Thanks to all of you for your positive responses
especially since I crammed a lot of stuff in to a rather brief
overview.
colin McDonald writes:
> another excellent article Martin.
> thanks, this explains an aspect of technical jargin that I always wondered
> about but never actually researched.
> In HF radio specs they always talk about first and second IF frequencies,
> and I wondered how that effected the quality of the receiver.
> If I understand you, the higher the first IF, the better the receiver is
> with regard to rejection, selectivity and all that stuff?

	Not necessarily, but generally true. When the first IF
is high such as 70 MHZ for a HF receiver, then it is easier to
design tuned circuits to filter out the image frequencies since
they are 140 MHZ away. One thing that might happen in a
poorly-shielded receiver is that 70 MHZ, for example, is within
the pass band of TV Channel 4 in the United States, Canada,
Mexico and I think everybody else in North, Central and South
America. Analog TV channels are full of birdies from the video
and then there is the wide-band FM audio near the highest part
of the pass band. If you had a receiver with a first IF of 70
MHZ, and you had the misfortune to live next door to a
television transmitter on Channel 3 or 4, you might get some of
that energy leaking in to your IF chain. If it leaks in via some
other path than the antenna, a low-pass filter on the antenna
will do no good at all. If it gets in through the air by pure
brute force, you might have to move underground in to an
abandoned missile silo to get away from the noise. If the
transmitter is digital, you'll get high background noise but no
birdies, but that would still be bad news.

	Butch Bussen was describing the converted CB radio with a
1.65 MHZ first IF. That brings back memories as a number of
receivers used 1.65 MHZ first IF's which is a pretty good design
for CB's and amateur receivers. The only problem is that when
you get close to 160 meters or the AM broadcast band, some of
that radiation is close to the first IF and outside signals
start leaking right in to that stage.

	I used to have a Halicrafter SX100 whose first IF was
1.65 MHZ and I could start hearing a little bit of leakage near
the high end of the AM broadcast band as if the BFO was faintly
on. 1.65 is now in the AM broadcast band so I pity those who
live in a town with an AM radio station on 1.65 and a piece of
otherwise beautiful gear that has a 1.65 MHZ first IF.

	By the way, the SX100 had a second IF of 50 KHZ.
> I don't think I've ever seen an amateur rig that has a first IF of higher
> than 455KHZ though? usually second IF is 10 or 12MHZ right?

There are about as many possibilities for choosing the IF
frequencies as there are manufacturers and most of them can
result in good receivers if done right, but almost all of them
can stink on ice if not shielded well from leakage or if the
quality of the tuned circuits is poor.

	The normal design of an FM broadcast receiver, for
example is to use high-side injection and a 10.7 MHZ first IF.
The image frequencies are 21.4 MHZ above the desired frequencies
which puts them in the 99-129 MHZ band meaning that if your
image rejection is poor, you will sometimes hear aircraft radios
in the middle of your music. A few FM broadcast radios, however,
use low-side injection putting the images you receive in a band
of frequencies that includes television channels 4-6. With the
analog TV switch, this is not nearly the problem it once was,
but I remember that FM radios in Oklahoma City that were
designed this way were almost useless because birdies and sound
carriers from Channels 4 and 5 littered the FM broadcast band.
> I've never seen a first IF stage of 70MHZ though...

	I now have an ICR75 receiver whose first IF is high but
I am not sure what it is, but I did own an ICR71A whose first IF
was 70.455 MHZ. It had a second IF of around 9 MHZ and a third
IF of 455 KHZ. Most modern gear has crystal filters on at least
the second and third IF's

	One reason that newer receivers have higher first IF
frequencies is that it is easier these days to design quality
circuits that work well in to the VHF and UHF ranges. If the
engineers at Halicrafter and National in the fifties and sixties
had had tubes or transistors that were efficient at 70 to 100
MHZ, they would have built radios then with those higher IF's.
It would have made a lot of the rest of their design easier.