|RF Board PCB Layout - RF Cafe Forums|
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Post subject: RF Board PCB Layout
Unread postPosted: Mon Jun 07, 2004 4:06 pm
I am about to enter into a PCB layout of RF board, which is basically an Up/Down Converter that operates between 700-2000MHz.
I have few questions regarding the PCB layout:
1) Supply voltage distribution:
Should I allocate voltage supply planes in an internal layer of the PCB and connect these planes to the relevant components through plated via holes?
2) Use of via holes:
The componenet side (Top layer) has GND plane (all the area which is not used for microstrip traces), should I connect this GND plane to the internal GND plane with plated via holes ONLY in the circumference of the board, or in addition to these via holes should I also add via holes near GND pins of RF components like Mixers, Couplers LC Filters etc?
The sensitive circuits are in the down conversion chain, should I avoid routing the voltage planes beneath these circuits? Should I use traces in the top layer to route the voltages to the components of thes circuits?
Your inputs are highly appreciated.
- IR :)
Unread postPosted: Tue Jun 08, 2004 11:28 am
1) Do NOT use voltage supply planes. They are a great way for providing leakage between circuits. The inductance of a series meandering line (sometimes augmented with chokes) is your friend. This inductance in conjunction with well placed decoupling caps will decouple your circuits from each other.
Double caps are generally a bad idea. They will tend to produce a glitch in the decoupling response. Instead use the largest cap you can get away with.
2) Stitch the ground planes together with vias at less than .3" spacing. You do not want significant runs of ground plane not stitched.
Unread postPosted: Tue Jun 08, 2004 3:54 pm
Many thanks for the input, however I still have uncleared questions: Do I need to put additional via holes near RF components beside the stitching in the circumference of the board?
Many RF devices on the board are Gain Blocks, most manufacturers of these kind of devices recommend to put a RFC (RF Choke) that will produce reactance of few hundreds of ohms in the desired frequency band (lowest frequency of course), bypassed with few values of decoupling capacitors. You advised not to use several values of capacitors, suppose I can live with that, but how should I connect these gain blocks to the same voltage? I need to distribute the voltage to them in some way and they are scattered all over the board...
Many thanks, I will be happy for more inputs.
Unread postPosted: Tue Jun 08, 2004 4:57 pm
Yes, you should have at least one via very near each gnd of a component. I meant to stitch vias based on a grid, not just around the edge.
The use of double caps would be OK if it weren’t for the small amount of inductance between them (1 to 2nH). Check it out on a simulator or try it in the lab.
The series inductors are typically chained in series with a cap at the VCC pin of each device. I generally do not recommend the star arrangement.
Post subject: Using a large single capacitor
Unread postPosted: Thu Jun 10, 2004 8:15 am
One person suggested that using multiple caps is a bad idea. This person obviously lacks experience in high frequency design. The biggest mistake that designers make is to use one large capacitor for decoupling thinking that it will de-couple all frequencies as long as it is large enough. There is something call "ESR" in capacitors, it is a resonance frequency where the capacitance is optimum. Below this point you are fine, using a capacitor above this point and you mine as well put in an inductor. Use multiple capacitors of various values , including a large value, to de-couple across the entire frequency band.
Also, place a minimum of two ground vias as close to the termination of any shunt component. Remember, that the RF ground is below and vias connect to the RF ground. Vias also have inductance, the higher the frequency, the higher the inductive reactance. Using more vias, as close as possible will minimize this inductance.
Unread postPosted: Thu Jun 10, 2004 12:08 pm
To the above post, I think you mean SRF not ESR for the cap. All the post I have read hear have metrits to certain applications.
The user must understand a component, there are almost no general rules for PCB layout. Some components are specifically used at their SRF.
If good RF work could be summed up in a few rules, then I am out of a job.
Unread postPosted: Thu Jun 10, 2004 12:29 pm
I am the person that said that double caps in decoupling networks are a bad idea. I will thank you not to make comments about my experience, which exceeds 30 years.
Just sweep your double caps and you will find a parallel resonance in the middle of it all. Try it you might learn something.
The last time I looked ESR was equivalent series resistance. It is not a resonance. Caps do have a self-resonance. That is the frequency above which the reactance is inductive.
Unread postPosted: Fri Jun 11, 2004 10:09 am
One person doubted that using multiple caps is a bad idea. This person obviously lacks knowledge in fundamental theory. The Foster’s theorem claims that the derivative of input impedance is positive function for any reactive one-port circuit. Which in turn means that between any two zeros there is pole, and visa versa. By other words, any reactive one-port circuit which has low input impedance at frequencies f1 and f2 inevitably has high impedance in-between.
My two backs
Unread postPosted: Mon Jun 14, 2004 1:48 pm
Well said Oleg.
Good point about the inverse. The inverse of decoupling is the coupling cap. We don't see any one paralleling coupling caps because the zero would be immediately apparent in a wide band circuit.
Post subject: PCB's for VHF/UHF
Unread postPosted: Wed Jun 16, 2004 3:09 am
IMHO, the use of both power plane layers and decoupling capacitors is recommended if it is a multi layer board design. Using simple traces as supply lines will make them act as microstrip transmission lines. Also the use of chokes must be verified as a choke together with a decoupling capacitor will have a resonance somewhere in the spectrum. If this resonance falls within a multiple of the operating frequency there will be problems. If you plan to use a choke, make sure that the Q of the decoupling network is low enough not to cause ringing effects.
Use power planes where possible but make sure that the ground plane layer is SOLID! with absolutely no slots in it and place the power plane underneath the GND layer. If the power plane layer is properly designed, it will act as one very good chip capacitor with exellent decoupling properties - but only if the connection to the top (signal layer) is made using very short connections. 1 mm of trace or via is equivalent to 0.7-1 nH which may be enough to impair the decoupling function.
Use minimum two via holes per decoupling capacitor.
Use copper fills all over the top layer, keeping a distance to the RF traces big enough not to make the copper/trace forming a co-planar waveguide, since this will result in changes to the caracteristic impedance of RF traces. Stitch the area fills with via holes so there is no possibility that any copper could act as a patch antenna.
The use of heat relief tracks between decoupling capacitors and the copper fill requires special attention. It is best to avoid this all together, but such boards are often impossible to assemble due to the amount of heat required during the soldering process.
Then it is easy enough to specify one trace and work with this, but since the inductance is 0.7~1nH/mm the series inductance will impair the decoupling seen over a broad bandwith, so try to use minimum two trace s per capacitor.
Use decoupling capacitors with NP0 material for VHF/UHF if you are using off the shelf components.
It is recommendable to paralell a eg 1nF capacitor with a ~27-100pF for VHF/UHF use.
Sure, you can make use of the series resonance formed by the capacitans and the internal inductans of the capacitors, but performance change as manufacturers improve their processes. Never rely fully on such effects although they may seem clever at first.
For lower frewquency decoupling, use X5R or X7R material. Do not even consider others as Y5V. Theese materials are voltage and heavily temperature dependant. Some of the moderns high efficiency materials also exhibit piezo electric effects and may actually generate noise if the circuit board placed in a vibrating environment(!) We used such capacitors in an EL high voltage driver and we could hear the high voltage oscillator...
Unread postPosted: Wed Jun 16, 2004 12:42 pm
I wish to thank you all guys for the useful advice. I will take it all under consideration in the board layout.
- :-D IR
Unread postPosted: Wed Jun 16, 2004 3:32 pm
You have gotten a lot of different advice. I suggest Looking at the swept responses AND nodal impedances of ANY candidate decoupling approach before you commit the success of your design to it.
While the double cap is a poor idea, it does not significantly hurt you most of the time due most decoupling networks being over built. For RF, the VCC plane is another matter. Given significant complexity and gain it will hurt you more often than not.
Post subject: Double Caps
Unread postPosted: Fri Jun 18, 2004 12:14 pm
You are correct I meant SRF although many vendors will only give you graph of ESR versus frequency making it so you have to determine the SRF by finding the lowest resistance. I did not have a problem with Tonys comments. He said that double caps are OK as long as you are aware were the resonances are. The problem I had was from the suggestion that double caps are "bad" and use the biggest cap you can find. This may be fine at lower frequencies but you will probably have a nice inductor at the higher frequencies.
Post subject: Double Caps
Unread postPosted: Fri Jun 18, 2004 12:18 pm
You are correct I meant SRF although many vendors will only give you graph of ESR versus frequency making it so you have to determine the SRF by finding the lowest resistance. I did not have a problem with Tonys comments. He said that double caps are OK as long as you are aware where the resonances are. The problem I had was from the suggestion that double caps are "bad" and use the biggest cap you can find. This may be fine at lower frequencies but you will probably have a nice inductor at the higher frequencies.