power supply for Rx board

Thread Starter

ryan good

Joined Oct 4, 2019
6
Hello everyone, i am designing a rx board utilizing an SI4362 board in the 450-470MHz. it is a design that was already in the field but is very outdated so i am working on a new age replacement to the radio. since it is RF i would assume Linear regulators are the best choice. please correct me if im wrong but my only question at the moment is about these in general.

the original circuit on the first prototype utilized a couple 78XX chips that both connect to VIN (12v from a 7812 on a distribution board).
1651540243036.png

is there a reason not to connect them in "series" like this. 5v will pull only about 400mA and the 3.3v around 120mA. those currents are estimated high for the currents as well. im open to switching regulator recommendations i mostly use TI parts and i use the Webench to develop them. i then mostly select parts available for pick and place from JLCPCB to keep life simple for me.
1651540099199.png
 

Ya’akov

Joined Jan 27, 2019
5,642
Dropping 7V though the 7805 is going to produce a lot of heat and waste a lot of power.

This TI part (which is in stock at TI) seems a good choice for you. It’s designed to be simple with minimal passives, handles 1A (2A and 3A versions are also available) and you can use two hanging off the 12V rail to get your 5V and 3.3V rails.
 

DickCappels

Joined Aug 21, 2008
8,704
Do some experimentation before you commit to a switching type power supply (especially one that switches at 580 kHz). You will probably want to follow the recommended layout religiously and probably add some extra glitch filtering after the output, for starters.

The type of demodulation and the coding of signals in your system will affect the susceptibility to such noises.
 

Ya’akov

Joined Jan 27, 2019
5,642
Do some experimentation before you commit to a switching type power supply (especially one that switches at 580 kHz). You will probably want to follow the recommended layout religiously and probably add some extra glitch filtering after the output, for starters.

The type of demodulation and the coding of signals in your system will affect the susceptibility to such noises.
This is very good advice and of course these devices are used in radios operating in the 450MHz land mobile band so it’s a matter of doing it right not if it will work.
 

Thread Starter

ryan good

Joined Oct 4, 2019
6
Thank you so much for both of your replies. another thought I had was to use an older device like a lm2575 that switches very low frequency into 6.5v and then stepping that into a 7805 / 5912-3.3. my primary question, is it frowned upon to run the regulators in series unlike our older design? i have not had problems with this in the past on other designs but i do not do alot of wireless RX. my concern with this regulator is the status of it will be obsolete in the near future and i want to make sure this design can be used on future similar products.

the board receives a POCSAG tone signal like old beepers. this first revision is more of a true prototype so i can place test points all over it to make sure i have sufficient data using my scope. i will also confirm the signal is pocsag at that time, but thats my take from the relevant information in the manual for the people finder.

quick overview of board design:
a "People Finder" is the tx at a central location that transmits signals to these rx decoders. the decoders are irrigation controllers. they are told to turn on a station for an allotted amount of time. the "decoder" portion of the board uses a 68hc11 and 27c512 firmware with the code. a radio (my design is a si4362) receives the signal sends the data to an input on the hc11 and the mcu does the rest.
obviously its a little bit more complicated than this but you get the idea.

I am controlling the SI4362 with a pic controller for the frequency control and radio settings as well as stepping the raw output of the radio to a clean digital signal using 2 I/O pins of the PIC controller.

as far as design, I will be following the radio recommended layout to a "T" as our current prototype although works, does work intermittently.
 

Thread Starter

ryan good

Joined Oct 4, 2019
6
i barely missed the edit time window i am sorry. If low frequency switcher is the only goal when using a switcher for low noice i could use a circuit like this in front of the 7805 LDO for a 6.5V its a TPS54335A with these values its switching at 50KHz1651593764467.png


the board size is roughly 7" x 4" so not an insane amount of space but i have room for this no problem
 
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