Thank you Sir, both for your masterful presentation, and the correction you 
later posted. Danny.
----- Original Message ----- 
From: "Martin McCormick" <[log in to unmask]>
To: <[log in to unmask]>
Sent: Wednesday, January 19, 2011 11:04 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