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Is "too many RF filter in the signal chain" a thing?


Ben B
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I'll explain myself. I am on the brim to update my antenna to some Wisycom LFA. I am currently using RF Venue Dipole antenna into two Professional Wireless System 470-616 passive filter and then into a Sound Devices SL2 with the 470-614 MHz filter activated.

I was wondering if adding the LFAs with filtering as well would then be "too much filtering". In my train of thought and logic it shouldn't be a problem as every filter would then filter the already filtered signal some more, but i thought i'd ask if there was some detail i didn't know about.

 

For info I am using Lectro block A1-B1 for all my booms and wireless and my IFBs and Camera hop are in the Sennheiser B Band (626-668) where i'm trying to keep the frequencies i use in the upper B Block to have as much spacing as possible between the Rx and Tx.

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I’m far from an expert here (hopefully Larry, Karl, Gordon, Glenn, etc. will chime in) but I know filters are additive and will add noise into the signal. While a well designed and built filter from a reputable manufacturer probably won’t have any noticeable noise I imagine adding several will increase your noise floor.

 

I have found that a single, well made filter provides plenty of attenuation on either side and don’t need anything more from it. I don’t think “doubling” it will yield any benefit. I would use the Wisy filters and select the “wideband” filter in the SL-6.

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Good to know about the noise floor. I didn't think about that. That's exactly why i posted this question here. :)

 

I also did some test with the bag rig have (the other rig with the Wisycom antenna would be for my cart rig). The bag rig is also an SL2 with the same PWS filters but going into either whips or the Betso Bowtie depending on what kind of job i do. Obviously the Tx is pretty close to the whips as it's all in a small bag.

During my test i found out that bypassing the PWS filters and going with the whips directly on the SL2 would overload the SL2 RF input with the cam hop TX. when putting the PWS filters between the whips and the SL2, it would stop the overloading. So i guess they do help somewhat to filter out the cam hop frequencies.

 

I guess it's an either this or either that situation where none of them is a perfect solution.

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Every rf component you add to your signal path causes a reduction in you rf signal, I think it’s at least 3dB per component (like even a BNC to SMA adapter), unless it’s a passive splitter which can have a higher loss. I don’t think it‘ll raise the noise floor, that only happens if you boost the signal at some point, but if you’re not then you should be fine as long as the rf loss  is ok with you. I‘m guessing the SL-2‘s filter won’t cause any loss, though, as it‘ll compensate for it.

I agree with Trey though that you probably won’t need all those filters. It’s definitely helpful to have some sort of filter before the SL-2, but one should suffice. Get the LFA which is great and ditch all the passive guys

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  • 2 weeks later...

Also, bear in mind that most RF filters are reflective, just “bouncing back” the energy present at the frequencies they reject. Their impedance on those frequencies is different than the design impedance, say, typically 50 or 75 ohms.

 

If you set up a filter connected to the output of an active RF component such as an amplifier you can cause distortion in case the amplifier is feeding a strong, unwanted signal to the filter. So, filters should be installed between an antenna and the amplifier input. 

 

Mini-circuits is now selling a special kind of absorptive filters that dissipate that energy turning it into heat instead of reflecting it but I guess they are only useful in very specific situations. 

 

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  • 2 weeks later...

This is certainly outside of my wheelhouse as I’m not an RF or electrical engineer, but in my mind the following makes sense. 

 

The receiving antenna are tuned to a certain range of frequencies which it will happily chuck down the line to the receiver. If you have a very close transmitter that falls within that range, the antenna will happily send that down as well. Too much energy hitting the receiver can overwhelm it and it won’t be able to properly tune the desired frequency. So to me, having a filter earlier in the chain makes a lot of sense. It’s akin to getting a HPF early in your audio chain so that the extra low  end doesn’t suck up available headroom on the headamps. The math behind these filters, I believe, is the same so that’s where my reasoning comes from. Continuing that line of thought, it also seems reasonable that additional filters down the line will stack and could potentially cause unwanted artifacts due to phase shifts and the like. 

 

So my view would be to filter early in the chain (with a quality filter) before the receiver and let it go after that unless there seems to be a problem that only additional filters can solve. 

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On 9/11/2022 at 9:04 AM, borjam said:

Also, bear in mind that most RF filters are reflective, just “bouncing back” the energy present at the frequencies they reject. Their impedance on those frequencies is different than the design impedance, say, typically 50 or 75 ohms.

 

If you set up a filter connected to the output of an active RF component such as an amplifier you can cause distortion in case the amplifier is feeding a strong, unwanted signal to the filter. So, filters should be installed between an antenna and the amplifier input. 

 

Mini-circuits is now selling a special kind of absorptive filters that dissipate that energy turning it into heat instead of reflecting it but I guess they are only useful in very specific situations. 

 

Thank you for this insight. I never realized rf filters were reflective, but this is very good to keep in mind. My current setup indeed runs filter & amp at the antenna, then again at the distro on my cart and recently I struggled with some overloading that i didn’t understand even though my antenna amp was set to 2db below my cable loss.  Additional attenuation ended up being my solution.    This explanation fits exactly to what i was experiencing. 

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On 9/23/2022 at 11:31 PM, Wandering Ear said:

Thank you for this insight. I never realized rf filters were reflective, but this is very good to keep in mind. My current setup indeed runs filter & amp at the antenna, then again at the distro on my cart and recently I struggled with some overloading that i didn’t understand even though my antenna amp was set to 2db below my cable loss.  Additional attenuation ended up being my solution.    This explanation fits exactly to what i was experiencing. 

 

Where did you place the attenuator? Between the amplifier output and the filter input?

 

An attenuator should definitely help to prevent distortion at the amplifier output because it will help correct the impedance presented to it.

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10 hours ago, borjam said:

 

Where did you place the attenuator? Between the amplifier output and the filter input?

 

An attenuator should definitely help to prevent distortion at the amplifier output because it will help correct the impedance presented to it.

In my current setup I’m using Wisycom active antennas which have a bandpass filter and amplification/ attenuation built in. Those feed into a micplexer which also has a filter and amplifier with fixed gain. 

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20 hours ago, Wandering Ear said:

In my current setup I’m using Wisycom active antennas which have a bandpass filter and amplification/ attenuation built in. Those feed into a micplexer which also has a filter and amplifier with fixed gain. 

 

I see. Anyway unless the active antenna is passing a strong interfering signal to the Mixplexer filter there should be no problem. 

 

Is that the case? Is the Mixplexer filter narrower than the filter at the active antenna? 

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53 minutes ago, borjam said:

 

I see. Anyway unless the active antenna is passing a strong interfering signal to the Mixplexer filter there should be no problem. 

 

Is that the case? Is the Mixplexer filter narrower than the filter at the active antenna? 

I believe the micplexer filter is 5 or 10Mhz narrower. I’m not 100% sure the filter was my issue, the behavior of the system acted just like when you overload a receiver and attenuation was the solution. I’m also in a fairly mild rf environment at the moment with not much broadcasting except the local tv stations so it took me a minute to figure out why i was getting poor performance. 

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18 hours ago, Wandering Ear said:

I believe the micplexer filter is 5 or 10Mhz narrower. I’m not 100% sure the filter was my issue, the behavior of the system acted just like when you overload a receiver and attenuation was the solution. I’m also in a fairly mild rf environment at the moment with not much broadcasting except the local tv stations so it took me a minute to figure out why i was getting poor performance. 

 

Then I doubt my explanation is the good one. Maybe the signal received by the Micplexer was too strong and it was overloading? That would be the simple explanation.

 

"My" theory would apply in a case in which the first element was amplifying a strong signal that would be rejected by the second filter. The impedance mismatch between the input of the second filter and the output of the first amplifier might cause overload on the amplifier, which would cause distortion. 

 

But, was there a strong signal within that 5 or 10 MHz difference between the two filters? 

 

Not intended as a interrogation or course!

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Nothing comes close to the 2 MHz filters inside Lectro´s VRT modules.

I was driving some hours through RF traffic spots in Berlin with 10 VRT´s inside 2 VR Fields  and amplified Betso´s at +4 db without running transmitters and distros.And  I was permanent watching the display,- No reception of anything. Colleagues tell me that the have to change frequencies on a daily base. This system stays on top for me for years now.

My other system: audio LTD/Sound Devices digital fail, even with handmade french multiplexers as soon as antennas have to stay behind the moving Camera when actors are more than 40 meters away.

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Just saw this topic. Here are some comments.

 

Filters should be passive devices and will not add any noise to the RF signal. Active antennas with filters are a different story as the manufactures of such devices don't tell you enough about them in most cases to be useful.

 

The micplexer has a great filter before its buffer amp that feeds the splitters so that the output of the micplexer has only a few dB of gain with the protection of our tunable filter before the amp. Its bandwidth is about 35 MHz +/- 17.5 MHz.

 

Do not use active amplified antennas. Use low loss coax instead . Active antennas are the industry's version of snake oil and will often do much more harm than good by creating intermodulation interference ahead of a good receiver front end that can not be filtered out.

 

I will look for my RF tips to post here that cover a lot of this.

 

Glenn

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14 hours ago, borjam said:

 

Then I doubt my explanation is the good one. Maybe the signal received by the Micplexer was too strong and it was overloading? That would be the simple explanation.

 

"My" theory would apply in a case in which the first element was amplifying a strong signal that would be rejected by the second filter. The impedance mismatch between the input of the second filter and the output of the first amplifier might cause overload on the amplifier, which would cause distortion. 

 

But, was there a strong signal within that 5 or 10 MHz difference between the two filters? 

 

Not intended as a interrogation or course!

You make a good point. I was likely not receiving a strong signal in the small gap between the two filters.  I do have a uhf transmitter on my antenna mast that feeds my boom ifb, but i know that’s not the source of my issue since it persisted with it off and i run it about 25 Mhz below the bottom end of my filters. 

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