Posted by
N2EY on
Aug 09, 2006; 12:27am
URL: http://elecraft.85.s1.nabble.com/Re-Wha-ts-Wrong-With-Our-Radios-WAS-NewProducts-Building-Demo-tp392768p392777.html
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.
>
73 de Jim, N2EY
_______________________________________________
Elecraft mailing list
Post to:
[hidden email]
You must be a subscriber to post to the list.
Subscriber Info (Addr. Change, sub, unsub etc.):
http://mailman.qth.net/mailman/listinfo/elecraft
Help:
http://mailman.qth.net/subscribers.htmElecraft web page:
http://www.elecraft.com