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 Post subject: Re: Reading the IF frequency at T3 can on a HW-32
PostPosted: Mar Sat 28, 2020 9:14 pm 
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And there you have it!!

Maybe anticlimactic, but keep in mind how "mixing" works. Ideally, it is the multiplication of 2 signals, which produces new signals at the sum and difference frequencies. The typical radio apparti, do not do simple multiplication, and therefore the original frequencies also come through. In basic circuits, any non-linear element will result in mixing--eg a diode, correctly-biased vacuum tube, etc.

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 Post subject: Re: Reading the IF frequency at T3 can on a HW-32
PostPosted: Mar Sat 28, 2020 9:24 pm 
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Location: Corinth, TX
That's one reason I like double balanced mixers. All you get in the output is the sum and difference.

John


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 Post subject: Re: Reading the IF frequency at T3 can on a HW-32
PostPosted: Apr Fri 03, 2020 3:09 pm 
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Joined: Jan Sun 06, 2013 2:14 am
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Location: Butler, PA
Thanks steve that is a hard read for me. Great info if you can follow it.


Attachments:
Hw-32 Receiving and transmitting.docx [17.79 KiB]
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 Post subject: Re: Reading the IF frequency at T3 can on a HW-32
PostPosted: Apr Fri 03, 2020 4:04 pm 
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Maybe tell us which part you don't understand?

For example, are you OK with the concept of mixing 2 signals to get 2 new ones?

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"Even if you don't understand Ohm's Law, you are still required to obey it."


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 Post subject: Re: Reading the IF frequency at T3 can on a HW-32
PostPosted: Apr Sat 04, 2020 12:53 am 
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Location: Corinth, TX
Cadmandu wrote:
In an AM receiver the IF is created in a dual var cap why do it differently


No, it isn't. The IF is created in the MIXER. The variable caps only tune the inputs. You COULD use two separate caps, but it's much more convenient to "gang" the variable caps together so you only have one tuning control.

For instance, let's say you have a 5 tube broadcast band receiver. and you want to receive a signal at 1000 KHz (1.000 Mhz). You tune the antenna circuit to 1000 KHz. You tune the LO (local oscillator - the second input to the mixer) to 1455 KHz. This is called high side injection, by the way. You could use lowside injection by tuning your LO to 545 KHz, but nobody does that because that would require larger coils and caps. The increased cost would add up over millions of radios.

So you feed 1000 KHz and 1455 KHz into mixer. In a conventional mixer, you get those two freqs and the sum and difference - 455 KHz and 2.455 KHz. The 455 KHz signal is far enough away from everything else that it is easy to separate and amplify.

In a garden variety receiver, you want the RF input and LO freq to track, so you need a 2 gang capacitor. A more elaborate receiver would have an RF amp feeding the mixer so you should have a 3 gang (section) capacitor because you have three circuits to tune.

The Heathkit transceiver has a fixed, crystal controlled LO (no tuning required) so it needs only one variable tuning control to make it work.

You can learn a lot by by studying the frequency conversion scheme of a receiver. Once you understand what is going on here, look at the freq conversion scheme of a Hammarlund HQ-180 (four band, double and triple conversion - two or three mixers depending on the frequency) receiver. Then if you are feeling REALLY adventurous, tackle a Collins R-388.

John


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