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Help- RF Transmitter Layout - RF Cafe Forums

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Amateur Radio | Antennas | Circuits & Components | Systems | Test & Measurement

Post subject: help- rf transmitter layout Posted: Sun Jun 22, 2008 1:28 pm


Joined: Sun Jun 22, 2008 3:57 am
Posts: 4

i have joined this forum recently. i want to design pcb for a video transmitter schematic. please tell the rules to follow to design the lay out for the schematic.like width or length of the tracks,ground plan etc. i heard that there is relation between frequency and track width.the transmitter operates in channel 7 to 13 of vhf band. i am enclosing the url for the schematic. please help me.
http://www.newcircuits.com/chimage.php? ... rfr007.gif


Post subject: Posted: Mon Jun 23, 2008 7:20 am


Joined: Fri Feb 17, 2006 12:07 pm
Posts: 218
Location: London UK
I have looked at the circuit and would comment as below:
You need to add the same C7 decoupling capacitor at the top end of L2 as you have at the top end of L1, and this should be a solder-in feed-through type.
All elements around the oscillator Q3 and the PA Q4 should be kept as close together as possible to the respective transistors.
Q3 and Q4 should have a screening metal wall between them.
Q4 might need a "flower" type heatsink clipped onto the can to keep the temperature within permitted limits.
For C3 I would be inclined to use a piece of miniature coaxial cable with the braid to R8/Q4 and the center conductor to CV1/Q3, length about 4 to 5 cm. This is to enable a physical separation of the VCO from the amplifier.
Build the unit on double-sided board with shorting pins through to ground large areas on both sides of the board. The component side layout should be designed to maximize the ground areas, but junctions of RF hot components, such as C9/R6/R7/base of Q3, plus the collector group of Q3 should be fairly large pads about 5 mm square. There needs to be a balance struck between having too much stray cap to ground (large pads) or having too much series stray inductance (small pads).
The video side is much more tolerant of layout.
Hope that helps.


Post subject: rf -layoutPosted: Mon Jun 23, 2008 2:17 pm


Joined: Sun Jun 22, 2008 3:57 am
Posts: 4
thanku very much sir! i am a beginner to rf circuits. could you explain briefly about layout of a double sided board. can you please, suggest any links/books on rf pcb design and layout?


Post subject: Posted: Mon Jun 30, 2008 6:48 am


Joined: Fri Feb 17, 2006 12:07 pm
Posts: 218
Location: London UK
Hi deepak
The techniques I aquired were from reading Application Notes, like HP/Agilent, M/A Comm, Texas Instruments, and the old Plessey Company semiconductor units (let's hear a faint cheer from any Plessey refugees with long memories). Many of these can be down-loaded off their web sites.
Sorry, I don't know of any textbooks that cover this practical topic.
If you are proto-typing a layout, a good practical starting point is to take double-sided board and start by measuring its permittivity using an RF bridge (divide the measured capacitance by the area). This enables you to design any microstrip track widths, using published tables, for any deisred Z nought (like 50 ohms).
Next do some paper and pencil rough layouts to suit the physical sizes of components you have selected for the circuit build.
When you are happy all elements are incorporated, use the paper layout as a drilling template, by glueing it to the board with water-soluble paper PVA glue. Use a drill suitable for the wire parts of any components or the through-pins used as "vias". Using a model-maker's milling attachment in a hand-held low voltage drill, mill out any pad areas. Any locations where you have a component grounded at one end but floating at the other, counter-bore at the floating end using the previously drilled small hole as a pilot, and a drill bit that has been ground down to present a flat end with a central spike. Thus an earea is milled out centered on the pilot hole, that is free of copper. Connect any such components together with any others at node points using teflon sleeves over 16 gauge wire with short direct lengths. This strictly DIY stuff is if your budget is very very limited. There are software tools for prototyping chemically etched boards if your budget is big.
I hope that helps.

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