Martin's explanation is good.  

Forty years ago, as a blind person, I was working for Collins Radio in
receiver front end design.  The Signal One CX7 was the first transceiver I
heard of which was using a first IF higher than any of the frequencies it
would receive.  The purpose of doing this is to get good image rejection,
and in some cases also to permit use of a series of broad-band filters in
the front end instead of tuned circuits which needed to track accurately as
you changed frequency.  So even some of the cheapest shortwave receivers
made now have a high first IF.  But good image rejection does not guarantee
good selectivity.  For this you need a second and probably a third IF.  

Most transceivers that have a general coverage receiver, made in the last 10
or 15 years, have a high first IF.  Even if they don't, it will probably be
in the 8 or 9 MHz range, because good crystal filters can be made there.

I think the TS480's first IF is near 70 MHz.  The FT9000, 2000 and 950 have
a first IF of 69.450 MHz, a second IF of 450 KHz, and third IF of 30 KHz
(where the digital signal processing and demodulation take place).  It's
very hard to make roofing filters narrower than 6 KHz at 70 MHz, although
one company claims to have a 3 KHz filter available as a mod.

More later if people want to discuss this stuff further.
73,
Lloyd Rasmussen, W3IUU, Kensington, Maryland
Home:  http://lras.home.sprynet.com
Work:  http://www.loc.gov/nls
 
> -----Original Message-----
> From: For blind ham radio operators [mailto:[log in to unmask]]
> On Behalf Of colin McDonald
> Sent: Thursday, January 20, 2011 5:54 PM
> To: [log in to unmask]
> Subject: Re: Double and Single Conversion (was bitter disappointment! )
> 
> 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?
> 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?
> I've never seen a first IF stage of 70MHZ though...
> 73
> Colin, V A6BKX
> ----- Original Message -----
> From: "Martin McCormick" <[log in to unmask]>
> To: <[log in to unmask]>
> Sent: Wednesday, January 19, 2011 9:04 PM
> Subject: Double and Single Conversion (was bitter disappointment! )
> 
> 
> > I will briefly explain why some receivers are double and
> > even triple conversion and why that has nothing to do with
> > selectivity.
> >
> > The superheterodyne design mixes a local oscillator with
> > the desired frequency to make the first IF or conversion. You
> > can either feed the local oscillator in to the mixer below the
> > desired frequency which is called low-side injection or you can
> > feed it in above the desired frequency for high-side injection.
> > If, for example, you have a receiver tuned to 28 MHZ with a 455
> > KHZ first IF, the local oscillator can either be 28.455 MHZ or 27.545
> > MHZ and you still hear 28 MHZ.
> >
> > Now for the bad news. Let's say your 28 MHZ receiver
> > uses low-side injection. The mixer is built to allow 28 MHZ in
> > from the antenna by means of a tuned tank circuit set to 28.0
> > MHZ exactly. There is, however, a nasty fly in the ointment.
> > Remember studying about the Q factor in tuned circuits? That is
> > the way the quality of a tuned circuit is measured and the
> > technique is to sweep a signal through the pass band of the
> > tuned circuit and determine how far off frequency you have to be
> > for the circuit to be down 3 DB from what it is in the center.
> >
> > One of the laws of physics is that in a tuned circuit,
> > the Q factor is a percentage of the frequency so a LC tuned
> > circuit at 1.8 MHZ will be rather sharp compared to the pass
> > band of a voice signal and it will not pass much more than one
> > AM signal which is not too bad.
> >
> > By the time you get to 28 MHZ, however, that tuned
> > circuit may pass frequencies between 26 and 29 MHZ fairly well.
> >
> > So, what's wrong with that? Everything! You have a 455
> > KHZ first IF. You are listening to 28.0 MHZ by injecting a
> > 27.545 MHZ signal in to the mixer so here's what else happens.
> >
> > If you didn't have that tuned circuit in the mixer to
> > select 28.0 MHZ, you would still hear 28.0 MHZ and you could
> > also hear 27.545 - 455 KHZ or 27.09 MHZ which is right in the
> > Citizens' Band. In other words, that is the potential with every
> > superheterodyne receiver. The first mixer converts the local
> > oscillator + the desired frequency to the IF and it can also
> > convert the local oscillator minus some other frequency to that
> > same IF. That other frequency is the image.
> >
> > okay, let's put back that tuned circuit in the mixer. If
> > we tuned the receiver down to the AM broadcast band, it's not
> > too bad. We hear pretty much only what we wanted to hear. Now,
> > let's go back to ten meters and 28.0 MHZ. If you are listening
> > in AM mode, you should maybe hear a few CW signals as popping
> > sounds but there's something else. There is a strong image of
> > whatever CB channel is at 27.085. That darn tuned circuit in the
> > mixer is worthless by the time you get to ten meters or VHF.
> >
> > Okay. That's a single-conversion receiver. Let's make it
> > double-conversion by putting a second oscillator and mixer at
> > the 455 KHZ IF. The second conversion oscillator is at 355 KHZ so
> > our second IF is at 100 KHZ. This would work just fine except
> > that we still have that crappy front end in which the images
> > from the CB channels are all over ten meters. We will get a
> > little more selectivity because we combine the selectivity of
> > the 455 KHZ if and the 100 KHZ second IF but the receiver is
> > still a piece of junk.
> >
> > Now, let's make a decent receiver.
> >
> > The first mixer mixes a local oscillator that is maybe
> > 70 MHZ higher than the desired frequency with that frequency to
> > produce a first IF of 70 MHZ. The difference between the image
> > frequency and the desired frequency is now 140 MHZ which is
> > much, much easier to build a tuned circuit for.
> >
> > If you put a crystal filter at the 70 MHZ first IF that
> > has a fabulous Q factor, then your second conversion mixer
> > doesn't have to try to filter everything as much. You end up
> > with good selectivity and virtually no images over the whole
> > tuning range.
> >
> > There are a lot more problems in building a good
> > receiver and I don't even begin to know them all, but I hope
> > this helps in the discussion as to whether single conversion or
> > double conversion receivers are best. There are some good ones
> > of both kinds but generally a double-conversion receiver with a
> > high first IF beats the pants off of a single-conversion
> > receiver with a fairly low-frequency first IF.
> >
> > Everything I have said here is generalities and some of
> > the older gear used lower first IF's with higher-Q multiple
> > tuned circuits in the first mixer and achieved beautiful
> > results.
> >
> > Martin McCormick WB5AGZ  Stillwater, OK
> > Systems Engineer
> > OSU Information Technology Department Telecommunications Services Group