Local oscillator below the station frequency--why?

Local oscillator below the station frequency--why?

Post by da.. » Fri, 20 Feb 2009 09:26:05

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

 
 
 

Local oscillator below the station frequency--why?

Post by COLIN LAM » Fri, 20 Feb 2009 10:57:56


"Why engineer a tuning system for the high end
of the shortwave spectrum to place the LO below the station
frequency?"

One good reason is stability.  The high frequency oscillators in these
receivers are less than perfect.  So, by using the lower frequency for the
oscillator injection, rather than the upper frequency, you will achieve a
slight, but noticeable improvement in stability.

For example, if you have a 15 MHz receive frequency and a 500 kHz i.f., you
could use either 14.5 MHz as the oscillator frequency or 15.5 MHz.  Using
14.5 MHz would yield a 7% inprovement in stability.  In addition, the coils
in the oscillator, the tube and the tube socket, like the lower frequency.
One other really minor advantage is that some tubes, particularly the 6C4,
drops off on the high frequency end as emission decreases - so the
oscillator tube might just work a bit longer.

73,  Colin  K7FM

 
 
 

Local oscillator below the station frequency--why?

Post by Tio Pedr » Fri, 20 Feb 2009 11:03:41


Quote:>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

 
 
 

Local oscillator below the station frequency--why?

Post by Bill » Fri, 20 Feb 2009 12:16:57




>> 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

 
 
 

Local oscillator below the station frequency--why?

Post by COLIN LAM » Fri, 20 Feb 2009 13:24:18

"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..."

Right.  I recently went through just this problem.  Radio had been twiddled
with.  The service information said nothing about whether low or high side
injection was used.  I used the trial and error method to figure it out.  I
looked through two different service instructions and both were silent.

The good news is that single conversion radios, with 1 rf amplifier stage,
are rarely used above 20 meters anyway, these days.

73,  Colin  K7FM

 
 
 

Local oscillator below the station frequency--why?

Post by Tim Wescot » Fri, 20 Feb 2009 13:34:44


Quote:>>> 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

 
 
 

Local oscillator below the station frequency--why?

Post by Michael Blac » Fri, 20 Feb 2009 14:10:43


> "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..."

> Right.  I recently went through just this problem.  Radio had been twiddled
> with.  The service information said nothing about whether low or high side
> injection was used.  I used the trial and error method to figure it out.  I
> looked through two different service instructions and both were silent.

I remember one review for a low end receiver, and I can't remember which
receiver or which magazine, and they outright said there was so little
image rejection that they couldn't tell which was real and which was
the image.

On the other hand, I recently reread a review for the Radio Shack sw
receiver from the late sixties, transistorized and something like the
DX-150, and it praised the receiver for it's image rejection even on the
highest band.  I suspect it was thus a very generous review.

Quote:> The good news is that single conversion radios, with 1 rf amplifier stage,
> are rarely used above 20 meters anyway, these days.

Of course, that's apples and oranges.  The frequency of the IF factors
in, and it's not the single conversion that matters, but the signal
frequency versus the IF frequency.

   Michael  VE2BVW

 
 
 

Local oscillator below the station frequency--why?

Post by Tim Wescot » Fri, 20 Feb 2009 14:37:00



>> "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..."

>> Right.  I recently went through just this problem.  Radio had been
>> twiddled with.  The service information said nothing about whether low
>> or high side injection was used.  I used the trial and error method to
>> figure it out.  I looked through two different service instructions and
>> both were silent.

> I remember one review for a low end receiver, and I can't remember which
> receiver or which magazine, and they outright said there was so little
> image rejection that they couldn't tell which was real and which was the
> image.

> On the other hand, I recently reread a review for the Radio Shack sw
> receiver from the late sixties, transistorized and something like the
> DX-150, and it praised the receiver for it's image rejection even on the
> highest band.  I suspect it was thus a very generous review.

>> The good news is that single conversion radios, with 1 rf amplifier
>> stage, are rarely used above 20 meters anyway, these days.

> Of course, that's apples and oranges.  The frequency of the IF factors
> in, and it's not the single conversion that matters, but the signal
> frequency versus the IF frequency.

>    Michael  VE2BVW

Kinda yes, kinda no.  It's much easier to get a high 1st IF and a nice
narrow overall response in a double conversion superhet than a single.

Having said that, my Galaxy 5 worked pretty well with a 9MHz IF and
single conversion.

--
http://www.wescottdesign.com

 
 
 

Local oscillator below the station frequency--why?

Post by Bill » Fri, 20 Feb 2009 16:07:59


> Having said that, my Galaxy 5 worked pretty well with a 9MHz IF and
> single conversion.

Apples and oranges since the Galaxy made no attempt to cover BCB range
ad certainly had no BCB parallel.

3200 kc Infradyne comes to mind as a "high IF  scheme" for BCB but I
can't think of any older single conversion rig that used a high IF freq
for BCB?  Ok, theres some 1600-17oo kc schemes. The older mindset was to
go lower in freq for improved Q.

 
 
 

Local oscillator below the station frequency--why?

Post by Tio Pedr » Fri, 20 Feb 2009 20:24:09





>>> 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

It is not a moot point... Usually the bottom tracking is fixed on
the higher bands (no padder or slug for the LO coil).  If the LO on
the  high end of the dial is set to the wrong side,
the mid band sensitivity will
be Zilch. Usually checking LO frequency on the lowest end
of the dial will show if it is supposed to be hi or lo side injection.
 
 
 

Local oscillator below the station frequency--why?

Post by da.. » Fri, 20 Feb 2009 23:07:20


Quote:> 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.

I had this exact problem with the Hallicrafters 5R10A.

With my signal generator set to 30 MHz, the image at 30.91 MHz was
maybe 1 dB lower.  However, when I set my generator to 15 MHz (the low
end of Band 4), the image at 15.91 MHz was much less noticeable. So,
the set is a reasonably good performer so long as you avoid using the
high end of Band 4.

-Dave Drumheller, K3WQ

 
 
 

Local oscillator below the station frequency--why?

Post by Richard Knoppo » Sat, 21 Feb 2009 04:12:20



Quote:> 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.

I had this exact problem with the Hallicrafters 5R10A.

With my signal generator set to 30 MHz, the image at 30.91
MHz was
maybe 1 dB lower.  However, when I set my generator to 15
MHz (the low
end of Band 4), the image at 15.91 MHz was much less
noticeable. So,
the set is a reasonably good performer so long as you avoid
using the
high end of Band 4.

-Dave Drumheller, K3WQ

     I had this same problem tuning up a Hallicrafters
S-20R, this RX goes to 44 mhz with only one RF stage so the
image rejection at the top frequency is almost nil.
Hallicrafter's did use an LO at lower than signal frequency
in some cases, for instance the S-36, a receiver that goes
up to about 143 mhz has the LO lower than signal frequency
on the top band only. The IF is about 5 mhz so the 10 mhz
difference probably made it easier to get sufficient LO
output. The S-20R appears to have the LO at higher than
signal frequency on all bands. The performance and
calibration are slightly, but noticably, better with the LO
on the correct side. Hallicrafters made some receivers with
no RF stage but tuning to the 40 mhz range, they probably
have no detectable difference in response between desired
and image signals. There should be a slight difference in
the dial calibration but it may be masked by drift in the
oscillator coil.
     BTW, while many of these receivers had no adjustment
for the low end of the oscillator or RF stages its usually
possible to tune them by adjusting the coils. For instance,
on the S-20R the turns are held in place by wax so its
possible to move them around a bit and fasten them back down
again by either re-heating the existing wax (with the tip of
a soldering iron or a concentrated heat gun) or by melting a
bit of new bee's wax (can be had from art supply stores)
onto the coil. Some RX may be found to have an adjustment
section on the coil or sometimes an isolated shorted turn
that can be moved around to adjust the inductance.

--

--
Richard Knoppow
Los Angeles
WB6KBL