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help- rf transmitter layout - RF Cafe Forums
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deepak007
Post subject: help- rf transmitter layout Posted: Sun
Jun 22, 2008 1:28 pm
Lieutenant
Joined: Sun Jun 22, 2008
3:57 am
Posts: 4
hello
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
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nubbage
Post subject: Posted: Mon Jun 23, 2008 7:20
am
General
Joined: Fri Feb 17, 2006 12:07 pm
Posts:
218
Location: London UK
Hi
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.
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deepak007
Post subject: rf -layoutPosted: Mon
Jun 23, 2008 2:17 pm
Lieutenant
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?
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nubbage
Post subject: Posted: Mon Jun 30, 2008
6:48 am
General
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.
Posted 11/12/2012
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