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Filters, isolators, bias

Posted: Sun Aug 04, 2019 1:59 pm
by rgp415
I’m looking at all these attachments to a sdr to decrease noise. Should I have a galvanic isolator or a bias tee or both implemented in the system?

Re: Filters, isolators, bias

Posted: Sun Aug 04, 2019 3:42 pm
by radiolistener
bias tee already has DC block (isolator), but bias tee is just a power source for LNA on the second end of the coax cable. If you don't have LNA then you're don't need a bias tee.

Filters is very useful, but they are expensive. Also you can use attenuators in order to reduce strong signals from antenna, because receiver has limited dynamic range and cannot handle too strong signals on the input.

If you're talking about USB isolator, you're don't need it. It will make a lot of noises.

Re: Filters, isolators, bias

Posted: Sun Aug 04, 2019 8:16 pm
by rgp415
That’s the kind of information I’m looking for. Thank you radiolistner

Re: Filters, isolators, bias

Posted: Sun Aug 04, 2019 8:32 pm
by alanzfq
Noise can be due to many things, including local interference.
You need to identify the problem.
If it is a RTL dongle keep gain as low as possible, it overloads.
Filters for the frequencies you want can be very useful depending on your circumstances.
There is no easy answer.

Re: Filters, isolators, bias

Posted: Mon Aug 05, 2019 9:43 am
by Username
a) Put the Antenna Outside of the Window
b) Get a Sat Flat Window think and put alu tape around for isolation
c) Get an USB Hub who I posted here in the Forum the are great to keppe the noise away.

Re: Filters, isolators, bias

Posted: Mon Aug 05, 2019 2:04 pm
by rgp415
Thank you @alanzfq and @username for the information

Re: Filters, isolators, bias

Posted: Mon Aug 05, 2019 2:13 pm
by Username
you need an hight gain antenna and an lna without you will receive not much.

antennas with limited bandwidth, HP- LP-,BP,BR- Filters, vs. isolators or bias

Posted: Sat Aug 10, 2019 7:47 pm
by snn47
It depends on the frequency range you want to receive and your RF enrionment, what is the best (affordable) option to reduce interference or in most cases it is overloading of the receiver.
Interference are signals or spurious emmisssions that fall within the receiver RF bandwidth.

Since RTL-SDR do not have any RF selectivity like a normal receiver design they will, unless you use HP-, LP-, BP or Bandreject-filters, receive all signals that you get from the antenna.

If a receiver is not overloaded it will amplify all signals by the same factor e.g. 20 dB. When reaching overloading not all signals will/cannot be amplified by 20 dB any more and as consequence due to intermodulation (IM) additional signal (interference) will be generated. If you add an attenuator of e.g. 20 dB and many signals that were stronger than 20 dB are gone you had Rx IM.

If you live in an area with transmitter having high EIRP (e.g. TV, BC) the only option for you are filter to reduce those frequency ranges with the strong signal or using antennas that be design are like a bandpass on for the frequency range they are designed for and in most designs also reveive multiple of these center frequency.

These are physical limitations and not a question of cost. A spectrum anylyzer for 200 000 € or more is basically just a very elaborate SDR with 16 dB or better AD and can provide receiving bandwidth of up to 512 MHz at a time, for frequencies up to 2 GHz or more. Even in those "receiver" you will get intermodulation and need filter when subjected to strong signals nearby.

Both Isolators e.g. Transformer just avoid DC-pass through and do not filter. While a BIAS T is intended to allow injection of the DC e.g. for a preamp if the DC pass is interrupted.

Re: Filters, isolators, bias

Posted: Sun Aug 11, 2019 1:26 pm
by rgp415
@snn47...... thank you