[Drake] Was Roofing Filters, Now High Perf RX?

Sun Aug 15 21:01:25 EDT 2004

Speaking of high performance receivers...

I've been looking around on Google for plans, circuit board kits, or 
complete description on how to build a high performance single band 
receiver that would be = to or bettern than a hevily modified R4C ?

Anyone know of one in existance?

73
Jim W7RY

At 11:05 AM 8/15/2004, Bob Camp wrote:
>Hi
>
>Keep in mind that the filter is not the only thing that determines the 
>dynamic range of of a radio. The phase noise of the local oscillator also 
>has a major impact on close spaced third order dynamic range.  For a lot 
>of years the R-4 series has had a ranking near the top of the list for 
>phase noise performance. Mixers, filters, and switching also get into the 
>act. Sometimes they do it  in a big way, sometimes not.
>
>Another item that you can get hung up on is a specific dynamic range 
>measure. You can focus on a third order close spaced measure or on wide 
>spaced second order number. They are radically different things but booth 
>evaluate the performance of a radio. A radio with a tuned front end like 
>the R-4's or an R-390 will do significantly better on second order 
>intercept than a broad band front end radio.
>
>Unfortunately when you hook a real radio up to a real antenna a whole 
>bunch of things happen all at the same time. Depending on which antenna, 
>which radio, and what time of which day you may be hit by one thing or by 
>another. In some situations a radio with a negative intercept for both 
>second and third order will do a better job than a radio with 80 and 40 db 
>intercepts. It all depends on what the limiting factor is right now with 
>your antenna listening to one specific signal.
>
>Even if you are talking about a CW contest situation with six foot tall 
>helical front end filters on the antenna, third order may not be all you 
>have to worry about. I am always amazed at how well some radios hear 
>things with no antenna attached.  The IM performance of your radio may 
>actually be better than the performance of that ten year old  coax 
>connector on the antenna ....
>
>A simple real world check on all of this:
>
>Pop the antenna on the radio and make sure the noise out of the radio goes 
>up when you are tuned to a quiet part of the band you want to use.
>Stick in a 10 db pad. Does the noise still go up? if so keep stepping up 
>the pad.
>
>Once you have a pad that keeps you from hearing antenna noise back off by 
>10 db. Most of the time you will find that you have a 10 or 20 db pad on 
>the lower HF bands and a 10 db pad on 20 meters. Of course this trick only 
>works if the band is open....
>
>At this point you are still noise limited by the antenna and not by the 
>pad and receiver combination.
>
>Next tune over to a busy part of the band and try to tune a weak signal. 
>Drop the pad down by 5 or 6 db  and see if that helps pick out
>the weak signal. If the signal is easier to pick out with the pad in then 
>you are dynamic range limited. If the pad has no effect then dynamic range 
>is not an issue. You have to do this with care because you will naturally 
>favor a louder signal. When the guy at the stereo store switches to the 
>expensive speakers he always cranks up the volume .....
>
>Finally find a signal that sounds like distortion. If the distortion signs 
>with KB8TQ it's just my normal signal ... Watch the S meter as you cut the 
>pad in and out. A 10 db pad will knock in radio second or third order 
>distortion down by significantly more than 10 db. If it only has 10 db of 
>effect then the distortion is from that coax connector and not from the 
>receiver. Or it may be the splatter from my 200KHz wide signal ...
>
>No need to fix what isn't broke .......
>
>If it is broke then the question is weather you want to spend major bucks 
>on a compromise multi band radio or build a cheap single band radio that 
>will blow any multi band away. It's not clear to me that doing heavy mods 
>on an R-4C is really any easier than building a simple radio from scratch ....
>
>         Enjoy!
>
>                 Bob Camp
>                 KB8TQ
>
>
>
>
>On Aug 15, 2004, at 12:21 PM, dmartin wrote:
>
>>Been doing some reading lately on the concept of roofing filters and their
>>influence on near-in blocking dynamic range, etc., performance. The TT Orion
>>seems to be the first factory offering to take advantage of a variety of
>>various bandwidth front end roofing filters. Of course, Rob has long offered
>>his Sherwood 600 Hz roofing filter option for the 4C. Tom Rauch, W8JI, has a
>>great article in the current WorldRadio where he gets into receiver
>>performance concepts. He pretty much disses the stock R-4C on close-in
>>performance and concludes that a Sherwood 600 Hz roofing filter makes the 4C
>>acceptable, at best, on CW performance. However, my confusion over
>>the "acceptable on CW" is this: if you check the performance tables on Tom's
>>website at http://www.w8ji.com/receiver_tests.htm, you'll note that although
>>the close-in performance of the stock 4C is as poor as several references
>>state, the "R-4C heavy mod" specs at least equal to >exponentially exceed<
>>anything else tested, including the Icom 7800 and current Orion. Along with
>>the addition of Sherwood's 600 Hz however, the "R-4C heavy mod"
>>includes "solid state double balanced high level mixers". I'm quickly aware
>>only of Rob's IC third mixer mod. Anyone know any details on the 1st and 2nd
>>mixer mods on Tom's test 4C?
>>
>>Not considering such mods, mind you, just want to better understand how such
>>apparently superior performance, equal to and exceeding $3,000-$10,000
>>radios, could have been squeezed out of what began life as a 4C? From this I
>>would conclude that an otherwise stock 4C but with Rob's 600 Hz roofing
>>filter and possibly his 3rd mixer mod might make for a CW receiver very
>>competitive with anything currently offered today, no?
>>
>>Dan
>>WB4GRA
>>
>>--
>>
>>--
>>
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