Connecting Multipule Radios via Cable 3

[daKlone]

Hiya,

This is really outside my area, so I'm hoping you guys can come up trumps for me.

I have a project where I need to connect a number of radios together via cables (for training purposes they are not allowed to actually transmit over air to each other).

Now, I'd be quite happy connecting two together using appropriately rated attenuators between them, but I'm a bit stuck once it gets to more than one. To make matters worse, we have to control the signal level between any two radios independantly.

Now, I've come up with what looks like an extremely complicated design, with fixed attenuators, variable attenuators and ampilfiers wired up in a right old mess. I can't help thinking that there must be an easy way...and you guys must know it!

Any advice appreciated.

 

[daKlone]

Peter - Thanks, you have pretty much described exactly what I had in mind - even down to the 30dB fixed attenuators. Now either we're both crazy or I'm on the right track!

I'm a bit concerned that all the components I come across on the Internet are intended for 0.8-2.0Ghz or more - does anyone do a range for DC to 500Mhz?

VE1BLL - The "dynamic variable element" is indeed just that, its needed to be able to create a required path loss between any two radios at any time to simulate the distance between them, under the control of a PC doing the calculations.

 

[Higgler]

Try

http://www.minicircuits.com

for low frequency items. Hittite has specialty items if minicircuits doesn't have them.

http://www.pasternack.com

always has stock on items, same day shipping when you order.

kch

 

[VEBill]

Spilt (again) the required attenuation between the first attenuator to dissipate the power down to something reasonable, and a second digitally-controlled attenuator to feed the summing point.

Your PC would probably have to solve the matrix to distribute the dBs as required. If the matrix can't be solved perfectly then do the best you can.

If you're simulating the real world, then perhaps your system really doesn't have to always be "accurate" (compared to your math model) because the real world has unexpected variations anyway. This depends on your purpose.

Timing might be an issue when several variable need to all change at once (short pulses of too much or too little attenuation as things are updated).

There are many specialize RF path simulators for various specific purposes (especially satellite), but I don't know of any off-hand to meet your requirements. But it's likely there is one out there somewhere.

 

[pstuckey]

It sounds like you are indeed on the right track. Mini-Circuits

http://www.minicircuits.com

is a good source for low power splitters, and Hittite

http://www.hittite.com

has many digitally controlled attenuators (available on connectorized eval boards). For cables and fixed attenuators I often use Tensolite

http://www.tensolite.com

and Inmet

http://www.inmetcorp.com

respectively (both are typically in stock at Richardson

http://www.rell.com),

although there are many other suppliers of these items out there.

Good luck, let us know if you run into any problems.

Peter

 

[daKlone]

Thanks again to all you guys - I knew I'd come to the right place!

Just one more question: Do you think it would be possible for me to get a PCB designed that would integrate the functions of the splitter, combiner and variable attenuators into one assembly?

 

[VEBill]

Another option is to use the Push-to-talk (PTT) line to switch additional relays in and out. With this approach there is no limit to what can be done since the attenuation can be set differently for transmit versus receive.

In this sort of scheme, it is common to interrupt the PTT line so that the antenna relays are given time to switch before passing the PTT along to the radio. 50ms is one delay that I've seen used. The transmit load needs to be settled before the RF arrives and remain for a brief period at the trailing end as well.

Such an approach can work with voice radios, but is asking for trouble with anything other than voice. Most data modes aren't expecting an additional delay in the PTT response.

 

[VEBill]

Quite a while ago, daKlone asked:

"For instance, if I need (simultaneously)...
120db between A and B,
100db between A and C,
150db between B and C, ..."

Higgler already noted that these sort of values (150dB) would require pretty good shielding. But, ignoring that excellent point for now and just doing the math, it is very easy to solve the equations for these example numbers:

On A use a 35dB attenuator
On B use a 85dB attenuator
On C use a 65dB attenuator

Join everything together in the middle (huge solder blob).

A+B=35+85=120dB
A+C=35+65=100dB
B+C=85+65=150dB

This simple math ignores the slight mismatch at the three-way junction that would cause the actual end-to-end losses to be slightly different (probably just a bit higher) than the overly-simple calculation indicates. But, it would be relatively simple to adjust things if required.

In practice, you'd use appropriately-big power attenuators on the back of each radio, and then run something like about +10 dBm transmit power (+/- maybe 10dB at your option) into a chassis with all the adjustable and digitally controlled attenuators and 'a huge solder blob' (slight exaggeration) where everything is joined together.

With most voice radios, you're only going to see anything interesting in the last dB before the signal drops out. The radios' AGC will fight all other signal level changes over a very wide range.

You should run a quick experiment to make sure that the radios provide adequate shielding to drop the link when they're all running into dummy loads. Otherwise the whole thing would be a waste of time.

If this is 'just' for training radio operators, then it seems like overkill to provide variable attenuation if you're not also going to simulate all the other stuff found on real over the air links (several types of noise, multipath, etc. etc. etc.).

You shouldn't need circulators (depending on your exact purpose), and you definitely shouldn't need any amplifiers.

And after something like 30dB of loss directly on the back of each radio, you probably don't even need to bother with matching (matched splitters) unless you really need to be more precise than could be achieved with a 'huge solder blob'.

 

[VEBill]

Should read "...last FEW dB before the signal drops out" not just the last 1 dB. And 'few' is relative.