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# RF 5 GHz circuit design [Beginner]

## RF 5 GHz circuit design [Beginner]

(OP)
Hi all,
very interesting forum!!
I hope you can solve my problem:
I have to design a small pcb working at about 5 GHz, and I don't know how to start exactly...
It's very simple, 2 sma connectors and 1 resistor. Here is the schematic:

[img]http://img690.imageshack.us/img690/9175/immagineja.jpg[/img]

And here is what I've done sofar in Microwave Office... but in this step there are some problems for sure...

[img]http://img80.imageshack.us/img80/8898/immaginelo.jpg[/img]

Anybody knows how to proceed in microwave office? I need help because this topic isn't covered so much on the web...

thanks

### RE: RF 5 GHz circuit design [Beginner]

(OP)
Sorry, I haven't provided enough informations...

The goal of my little project is to design a load for an uwb switch.
I've made some changes since the first post.
Since the switch has to "switch" from an open position and a short-circuited position, I've to design this 2 loads, considering the (about) 5 GHz frequency and the 50 ohm impedance.

And since the switch has 2 sma connectors as outputs, they are the inputs of my circuit.

I've chosen to use coplanar wave guide instead of microstrip, and after calculating length and width of the line I've done this:

short circuited:
http://img682.imageshack.us/i/smacc.jpg/

and open:

http://img227.imageshack.us/i/smaopen.jpg/

dimensions are not real and connector picture is only to understand where it is the sma connector...
what do you think about it?

thanks

### RE: RF 5 GHz circuit design [Beginner]

A number of companies make inexpensive SMA opens and shorts, why not just buy a couple of those?

For instance, see model 7006 (open) and 7008 (short) both are good to 18 GHz.

http://www.aeroflex.com/AMS/inmet/micro-inmet-imprdmisc.cfm

Peter

### RE: RF 5 GHz circuit design [Beginner]

I assume you remember that at 5 GHz the difference between short and open is only a few cm. I'm mentioning this because you mentioned SMA connectors, which implies that you intend to connect these shorts and opens to transmission lines of non-zero length.

Or perhaps I misunderstand.

### RE: RF 5 GHz circuit design [Beginner]

(OP)
@ pstuckey:
You're right, but here in Italy is very hard to find these things... I'll try to contact their distributor, thanks.

@ VE1BLL:
since I am a complete beginner in RF circuit design, I don't know exactly if I'm following the correct way.
I need a sma open and a sma short-circuited, and I've done this 2 pcb. I thinked that the line length could be the same but I don't know... I'm wrong?

Thanks

### RE: RF 5 GHz circuit design [Beginner]

When you have (for example) a short circuit at the end of transmission line, there will be complete reflection at the infinite mismatch. This will create standing waves. As you (in your imagination) move along the transmission line examining the ratio of V to I, it keeps changing with distance. At the short circuit there is maximum current and essentially zero voltage. But one-quarter wavelength further along the line, taking into account velocity factor, the voltage will be maximum and the current minimum which is an open circuit.

At 5 GHz, these distances are on the order of 1 or 2 cm.

If after digesting this, and reviewing the Smith Chart, you still require shorts and opens, then you may have to make the transmission line length an exact multiple of 0.5 wavelength, including consideration of Vf.

If you don't understand Smith Charts, then search for the webpage that explains them in terms of speedometers. Makes it very simple.

### RE: RF 5 GHz circuit design [Beginner]

"...I need a sma open and a sma short-circuited..."

I think you need to back up a bit. There's too much here that doesn't make any sense.

You have an SMA connectorized switch. Presumably SPDT (single pole, double throw). And you "need" an open on one of the ports. Hmmm... It's a bit like being sent off on an errend to find a left-handed crescent wrench. The open SMA connect is already open. Put a plastic or (non-shorting) metal cap on it simply to keep out the dirt.

And if you're purchasing the non-shorting cap, then might as well order up a shorted version at the same time. In other words, exactly as suggested by Peter. Or make your own using SMA connectors.

But you also mentioned that this project is to "...design a load for an uwb switch..."

Ultra Wide Bandwidth at 5 (5.8?) GHz, depending on the application, isn't going to work with intentional mismatches and reflections in the transmission lines.

You're not using the switch to modulate the carrier are you? To create the UWB signal?

### RE: RF 5 GHz circuit design [Beginner]

(OP)
uhm...

"The open SMA connect is already open"...
I don't know... if it's true, so what's the utility of the "open sma connector" that some company sell (like the one that peter said).
In addiction, someone told me that if I leave only the sma open, the impedance wouldn't be good. He told me that I have to create a line that connects itself to ground.
(sorry for my bad english). And that's what I've done...

At this moment, there isn't any modulation, maybe in the future.
Detail of the project :)
an uwb signal arrives to the antenna. I have to create 2 situations:
1. signal reflected with the same phase.
2. signal reflected with opposite phase.
To make this 2 situation I switch the antenna load from open to short and viceversa.

Any advice or modification to my design would be very appreciated!

thanks

### RE: RF 5 GHz circuit design [Beginner]

"...what's the utility of the open SMA connector...?"

If you think about it you'll realize that an open SMA is nothing but a cap.

The utility is:
A. Provides shielding (metal versions).
B. Keeps out dirt.

You have a system architecture problem.

1) You have non-negligible transmission lines length.
2) You have a UWB signal (very wide bandwidth).

Item 1 means that your "short" and "open" are going to roll around the edge of the Smith chart and land who-knows-where at the far end of the transmission line. You would have to trim the transmission line to make it work at ONE (!) frequency. It's more likely to be a pair of random complex impedances.

(To be complete - perhaps even a messy random solution of the sort you were attempting would provide some sort of effect on the UWB signal that could be detected. But you should still be aware of these transmission line / reflections / bandwidth issues.)

Item 2 means that this effect will vary with frequency EVEN WITHIN the bandwidth of your UWB signal. The longer the line length, the tighter the resultant "comb" effect (like "/\/\/\/\/\" along the frequency domain).

The solution to this specific problem (Item 2) is to install the raw, unpackaged switch itself directly at the feed point of the antenna. Then the bandwidth of the 'switch system' is essentially unlimited. Basically you would need near ZERO line length, at 5 GHz that is measured in millimeters.

This is a very challenging project for a beginner. Maybe you should turn off Microwave Office and study the Smith Chart until the related transmission line concepts are instinctive. Computers should be for tweaking, not planning.

http://www.ac6la.com/stss.html

Book: Reflections I and II, Walter Maxwell W2DU

### RE: RF 5 GHz circuit design [Beginner]

(OP)
yes, you're right, there are a lot of things that I should study...

I'll give a look at smith chart.

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