Projects‎ > ‎Digital Ham Radio‎ > ‎

RF_Envelope

A brief description of RF Envelope



Stations operating under standard conditions should be able to space themselves every 3 Kilohertz (KHz) or so on Lower Side Band (LSB) and/or Upper Side Band (USB).  As illustrated, this allows amateur radio stations to operate without interference on, for example,  3.919 Megahertz (MHz), another at 3.916 MHz and another at 3.913 MHz in 3 Khz steps.


Amplitude Modulation or AM is twice the width of a Single Side Band (SSB) signal.  AM has a "carrier" tone transmitted at center frequency.  In receive audio, the receiver filters this carrier out of what is heard.  This is known as a "suppressed carrier".  As one "tunes in" an AM station the carrier is suppressed.  The technique also allows "zero beating" Continuous Wave (CW) signals.

Zero Beating

To "zero beat" a CW signal, tune down the band in LSB mode or up the band in USB mode unill the desired CW signal fades away.  At the point in which the signal disappears, the receive signal is at center frequency.  Once on frequency, the receiver can be switched to CW mode and the transmitting station is on frequency and has been "zero beat".

Interference occurs when a signal is:



1) Incorrectly spaced (transmitting within 2.5 KHz of another station)






2) Too wide (transmitting a signal greater than 2.5KHz)





3) Splattering: When voice peaks carry beyond the 2.5KHz RF Envelope either up or down the band.  Splattering up and down the ban can be caused by overmodulation and spurious emissions being generated by the transmitting station.  The sketch shows a signal that is both wide (5KHz in the intended side band) and splattering above (outside of the intended side band), where all transmitted signals should be suppressed. 

IF Shift

"IF shift" allows one to shift the segment of the band you are hearing around.  If the center frequency was set to 3.916 MHz as in the previous examples and the IF Shift was turned to the right, it would minimize any back splatter coming from 3.913 MHz, but would capture more of the transmitted signals from 3.919 MHz.



Turning IF Shift the opposite way would reduce the signals or splatter from 3.919 MHz, but would catch signals being transmitted at 3.913MHz.


Filtering

Filtering cuts the RF Envelope down, generally from 2.5 KHz to something less.  In combination with RIT or "Shift", one is able to focus on weak data (or CW) signals in the midst of strong adjacent signals.

The NULL

There is a difference in perspective between your transmitter and your receiver regarding the radio signal waveform as illustrated below. 

AM has a full carrier when transmitted, but that carrier is suppressed or inaudible at the receiving end.  If the radio is flipped from AM to USB or LSB and tuned up or down, the carrier becomes audible as it moves through the passband of the selected mode.



The same is true with USB and LSB:


Center Frequency or the Frequency of the VFO (Variable Frequency Oscillator) is what one sees on the radio dial or display, on most radios, most of the time.  On the radios I use, this is mode dependent. For instance, on the Yaesu FT-920, switching from SSB to DATA shifts the VFO display, but does not change frequency.
Comments