band of signals, you improve the dynamic range (performance) of the receiver.  You also block
strong signals just outside the band from causing trouble.

If you recall the release timeline for the Icom 7610, they were initially showing off the radio at
open house events, and claiming release near summer of Dayton that year.  Then, after Dayton, and rumors of undefined problems, Icom
went strangely radio silent for months and months, then emerged with glossy ads about their new
relay driven pre-selector box, and a few months later announced ship dates for the Icom 7610.
I suspect they found the hard way that leaving out preselectors results in rather poor performance,
and the input to the ADC chip getting swamped with what I will loosely call "pollution of irrelevant
signals" outside the band of interest, that need to get digitized like everything else on the input.

[Technically, the ADC chip does not get swamped at all;  you simply get less dynamic range, less performance, since all the unneeded signals have to get digitized also.  It's very good to block the unneeded signals.]

So the preselectors help with the following;

1.  Isolation between the antenna inputs, good for contesting and helps with full duplex.

2.  Improve dynamic range

3.  help block irrelevant signals outside the band of interest  (really part of #2 :-)  )

Cheers,

Neal
(Edited)
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Bill -VA3WTB

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Back to his original question. why must the band filters be analog and not built in software?
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Paul

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The title of his post is "Why does Flex 6600 need band preselectors?" Seems like it has been answered comprehensively Bill.
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Duane, AC5AA

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Maybe because software filters would have to FOLLOW the ADC, not precede it.  You're trying to protect the signal coming into the ADC, not deal with the aftermath of overly compressed dynamic range.  Pretty much what Neal said, above.  That's what I'm understanding.
(Edited)
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Ken - NM9P, Elmer

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My simple answer is that it is NOT to protect your receiver from all the OTHER stations on the bands, but to protect your receiver dynamic range from your OWN signal if you are running SO2R or multi-transmitter contest stations where you are only feet away from other KW transmitters, or from your own running full duplex.  It is a "competition grade" preselector that will generally only be essential for those in competitive situations.
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Varistor

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Ain't happening. You need a lot more filtering to protect two KW stations from interfering with each other. I use stubs at the feed point of each antenna, high power BPFs after the amps, low power BPFs between the radios and the amps, common mode chokes on each and every feed line, and AC and DC lines RFI filtering. There is no silver bullet that cures all interference.

Running low power is a different story.
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Stan - VA7NF

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The replies above covered many of the circumstances where the filters are needed.  Three items:
* In a multi-multi environment, a standard piece of hardware is a stack of band pass filters that are placed before the receiver(s).  They block the KW other band signals from damaging the Rx front ends (SDR or other).  I have seen the SWR meter jump from other band signals, and some auto-tune tuners jump into tune mode on the wrong band's signal.
* AM broadcast signals on a 160M dipole can drive a 6700 and 7610 into ADC overload which causes mixing products and noise at the Rx.  Even a 160M filter does not filter it enough from a S9+70 db signal at 1600Khz (not a typo) without turning on the attenuator. Our FD antenna is an extended OCF on 160/8040 and is resonant at 900Khz - talk about out of band signals.
* The new 6000s band pass filters make my 6700 grey case turn green with envy and eliminate the need to use two or switch band pass filters in multi-multi opening up SO2R
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Bill -VA3WTB

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A 6700 will not suffer from overload.
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SteveM

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Bill,

I've got a 6500 and a 43' vertical positioned ~1/4mi from an 100kW ERP AM station. Apparently the AM blew right past the preselector because AM station images appeared throughout the spectrum. I could see the AM station flat-topped at 0dB in the pan adapter. I added an external BBF and it solved my overloaded 6500 issue.


So, you're saying a 6700 would not overload at my QTH?
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SteveM

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Stan,


The main purpose of external high power BPF are to reduce the out of band transmission power. They are essential in order to reduce the interference your transmitter causes to your neighbor who may be listening on a different band. Don't get rid of them just because you have better receive-only filters in your radio.
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Stan - VA7NF

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Bill,  Oh yes the 6700 overloads.  Just try listening below the BC band with a 160M antenna or longer without a BC filter; noise and mixing products everywhere.

SteveM, the topic was preselectors and (200W) filters between rig and amp.  Those high power (3KW) filters are normally associated with a triplexer to protect receiver front ends from alternate band fundamental frequencies that the naked triplexer cannot suppress.
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SteveM

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K, last comment on this. When I used the term "high power", I meant any power over the level that small surface mount components can provide. Our club uses the 200W filters on barefoot rigs in our multi-op field day setup. They are absolutely essential to running multi-op. No amount of preselectors or receive filters matter because any one of the sideband or digital transmitters produce enough hash across the other bands to kill the other ops. This is the same issue that repeaters face and is the reason why they require a duplexer.

If it were possible for receive-only filters to solve the multi-op problem, then no one would buy/sell the high-power BPFs. Instead, they would sell much cheaper (but higher # of stages), milli-watt power lever filters and use a T/R relay to bypass during transmission.


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Bill -VA3WTB

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Stan, ready the info I posted, as Steve H explains the overload mith.
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SteveM

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Bill,


Just an FYI. The link you posted does not argue that overload is a myth. I'm not sure why you want us to read it. The post is quite long so please just quote from it what you would like us to see.
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Bill -VA3WTB

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The article is a wonderful explanation as to how over load is not a issue in direct sampling radios.
Even for anyone interested in understanding the technology in our radios it is a great read.
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SteveM

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The article is a response to some critics of direct sampling radios who made wild claims about the applicability of the technology to radio signals. It does not state that overloading can not occur on a 6700.


For example, I provide a thought experiment. Connect a 100W transmitter directly to ANT-A of a 6700, and then PTT the transmitter. Do you think the ADC of the 6700 will be overloaded, Bill?


I'm just saying that there are many Flex-6k owners who have experienced overloaded front-ends. It's strange for someone to dispute it and claim it's a "myth".


I'm not sure why the broadcast band causes trouble for my 6500, it really shouldn't given that the 6500 has a preselector. Tim was even sending me modified TURF files with tweaked preselector settings, but to no avail. I suspect that the preselector is an LPF for certain panadapter settings rather than the expected BPF.
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Varistor

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Of course the 6000 series can overload, just like any other SDR. It’s just that SSDR doesn’t tell you that the radio has overloaded.
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Bill -VA3WTB

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Steve H said that a direct sampling radio even if it did over load will not have any bad effect on the receiver performance. He points out that ADC over load is highly unlikely. And he points out that an ADC with direct sampling, the more signals on the band and the stronger they are helps the ADC signal to noise. As he explains why.
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SteveM

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SteveH also said "If we're talking about a large number of signals across a wide spectrum..."

The problem, Bill, is that we are not. We're talking about a single signal within a very narrow spectrum - a broadcast station.

Going back to my example, do you honestly think the 6700 ADC could handle a 100W signal directly injected into the antenna port? (Hint: something will probably explode.)


(Edited)
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Bill -VA3WTB

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Is that what you had,,a 100w signal in your antenna port?
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SteveM

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My mouth is agape. No! That's why I called it a thought experiment.

But I do have a relatively powerful broadcast signal coming into my antenna port. It's the same situation except that the broadcast signal is not powerful enough to cause an explosion, but it is powerful enough to cause ADC overload.
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Bill -VA3WTB

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Steve, there is a difference between what the pre selectors can do,,and ADC overload.
For technical reading on ADC overload  
https://community.flexradio.com/flexradio/topics/adc-overload-myths-debunked
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Steve - N5AC, VP Engineering / CTO

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Official Response
Analog to Digital converters (ADCs) are voltage devices.  They essentially measure the instantaneous analog voltage on the input to the ADC hundreds, thousands or millions of times per second and report those findings to the digital device that is connected to them.  Like all integrated circuits, they are designed to operate in a specific voltage range.  Above that range, their measurement accuracy either degrades or, given enough voltage, will fail catastrophically, destroying the part.  

In a radio, the designer understands that the radio may be subjected to different operating environments from an RF field standpoint.  In other words, some RF environments the receiver is in may be benign (no strong signals) whereas other environments may be sufficient to cause issues or damage in the receiver.  Before we explore the rationale for preselectors, let's talk about what damage or performance degradation looks like.

In what I will call a "legacy" receiver (a superheterodyne receiver), a series of mixers convert the frequency of interest to one specific frequency where the actual receiver in the radio operates.  If this receiver is an ADC the manufacturer might call the radio an "SDR," and certainly the radio would have software if takes data from an ADC and then demodulates it, but it does not meet the traditional radio industry definition of an SDR where the radio is "defined" by software and can be changed with a software load.  In legacy radio, the performance characteristics are only partly defined by the ADC -- the larger component of performance is often the limitations in the non-linear mixers in the superheterodyne receiver.  In these radios, protection of these devices from other signals which can pollute the receive passband with reciprocally mixed signals is paramount if you care about performance.  Large out-of-band signals degrade receiver performance in a radio like this and so band filters (preselectors) are an important, and required, component of the radio.

In the direct sampling world, however, the things that degrade performance  are different.  The key things to "avoid" are 1) poor phase noise in the sampling clock and 2) anything that would result in sending too much voltage to the ADC (otherwise known as an overload).  (1) requires careful design and component selection, but isn't really the topic at hand.  For (2) to be true, we just want to limit the total RF input to the radio and ensure it is below the overload threshold.  But, interestingly, in a true direct sampling SDR, additional signals in the receiver are actually BETTER for the receiver.  Because the risk of reciprocal mixing is low if (1) is done properly, the additional signals (especially large ones) linearize the converter.  Understanding this concept requires at least a rudimentary understanding of quantization noise.  The brief explanation is that in the process of converting from analog to digital, any patterns that emerge can cause repetitive frequency components to emerge from the spectrum resulting in undesired noise (which was not an input to the ADC, but is an artifact of conversion).  For a more thorough explanation, there are a lot of good papers on the Internet such as http://www.analog.com/media/en/training-seminars/tutorials/MT-001.pdf.  Statements above from other posters that reference limiting the available signals to a particular band of interest through a preselector are actually not correct.  These extra signals help as long as they don't appreciably overload the converter.

So what do I mean by "appreciably overload the converter?"  There is a point in an ADC where the voltage can cause some degradation of performance.  This is generally referred to as "0dBFS" or 0dB with reference to Full Scale or just Full Scale.  To avoid tripping over anything above this level, most converters are tested at -1dBFS or one full dB below full scale.  But the converter can handle voltages above this level before exhibiting damage.  For example, in the FLEX-6000 Signature Series radios, the radios end up at 0dBFS around +5dBm to +10dBm with no preamp on but only experience damage at something around +25dBm (as I recall).  For good reception, you want to keep the total input below the stated overload level of +5 to +10dBm.  Since the converter is a linear voltage device, it responds using the superposition principle.  This principle says that, in the converter world, the received voltage is the sum of the instantaneous voltage of each individual signal (as a side note, if this wasn't true we could never separate one signal from another in the DSP on the back end and just allow you to listen to a single signal).  While this is easy to explain in pictures, it's a little harder in text.  Briefly, if you are injecting a signal generator into the radio and the power level is above the stated overload value (+5 to +10dBm) then you will experience distortion of some kind in the radio.  But if you have 100 signals all across the bands and they very occasionally sum to be greater than the overload value (since they are not harmonically related in any way, come from different clock or signal sources, this is almost certainly a truism), they will only serve to linearize the converter and reduce quantization noise even if they occasionally overload the converter.  How can this be?

If you are listening to a conversation and someone drops a PA microphone and it produces a momentary loud "bang" in the room, you can probably still understand the conversation.  Similarly, if you overload for a few samples out of the 245,000,000 samples produced by the radio each second, it will not be detectable in the 3kHz bandwidth you are listening in.  It will be too brief to affect anything (and you'd never hear it, unlike my imperfect analogy of a mic drop).  This is why overload is not really a problem as long as it's only occasionally.  If you overload 0.005% of the time (this would be over 10,000 samples a second), you're probably not going to notice it while listening to the radio.

Back to the question at hand: why do we need preselectors?  Well, it's all about the total signal level in the receiver (NOT which bands of signals are in the receiver) and whether it is likely to 1) be a significant amount of overload (from a time perspective) or 2) produce damage.  This is why the preselectors are present.  At a contest station or field day, you may have antennas close together that could couple more than a few mW worth of signal into your receiver.  To prevent this from causing damage, the preselector filters are present.  They absorb the out-of-band energy from a co-located transmitter and keep that energy from the receiver.  If you are not in this kind of environment, they are not necessary.  The FLEX-6300, for example, does not have band preselectors at all.  It does have broadcast band filters, but no amateur band filters.  It would not be suitable for operation at a multi-multi contest station, but for a ham in an environment where he has no strong ham neighbors, etc. it's a great radio.  In the FLEX-6600 (the other end of the spectrum), it has filters that can take power approaching 10W from an out of band signal and filter that before it hits the receiver.  Obviously, this is not typical for amateur radio receivers, but the FLEX-6600 is designed to work at serious contest stations.  It's worth noting that a +10dBm signal which would not hurt any FLEX-6000 receiver is an S9+83dB signal.

Kind of a long-winded answer, but I hope it clarifies why and when the filters are necessary.
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Bill -VA3WTB

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Still yet more ongoing education, thanks Steve.
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Ken - NM9P, Elmer

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Excellent lesson, Professor Hicks!
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Jon, EA2OT

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Thank you for the nice explanation Steve.

Where could I find the adjacent band rejection (dB) for the band preselectors of the 6600 (7th order) and 6400 (3rd order) series?

73! Jon
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Steve - N5AC, VP Engineering / CTO

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It's greater than 25dB for the 6400, 6500 and 6700 and greater than 50dB in the FLEX-6600.
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Jon, EA2OT

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Thank you for the fast answer Steve!

73! Jon
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Steve - N5AC, VP Engineering / CTO

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I should point out that the FLEX-6600 is >50dB on the contest bands (80,40,20,15,10) and >25dB on all other ham bands where you wouldn't be likely to have a strong transmitter adjacent while you are operating.