Help- RF Transmitter Layout - RF Cafe Forums
Post subject: help- rf transmitter layout Posted:
Sun Jun 22, 2008 1:28 pm
Joined: Sun Jun
22, 2008 3:57 am
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.
Post subject: Posted:
Mon Jun 23, 2008 7:20 am
Feb 17, 2006 12:07 pm
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
All elements around the oscillator Q3 and
the PA Q4 should be kept as close together as possible to the respective
Q3 and Q4 should have a screening metal wall between
Q4 might need a "flower" type heatsink clipped onto the
can to keep the temperature within permitted limits.
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
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
Hope that helps.
Post subject: rf -layoutPosted: Mon Jun 23, 2008 2:17 pm
Joined: Sun Jun 22, 2008 3:57 am
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
Location: London UK
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
Sorry, I don't know of any textbooks that cover this
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.