Why does the Flex 6600 need band preselectors?

Jeremy Gilbert KC1JZE

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-us/articles/205527943-ADC-Overload-with-Direct-Sampling-SDRs-Myths-Debunked). 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.

Bill -VA3WTB

Greg N8GD

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.

Neil D Friedman N3DF

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

Geoff_W8GNM

Geoff_W8GNM

Robert Lonn

Neal Pollack, N6YFM

Bill -VA3WTB

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

Duane_AC5AA

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.

Ken - NM9P

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.

Varistor

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.

Stan VA7NF

Bill -VA3WTB

A 6700 will not suffer from overload.

SteveM

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.

SteveM

Varistor

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

SteveM

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.

Bill -VA3WTB

SteveM

A relay box for them, too? Nice!

Stan VA7NF

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 **** triplexer cannot suppress.

Bill -VA3WTB

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...

SteveM

Bill -VA3WTB

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

SteveM

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.

Bill -VA3WTB

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.

SteveM

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.

Varistor

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.

Bill -VA3WTB

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.

SteveM

Bill -VA3WTB

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