Electronics > RF, Microwave, Ham Radio

AM Radio How to equalize upper and lower IF BW

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watchmaker:
I am using the Elenco AM/FM radio kit as a learning platform.  Leave aside it has the same endearing surprises as the ThaiKit CT.  I am over coming them (like replace the provided LM385 with a real one).

I have an IF BW of 462 and 439 kHz.  The instructions say the BW is acceptable.

However, I would like to get the response equal on both sides (this is a learning exercise after all).  I recorded the measured values of my resistors so that I can calculate expected voltages and currents for example.

What would need to be changed.  Or is it predetermined by the choice of IF xformers?  I do have a decadal capacitance substitution box.

I have found voltnut's youtube on this and he encountered many of the obstacles I encountered; and his ckt analysis helped me to extend my calculations.  But he accepts the BW and moves on. 

THANKS!

wasedadoc:
I do not understand.

The IF frequency in an AM radio (530 to 1600 kHz or so) is typically in the region of 450 kHz. The IF bandwidth is about 10kHz.

For FM broadcast band (88 to 108 MHz) conventional design uses an IF of 10.7MHz and a few hundred kHz bandwidth.

What do you mean by "upper and lower IF BW"?

BrokenYugo:
BW? Usually you just adjust the IF strip for max signal out of the detector or speaker with a modulated 455kHz test signal very lightly coupled in (you want the AGC at max gain), so every can is peaked to 455 in circuit. Bandwidth is baked in, I think in a single tuned transistor radio IF can this is set by the Q of the tuned circuit on the collector side, but that's about the limit of my understanding.

IIRC some old HiFi consoles had you optionally do a sweep alignment of the AM IF, so you could see and shape the passband by slightly detuning the cans or something to that effect. Something I've only read about, mostly an academic exercise with how little music is on AM anymore.

CaptDon:
For best sensitivity you peak the cans for maximum recovered 455KHz. Bandwidth will be whatever it is. If a balanced sideband response is important (and it probably isn't) then you 'stagger tune' the I.F. with a sweep generator to give the desired I.F. passband. Usually requires an R.F. Detector probe to feed a D.C. analog of the I.F. output and a sweep generator that provides a sawtooth sweep voltage output. The sweep gen sawtooth output drives the horizontal input of the scope and the R.F. probe drives the vertical input. When all is set up correctly you get a trace on the scope that looks like the side view of an upside down bucket sitting on a floor.

watchmaker:
5 kHz above and 5 below.

I think BrokenYugo and CaptDon gave me the answer.  After you align the 3 IF stages it is what it is.

Yes AGCi s peaked.

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