PLL question

[Crambone]

so I figured since so many knowledgeable individuals are on here someone might be able to enlighten me on how a PLL works. Let’s use a President Madison I guess as an example.
So as someone that doesn’t know much about how things work in a Transceiver I’m trying to learn.
When I have the radio on channel 34 let’s say or 27.3450 does that come from the PLL itself or does it get mixed somewhere? If I were to take a frequency counter and hook it to one of the PLL outputs would I see 27.3450? And does the PLL manage both RX and TX?
Or am I getting this all wrong?

 

[Dmans]

Crambone,
That is a question that can't be answered quickly.

When I have the radio on channel 34 let’s say or 27.3450 does that come from the PLL itself or does it get mixed somewhere?

The PLL causes the mix from an internal frequency generated within it (divided from an external 10.240 Mhz crystal typically) and from external sources-channel selector, crystals usually 2 or 3 in total (instead of 12 or 15 as used in the 23 channel era- A major cost savings. The driving factor in PLL development (as well as FCC efforts to stop illegal modifications)

If I were to take a frequency counter and hook it to one of the PLL outputs would I see 27.3450?

You won't find a direct channel frequency from a PLL pin.

And does the PLL manage both RX and TX?

Yes.

Google up components of the PLL such as:

Reference Oscillator & Divider
Programmable divider
Phase Detector
Loop or Low Pass Filter
Voltage-Controlled Oscillator or VCO


Try and get a copy of Lou Franklin's "CB PLL DATA BOOK". It has plenty of information presented in an easy to understand format.

73
David

 

[nomadradio]

I would start with Lou's "Screwdriver Expert's Guide".

Your PLL question's answer should start with why the radio generates a carrier frequency that determines the channel the receiver will hear, and the transmitter will produce.

Lou explains the superheterodyne radio better than I could in a forum post.

Long story short, you mix two frequencies together, one of them is the carrier from the radio's PLL. The other is the soup of frequencies coming down the antenna. This feeds into a fixed-frequency amplifier, filtered to accept only one frequency, the Intermediate Frequency, or IF.

The oldest CB radios used a single crystal to feed into the mixer and select the receiver frequency. A radio with an IF frequency of say, 1 MHz would need a crystal that oscillates 1 MHz below the channel frequency desired.

And here's where it gets ambiguous. You can also use a crystal that is 1 MHz above the channel frequency. What matters is the difference between two frequencies when you mix them together.

The IF in older Uniden SSB CB radios is 7.8 MHz. In most AM-only CB radios, it will be 10.695 MHz.

In a radio that has an IF of 10.695 MHz, the local oscillator (LO) for channel 1 will be 16.270, if you use the low-side mixing frequency. And if you use the high side, that's 26.965 plus 10.695, or 37.66 MHz. This is what a straight frequency counter would read, if you tap into the radio's PLL. A straight counter, meaning one that counts up from zero. The built-in counter display in a radio doesn't count up from zero, but counts up from that IF frequency. This does the math to add both frequencies together and display the operating frequency.


There's more than one way to skin a cat. 23-channel CB radios would do this mixing process more than once, mixing ten crystals in different combinations to get 23 separate LO frequencies. Around the time 40-channels became the rule, chip technology had developed to a level that allowed putting an entire PLL circuit in one or two chips. Turned out to be a lot cheaper to manufacture than the big pile of crystals that would be required to do the same thing. A very few of the first 40-channel CB radios used crystals, but every one of them since about 1978 uses a PLL.

The transmit frequency gets produce pretty much the same way.

And if that story doesn't neatly add up for you, Lou's version is a lot better.

73

 

[Crambone]

Again thank you for all the great info!