Noob question: Adjustable Low-/High-/Band-Pass Filter - do they exist?

[Bicurico]

Hi,

When using cheap SDR like the HackRF One or ADALM Pluto, the TX output is fairly harmless, even with an antenna attached. Typically after a range of 3-4m the emitted signals cannot be measured anymore.

This allows me (or anyone else) to play at home with any frequencies, including non amateur radio radio frequencies, and emulate for instance a GSM network cell or whatever.

But, even as a radio amateur, transmitting in the reserved ranges, one should not attach the ouput of a HackRF or ADALM Pluto (or any other cheap SDR) to an amplified antenna, because the TX signal on the output is not filtered in any way and there are lots of harmonics, which might (read "will") cause disturbances in other reserved RF bands.

So the common advise is: "you need to filter the TX output"!

So far, so good, all understood.

My questions are regarding the required filter and how to use them...

Ideally I imagine a device, where I can set the HIGH-Pass frequency (i.e. 472MHz) and the  LOW-Pass frequency (i.e. 473MHz), which would be equivalent to a BAND-Pass filter (472-473MHz).

Then I could attach the output of the SDR to an amplified antenna.

Question 1: Would I place the filter at the output of the SDR or at the output of the amplifier? I imagine that I would want to filter the output of the amplifier, as it will amplify the noise, too? But then it might be amplifying harmonics at frequencies for which the antenna is not tuned - would/could the reflected amplified harmonic kill the amplifier?

Question 2: Does a "tunable" Low/High-Pass filter even exist? Or are filters, due to their natural (coils) always tuned for a single frequency?

Question 3: If a filter is tuned for a single frequency and "tunable"/"adjustable" filters don't exist, what is an owner of a TX capable SDR supposed to own? Buy a set of different filters for every possible application one wants to try out? What is thre recommended attenuation of such filters for SDR use?

Question 4: How does amplification even work? Imagine the example: I want to output 2W (or 20W - just asking) to amplify my 472.5MHz signal, generated with the SDR. Do I use a specific amplifier tuned for this frequency or do I use a broadband amplifier that amplifies 50MHz up to 3GHz by so many dB? Never thought of this before and so I really don't know how this normally is handled...

Thanks for any responses and sorry if the questions seem silly...

Regards,
Vitor

[radiolistener]

Question 1: Would I place the filter at the output of the SDR or at the output of the amplifier? I imagine that I would want to filter the output of the amplifier, as it will amplify the noise, too? But then it might be amplifying harmonics at frequencies for which the antenna is not tuned - would/could the reflected amplified harmonic kill the amplifier?

you're needs to filter both. For short wave you can make your own or buy something like that: https://www.aliexpress.com/item/12v-100W-HF-low-pass-LPF-unit-DIY-KIT-3-5Mhz-30Mhz/32780614543.html

it allows to select band by putting voltage on corresponding pin.

For VHF and UHF it will be more complicated and expensive.

Question 2: Does a "tunable" Low/High-Pass filter even exist? Or are filters, due to their natural (coils) always tuned for a single frequency?

it's hard to implement. Usually there is a switch for several different filters. But for VHF and UHF even switch is hard to implement.

Question 3: If a filter is tuned for a single frequency and "tunable"/"adjustable" filters don't exist, what is an owner of a TX capable SDR supposed to own? Buy a set of different filters for every possible application one wants to try out? What is thre recommended attenuation of such filters for SDR use?

You can find general requirements for transmitters in your country laws, it depends on the country. Usually it requires attenuation about 40-60 dB outside working bandwidth. For example, in my country, laws requires at least 40 dB attenuation.

Question 4: How does amplification even work? Imagine the example: I want to output 2W (or 20W - just asking) to amplify my 472.5MHz signal, generated with the SDR. Do I use a specific amplifier tuned for this frequency or do I use a broadband amplifier

you're needs specific amplifier with filter for specific band. But first, you're needs to get government license of ham radio amator with call sign. In order to get it, you're needs to learn basic radio things and take exams.

This is illegal to use power amplifiers with no license and permission from government 

[Bicurico]

Thanks for your replies.

I do have a valid amateur radio license. However, my interest is not buying some amateur radio transciever and collect QSl cards. Also, I do not belong to any club or so. I just studied the laws and techs by myself, went to the exam and passed.

I was just wondering how to use a HackRF One (or similar) as an amateur radio to transmit within the reserved frequencies (for amateur radio, of course) using more TX power to cover a bigger distance than the 3-4m it does right now.

This had led me to conclude that I know little about filters or RF amplifiers. Hence the questions.

Regards,
Vitor

[jujun]

The best thing to do is to design yourself a filter when you need it, and it's fun and cheap.
I suggest you to use Qucs to generate it and simulate it (open source and works on linux/windows).
But there is also lot of others softwares.
For HF filters there is lot of cheap kits avaiable, but it's always more fun to make your own.

You will even be able to check them with your SDR.

[Bicurico]

I guess this is what I was asking about:

https://www.ebay.com/itm/NF-3628-Dual-Channel-Programmable-Filter-8128/153569474385

A bit expensive, though... 

Regards,
Vitor

[Bicurico]

I think I found the correct search terms for eBay now:

https://www.ebay.com/itm/HMC1044LP3E-Programmable-Harmonic-Low-Pass-Filter-Module-1-3GHz-STC-control-brd/283280682282

Any thoughts on this?

Regards,
Vitor

[paul002]

Tunable filters do exist google on Cohn filters, invented somewhere in 1950. I am building my self a set of selectable filters using relays to switch between bands. Anyway all the vinage communication receivers used tuned filters, only to implement it on 472 mhz can be a challange. Look at http://tonnesoftware.com for free filter design software. You can try https://coil32.net/ to calculate inductors and and tuned circuits. It has kept me busy for a number of weeks . Good luck

[hagster]

Firstly, the approx 0 to 10mW from a HackRf is detectable much further than 2m away. You can send signals to the moon with that kind of power if you know what you are doing. GPS signals at ground level are about -130dBm for example. That's 140dB below the output power of a HackRF. So yes, you can cause real problems if you don't know what you are doing even without a power amp. Mostly you will be fine, just be a little careful what frequencies you choose.

Yes you can cascade high and low pass filters, so long as the input to output impedance is matched. E.g. 50ohms.

It is a good idea to add a filter after your amplifier, unless you know for certain that it's very linear and thus not inducing its own harmonics.

Tunable filters do exist, for VHF and UHF tunable cavity filters are great stuff you can find one. Not too cheap though.

[Dulus]

Hello,

RF design engineer here. Will try to comment on the points i know about.

Would I place the filter at the output of the SDR or at the output of the amplifier?

Both would be better. You dont want to amplfy the harmonics, waste the output power of your amplifier. Also if the harmonic levels of the SDR output is high, you can distort your signal too much during amplification.
But you dont want to broadcast the amplifier output harmonics either.
This is a quite deep topic involving power levels, amplifier specs and used modulation type.
My suggestion is to copy the existing work at first, and learn while doing it.

would/could the reflected amplified harmonic kill the amplifier?

It could in general terms. But i doubt it at the mentioned signal level.

Does a "tunable" Low/High-Pass filter even exist?

They do. They are big, expensive and unwieldly. In my opinion, it is best to use switched filter arrays as others mentioned.
A 1MHz bandwidth BPF at 470MHz is quite hard to design let alone building a tunable one.
As I understood from your comments, you dont own/have access to a network analyzer or equivalent equipment.
After 100MHz, it is unlikely that you would get a succesful result at designing an RF filter higher that 3rd degree without RF test equipment.
I even dare say 'dont bother trying'.
I suggest buying couple filters as your wallet allows, and staying in their passband.

How does amplification even work? Do I use a specific amplifier tuned for this frequency or do I use a broadband amplifier that amplifies 50MHz up to 3GHz by so many dB? Never thought of this before and so I really don't know how this normally is handled...

Both works. A tuned amplifier requires design, so designers mostly go with mimics(otherwise known as MMIC).
You can get a variable gain mimic, to be able to control the gain (duh). Below an example:
https://www.qorvo.com/products/p/TQM879006A
Lets assume SDR output level is 10dBm (10miliwatt). You want 10watt output (40dBm). So you need 30dB gain.
However, not all 30dB gain mimics can be used. The Qorvo part above can output 0.25W for example.
And so you need a high power amplifier. And its design is whole other business.

I dont like to say it but, designing every part in a VHF or upper band comm. equipment is a very big challenge, even for an experienced designer. Keep it small at first so you dont get frustrated.

Would be happy to help if you decide to go further.

Deniz

[vk4ffab]

Hi,

Question 1: Would I place the filter at the output of the SDR or at the output of the amplifier? I imagine that I would want to filter the output of the amplifier, as it will amplify the noise, too? But then it might be amplifying harmonics at frequencies for which the antenna is not tuned - would/could the reflected amplified harmonic kill the amplifier?

Question 2: Does a "tunable" Low/High-Pass filter even exist? Or are filters, due to their natural (coils) always tuned for a single frequency?

Question 3: If a filter is tuned for a single frequency and "tunable"/"adjustable" filters don't exist, what is an owner of a TX capable SDR supposed to own? Buy a set of different filters for every possible application one wants to try out? What is thre recommended attenuation of such filters for SDR use?

Question 4: How does amplification even work? Imagine the example: I want to output 2W (or 20W - just asking) to amplify my 472.5MHz signal, generated with the SDR. Do I use a specific amplifier tuned for this frequency or do I use a broadband amplifier that amplifies 50MHz up to 3GHz by so many dB? Never thought of this before and so I really don't know how this normally is handled...

Thanks for any responses and sorry if the questions seem silly...

Regards,
Vitor

Q1 Always filter after the final amplifier because that is the final place where harmonics are generated. At every other stage before that, whether to have filters or not really depends on if they are needed or not. A transmitter I am currently designing and building has 3 Class A gain stages to bring the power level up to a couple of watts, then a Class AB amp producing 20W. 80% of the harmonics are created in that final amp stage.

Q2. Yes they exist, but what, who and how depends on what bands of interest you are using and how much harmonic attenuation you need. 

Q3. Most of the convenient filters for VHF and up like SAW Filters etc are only going to be good on receive as they are not designed to take much power. For TX you are stuck like the rest of us designing and building LC filter networks. Much easier at HF where sloppy design is not punished so much, but certainly doable up to UHF at home with some basic measurement tools.

Q4. Ok, I will be very specific here to your needs. Using the HackRF as a transmitter is trivial, throw gain stages after it, filter and bobs your uncle. I have been playing around with my HackRF One to do the very same thing. I have a couple of videos up on youtube. I used a small ebay amp to take the 10mW out to 250mW drive level i needed for the input to a 10W HF amp.

There is no free lunch here, to go from a tiny signal to a big one has to be done in stages. In this case there are 4 stages, the hackrf, ebay mmic amp, preamp and final PA followed by a 7th order low pass filter to clean up the harmonics. (Not present in the video)



Again, this is easier at HF because it is forgiving, but, with care you can do the same thing at VHF and UHF. Places like https://www.minikits.com.au/ have VHF/UHF amps and filter kits that take the design work out of it for you. At HF QRP_Labs for the same kind of things. Mostly have a go, use a Dummy Load often and have fun. Its not all as hard as some make it seem.

Oh and I would not use the HackRF for anything serious and for not more than say 10 to 50W. The reason being, its signal integrity is not the best. Its not the worst by anyone imagination, but just be aware that its not really suitable for high power outputs and some neck beard with a 40 inch screen is going to whine to buggery that your signal is not pure enough looking on his waterfall. Spec-An images attached below.

[Bicurico]

Thanks to all for the valuable information.
I am away for a few days and without internet. I will study the links, soon.
I do own quite some test equipment, actually, including spectrum analysers.
Because I am an autodidactic in terms of electronics and RF I am aware of some voids in my knowledge - hence the apparently silly questions...
I don't want to do anything specific at this point but was rather generally thinking out loud how I would output a stronger signal if required. Because I never actually considered TX as a radio amateur, I am illiterate in this.


Thanks,
Vitor

[vk4ffab]

Thanks to all for the valuable information.
I am away for a few days and without internet. I will study the links, soon.
I do own quite some test equipment, actually, including spectrum analysers.
Because I am an autodidactic in terms of electronics and RF I am aware of some voids in my knowledge - hence the apparently silly questions...
I don't want to do anything specific at this point but was rather generally thinking out loud how I would output a stronger signal if required. Because I never actually considered TX as a radio amateur, I am illiterate in this.


Thanks,
Vitor

I would go the opposite route you are thinking and use the Hackrf as a receiver and build some simple transmitters and TX/RX switching to go with it. This is currently something I am working at for myself. CW transmitters are easy enough to design and build, obviously to use it you are going to have to learn CW LOL, but also the next step is DSB which is simple enough to do also. Getting to around 10w, a very usable power level is kind of trivial and cheap also at HF, 80 40 and 20m being the main bands people use daily are the bands to start with depending on what antenna you can erect.