[Drake] Was Roofing Filters, Now High Perf RX?
Bob Camp
ham at cq.nu
Sun Aug 15 22:14:13 EDT 2004
Hi
I doubt you will find a single band high end radio as a kit. There just
isn't enough of a market for them.
There are a lot of internet sites out there with information on fancy
mixers. Currently top of the hill honors start out around +45 dbm for
third order intercept and go up from there.
For the IF filtering I would use the same tried and true Sherwood parts
that we already know work very well. They are an available and proven
part. Why mess with success .....
For the local oscillator start out with something up at microwaves and
run it through an expensive divider chip. That and a reasonable
synthesizer design should get things fairly quiet.
Each of the blocks is out there and various people have put them
together with more or less success.
Enjoy!
Bob Camp
KB8TQ
On Aug 15, 2004, at 9:01 PM, Jim W7RY wrote:
> 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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>>
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