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Ceramic capacitor cost reduction - RF Cafe Forums

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 Post subject: Ceramic capacitor cost reduction
Posted: Sun Jul 25, 2010 2:30 pm 

Joined: Mon Apr 20, 2009 2:04 pm
Posts: 14
Hello again,

Been awhile since I've been on the forum, and in that time, I had to take a new job. Instead of designing cutting edge products for the semi industry, I'm performing cost reduction measures....
Easy at first, but now I'm faced with the mother of all cost reductions - All the semis are sucked into an asic, and all that surrounds it are passives.
Anyway, the boss want's his pennies, and that's what I'm paid for, so here's the question:

This device uses a 430MHz RF link with one end in the ASIC and the other in a bottom loaded antenna that's comprised of a descrete inductor and PCB trace.
Between, there's a fourth-order, LC, underdamped low pass / matching network with the (~4dB) bump centered on the desired frequency.
The filter network is comprised of discretes, and the PCB lacks space for much in the way of resonant networks.
I'm curious whether it would be a worthwhile to use PCB inductors (15-90nH) and run-of-the-mill capacitors - currently, the designer has Johanson designed in.
Also, I'm curious whether the entire scheme can be scrapped for a tuned loop antenna - like you find in car alarm remotes.



RF Head
 Post subject: Re: Ceramic capacitor cost reduction
Posted: Tue Jul 27, 2010 12:00 pm 

Joined: Mon Feb 09, 2009 12:26 pm
Posts: 11
Looking at this from a power loss point of view, a lot will depend on the antenna impedance you are trying to match and required bandwidth. In the nasty case of a very low antenna impedance and narrow band the majority of the power loss in your matching network will be due to the inductors at 400 MHz. So from a pure component power loss point of view you can probably get away with much cheaper capacitors (I have done so many times). However, the higher quality (not just higher quality factor) capacitors are often also much tighter tolerance which may also be required.

If it were me I would proceed as follows:-

Measure the input impedance of the antenna. Run some simulations of your matching network to determine where the losses are occuring in the network and what the affect of looser tolerance is. If you are lucky you may be able to get away with cheaper caps. You could also get the figures you need by measurments of your matching network by by building the network then connecting it to another matching network which is designed to take you back to 50 ohms. You can then mess around with values etc and see what the relative change in the power loss through the network is. This is probably what I would do first. You will be able to see exactly how the cheaper components compare to the expensive ones.

"PCB inductors" might add too much loss. However, running a transmission line match might work.

I would look at improving the input impedance of the antenna. Unless you are really tight for space it would be much nicer to have a antenna impedance closer to 50 ohms, you may then be able to do away with the matching network entirely. If your antenna is a thin PCB track you may be able to improve the Zin by adding capacity to the open end of it, like top loading a short monopole with a top hat. However, if you try to do this with part of the end of the track working through FR4 it might proove to be lossy.

Loop antenna might not be great in terms of performance.


 Post subject: Re: Ceramic capacitor cost reduction
Posted: Wed Jul 28, 2010 9:34 am 

Joined: Wed Apr 08, 2009 11:07 am
Posts: 33
Chip inductors cost a lot. So eliminating them would be my first priority.

You need to set up a spectrum analyzer, calibrated broadband antennas, and measure the spurious signals. FCC has limits to what 2nd, 3rd, etc harmonic levels can be. Since they redesigned the circuit to be an asic, it is fairly possible that you now meet the spurious requirements without the lowpass filter, and can throw away those 5 chip parts.

If there is no filter, there is no need for high tolerance chip capacitors, and common 2 cent chinese chip caps will do the job.

As far as the bottom loading inductor on the antenna, you need that, AND it needs to be high Q (or you will sacrifice transmit range--there is a lot of RF current resonating in it). If you have the size, you can make the antenna bigger and not use it! If you still need it--just get a bunch of chip and aircoil inductors, and use the cheapest one that gives you the same transmit range as the current high-priced one.

Maguffin Microwave Consulting

 Post subject: Re: Ceramic capacitor cost reduction
Posted: Thu Jul 29, 2010 1:32 am 

Joined: Mon Apr 20, 2009 2:04 pm
Posts: 14
Hey guys,

Thanks. That's some really good stuff. I especially appreciate the heads up about the spiral loading coil. I was enamoured with the cost aspect of this. Being from another age, I find it difficult to think of 433MHz as a "low frequency," but it's a terribly long wavelength to deal with when you only have a few inches of trace for antenna.

Fortunately, this thing only has to reach a couple of meters, so the xmit power can be set very low. I'm thinking of doing as you say and chunking the bottom loading on the antenna for more antenna / top loading.
Also, using a single inductor at the bias-T to serve as current source / tuning element. Then following with capacitor divider accross it to act as the other tuning element / impedance match. The chip has about 300ohm output z, so this follows naturally to drive a lower z antenna.
With a fairly low Q, I'm thinking the cap tolerance won't be a terrible issue and I'll still be able to knock down the second order that I was left as a present from the single-ended output.

Thanks again guys,

Mike in Plano

Posted  11/12/2012
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