Am Frequency Response and Linearity -


What's the reason for checking AM transmitter frequency response and linearity in the first place?

The purpose is to insure that the transmitter is clean at ALL voice frequencies, EVEN if they are not going to be used all the time!

If the rig is non-linear, it will create and produce products (splatter) that are much wider than the voice frequencies it was designed to amplify....or the voice currently being amplified. Not to mention sounding bad.

** The idea is to have an AM transmitter that can CLEANLY reproduce 40-7,000hz audio, but it is up to the operator to decide how wide he chooses to run his audio at ANY particular time.

How is this done?

For example: Using a "wide body capable" and clean AM transmitter with an audio equalizer rolled off sharply starting at 4khz with an additional brute force 4kc audio low pass filter will produce extremely clean side products as well as a narrow +- 4kc signal.

However, a transmitter that has linearity problems, phase distortion, flat topping etc., may generate products out to +-10kc even though a 4kc wide voice as described above is being inputted.

The point is to make your transmitter a transparent audio platform that will CLEANLY reproduce audio frequencies FAR GREATER than you will ever need. Then roll off the audio response at the LOW LEVEL point to match crowded band conditions or open it up wider when not.

Having a wide body AM transmitter doesn't mean it has to be run that way all the time, just like owning a big linear or race car doesn't mean balls to the walls all the time.

*** I once used a homebrew variable eight stage Butterworth audio filter in the low level audio chain. I could control the roll off "on the fly" from 2.5khz up to 8khz. It looked very good on the scope when swept. When in the 2.5khz position my 813 plate modulated rig was as narrow as any ricebox and I would get guys asking me what the hell happened to the highs... :-) Think I will put it back on line as a demo.

Bottom line...perhaps we can consider another dimension of dynamic control for our rigs. When there's five AM QSOs all jammed in a row 5kc apart, we simply set the audio filter to satisfy conditions. If the transmitter is CLEAN in the first place, what you dial is what you get!

If anyone is interested, I will make up a schematic of my variable Butterworth audio filter and get it posted. Pete, WA1SOV sent me the basic Butterworth design and I simply added eight variable pots to the actual unit with marked presets for various roll off frequencies.

When set correctly the roll off was brick wall...far sharper than an EQ that has its high frequency sliders shoved all the way down.


Tom, K1JJ

                                            Back To East Coast Sound        Home