Thank you for visiting RF Cafe! Electronics World Cover,TOC,and list of posted Popular Electronics articles QST Radio & TV News Radio-Craft Radio-Electronics Short Wave Craft Wireless World About RF Cafe RF Cafe Homepage RF Cafe in Morse Code Google Search of RF Cafe website Sitemap Electronics Equations Mathematics Equations Equations physics Manufacturers & distributors Engineer Jobs Twitter LinkedIn Crosswords Engineering Humor Kirt's Cogitations Engineering Event Calendar RF Engineering Quizzes AN/MPN-14 Radar 5CCG Notable Quotes App Notes Calculators Education Magazines Software,T-Shirts,Coffee Mugs Articles - submitted by RF Cafe visitors Simulators Technical Writings RF Cafe Archives Test Notes Wireless System Designer RF Stencils for Visio Shapes for Word Search RF Cafe Sitemap Advertising Facebook RF Cafe Forums RF Cafe Homepage Thank you for visiting RF Cafe!

Help with Smith Chart for Matching at 433MHz - RF Cafe Forums

The original RF Cafe Forums were shut down in late 2012 due to maintenance issues. Original posts:

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


PaulMetcalfe
Post subject: Help with Smith Chart for Matching at 433MHz Posted: Sat May 16, 2009 3:58 pm

Captain

Joined: Sat May 16, 2009 3:16 pm
Posts: 5
Hi,

I'm new to Smith Charts, I’ve done a lot of reading and understand the concepts but am having a little difficulty navigating my first problem.

I’d like to design a circuit to match a BAT54 diode to a 50ohm source for the purpose of creating a low-cost, ASK detector at 434MHz. Hoping to achieve useful sensitivity down to about -50dBm.

I’ve forward biased the BAT54 with 10uA and measured the S11 using my network analyser. It looks mostly open circuit and slightly inductive, see attached plot which represents 400 to 450MHz.

Is there anyone out there who can help me? If someone’s got the time, I’d prefer to get my head around the approach and method rather than just be given the solution.

Many thanks


Paul.


http://www.lightsymphony.com/pauls/smith.gif


Top

IR
Post subject: Re: Help with Smith Chart for Matching at 433MHzPosted: Sun May 17, 2009 2:33 am

Site Admin


Joined: Mon Jun 27, 2005 2:02 pm
Posts: 373
Location: Germany
Hi Paul,

Is it possible in your application to increase the forward bias to 100uA? It should give you an easier starting matching point.

Regarding matching, if the diode is meant to work as detector, use only reactive components (L,C) for matching network and not any resisitive attenuators. If you wish to find the network yourself and optimize it, there are many free softwares in the Internet for impedance matching, in which you can define the type of network, frequency range, input and output impedances, Q etc.

_________________
Best regards,

- IR


Top

PaulMetcalfe
Post subject: Re: Help with Smith Chart for Matching at 433MHzPosted: Sun May 17, 2009 3:07 am

Captain

Joined: Sat May 16, 2009 3:16 pm
Posts: 5
Thanks IR,

Increasing bias is no problem, I picked 10uA because it seemed to give the best sensitivity around there, but I'm sure I'm loosing most the power due to lack of matching anyway.

I understand R's will add loss.

Could you recommend a free CAD prog for this type of problem?

But if possible, this time, I'd prefer to do it on paper to help my understanding. Or is this making my life unnecessarily complex?

My biggest problem is knowing what type of network to build. Can you give me any guidance here? Although I've been designing electronics for 20+ yrs, I am very new to this stuff and I was hoping someone could walk me through the solution so I can learn then approach.

Thanks.

Paul.


Top

IR
Post subject: Re: Help with Smith Chart for Matching at 433MHzPosted: Sun May 17, 2009 3:36 am

Site Admin


Joined: Mon Jun 27, 2005 2:02 pm
Posts: 373
Location: Germany
Hi Paul,

Yes indeed, most of the power is lost due to power mismatch.

At first search, I found this SW - http://www.zzmatch.com/ you can download it for 20 days free.

Doing the calculation on paper is possible although extremely inconvenient and prone to errors. I don't think it will bring much to your understading.

Regarding the network topology, from the given S11 (At 10uA), it looks like the network would have at least 3 elements: series L, shunt C and another series L, but of course there are other cominations which would yield 50 ohm matching.

_________________
Best regards,

- IR


Top

darcyrandall2004
Post subject: Re: Help with Smith Chart for Matching at 433MHzPosted: Mon May 18, 2009 12:57 am

Colonel


Joined: Tue Feb 27, 2007 6:16 am
Posts: 46
Try this

http://bwrc.eecs.berkeley.edu/Research/ ... Match.html

_________________
Regards, Darcy Randall, Perth, Western Australia


Top

PaulMetcalfe
Post subject: Re: Help with Smith Chart for Matching at 433MHzPosted: Tue May 19, 2009 1:54 am

Captain

Joined: Sat May 16, 2009 3:16 pm
Posts: 5
Thanks guys, it's been very helpful. I have downloaded ZZmatch and Microwave Office, which are fantastic tools but I am still struggling.

The diode looks more or less like an open circuit even with 100uA or more of bias.

The obvious answer seems to be a length of transmission line but I don't have space for it at 434MHz.

Is it possible to transform to an open circuit with lumped elements whilst avoiding R’s? Everything I try just seems to move the match point around the outside edge of the smith chart, which is obviously the last place I want to be!

I’m assuming the BAT54 diode looks like 1-2pF’s but the Network Analyser shows the unmatched S11 to be slightly inductive (below the centre) but its so slight that this measurement could be being influenced by my test board etc. It’s really so close to an open-circuit that I’m assuming it is, but if that’s true, how can any power ever couple into it. HELP, what am I doing wrong?!?

I only need a very narrow bandwidth at 434.0MHz, which I’m sure must help.

This is my first attempt at building a matching network, so I’ve got no experience on my side and it looks more challenging than any of the text book examples I’ve researched.

Many thanks if anyone can guide me through this.


Top

nubbage
Post subject: Re: Help with Smith Chart for Matching at 433MHzPosted: Wed May 20, 2009 9:41 am

General


Joined: Fri Feb 17, 2006 12:07 pm
Posts: 218
Location: London UK
Hi Paul
I have been lurking on this topic since you posted it, because I am building a simple (I hope) diode power meter, and also need to optimize its sensitivity. This inevitably involves matching optimally to the diode.
The data sheet for the BAT54 states the capacitance is around 10pf but shows it critically depends on bias.
Looking at the Smith Chart, it looks like you have an impedance of around 300 ohms in parallel with about 3pf capacitance. To match this roughly to 50 ohms, some formulae I have seen suggest a simple PI matching circuit might get you in the ball-park. At 434MHz you would need to model into any software you find, an input (ie 50 ohm port) shunt capacitor of around 8pf, a series L of about 200nH (quite feasible at 434MHz) and a shunt capacitor at the 300 ohm end of about what the diode is showing, 2 to 3pf. To improve the match without really decreasing the sensitivity too much, I would terminate the assembly at the diode end by a shunt 390 ohm chip resistor.
The whole subject of matching to detector diodes under widely varying drive levels occupied the best non-linear theory gurus (like Prof D.G.Tucker) from the start of World war II right through to the present. It is a knotty problem that does not seem to have a simple circuit to do it, so far as I can see. But over a narrow band you might find that matching technique is good enough.

_________________
At bottom, life is all about
Sucking in and blowing out.


Top

PaulMetcalfe
Post subject: Re: Help with Smith Chart for Matching at 433MHzPosted: Fri May 22, 2009 2:16 am

Captain

Joined: Sat May 16, 2009 3:16 pm
Posts: 5
Thanks for all your help guys! I think I'm there now.

I think my initial measurements were wrong and the smith chart I've posted should be ignored, although I'm not sure how I managed that!

Thanks 'nubbage' for the big clue that the BAT54's datasheet says it's C is around 10pF, this made me realise my readings and assumptions must be wrong and the diode shouldn't be looking like an open circuit. I went back to the beginning, re-cal'ed everything and started again. Sure enough it looked totally different; I actually measure around 21pF.

I calculated that a shunt inductor of 6.4nH should bring it to resonance. With a little experimentation I found 3.3nH actually created a nice tank circuit at 434MHz and gives me a nice 50ohm resistive impedance. By adjusting the bias from 10 to 40uA I can fine-tune its resonance to end up with a fairly decent S11 match of -24dB.

I never did manage to model it successfully, but I think its probably my ignorance with Microwave Office that's causing the problem, not the design. This looks like an amazing package but will obviously take some time to master!

I'm now getting usable DC sensitivity of >5mV with -40dBm input, which is a 25dB improvement on where I started, so I’m very happy!

Once again, thanks for all your help

Paul.







Posted  11/12/2012

RF Cafe Software

   Wireless System Designer - RF Cafe
Wireless System Designer

RF & EE Symbols Word
RF Stencils for Visio
Calculator Workbook
RF Workbench
Smith Chart™ for Visio
Smith Chart™ for Excel

About RF Cafe

Kirt Blattenberger - RF Cafe WebmasterCopyright
1996 - 2022
Webmaster:
Kirt Blattenberger,
 BSEE - KB3UON

RF Cafe began life in 1996 as "RF Tools" in an AOL screen name web space totaling 2 MB. Its primary purpose was to provide me with ready access to commonly needed formulas and reference material while performing my work as an RF system and circuit design engineer. The Internet was still largely an unknown entity at the time and not much was available in the form of WYSIWYG ...

All trademarks, copyrights, patents, and other rights of ownership to images and text used on the RF Cafe website are hereby acknowledged.

My Hobby Website:
 AirplanesAndRockets.com

Try Using SEARCH
to Find What You Need. 
There are 1,000s of Pages Indexed on RF Cafe !

height-line