1.8 GHz Wireless Microphones

[sunray]

32 minutes ago, cedd said:

I had a long chat with Sennheiser about this at Plasa. They have circulators on their antenna inputs (that'll be the ferrite directional couplers) which they said means they're essentially intermod-free. Just space the channels correctly and get on with it. I was really quite amazed by the claim, but am not an RF circuit design expert, so can only take them at their word. 

Received strong signals present on a transmitter aerial can go down the feeder and enter the transmitter and there mix with the transmit signal to generate intermodulation. That intermod could then be transmitted as as a signal every bit as strong as the correct signal.

As an example TX = 600MHz, received signal = 100MHZ will mix to produce 700MHz & 500MHZ

RF circulators are normally used between transmitter and aerial to prevent that received signal entering the transmitter, They usually consist of 3 ports (3 physical sockets) labeled; TX, AE, Dummy Load (sometimes 1,2,3) with tuning arrangements between then around a ring of ferrite.

The path between TX & AE is tuned to pass the spot frequency of the transmitter 600MHz. So the transmit signal passes unhindered to the aerial

The path between AE & dummy load is tuned to notch (Stop) the spot frequency of the transmitter 600MHz. Any signal apart from the transmit passes unhindered to the dummy load

The path between dummy load & TX is tuned to pass the spot frequency of the transmitter 600MHz.  Anything left over getting back to the transmitter may only be permitted to be the original transmit.

The bandwidth of the filtering is wide so don't expect these to filter out one channel within a band.

 

How such a device would be useful for a receiver is hard to see as the main route (AE to RX) would be tuned for the correct frequency and there should not be anything generated within the RX to mix with and radiate to the aerial. Anything already in there will have to be dealt with internally. Any filtering before a receiver would generally be a band pass or a notch (band stop), a ferrite circulator can serve those functions

I know some have advocate using a ferrite circulator as part of the signal separation between transmitter and receiver in a repeater system (where ports would be TX, AE, RX) but even that is not how they are designed to be used.

[Lamplighter]

Ray

We are referring to the circulator in the transmitter aerial path, this reduces the level of other transmitted signals getting into the output stage and mixing with that transmitter frequency. The reduction in spurious output is pretty dramatic, it can be as much as 60 dB if well engineered. The first 30 dB is directly due to the isolation given by the circulator the rest by the reduction of mixing products. As previously mentioned the main source of intermod problems in radiomic systems is interaction between the transmitters. 

[sunray]

On 1/28/2023 at 12:36 PM, Lamplighter said:

That makes sense, fitting circulators in the transmitters will drastically reduce the intermods between them, the major source of the problem. It is interesting that they have developed them to be small enough to do this.

That's assuming it's where the intermod is being generated.

One also has to understand that a RX DA is also handling/mixing all of those frequencies then amplifying them using non linear semiconductors. There is big potential for intermod there too. We had exactly this problem on a base mult-iuser aerial mast some years back and never did resolve it, other than moving our kit.

Blindly saying fitting circulators in our TX's eliminates intermod (which I know they haven't said) is rather short sighted in my view.

[sunray]

On 1/28/2023 at 10:06 PM, Lamplighter said:

Ray

We are referring to the circulator in the transmitter aerial path, this reduces the level of other transmitted signals getting into the output stage and mixing with that transmitter frequency. The reduction in spurious output is pretty dramatic, it can be as much as 60 dB if well engineered. The first 30 dB is directly due to the isolation given by the circulator the rest by the reduction of mixing products. As previously mentioned the main source of intermod problems in radiomic systems is interaction between the transmitters. 

Ah! reading

On 1/28/2023 at 10:50 AM, cedd said:

They have circulators on their antenna inputs.

I took to mean the receivers...

And yes I have seen circulators fitted on RX's on a number of occasions and yes the intermod was most definitely still there.🤣

Edited January 30, 2023 by sunray

[Lamplighter]

And now we are on the same wavelength (pun intended)

 

[sunray]

7 minutes ago, Lamplighter said:

And now we are on the same wavelength (pun intended)

 

Indeed but as in my edited last message I've seen them inserted in RX by people who really should know better.

 

The size of the circulator is directly controlled by power handling so for mW's they can be tiny and the magnetic shielding may not be required.

Edited January 30, 2023 by sunray
typos

[cedd]

I was talking about receivers above. Tx path makes most sense, but the Sennheiser rep was definitely referring to receivers. That said, he was a sales guy who admitted I needed to be talking to his technical colleague about these kinds of things, not him, and his colleague was busy on the trade stand with another customer, so I just went on with my day and forgot to go back. I think on reflection, he was probably meaning to refer to tx. 

 

[sunray]

5 hours ago, cedd said:

I was talking about receivers above. Tx path makes most sense, but the Sennheiser rep was definitely referring to receivers. That said, he was a sales guy who admitted I needed to be talking to his technical colleague about these kinds of things, not him, and his colleague was busy on the trade stand with another customer, so I just went on with my day and forgot to go back. I think on reflection, he was probably meaning to refer to tx. 

 

It can be so difficult at such events, bearing in mind the main purpose is to sell their products it tends to be flooded with sales staff.