The revision B schematic is available for viewing at [1]. I'm not super happy about needing 20 varactor diodes to get a turning range of 700kHz to 1600kHz (both preselector and LO get their own bank of 10 diodes). I could avoid the need for so many diode by using a 10x larger inductor, but then the Q of the preselector and the LO drop significantly. Based off of the fact that ZZRX-40 works and that my design is essentially a copy of that, I am optimistic that I will only need to alter a few component values (like the LO feedback capacitors) to have the design receiving audio. Work has begun towards the PCB (enclosure selection/footprint creation/etc.= ...) [1] https://drive.google.com/file/d/0BwP0qhqyaTIIcEdFMTkwQXlwWFk/view?usp= =3Dsharing On Mon, Oct 17, 2016 at 10:38 PM, Harold Hallikainen wrote: > >> Well, IR is RF in the terahertz range, so what was described was >> amplitude modulation of a signal using a frequency modulated signal as >> input? ;-) > > Yes, it's FM subcarriers on the terahertz "RF." This is the similar to th= e > DSB-SC subcarrier on FM used to carry the stereo difference signal. The > original definition of FM stereo used this frequency division multiplex > description. But, you get the same thing if you use time division > multiplex spending a little time on the left channel and a little on the > right. It's amazing how you can look at the same signal in different ways= .. > > Thinking about the direct conversion AM receiver, I'm thinking there will > be problems due to the presence of both sidebands unless the local > oscillator is phase locked to the received carrier. If the local > oscillator is phase locked at 0 degrees, you will get DC plus the audio > (plus two times carrier that is taken out by the low pass filter). If the > local oscillator is 90 degrees out of phase with the received carrier, yo= u > will get nothing. If the local oscillator is not the same frequency as th= e > received carrier, you'll get a tone corresponding to the difference in th= e > frequencies plus a mix of the audio shifted up a bit in frequency and the > audio shifted down a bit in frequency. If the frequencies are very close > (say 0.1 Hz apart), I suspect you will hear the audio disappear and > reappear over a 10 second period as the phase between the two rotates. > > The null at 90 degrees is an interesting situation. You can transmit two > signals on the same frequency without interference by setting the carrier= s > 90 degrees apart. This is the basis of QAM or Quadrature Amplitude > Modulation. This is a common digital modulation system where, for example= , > 8 levels are transmitted on the I (in phase) carrier and 8 are transmitte= d > on the Q (quadrature) carrier. For each transition of the carrier (a > baud), 3 bits are transmitted on I and 3 bits are transmitted on Q for a > total of 6 bits per baud. It turns out that when you add sine waves that > are 90 degrees apart and vary the amplitude of each, the result is a sine > wave whose amplitude is the square root of the sum of the squares of the > amplitudes and the resulting phase angle is the arcsine of the Q amplitud= e > over the I amplitude. You can plot the resulting signal on a > "vectorscope." With the 8x8 QAM mentioned before, you get dots in 64 > different positions (an 8x8 array). This is "64QAM." > > But, the QAM signal could carry analog signals. In NTSC color television, > the "baseband" carries a linear combination of red, green, and blue that > produces a good picture on a monochrome television. Another linear > combination of red, green, and blue modulates the I subcarrier at 3.58MHz= .. > Another linear combination of red, green, and blue modulates the Q > subcarrier at 3.58MHz. Through the baseband, I, and Q signals, we are > transmitting three signals that are "linearly independent." In the > receiver, we can multiply each of the signals by different constants, the= n > add them to yield the original red, green, and blue. Very clever! > > But then, a monochrome signal does not modulate the I and Q subcarriers a= t > all. As such, we can adjust the gain of the I and Q signal to vary the > color saturation (no gain gives monochrome). Further, adjusting the phase > of the local I and Q signals relative to that at the transmit end (and > transmitted in the color burst for reference) varies the "hue" of the > resulting color. So here we have a QAM signal, but the amplitude and phas= e > have their own meaning. When you put up color bars, you should see dots i= n > specific locations on a vectorscope. > > Modulation... It's fascinating! > > Harold > > > > > > > -- > FCC Rules Updated Daily at http://www.hallikainen.com > Not sent from an iPhone. > -- > http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive > View/change your membership options at > http://mailman.mit.edu/mailman/listinfo/piclist --=20 Jason White --=20 http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .