Rather than using banks of varactor diodes, I'm wondering if there's some way of using a capacitance multiplier circuit and just the one diode? I haven't seen it done anywhere but your frequency is low so opamps and/or "standard" transistors etc should be up to the task. Maybe something to look at after the first version is working OK? RP On 20 October 2016 at 12:31, Jason White wrote: > 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/et= c...) > > [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 t= he >> 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 way= s. >> >> Thinking about the direct conversion AM receiver, I'm thinking there wil= l >> 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 th= e >> local oscillator is 90 degrees out of phase with the received carrier, y= ou >> will get nothing. If the local oscillator is not the same frequency as t= he >> received carrier, you'll get a tone corresponding to the difference in t= he >> frequencies plus a mix of the audio shifted up a bit in frequency and th= e >> 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 carrie= rs >> 90 degrees apart. This is the basis of QAM or Quadrature Amplitude >> Modulation. This is a common digital modulation system where, for exampl= e, >> 8 levels are transmitted on the I (in phase) carrier and 8 are transmitt= ed >> 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 sin= e >> 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 amplitu= de >> 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 3.58MH= z. >> 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, th= en >> 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 = at >> 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 phas= e >> 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 pha= se >> have their own meaning. When you put up color bars, you should see dots = in >> 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 > > > > -- > Jason White > -- > 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 http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .