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the "F" in IF - Printable Version

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the "F" in IF - fenbach - 08-22-2021

IF transformers are [surely?] designed to be as efficient as possible at the rated frequency. at least, i see max output at or near the IF when i apply a signal to the primary and attach a scope to the secondary. why is that?
more specifically, what determines the frequency at which power transfer is a maximum? for example i had a transformer rated at something like 4.5MHz and it didn't look all that different from a 455KHz transformer from an AA5 radio.

RE: the "F" in IF - RodB - 08-22-2021

The IF transformers have a primary and secondary with specific inductance properties when combined with a specific value of capacitor connected in parallel form a tank circuit that oscillates at the desired frequency.

RE: the "F" in IF - morzh - 08-22-2021

IF was chosen low for a reason. It has to do with selectivity and not really the efficiency. The idea is, the selectivity is way better at lower frequency, as the distance between station (10kHz) is 10% at 100kHz, or 2% at 450kHz, but 0.2% at 4.5MHz. THis is why the TRF radios did not really go into the short wave bands.

Efficiency in RF transformer is not that important as not much power is transferred: it is pretty much the voltage conversion, and is chosen out of consideration for the type of tubes or detectors etc used.
Efficiency is important where the power is transferred. NOt only voltage or only current.

And in IF transformer the most important part is the resonant one. But the frequency choice was the compromise between selectivity and quality (too low a frequency will not give good sound quality, itself eventually venturing in the sound frequency band). Plus other considerations. Like triodes could not amplify high frequencies well, but being able to oscillate at them. And then to amplify you need to lower the frequency.

RE: the "F" in IF - mikethedruid - 08-22-2021

Look at the schematic diagram for an IF transformer. You will see that both the primary and secondary are LC tuned circuits, with an inductor in parallel with a condenser. The inductor and condenser are in parallel and form what is called a "tank" circuit. It will resonate most efficiently at one particular frequency. In aligning a superheterodyne radio one tunes each side of the IF transformer so that it is peaked exactly at the desired IF frequency. This maximizes the signal at that signal being passed through the transformer, and cuts back the signals at all frequencies above and below the IF frequency. A single intermediate frequency is used because one can design an amplifier to be most efficient at one particular chosen frequency. If you design it for a broad range of frequencies it cannot be designed to be as efficient. In many superheterodyne sets there is only one stage of intermediate frequency amplification used because even this one stage can amplify the incoming radio signals enough so that the sensitivity of the receiver is sufficient. In some receivers, however, there can be a second stage of IF amplification which tunes the radio even more sharply, giving even greater selectivity, and rejecting adjacent signals.

The intermediate frequency is achieved by mixing the incoming radio signal tuned by the antenna circuit with a frequency generated by the local oscillator, which is above (or below) the incoming radio frequency by exactly the desired intermediate frequency. The two signals "beat" against each other and produce other signals which are the oscillator frequency above, and also below the incoming radio signal. One can use either of these as an intermediate frequency, but most often the lower is used. It is then amplified as explained above.

In some expensive "communications receivers" a second local oscillator is used, and mixed with the amplified IF frequency resulting in a second IF frequency. This greatly improves selectivity and rejects the false signal above (or below) the proper tuned signal. These are known as "dual conversion" sets. In some sets a third conversion is used to cancel both the image above and below the proper signal. These are known as "triple conversion sets," and are only used on VERY expensive communications receivers which have great selectivity and sensitivity. Almost all the antique radios we find ourselves restoring will be single conversion sets, Single conversion provides quite adequate selectivity for most listening purposes.

I hope this answers your questions.

RE: the "F" in IF - fenbach - 08-24-2021

whew. lots of good info. and yet. the IF transformers in the crosley AA5 i redid had tuning slugs. with fixed caps. and i'm still wondering how the design of the transformer determines the frequency at which it operates best. number of windings?, diameter of coils?, gauge of wire? as i mentioned, a transformer rated 4.5 MHz looked much the same to me as the 455 KHz ones from the AA5.

RE: the "F" in IF - mikethedruid - 08-25-2021

As I told you, the two sides of an IF transformer are BOTH TUNED LC TANK CIRCUITS. One tunes them to peak at the desired frequency, on both sides. Tuning can be accomplished in an LC tank circuit by varying either the inductance, or the capacitance. In your case the capacitance is fixed, and the inductance is varied. This still tunes the tank circuit.

In a resonant tank circuit both the capacitance and the inductance affect the frequency at which the circuit "peaks" Inductance is affected by the diameter of the coil, the number of turns, the length of the winding, and the magnetic properties of the coil core. The gauge of the wire is of little importance except in that it can affect the length of the winding, and must be heavy enough to handle any current which will pass through it without overheating. The slug which moves in and out of the coil tunes the inductance, increasing or decreasing it by varying the magnetic properties of the core.

I STRONGLY suggest you invest in a copy of Audel's Radioman's Guide. Get an old one from before the mid 1960s. You can find them cheaply enough on Amazon or even eBay sometimes. READ IT CAREFULLY. You will find the answers to these, and many other questions you may have along the way.

Alternately, if you don't mind reading from a screen, or investing in several reams of paper and a lot of ink cartridges to print it out, you can download the entire old National Radio Institute course for radio repair men here:

https://worldradiohistory.com/Archive-Courses/National_Radio_Institute_Practical_Radio_1930.htm
It is a series of PDFs, 49 in all, and will give you an even more thorough grounding in electronics than Audel's

RE: the "F" in IF - John Bartley - 08-25-2021

Here's another good series, originally used (as I recall it) by the U.S. Navy as a training manual. I have the hard copy single volume and it's still my go-to reference when I run into trouble.

https://worldradiohistory.com/Archive-Rider/BASIC%20ELECTRONICS/