Re: Local Oscillator Convention for Single Conversion?

Abel Pranger wrote:
On Wed, 21 Mar 2007 10:24:12 +0000, The Natural Philosopher <a@xxx>
wrote:


I thought you meant that having the LO above or below made an impact.

Well, it does have an impact, and that goes back to my original
question.


ye, but not on audio distortion etc. etc...howvere, back ON topic

My concern is image frequency interference. A superhet rx will be
receptive to transmitted signals at the LO frequency + and minus the
IF. Only one of those + or - offsets is the desired receive window,
the other being the 'image' frequency where interference may be
received from another transmitter. As I have seen no figures cited
by R/C vendors for image rejection, I presume they are not proud of
the performance of their products wrt this parameter (or they think we
are too stupid to understand what it means).

Single conversion sets are truly dire in this respect and rely on the image being 'uninhabited'. To get an RF tuned circuit capable of rejecting the image would mean that sets would need to be tuned up every time a crystal change was contemplated. Its probably true to say that the image rejections is virtually zero. HOWEVER if you do the sums, yoiu will at least find that the difference is 910kHz, and IIRC at least in Europe we use 20Khz channel spacing, so no other transmitter even on an image, will produce a signal that is closer than 10KHz to the wanted signal in the IF. That is within the bounds of what a decent IF can block pretty well.Also I think our channels do NOT extend to 910KHZ in total. So there are no signals on the images at all. Or should not be.

In LW/MW it is advantageous to place the oscillator above, if for no other reason than that trying to tune into a station on - say 455KHZ with a 455KHZ would place the LO at zero!

Up at our sorts of frequencies there is no really sound reason for either, but the convention seems to always been to place the LO above incoming..its just a habit. Sometimes in specialized applications you may have a reason to do the reverse..if there is a particular need to block a particular out of band signal for example.


Based on a sampling of SC crystals that I have on hand, I know that at
least some R/C mfgrs use a LO offset that is above the assigned
channel frequencies in the US band allocation. From that, it is
simple math to determine that the 4 channels at the low end of the
band have images that are interstitial to the 4 channels at the upper
end, hence there is a potential for image interference from sources
within the allocated band.

What I don't know, and this was/is the essence of my question, is
whether all SC rx's use an LO offset that is above the carrier freq,
or if there are some in common use that employ an LO that is below the
carrier they are intended to receive. If so, there would also be a
potential for the channels at the upper end to be interfered with by
transmitters at the lower end.

I am fairly sure they are all above.

In DC sets sometimes its possible to replace the second LO with instead of (10.7MHz + 455kHz) with (10.7MHz -455kHz) and turn a negative shift to a positive shift set or vice versa.

But by and large most SC sets are simply 'above' ..its pure convention..

Its worthwhile looking up waht is on the image..IIRC in the USA you may have trouble from high power pager TX's that may militate against the use of certain channels in certain locations.


Abel
.

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