RadioBanter - View Single Post - Local oscillator below the station frequency--why?

Tim Wescott · February 19th 09, 05:34 AM posted to rec.radio.amateur.boatanchors

On Wed, 18 Feb 2009 20:16:57 -0700, Bill M wrote:

Tio Pedro wrote:
wrote in message
news:eafd0032-e112-40cb-ab88-
...
I recently refurbished a Hallicrafters 5R105 and, while performing a
re-alignment, discovered that on the Band 4 (14 to 31 MHz) the local
oscillator (LO) is below the station frequency, whereas on the three
lower bands, the LO is above.

So my question is this: Why engineer a tuning system for the high end
of the shortwave spectrum to place the LO below the station frequency?

-Dave Drumheller, K3WQ

It would be unusual. A quick check is to see if the LO is above or
below the signal at the LOW end of the tuning range. I've quite a few
shortwave sets that were properly aligned for high side injection at
the low end, and improperly set for low side injection at the upper end
of the band. Needless to say the midrange sensitivity was practically
nil.

It's pretty easy to set the
high end of dial for the wrong side LO injection because of the wide
authority range of the LO trimmer.

I can't think of any advantage--usually high side inj. is beneficial on
the lower ranges to keep LO harmonics from falling the tuning range or
the RX.

Pete

Its a real gripe of mine! The image rejection is so poor on the top
band of many consumer grade radios that its really a moot point and the
only way to guess which side the injection goes is by evaluating which
side suffers the worse dial tracking...and often thats so close a call
its difficult to say which was intended.

-Bill

All of my handbooks from the '40s through the '60s have converters to
take the signal down to 40m or below, for just that reason.

20MHz down to 0.455MHz is just a conversion too far.

--
http://www.wescottdesign.com