Local Oscillator Convention for Single Conversion?

On Tue, 20 Mar 2007 13:48:21 -0700, I said, "Pick a card, any card" and Abel Pranger instead replied:

I've never attended that convention. Where is it held?

Ok, all kidding aside, the reason for the initial choice many, many, many years ago for the LO frequency had to do with the issue of fidelity in removing the modulated information from the carrier. That particular frequency yielded the smallest amount of distortion due to harmonics when the information being transmitted was in the audio range. Today, the reason is far more practical in that the components have a standard value and availability making the overall cost less for the end item. In reality, you could choose any LO frequency you wanted but finding the components would be difficult.

As to the LO being above or below, it is a convention for Single SideBand, or SSB transmissions. The DSB or Double(Dual) SideBand signals go both above and below. The USB and LSB signals, Upper and Lower SideBand respectively, are demodulated from a carrier that has both upper and lower modulation with the actual carrier removed. Don't ask about that.

Before all you radio heads and techies out there go ballistic on me, I answered this for the casual reader and as you know left out a great deal of highly technical reasons and theory.

-- Ray

Actually in AM radios in the broadcast sense you HAD to have it above otherwise long wave (100-200Khz) didn't work well with a 455KHZ IF!!!the range (255-355KHZ) was a big percentage spread for a tunable radio..a lot easier to run from 555Kh-655Khz

I think the convention ran on from there.

Switching the LO from below to above will of course reverse the shift on an FM set. And vice versa.

The relative interchangeability of crystals between manufacturers suggests that that they all use the same convention.

I have seem them work BADLY when speed but never not at all..which an inadvertent reversal of shift would show up as.

That does not compute, Jim. with full double sidebands there is no difference in audio quality between HIGH L.O. and LOW L.O.

On Wed, 21 Mar 2007 00:44:29 +0000, I said, "Pick a card, any card" and The Natural Philosopher instead replied:

Make it smaller and see what happens. Trust me, that was the reason for the minimum of 455khz. Remember, it was the harmonics causing the distortion, not the principal frequency of the LO. There have been experiments with other frequencies over the years but the old standard seems to keep shining.

As I stated in my original post, I am NOT going to start arguing here as it would not benefit anyone who is suddenly alienated by the technical-speak that would ensue. Go ahead and have the last word while I have the last laugh. You may now attempt to snow everyone under with information that has absolutely no bearing on the question but serves only to salves your ego.

-- Ray

Oh..if you mean that the IF FREQUENCY has an impact, i agree.

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.

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).

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.

Abel

Hello,

Have a look around "Multiplex" brand and you may have a good contender for LO below carrier.

By experience you cannot mix their receiver quartz with Futaba or JR.

Bye,

Marc

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

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.

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.

I am surprised, but it would possibly make sense..Multiplex do things differently in many cases.

Or it may simply be that they cut their xtals to suit different loads, and that is why they do not work in other sets.

A frequency counter would be all it took to establish the real situation.

I have the solution for the Multiplex issue:

A techie at the Multiplex factory confirmed that the MPX transmitter is compatible with Futaba PPM receivers as long as the relevant manufacturer's crystals are used! don't put a Futaba crystal in the MPX Tx or vice versa (in any case this won't work, as the MPX Tx crystal oscillates at half the nominal frequency).

Have a nice flight,

Marc.