FM PLL. Dip Switched VS microcontroller based. Noise issues

Hello, I have seen several broadcast FM PLL designs on the internet.
I have seen two basic types. One that uses a microcontroller and LCD and another one which uses DIP switches and a PLL chip like the MC145151 to set the frequency.

I have red somwhere in the past that DIP switched PLLs are better because the noise is lower, due to the absense of a microcontroller on the loop. Is this right?

Quick frequency adjusting is not an issue to my application. Which one would you recommend?

neazoi said:

Hello, I have seen several broadcast FM PLL designs on the internet.
I have seen two basic types. One that uses a microcontroller and LCD and another one which uses DIP switches and a PLL chip like the MC145151 to set the frequency.

I have red somwhere in the past that DIP switched PLLs are better because the noise is lower, due to the absense of a microcontroller on the loop. Is this right?

Quick frequency adjusting is not an issue to my application. Which one would you recommend?

Click to expand...

Is this for new design of a 1) product, or 2) project, or 3) personal (hobby) use?

Will it have to be certified to comply with any post or radio regulations for purity of emmisions (spectral purity)?

Bear in mind, too, that the MC145151 may not be available (IDK, but it is an old part) but there may be others suitable.

Jim

RF_Jim said:

Is this for new design of a 1) product, or 2) project, or 3) personal (hobby) use?

Will it have to be certified to comply with any post or radio regulations for purity of emmisions (spectral purity)?

Bear in mind, too, that the MC145151 may not be available (IDK, but it is an old part) but there may be others suitable.

Jim

Click to expand...

I would justify it as a personal use and as a project a bit, so strict regulations do not apply. my question refers to noise produced by the microcontroller if used in the pll system. The design I have seen is this FMtXSt.png (image)
I have the chips already.
It is the most simple stereo pll system I have seen

neazoi said:

I would justify it as a personal use and as a project a bit, so strict regulations do not apply. my question refers to noise produced by the microcontroller if used in the pll system. The design I have seen is this FMtXSt.png (image)
I have the chips already.
It is the most simple stereo pll system I have seen

Click to expand...

With careful PCB routing, parts placement (don't put uC next right to the PLL's VCO circuit for instance without at least some board copper there) and proper Vcc and Vdd bypasssing and use of a separate regulator to power the PLL and Uc, should not have much of an issue for your application ... on-channel noise and spurious are not the issue with a uC controlled little transmitter like this, it is when one tunes off-channel and one picks up products 'mixed' with the uC clock and various other 'rates' and clocks (that might be down only 30 dB from carrier) that it might become an issue, especially if one has to meet spectral purity requirements. For your own use, I would not be as concerned (unless you plan to boost output to 50 Watts that is!)

With the radio spectrum, it is a good idea to be clean and not bother adjacent stations (your neighbors, so to speak) lest they make/file an FCC complaint.

Jim

RF_Jim said:

With careful PCB routing, parts placement (don't put uC next right to the PLL's VCO circuit for instance without at least some board copper there) and proper Vcc and Vdd bypasssing and use of a separate regulator to power the PLL and Uc, should not have much of an issue for your application ... on-channel noise and spurious are not the issue with a uC controlled little transmitter like this, it is when one tunes off-channel and one picks up products 'mixed' with the uC clock and various other 'rates' and clocks (that might be down only 30 dB from carrier) that it might become an issue, especially if one has to meet spectral purity requirements. For your own use, I would not be as concerned (unless you plan to boost output to 50 Watts that is!)

With the radio spectrum, it is a good idea to be clean and not bother adjacent stations (your neighbors, so to speak) lest they make/file an FCC complaint.

Jim

Click to expand...

No that is basically more an experiment than an actual broadcasting. 5W or so would be just great. I am thinking of using the schematic supplied because I find it easy to construct. I do not see why someone would like to use an LCD in such a transmitter where the frequency does not need to be set quickly many times, other than for decoration purposes. In an amateur radio station this is another issue.
So, this mc free schematic will not have the chance to internal interference right? It still uses two clocks though one for the pll and another one for the ba1404

The thing is, the microprocessor has hundreds of thousands of transistors in the form of thousands of gates, D-latches, counters, shift registers and the like internally on the device die ... when active, the current 'draw' by the uC is dependent on the instruction executed, literally, by how many gates and latches etc are internally 'active', and CMOS gates draw current during state transition, and very little to none when 'static' (doing nothing).

If you intend to go to 5 Watts, you have a lot more to worry about to make the PLL happy! If your PLL sees _any_ of the RF you are trying to radiate, the PLL will try and 'phase' lock with that energy rather than the weak little sample that is fed from the VCO, the result shows up as 'funnies' in the modulation (moreso at low freqs within the PLL loop bandwidth where the PLL fill 'fight' any applied modulation directly to the VCO) ... here is where *shielding* is important and the uC is going to be much less of a problem ...

I experienced this many years ago in a 2-way design retrofitting several originally xtal-controlled Motorola commercial radios ... the design used painted aluminum covers that provided no 'shield' effect at the paint acted as an insulator, scraped away the paint and saw a big improvement, although the design still needed 'some work' to make it perfect.

I had good luck using the FM transmitter by North Country Radio a few years back, but, after several months on the air at about 5 Watts into a gain antenna (5/8 over 3/8 collinear) at about 30' I did get a visit from the govt authorities ... (do not know to this day how they became aware. Sometimes it is ppl you know who turn one in.)

Jim