its two inputs, until an exact match occurs again. While this appears to be just a trial-and-error
process, the whole thing happens in the time it takes you to change from Ch.1 to Ch.2 !
Receiver IF`s
We've now seen how the Ch.1 PLL mixer signal of 37.660 MHZ provides the RX first IF injection. Now
note from Figure that we can make even a third clever use of the 10.240 MHz Reference Oscillator. By
mixing that with the 10.695 MHz first IF, the result will be 10.695 - 10.240 = 455 kHz, the second RX
IF. (The sum product is ignored.) Pretty smart these engineers...
Almost all AM or FM CBs use this method of dual-conversion for their receivers. It's also commonly
used in car radios, scanners, FM stereos, etc. where a lot of the circuit hardware already existed.
Transmitter Section
In this example the TX carrier frequency is produced very simply. A local oscillator of 10.695 MHz is
also mixed with the 37.660 MHz Ch.1 PLL output. The difference is 37.660 - 10.695 - 26.965 MHz,
which is then coupled through various tuned circuits and the standard RF amplifier chain.
The Truth Chart is the most important first step in determining how a modification can be made. or if it
can be made. Let's examine it in greater detail now.
The exemple just described was a very easy PLL circuit using the binary type of programming code.
It's quite possible for the same chip to heve different N-Codes depending upon how many crystals are
used, or if it's AM or AM/SSB. The preceeding circuit is one of severel used with the PLL02A; this is
the "2-crystel AM" loop. It used N-Codes from 330 Ch.1 to 286 Ch.40, because those were the
numbers needed for exact division, correct IFs, etc. An earlier AM loop used 3 crystels and N-Codes
which went up, from 224 Ch.1 to 268 Ch-40. And in the ever-populer SSB chassis the N-Codes were
255 down to 211.
Notice that these N-Codes can go up or down with increasing channel numbers. It depends on the
VCO design.
Those Infamous Channel "Skips"
Meanwhile, let's return to a portion of Programming Chart to study some of its other feetures.
Programming Chart is e eimplificetion ehowing only the channel number, frequency, end N-Codes
from the original full chart.
Notice anything unusual in the N-Code sequence going from Ch.1 to Ch.40? The codes aren't all
consecutive and skip some points that aren't legal CB frequencies. For example, Ch.3 is 26.985 MHz,
end Ch.4 is 27.005 MHz. So what the heck heppened to 26.995 MHz? Gee, it's not e legel FCC
channel. This is known to CB`ers as en "A" channel, in this case, Ch.3A. There are also skips et
Chennels 7, 11, 15, end 19. And Ch.23, Ch.24, end Ch.25 of the FCC CB band are essigned out of
sequence. (Thet's left over from the old 23-chennel deys.)
What this means is that all the N-Codes es well as VCO end mixer frequencies ere also out of order in
the chart. Meny Europeen countriesthat originelly ellowed only 22 channels simply adopted the
Americen scheme exectly for those first 22 channels. Austrelie had 18 channels whose numbers didn't
correspond to American/EEC numbers, but meny of the actual frequencies were the same. And the
UK originelly assigned 40 consecutive channels with no skips at all. Remember these points when
studying en older model's Truth Chart, or you mey think your math is wrong when it really isn't.
LOOP MIXER MODIFICATIONS
Now let's examine the second possible conversion method, that of changing the Loop Mixer frequency
itself. This is one of the easiest ways to modify a PLL circuit having a downmix signal. A few chips like
the PLL02A can be modified by either of the programming pin change or downmix chang methods.
The choice depends upon the total number of extra channels desired, and how much modification
work you're willing to do.
Changing the mixer crystal is most commonly done when jumping up to the 10-Meter HAM band.
Since there's no intention of ever using the rig again for CB, it can be permanently retune at the higher
frequency. But many of you are still expanding from the CB band and adding an extra 40 or 80
channels. The European models like those from HAM International, Major, and SuperStar were
basically just American model with the extra mixing crystals already there.
To Previous Page
Loading...
