help-rf circuit groundplane

[deepak007]

hello

i am building a vhf transmitter circuit. if i increase the width/length of the negative rail or if i use the total area of the pcb except ,the positive lead and the components connected with it, as a ground plane will there be any increase in the inductance. also can i use the other side of the pcb completely as a ground plane? will it reduce noise?

 

[flatulent]

You should have a full ground plane as the next layer below the top one where the parts and interconnecting traces are. Do not try to "save" money with a two layer board.

 

[vfone]

To use solid ground plane on both sides of the PCB is a good idea. The two ground planes will behave as a shield, reducing the external noise influence.

 

[Azulykit]

While you are at it you may as well add a bunch of via's to connect the two groundplanes. Spacing of λ/10 is a good place to start.

 

[RF-OM]

Ground plane is definitely better than single ground trace, but keep in mind that high frequency current will flow only through narrow area in ground plane. Usually this area is located just under the forward path trace and repeats this form. High frequency current always flow by the way of lowest inductance, whereas DC will flow by way of lowest resistance.

For VHF it is desirable, but not mandatory to use upper ground plane. If you build one sample it is okay, but may be too expensive for mass production. When you use upper ground plane via become important part of design. Via spacing actually depends on high frequency current distribution and board thickness. When you need to get more equal potential through surface the best spacing for VHF is about 3 to 5 via diameters, for higher frequencies 2 to 3 diameters. When you need to isolate some area via row may be needed or even two rows. The least expensive via diameter is about 15 mils. It is helpful to check via spacing for possible cavity resonance, which is possible in transmitter’s circuits with relatively high power, especially on higher harmonics.
Best regards,
RF-OM

 

[RF-OM]

I received your question, but could not answer immediately, so I’ll do it here. I do not know these schematics, you may check Encyclopedia of Electronic Circuits or radio amateur magazines, may be web sites. You may find inexpensive kits for making such receivers. Regarding the PCB and layout books, I can recommend these books:
1. Wadell. Transmission line design handbook. Artech House. 1991. Excellent book, but unfortunately there are a lot of typos and errors, so be careful.
2. Archambeault. PCB Design for real-world EMI control.Kluwer academic publishers. 2002.
3. Morrison. Grounding and shielding. Circuit and interference. Wiley. 2007.
These are the book that I have in my office shelf and can see right now. A lot of very good books are available. You may check the authors Henty Ott, Howard Johnson, Montrose, Eric Bogatin, Brooks and others. Many of these books are uploaded to EDAboard and you may find them here. RF design and PCB layout are not easy stuff but it is very interesting and exciting. Moreover, if you learn it well and become a good RF engineer, you will have good monetary compensation for your work. RF and RFIC engineers have the best salaries in engineering today, at least in US.
Best regards,
RF-OM