[HomeBrew] Pll Idea, needs sanity check

[Steven Weber]

Two problems:

You need programmable divider chips for the VCO frequency. Assuming a 
standard 10.7 MHz 1st IF and 28.7 MHz operating frequency to make it easy, 
the VCO will be at 18 MHz. So to get the 1 kHz reference frequency for the 
PC, you need to divide by 18,000. Now to tune one step higher, you need to 
divide by 18,001.

Second problem: Using a 1 kHz reference, you will need a very aggressive 
loop filter to eliminate 1 kHz side lobe spurs in the VCO. This translates 
to very long settling times and getting it stable is a task. 5 kHz steps is 
the smallest which can be delt with without too much trouble.

If this is an AM CB rig, the transmit frequency is twice the VCO frequency, 
which means you have to swing it between 18 for receive and 14.35 MHz for 
transmit. This means the /N number has to change from 18,000 to 14,3500. In 
addition to keep the Tx and Rx frequencies the same, both /N numbers have to 
change in the same ratio, but one is changing in 1 kHz steps, while the 
other is in 500 Hz steps. So, you really need a 500 Hz reference frequency 
and that is not very practical at all.

The PLL in the CB has two sets of /N tables for each channel and works at a 
base reference frequency of 2.5 kHz.

While you can build a discrete logic chip PLL like this, it will take a lot 
of parts and be clumbersome to use. (unless you just want to operate on one 
specific frequency, but in this case you'd be better off using a much higher 
reference frequency) Your new PLL board will likely be larger than the CB! 
And all those divider chips will be generating all kinds of harmonics, so 
expectionally good power supply by passing and sheilding will be required so 
you don't hear them in the receiver.

A much simpler solution is to use a programmable PLL chip and mircoprocessor 
control. Unfortunetly, this requires learning how to write the control 
program for the processor to load data into the PLL chip and DIP versions of 
PLL chips are scarce today. Plus of course, you need a chip programmer to 
load the program into the processor chip.

One alturnative which still requires a uP, but not a programmable PLL chip, 
is to mix down the VCO frequency low enough for the uP to count it and use a 
programmable counter inside the uP to do the divider.

70's vintage CB's often used a PLL with extra inputs for the channel 
selection which would put them into the 10M band. Might be worth trying to 
find out if this is one of them or not. It is also possible that a rig of 
that vintage uses a different mixing scheme than the more modern ones I'm 
familer with.

73, Steve KD1JV