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 Post subject: how to check if frequency
PostPosted: Jan Wed 11, 2017 11:18 pm 
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Over the years I've just assumed the if frequency of any particular radio was whatever was published by the manufacturer. But how would you actually check it? Couldn't it drift due to deterioration of parts over time? And if it did drift wouldn't setting the if transformers to the published frequency not produce peak performance? Wouldn't just peaking them with a weak station instead of a signal generator be a more accurate way to peak performance in this case?


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 Post subject: Re: how to check if frequency
PostPosted: Jan Wed 11, 2017 11:34 pm 
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As radios age they do tend to drift due to resistors going up in value and leaky capacitors. Original capacitors and resistors were not that accurate. Did not really need to be. Each radio got all the adjustments needed at factory. After replace of old capacitors and resistors that are way off Will normally still need an IF touch up. If you don't have a signal generator you can tune to an upper weak station and tweak the IF trimmers a little for best reception. Same with the trimmer for antenna on tuner. If dial seems to track a little off you can adjust the oscillator trimmer a little too. Most times that's all it needs. Radio will work very good.


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 Post subject: Re: how to check if frequency
PostPosted: Jan Wed 11, 2017 11:49 pm 
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Manufacturers suggested shifting (re-tuning) the IF as a means of eliminating certain types of interference.

So the actual frequency may differ from the published value for individual examples.

The only way to accurately check the IF frequency is with a spectrum analyzer and a tracking generator.

- Leigh

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 Post subject: Re: how to check if frequency
PostPosted: Jan Wed 11, 2017 11:52 pm 
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Your question seems to imply that there is some other component in the radio that determines the IF besides the transformers. But there is not. The IF is whatever frequency the transformers have been aligned to.

Matt


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 Post subject: Re: how to check if frequency
PostPosted: Jan Wed 11, 2017 11:53 pm 
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IF frequency is determined mainly by the LC resonant circuits of the IF transformers themselves. Although drift in the values of neighboring components could in principle shift the IF frequency slightly, this is probably negligible. Aging of the caps and inductors within the IF transformer does cause some drift over time, often large enough that a readjustment of the IF transformers can improve sensitivity and selectivity substantially.

Having the center IF frequency super precisely on spec doesn't matter too much -- if it is off by a few kHz, most people wouldn't notice the effect (it slightly affects RF and oscillator tracking, as well as dial calibration). But what is quite important is that all the IF tuned circuits are tuned to the same frequency; here an error of more than 1 kHz on the various coils relative to one another can be noticeable.

Getting all the IF tuned circuits on the same frequency is not difficult. Simply tune in a weak station anywhere on the dial, and adjust all the IF trimmers or slugs for maximum signal strength.

Checking what the actual IF frequency is is a little harder. The best way to do that is to feed in a signal of known frequency (either using a very well calibrated RF generator, or one with a frequency readout via a frequency counter). As you adjust the frequency of the RF generator, the frequency at which the signal is strongest represents your center IF frequency.

One good way to both get all the IF transformers on the same frequency, and to have that frequency be the spec frequency, is to feed in a weak audio modulated signal (just strong enough to hear) at the spec frequency (455 kHz for most, but not all AM radios). Then adjust all the trimmers/cores for maximum signal strength. If you have a signal generator, this is the best procedure to use in general. For those that don't have a signal generator, aligning to a weak station is the next best thing. It gets all the IF tuned circuits on the same frequency, but not necessarily the spec frequency.

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 Post subject: Re: how to check if frequency
PostPosted: Jan Thu 12, 2017 12:10 am 
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When you adjust IF transformers, you are not setting them to a frequency. You are adjusting them to have maximum RF output at that frequency. You essentially are setting them to a center frequency. But there is actually output of frequency lower and higher than that center frequency. The IF transformer is a sharply tuned circuit. It will have high output at the currently set center frequency and greatly reduced output lower and higher than the center frequency.


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 Post subject: Re: how to check if frequency
PostPosted: Jan Thu 12, 2017 12:28 am 
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Dale Saukerson wrote:
When you adjust IF transformers, you are not setting them to a frequency. You are adjusting them to have maximum RF output at that frequency.


Well, Actually you are setting them to pass a variable frequency at peak efficiency. Take a radio that has an IF of 455 KHz for example, the converter tube will output a heterodyned frequency of 455KHz based on the received frequency. The IF transformer will pass the frequency it is tuned to much more efficiently while rejecting the majority of signals at frequencies above and below what they are tuned to. Because we have variable capacitors or slugs to change the frequency the IF transformer passes we can adjust for variances in the circuit.

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 Post subject: Re: how to check if frequency
PostPosted: Jan Thu 12, 2017 12:28 am 
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Dale Saukerson wrote:
When you adjust IF transformers, you are not setting them to a frequency.
You are adjusting them to have maximum RF output at that frequency.

How do those two statements differ?

- Leigh

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 Post subject: Re: how to check if frequency
PostPosted: Jan Thu 12, 2017 12:38 am 
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Well, I suppose one is what is happening and the other is why it is happening.

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 Post subject: Re: how to check if frequency
PostPosted: Jan Thu 12, 2017 1:28 am 
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Quote:
Your question seems to imply that there is some other component in the radio that determines the IF besides the transformers. But there is not. The IF is whatever frequency the transformers have been aligned to.


I could be wrong but I don't believe that. Isn't the actual if generated by the local oscillator? The way I understand it the if transformers are then tuned to pass this frequency. What I'm saying is that if you're tuning the if transformers to 455 kc for example and the if actually put out by the local oscillator is 465 kc or whatever then you're not really aligning for optimum performance. 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?


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 Post subject: Re: how to check if frequency
PostPosted: Jan Thu 12, 2017 1:34 am 
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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, respectively. All of these will be present simultaneously on the plate of the mixer tube. The IF transformers determine which one of these makes it through to the detector. If the IF transformers are too broad, you will hear two or more stations at once.

Matt


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 Post subject: Re: how to check if frequency
PostPosted: Jan Thu 12, 2017 2:34 am 
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The RF input to the mixer is tuned, so it won't pass all those stations with equal gain. However, the RF tuning is not very sharp, so indeed even though you could say that the oscillator frequency minus the frequency of the peak of the RF tuned circuit is the ideal IF frequency that the mixer produces, it does in fact produce a band of IF frequencies, and the particular narrow band passed by the IF selects what actually gets through.

So the resonant frequency of the IF transformers indeed sets the actual IF frequency. It is the job of the mixer and oscillator to produce that IF frequency properly when the dial is tuned to a particular station frequency.

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 Post subject: Re: how to check if frequency
PostPosted: Jan Thu 12, 2017 2:42 am 
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metzman wrote:
I could be wrong but I don't believe that. Isn't the actual if generated by the local oscillator?

No.

The IF signal is generated by the mixer. One of the inputs to the mixer is the LO, the other is the RF.

Any mixer has four frequencies present at its output, those being the RF, the LO, and the sum and difference of those two frequencies.

The IF chain is tuned to ONE of those four frequencies, amplifying it while discarding the other three.
This frequency-selective amplification is the job of the tuned transformers.

- Leigh

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 Post subject: Re: how to check if frequency
PostPosted: Jan Thu 12, 2017 3:05 am 
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Sometimes the mixer tube and oscillator tube are one in the same, Take the infamous 1L6 pentagrid converter for example.

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 Post subject: Re: how to check if frequency
PostPosted: Jan Thu 12, 2017 4:32 am 
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metzman wrote:
Quote:
Your question seems to imply that there is some other component in the radio that determines the IF besides the transformers. But there is not. The IF is whatever frequency the transformers have been aligned to.


I could be wrong but I don't believe that. Isn't the actual if generated by the local oscillator? The way I understand it the if transformers are then tuned to pass this frequency. What I'm saying is that if you're tuning the if transformers to 455 kc for example and the if actually put out by the local oscillator is 465 kc or whatever then you're not really aligning for optimum performance. 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?

Echoing what others have said.....

The mixer combines the incoming RF with the local oscillator. Depending on the RF front end, there could be several different stations in the mix---the bandwidth of the difference signal could be many 10s of KHz**. The center frequency of the IF chain determines what gets through.

To answer your original question: Alignment is done using a signal generator which, in turn, can be calibrated in at least two ways:
beating against known stations--using harmonics as needed
frequency counter connected to the output


**With no tuning in the RF front end, the mixer produces sum and difference signals for everything that gets to the control grid of the tube. The IF chain will pass both the sums and the differences---this is what causes images in an incorrectly aligned set.

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 Post subject: Re: how to check if frequency
PostPosted: Jan Thu 12, 2017 5:04 am 
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Many have said "tune by ear to a maximum output on a weak station" - unfortunately the Mk1 ear has a remarkably non-linear response. Much better to attach an audio voltmeter across the speaker, a 'scope across the volume control or a DC voltmeter across the AGC line. You can still use your weak station but a sig gen is much better, especially if your receiver has shortwave bands. The problem with the weak station is you must have the oscillator right on frequency otherwise you could end up with the IF not on its correct setting - as others have said, in most cases it won't be noticed but in the extreme misalignment, dial linearity will suffer - this is why the sig gen is better as you can set the IF right on specification, always assuming your sig gen is well calibrated. The IF amp after all is the first thing you should set up, then the oscillator tracking with the dial and finally the antenna or RF amplifier if there is one - work your way from the detector to the antenna.

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 Post subject: Re: how to check if frequency
PostPosted: Jan Thu 12, 2017 5:06 pm 
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Quote:
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.


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 Post subject: Re: how to check if frequency
PostPosted: Jan Thu 12, 2017 5:16 pm 
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Local oscillator can be verified with a frequency counter or by beating it against a known station.

A tuned IF Transformer wouldn't pass much signal 20 KHZ away from 455. In addition antenna section of a tuner is selecting frequency of the desired station.

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 Post subject: Re: how to check if frequency
PostPosted: Jan Thu 12, 2017 5:45 pm 
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metzman wrote:
Quote:
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.

I don't understand what you don't believe......the math that you quoted above is correct.

See if this helps: To pickup stations with a Superhet, we first tune the IF strip to a specific frequency---eg 455KHz**. Depending on the design of the receiver, the shape of the response curve will be different, but---in any decent design, there won't be useable response outside of ~445 to 465***.

Having picked the IF frequency, we then tune the LO to the desired station frequency, plus (or sometimes minus) the IF frequency. We also typically pre-tune the RF front end to the desired frequency. As already described, the output of the mixer is a jumble of signals. Ideally, the IF strip selects only the desired one, and has no response to the others.

To answer your most recent question:
You know that the LO is 1455, when your IF is set to 455, and you are receiving a station at 1000


**There have been several IF frequencies over the years, including 175, 262, and the most modern at ~455

***AM BC channel spacing is 10KHz. This means that the perfect IF designed for 455KHz, will have 100% response from 450 to 460, with zero response outside that. Unfortunately, there is no perfect IF.

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 Post subject: Re: how to check if frequency
PostPosted: Jan Thu 12, 2017 6:06 pm 
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If the LO frequency is off a bit at a particular dial frequency, it simply means that the station comes in a little higher or lower on the dial. It will only come in when the LO frequency minus the station frequency equals the IF frequency.

Another way to understand a superhet: Basically, at the output of the mixer, you have a single frequency radio with high gain and excellent sensitivity at the IF frequency. The job of the mixer and oscillator is convert whatever station you want to hear to this single frequency, which is the only thing the receiver can receive once the signal gets to the IF stage.

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