Hi all! I work on small project which will be embedded in old radio receiver (this is FM converter). However this receiver has unusual power supply system (I believe this is because of the huge amount of PNP transistors inside). The chassis of the receiver is tied with positive output of power supply in contrast to the modern devices where GND is usually tied with the external case and/or chassis. Negative output from power supply is single wire and all separate modules inside radio receiver get their power from the chassis and wire running to this negative output. Power supply gives 9-9.5 V of output and my project will work from 5 V so I’ve decided to use LM7905 linear voltage regulator. However I’m a bit unsure how to do schematics and layout properly.
If I got it right I should connect chassis to the GND pin of LM7905 and this power rail will become my ground plane so I will pour free space on my PCB with it and also connect aluminum case of converter to this “ground” too to create RF shielding thus improving working characteristics. Also I need to connect negative output from power supply (I believe that should be named VEE from that moment) to the LM’s input pin. The last output pin will be named -5V in that case.
Here are two points which worry me. First: I’m not sure if this is considered to be a good practice to fill free board space with positive wire network and connect it to the aluminum enclosure. Will it work as in intended shield? Second: all library components which require external power (e. g. MCUs and some another components) have power pins labeled VCC and GND (more often). In my case common sense tells me to connect all GNDs of these chips as well as diode’s cathodes to my new -5V rail and all VCCs to the LM7905’s GND rail. That seems a bit contradictory to me and I doubt if I understand all right.
There is nothing magical about the label GND. It’s just that modern components tend to have a positive supply voltage so GND is the negative return. If the majority of your circuit works on a positive return, then so be it for the minority components, the GND for those devices now goes to the negative supply.
Just be aware of the voltage levels in the interfaces between the older parts of the radio and the new components. The I/O levels of the new components will be specified with respect to its GND which is the negative supply rail. Level shifters may or may not be needed.
Anyway, the only signal lines of the receiver which will be touched are RF_IN and RF_OUT. So as I understand I just should remake linear regulator part and re-wire all component’s GNDs and VCCs.
Addendum 1. Here is my re-drawn schematics. Could you please say if there is something wrong with it? I’ve also changed 3-pole switch to three jumpers.
The thing I’m curious about is why the original circuit connected the crystals to +5V, even though it shouldn’t matter as +5V and GND are the same for AC signals due to the bypass capacitors.
I have to say your choice of colours for the schematic and background have to be the worst for my eyes that I’ve encountered.
I’m not the original author of this schematics. It was published on ham radio forums in Russian part of internet and I just tested it on perfboard and that worked just fine.
Regarding colors of the schematics: I’ve just exported it from EESchema main menu and saved via GIMP. During drawing it is drawn on usual white background. I don’t know why EESchema exports it with such colors.
Just to reinforce the power issue. A power plane is a power plane is a power plane, the impedance to the supply is the real question you should be concerned about. I would recommend stop using the label “GND” and use only “COM” to emphasize that voltages are all relative and get away from the idea that there is any such thing as an absolute.
Sorry for the wordiness, this is one of my standard rants…
Yep, I’ve used PDF exporting in the past. However, I thought that attaching simple image would be fine and just exported image. Regarding that only background became black I suppose this is bug with exporting to the clipboard
