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RF switch for cellular application

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EngIntoHW

Member
Hi,

I got two cellular mobiles.
I want to connect them to an external antenna, via RF switch.

Can you please advice on RF switches for cellular applications - for both WB and GSM bands?
I'd like to operate the switch via PC USB Port.

Thanks in advance.
 
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Reloadron

Well-Known Member
Most Helpful Member
Start with a Google of RF Relay and that will get you hits like this one as well as a dozen others. You need to decide if you want to roll your own circuit board or just go with a canned solution using SMA connectors. Most relays like this one series also work both ways meaning you can use the in as an out. You need to decide what is an acceptable insertion loss for your signal(s). I do not know of a single canned USB solution, someone else may.

As to USB control? I would suggest a simple cheap USB relay card. No matter how you slice and dice you will need some form of hardware to interface to the USB port that you can communicate with. That hardware in turn would drive the relay (switch). For just a single channel if the PC has an RS232 serial port it could be interfaced directly to the relay / switch fairly easy and simple software written.

Matter of how you want to go about it. These are just a few broad suggestions. Hopefully others have some thoughts.

Ron
 

EngIntoHW

Member
Hi Ron,

Thank you very much for your support! :)

I reviewed many RF switches by now - I'm looking for canned solutions.
I already got a PSU (I can use any 5V / 12V PSU) which I can control via PC USB port, therefore I'm looking for a mechanical RF switch as this:

View attachment 63098
This one is MSP2TA-18-12 (Mini-Circuits, https://www.electro-tech-online.com/custompdfs/2012/04/MSP2TA-18-12.pdf), but it costs 190$, while I'd prefer finding solutions that cost less than 100$ (its frequency range is DC to 18GHz and is too wide for cellular application).

I'll connect an external antenna to the input, and an antenna entry of cellular mobile to each output.
I don't want the not-connected output to reflect waves to the antenna and damage operation of my application, so I need it to be an absorptive RF switch, and not reflective, isn't it? (the MSP2TA-18-12 is indeed absorptive).

I only need a frequency range of DC to 3GHz, and it includes of UMTS and GSM bands (highest UMTS band is 2.7GHz).

The thing is that I'm not sure regarding the spec criteria of Insertion loss, VSWR.

Do you have a recommendation on a proper RF switch?

Thank you :)
 
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Diver300

Well-Known Member
Most Helpful Member
Cellular mobiles won't transmit unless they can receive from a base station, so if you disconnect the antenna from one it won't transmit and it won't damage itself. You can also just turn it off.

The insertion loss is likely to be far less than the cables that connect to the antenna, so I wouldn't worry about that.

At a guess, I would say that a VSWR of less than 1.2 would be fine.

What are you trying to do? Is there any reason why you can't connect an antenna to each cellular phone?
 

Reloadron

Well-Known Member
Most Helpful Member
While you only need a frequency range of DC to maybe 3 GHz you will find most RF relays and switches are designed for DC to 18 GHZ. They all carry a price tag. I have seen some rated for lower frequencies but few and far between. Yes, they are far from inexpensive. Also, if you read the data sheets for these relays you will see the VSWR and insertion loss specified by sections of their frequency ranges.

Sorry it took a while to post back but I have been traveling so my forum visits were infrequent. :(

On another note there are relay designs like this type, however, they require skill in making your own board. Thus, I discouraged their use early in the thread.

Ron
 
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EngIntoHW

Member
Hi mates.
Thank you for helping :)

I found several suitable RF switches in **broken link removed**

However, the ones I found don't have built-in DC block capacitors in their RF ports, while I cannot guarantee that the cellular mobiles don't output any DC voltage in their transmitter output.
For example: https://www.electro-tech-online.com/custompdfs/2012/04/ZX80-DR230.pdf

I don't know how to connect these DC block capacitors, and still be able to easily connect the RF cable to the RF switch (this RF cable connects the cellular mobile and the RF switch).
As I wouldn't wanna cut the RF cable and solder it to the DC block capacitor.

You have an idea how to do it?

--
I found PIN Diode RF switches, which have built-in DC block capacitors.
You see any reason why not using these?
I found this one, which is a little more expensive (90$ per unit): ZSDR-230+ https://www.electro-tech-online.com/custompdfs/2012/04/ZSDR-230.pdf



@Diver:
As for the purpose of this switching:

I'm testing a cellular chip in front of live network.
So what I do is connecting the cellular chip to an external antenna which is located on the roof of the building.
The cellular chip is also connected to a PC, so I can see how it communicates with live network in different scenarios - Idle, Voice Call, Internet connection, etc.
Sometimes, when I get a weird behavior (for example, the cellular chip doesn’t manage to register on a cell and therefore remains in No Service), I want to see how a Reference cellular mobile behaves, to determine whether the strange behavior is a live network issue, or an issue of the cellular chip that I test.


Thank you.
 
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Reloadron

Well-Known Member
Most Helpful Member
You have me on the block caps as I never used them or had a need for them in my applications. :(

Maybe someone else will have some thoughts. I don't want to give you bum information.

Ron
 

EngIntoHW

Member
You have me on the block caps as I never used them or had a need for them in my applications. :(

Maybe someone else will have some thoughts. I don't want to give you bum information.

Ron

Hi Ron,

Thanks for your comments :)

Do you know where I can look for other low cost (I'd say below 150$ per unit) RF switches - SMA connectors, SPDT? (not ICs).

I only found such at minicircuits.
 

Reloadron

Well-Known Member
Most Helpful Member
For US distributors I use DigiKey, Mouser and Allied Electronics. Any of those suppliers would likely have the connectors, I know Allied carries them. THey also likely carry the switches (relays) as I know Allied does. I only mention those three distributors as I have done plenty of business with them especially Allied Electronics through work as well as home.

Ron
 

EngIntoHW

Member
Hi,
Thanks to Ron I found what I was looking for at Allied Electronics.

The RF switch I found is CCR-33S3O - SPDT, 12V FAILSAFE, DC-18GHZ (https://www.alliedelec.com/search/productdetail.aspx?SKU=70020578#tab=specs)

This is its spec:
View attachment 63328 View attachment 63329

I'll connect 2 cellular mobile phones (DUT + Reference) to the RF output ports, and an external antenna (located at the roof) to the input port.

I actually need only DC - 3GHz (not DC - 18GHz), but haven't found one with that range.

I'd like to ask a few questions please:

1.
They don't mention in datasheet that DC should be blocked at the input/output ports, but the schematic doesn't include any DC block capacitors.
Does it make sense that the RF connector can absorb DC voltage that might be transmitted from the cellular mobiles or come from the antenna?


2.
Does the 60dB Isolation refer to Isolation between the 2 output ports or to the Isolation between the not-connected input port and the output port?

3.
Does the below graph mean that in the DC - 3GHz range, the switch can handle up to ~80W at each input/output port?
View attachment 63330

Thank you very much!
 
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Reloadron

Well-Known Member
Most Helpful Member
The power curve in the chart will not matter for your purposes. I may be wrong as this is far from my forte but a typical cell phone outputs about 600 mW (.6 watt). Additionally there is no DC so there is no need for DC blocking capacitors. Isolation is just a matter of how well the signals are isolated from each other. Rest assured that 60 db is way, way more than adequate. We won't get into isolation and db as it pertains to power but believe me it is more than enough for your purposes and power levels.

There are a few considerations starting with the cable run. While I understand your objective a cell phone only puts out so much power and that is relatively low power. The cable used, any cable will have loss. You also have not mentioned the planned antenna? Switching a RF signal is actually the easy part, there are a dozen other considerations to consider like cable type and run length.

Ron
 

KeepItSimpleStupid

Well-Known Member
Most Helpful Member

EngIntoHW

Member
Hi guys.
Thank you very much!
I find your comments really helpful and important!


- Power:
A typical UMTS phone can reach a max output power of 27dBm (600mW), however a GSM mobile can reach 33dBm (2W), which is quite much, isn't it?
The question is what the "1dB compression point" of this RF switch is, whether it's below or above 2W, but it's not specified, therefore I asked about the graph.

- Antenna Cable:
I got pretty thick qualitative RF cables.
Yeah they cause significant attenuation because the antenna is located at the roof, about 10m distance, but I already checked it and I can get a strong channel signal after the cable attenuation.

- Matching Network:
The datasheet specifies a max VSWR of 1:1.25:1 in the DC-6GHz range, which is considered efficient, isn't it?
How can I tell if I still need a matching network?
The cellphones measure the power level of the received signal.
If the cellphones get the same power level when using the RF switch, and when connecting the external antenna directly to each of them (separately), is it enough to say that no matching network is needed?

Thank you again dear fellows.
 
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