http://elecraft.85.s1.nabble.com/Re-Wha-ts-Wrong-With-Our-Radios-WAS-NewProducts-Building-Demo-tp392768p392781.html
Wow. Thanks Don!!
> Folks, (long philosophical response - delete if not interested in my
> opinion).
>
> Some very good points mentioned in this thread, but the reality of all of
> it
> is compromise.
> For any given price target, some tradeoffs must be made. If I can
> interpret
> the K2 design goals loosely, the dynamic range was considered uppermost,
> and
> good sensitivity and IP3 performance running a close second. To achieve
> that in a kit product, single conversion IMHO is the only realistic way to
> go.
>
> Yes, there ae very good designs out there for multi-conversion receivers
> and
> they have their costs and limitations, but the K2 is a compromise of all
> that. It is single conversion because that is the way to contain the
> dynamic range and good IP3 characteristics within the chosen price range.
> It is well known that one must get into the $3K+++ transceivers to achieve
> those receive parameters that K2/100 owners have achieved for less than
> half
> that price.
>
> Certainly, IF shift is not an easy acomplishment with a single conversion
> transceiver. Answering one question posed in the thread which asked why a
> BFO frequency knob would not do the deed - the answer is YES, BUT - if
> only
> the BFO frequency is changed, the displayed frequency would no longer be
> correct - the firmware currently corrects for both the BFO and VFO
> frequencies and displays the correct carrier frequency, so while passband
> tuning might be accomplished simply by changing the BFO, the VFO would
> have
> to be altered manually to compensate for the BFO shift, and the resulting
> frequency on the dial would be incorrect. We used to do exactly that on
> receivers with a variable BFO, but then we did not have digital dials that
> were good to the nearest 10 Hz (but I digress). When CAL FIL is run, all
> that is taken into consideration in the firmware, and the EEPROM values
> contain the result of that calibration run which produces correct carrier
> frequency dial readings - you have to give up one thing to gain another in
> any design process unless 'price is no object'.
>
> If the world were perfect, we would all have receivers with a 120+ dB
> dynamic range, straight sided selectivity curves (with perfectly flat pass
> band shapes and no change in group delay across the passband), MDS figures
> in the -160 dB range and all that at a cost of less than $100 - of course
> I
> am dreaming based on today's technology and price/performance criteria
> that
> would be within the budget of the majority of hams. There is no sense in
> developing a superior design for production that would sell only one unit
> because of the cost involved - that is the stuff that extreme designs can
> produce, but those are currently the stuff of advanced homebrewers - they
> are the designs of tomorrow when the price of such designs comes down to
> an
> affordable level suitable for production.
>
>
> 73,
> Don W3FPR
>
>
>> -----Original Message-----
>> From:
[hidden email]
>> [mailto:
[hidden email]]On Behalf Of
[hidden email]
>> Sent: Tuesday, August 08, 2006 7:27 PM
>> To:
[hidden email];
[hidden email]
>> Subject: Re: [Elecraft] Why no IF shift ?
>>
>>
>> In a message dated 8/8/06 6:41:46 AM Eastern Daylight Time,
>>
[hidden email] writes:
>>
>>
>> > One problem with single down conversion receivers is that their
>> > "stronger" spurious responses which, depending on the IF used,
>> can be close
>> > to or even in one or more of the frequency bands covered by the
>> receiver.
>>
>> How?
>>
>> In a single conversion superhet, the most important spurs are the
>> image and
>> IF feedthrough. In the case of a K2, the image is always about
>> 9.830 MHz from
>> the desired signal, and the IF is around 4.915 MHz. The bandpass
>> filters take
>> care of those spurs very well, in my experience.
>>
>> If
>> >
>> > bandpass filters are used between the antenna connector and the
>> mixer, their
>> >
>> > selectivity might offer some degree of protection against
>> signals coming in
>> > at spurious response frequencies outside but close to the bands
>> covered but
>> > obviously no protection against anything coming in at an
>> "in-band" spurious
>> > response frequency.
>>
>> What in band spurs exist in the K2?
>>
>> The choice of IF that reduces this problem in a single
>> >
>> > down conversion receiver is quite limited.
>> >
>> > In a double conversion receiver, up and then down, assuming
>> that sensible
>> > design and construction practices are followed, the close or in-band
>> > spurious responses (if they exist) are considerably reduced.
>> >
>>
>> That may be the case in a general-coverage receiver, but in a
>> ham-bands-only
>> design, the spurious responses are easily handled by good input filters.
>>
>> > Then there is the internal birdie problem, usually created by
>> one or more
>> > of
>> > the receiver's oscillators and /or their harmonics getting together to
>> > produce a signal at some spurious response frequency of the receiver.
>>
>> Again, this is dependent on the design. For a ham-bands-only
>> receiver, the
>> birdies can be placed outside the ham bands.
>>
>> If the
>> >
>> > Front End, LO and IF are not properly shielded in separate
>> "boxes" with all
>> > associated DC and control lines filtered, then expect birdies
>> in a single
>> > down conversion receiver. The same method of construction
>> should be used in
>> > a double conversion receiver. I suspect that commercial double
>> conversion
>> > amateur receivers have received a bad press because for reasons
>> of cost this
>> >
>> > is usually not done.
>> >
>>
>> There are some very good up/down double conversion amateur receivers. But
>> they all suffer from the same problem: The signal has to go
>> through several
>> stages and conversions before it gets to the sharp filters. In a
>> single conversion
>> design like the K2, the number of stages and conversions between
>> antenna and
>> sharp filter is minimized.
>>
>>
>> > It is true that every conversion degrades the performance, but
>> for several
>> > years the technology has been available that allows a double conversion
>> > receiver to be built which exhibits an IIP3 of +40dbm
>> > at an offset of 2 kHz while running at full gain, Noise Figure
>> of 8db on
>> > 10m. I have one here. The downside is that each one of the
>> three VHF roofing
>> >
>> > filters / embedded amplifiers selected draws 240 mA.
>> >
>>
>> There's also the issue of price....
>>
>> > With double conversion, in addition to true "IF Shift" a form
>> of continuous
>> >
>> > bandwidth control can also be introduced.
>> >
>>
>
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