[HBR] Swapping IF and 1st osc frequencies

[Walt Hutchens]

> Would it be possible to do a "switcherooni" with the IF frequency
> set at 1700 kcs and the 1st oscillator to be at 1600 kcs, the
> difference being, of course, 100 kcs? That should help address my
> earlier concern of placing the AM band at the first IF.

It's perfectly possible.  However, this places the first image
frequency in the hard-core (commonly high power) part of the AM band.

For example, if you use a 100 kcs 2nd IF then with a 1600 kcs 1st IF
and 1700 kcs 1st oscillator, the image is at 1800 kcs.  That's the
edge of the 160M ham band.  There won't be a station exactly on that
frequency and anyone in the lower part of the band (most likely to
cause problems) will be limited to ham power levels.

If you switch things around -- 1700 kcs 1st IF, 1st osc. at 1600
kcs, then the image is at 1500 kcs.  You might not have a nearby
station at that exact frequency, but there WILL BE strong stations
within 50 kcs or so, and you'll have only the selectivity provided by
your RF and mixer tuned circuits plus whatever your antenna setup does
for you.

Broadcast stations are up to what -- 50kW carrier? It's only 20-some
dB more power but they have better antenna systems than hams, too.

I think it's better to have the IF in the nearly unused upper end of
the BC band than the image in the heavily used high power region. Once
you get past the RF stage, there's no difference in the gain at these
two frequencies.

Don't overlook the possibility of a slight jiggering of the IF. 100
kcs IFTs will probably tune down to 90 kcs or so and if you move the
1st oscillator up to 1705 kcs then you have the 1st IF at 1615 kcs.
There won't be a BC station of any sort exactly on that frequency and
shouldn't be anything powerful even very nearby. This puts the image
at 1795 kcs which should certainly be clear.

One additional possibility is a fix-tuned trap to attack an unwanted
interfering station.  This can go in the antenna lead or even in the
cathode of the RF stage.

There's no substitute for using a general coverage receiver to check
out an IF before making a final decision.  Be sure to check both day
and night as the answers at these frequencies will often be different.

Walt
KJ4KV