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 Post subject: Re: how to check if frequency
PostPosted: Jan Thu 12, 2017 6:07 pm 
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metzman wrote:
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Let's say you have broadcast stations at 980, 990, 1000, 1010, and 1020 kHz. And the local oscillator is oscillating at 1455 kHz. The mixer will mix the station frequencies (all of them) with the local oscillator to give new frequencies of 475, 465, 455, 445, and 435 kHz,
I don't believe that either. According to the books the mixer is taking the tuned rf and combining it with the LO. So let's say the LO was at 1475 Hz and heterodyning with a tuned rf of
1000 Hz . The if transformers tuned at 455 Hz would be passing 475 Hz thru but not at peak. So how do you know or get the LO oscillator to be at 1455 Hz. I've never seen anyone show it varified or a method of varifying it.
The LO is settable (tunable). You put it where you want it.
The output of the mixer is both the sum and difference of the input frequencies, so for LO = 1455
Code:
1455 +  980 = 2435 and 1455 -  980 = 475
1455 +  990 = 2445 and 1455 -  990 = 465
1455 + 1000 = 2455 and 1455 - 1000 = 455
1455 + 1010 = 2465 and 1455 - 1010 = 445
1455 + 1020 = 2475 and 1455 - 1020 = 435

- Leigh

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 Post subject: Re: how to check if frequency
PostPosted: Jan Thu 12, 2017 6:12 pm 
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Maybe this will help also: In an AA5 there is only one tuned circuit (resonant LC circuit) between the antenna and the mixer. With this one tuned circuit you will get a broad response. If the circuit is tuned to 1000 kHz and there are stations at 980, 990, 1000, 1010, and 1020 kHz, they will all get through to the mixer because they are within ±2% of the center frequency and a single tuned circuit is not very selective. Yes, the stations at 980 and 1020 kHz will be attenuated somewhat, but not very much.

The receiver's selectivity comes primarily from the IF chain. If the radio has two IF transformers, that is four tuned LC circuits in a row. All of them tuned to 455 kHz. So if you set your LO to 1455 kHz (by tuning the radio dial to 1000 kHz), the above mentioned stations will mix with the LO to create new signals from 475 kHz to 435 kHz as stated before. At this point, instead of a ±2% frequency difference it is ±4.4%, and you have four tuned circuits, so the signal at 455 kHz will be selectively amplified. (As Leigh mentioned above, there will also be signals from 2435 to 2475 kHz created in the mixer, but they are so far away from the IF that they have no chance of getting through.)

In addition to the methods Norm mentioned, you can listen to the LO using an adjacent communications receiver in CW mode.

Matt


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 Post subject: Re: how to check if frequency
PostPosted: Jan Thu 12, 2017 6:52 pm 
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It get's worse....

In addition the various stated combinations that give a 455 IF signal, we also have the harmonics. Assuming that the signal gets through the front end, you will get 455 from:
<<S = signal, LO = local oscillator>>
<<2LO = 2nd harmonic of LO, 2S = 2nd harmonic of S, etc.>>

LO-S = 455
S-LO = 455
2LO - S = 455
S - 2LO = 455
2LO - 2S = 455
......ad infinitum, ad nauseum

Here is an Excel tool for playing with the combinations:
Attachment:
Image.xlsx [6.5 KiB]
Downloaded 15 times

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 Post subject: Re: how to check if frequency
PostPosted: Jan Thu 12, 2017 7:00 pm 
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pixellany wrote:
In addition the various stated combinations that give a 455 IF signal, we also have the harmonics.
While that's true on a spreadsheet, it's quite unlikely in a real radio.

LOs are generally pretty clean, so 2nd harmonics will be attenuated*.

Similarly, the 2nd harmonic of a broadcast transmitter is attenuated by law.

- Leigh

*Speaking here of single-band radios. Multi-band sets may have higher LO harmonics.

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 Post subject: Re: how to check if frequency
PostPosted: Jan Thu 12, 2017 7:50 pm 
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The question about how the LO is determined is that you set it during alignment.

Most alignment procedures start by injecting the IF frequency into the IF chain and then adjusting the caps on the IF transformers to maximize the signal. At that point, the IF chain will pass the IF frequency set on your signal generator.

The next step in alignment is adjusting the RF and LO frequencies. For example, suppose the IF were set to 455 kHz using the procedure above. You might set the dial pointer to a particular frequency, say 1000 kHz, and inject 1000 kHz into the front end of the radio. You then adjust the trimmer caps for the LO and RF to maximize signal through the IF chain. You will see maximum signal going through the set when the RF section tunes 1000 kHz and the LO tunes 1455 kHz (most sets use high side injection), and the trimmer caps allow you to fine tune those frequencies.

There is often more than one set of trimmers that get adjusted so that the RF and LO tune correctly across the band. One set is adjusted at the low end of the band, the other at the high end. As a result, the alignment procedure is usually a little more complicated than the above description. However, it accomplishes the same thing.

In practice, the exact value of the IF isn't that critical. For example, I once came across a set that had an IF of 460 kHz, but a previous owner had replaced one of the 460 kHz IF cans with a 455 kHz can. There was enough adjustment in the replacement can to set it to pass 460 kHz using the procedure above. The only thing that really matters is that all of the IF filter stages get set to the same frequency, and the RF and LO trimmers are adjusted so that the LO frequency is exactly 1 IF frequency above the RF tuned frequency all the way across the band.

If you want to measure the IF frequency, you have some options. I prefer to attach my oscilloscope probe on the IF chain and measure the frequency on the scope. A frequency counter will work as well. You could also tune the set to the low end of the band, say 600 kHz, then listen for the LO on a nearby radio. Just be aware that the LO won't have any modulation, so it will be a quiet spot about 1 IF above 600 kHz rather than a tone.

Roger


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 Post subject: Re: how to check if frequency
PostPosted: Jan Thu 12, 2017 8:27 pm 
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Leigh wrote:
pixellany wrote:
In addition the various stated combinations that give a 455 IF signal, we also have the harmonics.
While that's true on a spreadsheet, it's quite unlikely in a real radio.

LOs are generally pretty clean, so 2nd harmonics will be attenuated*.

Similarly, the 2nd harmonic of a broadcast transmitter is attenuated by law.

- Leigh

*Speaking here of single-band radios. Multi-band sets may have higher LO harmonics.

respectfully, I might be disagreeing. A great many LOs are class B circuits, which are guaranteed to have major even harmonics.
As far as the incoming signal, it's the harmonics generated in the mixer (and RF tube, if present) that are the issue.

I agree that images due to the harmonics are not common, but I have seen them

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 Post subject: Re: how to check if frequency
PostPosted: Jan Thu 12, 2017 9:22 pm 
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As far as harmonics are concerned, it mostly has to do with the way the mixer itself works. You can have an absolutely pure signal from the local oscillator and a pure signal from the RF section and still get harmonics coming off the mixer.

Suppose you have two signals entering a pentagrid converter, sin(at) and sin(bt) representing the RF signal and the LO. The tube will try to modulate the RF signal by the LO signal, although this will not always be complete. As long as the pentagrid converter is a perfectly linear amplifier, the output will be come combination of sin(at)sin(bt), sin(at) and sin(bt).

If you do some trig, you can show that

sin(at)sin(bt) = 1/2cos((a-b)t)-1/2cos((a+b)t)

so you end up with both sum and difference terms at the output, as well as some residual RF and LO frequencies as well. The role of the IF filters is to remove everything except the difference term, which is the IF frequency.

Harmonics arise when the pentagrid converter isn't a perfectly linear amp. If I input sin(bt), you can write an expression for an arbitrary non-linear output as a Taylor series, eg:

c1*sin(bt)+c2*sin^2(bt)+c3*sin^3(bt) + .... + cn*sin^n(bt).

where the c's are the various coefficients.

The second term can be written as the second harmonic using the relationship:

sin^2(bt)=1/2-1/2*cos(2*bt)

You can continue down the expression to get other harmonics as well.

In practice, the mixer is designed to minimize harmonics, although there will always be some present. The IF filters simply select the desired output of the mixer and reject the other frequencies present.

Roger


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 Post subject: Re: how to check if frequency
PostPosted: Jan Thu 12, 2017 11:56 pm 
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Last edited by AJJ on Jan Fri 13, 2017 12:12 am, edited 1 time in total.

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 Post subject: Re: how to check if frequency
PostPosted: Jan Fri 13, 2017 12:07 am 
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Not all RF sections are tuned to a specific frequency, some are wideband that cover all of the AM broadcast band and then are resistance coupled to the mixer. This allows the manufacturer to add an RF amplifier without having a 3 gang tuning capacitor.

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 Post subject: Re: how to check if frequency
PostPosted: Jan Fri 13, 2017 1:45 am 
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Hcompton79 wrote:
Not all RF sections are tuned to a specific frequency, some are wideband that cover all of the AM broadcast band and then are resistance coupled to the mixer. This allows the manufacturer to add an RF amplifier without having a 3 gang tuning capacitor.

???
Where is the benefit in adding an RF amplifier if it is not tuned?

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 Post subject: Re: how to check if frequency
PostPosted: Jan Fri 13, 2017 2:23 am 
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pixellany wrote:
???
Where is the benefit in adding an RF amplifier if it is not tuned?
It increases the tube count, and actually does something.

- Leigh

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 Post subject: Re: how to check if frequency
PostPosted: Jan Fri 13, 2017 2:23 am 
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Metzman,

I started to post a rather long explanation, but I think you're almost there.

Quote:
I don't believe that either. According to the books the mixer is taking the tuned rf and combining it with the LO. So let's say the LO was at 1475 Hz and heterodyning with a tuned rf of
1000 Hz . The if transformers tuned at 455 Hz would be passing 475 Hz thru but not at peak. So how do you know or get the LO oscillator to be at 1455 Hz. I've never seen anyone show it varified or a method of varifying it.


- You are correct that a correctly aligned IF section at 455khz (not Hz), would pass some 475khz signal, however weak or minuscule. That's why most alignment procedures include using a calibrated signal generator (not a weak station) to align the IF as the first step.

- Some of us actually do verify the oscillator frequency: At the end of the alignment procedure, I would place my newly aligned (broadcast band) radio next to radio #2 (with a digital readout I trust). I would carefully tune to a known 1600khz local station (not looking at the dial). If the oscillator and IF sections were both correctly aligned, radio #2 would pick-up the local oscillator radiating at exactly 2055khz (2055-1600=455); at this point, the dial should be correct too.

The postings I've seen here are mostly technically correct. Sometimes it takes some pondering to sort out how it all fits. This is a difficult theory to explain in an online forum. "Believe" it or not, you're actually on the right track. Hope this helps.

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 Post subject: Re: how to check if frequency
PostPosted: Jan Fri 13, 2017 4:01 am 
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Most of the responses assume that all the radio signals that hit the antenna come thru equally. This is only true if the radio does not have a tuned RF front end. Many do gave a tune RF frompnt end. In any case, tuning the IF really just sets the IF to pass a specific range of frequencies (typically 455 KHZ plus/minus 10KHZ for modern AM radios). The actual frequency picked up is a the Local oscillator frequency minus the IF frequency. The RF front end needs to track that sum or difference frequency or you will get extremely poor reception.


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 Post subject: Re: how to check if frequency
PostPosted: Jan Fri 13, 2017 4:09 am 
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Leigh wrote:
pixellany wrote:
???
Where is the benefit in adding an RF amplifier if it is not tuned?
It increases the tube count, and actually does something.

- Leigh


It makes the receiver more sensitive to weak signals but not as selective. In practical sense this allows the receiver to pick up weaker signals but also increases the noise floor. To determine whether a radio uses this method, it will have a RF amp tube but only a 2 gang tuning capacitor. While it does work better than a radio with no RF amplifier one with a tuned RF is better.

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 Post subject: Re: how to check if frequency
PostPosted: Jan Fri 13, 2017 4:16 am 
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metzman wrote:
This notion popped into my head last night when I was watching a restoration video on YouTube but it never really occurred to me before that. Am I crazy?
Of course you're not crazy. You've asked a set of sincere questions that has generated some remarkably thoughtful answers. Isn't this the true strength of this forum?

This is the kind of posting that I hope to find each time I visit this forum. Thanks for sharing your notions. :D


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 Post subject: Re: how to check if frequency
PostPosted: Jan Fri 13, 2017 4:21 am 
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BTW, noise is proportional to the square root of bandwidth. So that reduced bandwidth greatly reduces noise as well.


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 Post subject: Re: how to check if frequency
PostPosted: Jan Fri 13, 2017 2:06 pm 
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Gentlemen, these are certainly strong and interesting arguments, the knowledge that one can absorb from these AR forums is simply amazing.... but I wonder if the original question/thought process was perhaps touching on an IF circuit containing a crystal filter- which would better justify the asking? The crystal filter frequency (original and aged) and subsequent IF alignment process has been discussed several times on the forum, as the actual crystal frequency in effect becomes the new IF frequency that one would align to....


Todd
ka8gef

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 Post subject: Re: how to check if frequency
PostPosted: Jan Fri 13, 2017 2:20 pm 
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I think all the discussion here is applicable to any kind of IF strip......first you set the IF frequency, then everything else works with that.

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 Post subject: Re: how to check if frequency
PostPosted: Jan Fri 13, 2017 6:48 pm 
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Thank you for all the response and sharing the valuable knowledge. I understand the math and alignment procedures but forgot that there are more than one station bouncing around in the mixer. I was incorrectly assuming there was only one in there at a time and that was lousing me up. I forgot the rf antenna section was an LC circuit as well and allowed a bandwidth into the converter stage. It makes a lot more sense now.


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 Post subject: Re: how to check if frequency
PostPosted: Jan Fri 13, 2017 7:56 pm 
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Quote:
- Some of us actually do verify the oscillator frequency: At the end of the alignment procedure, I would place my newly aligned (broadcast band) radio next to radio #2 (with a digital readout I trust). I would carefully tune to a known 1600khz local station (not looking at the dial). If the oscillator and IF sections were both correctly aligned, radio #2 would pick-up the local oscillator radiating at exactly 2055khz (2055-1600=455); at this point, the dial should be correct too.


That was the way I finally nailed the highest frequency band on my Heath "Mohican" - actually plotting the frequency of the oscillator against the dial frequency and finding that I had high side injection at one end and low side at the other! The RF stage just didn't have the "Q" to prevent the image getting through.

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