BTT SELECT-78 770/850 MHz Public Safety Filter

[btt]

When the MicroP25RX was first being developed, it was planned to add pre-select filters to various versions of the product for different frequency bands. Several people pointed out that 770 and 850 MHz bands have been combined for systems in larger cities due to shortage of spectrum. At the time, I stated that the approach could be to either combine multiple receivers with a single filter each or to implement RF-switched filter paths. Since then, I thought of another approach for a dual-pass-band filter. This is the BTT SELECT-78 filter. The cell phone downlinks on either side of the 700/800 bands are not completely rejected at the edges. There is a small overlap in the roll-off. As you can see from the close-in and wide view plots, the filter has very high stop-band rejection from 30 MHz to 4 GHz.

Features:
Housed in aluminum enclosure
USB Type-C power supply
Power Indicator LED
SMA female IN / OUT connectors
Can be used with any receiver RTL, AirSpy, BlueTail, etc.
Integrated, ultra-high OIP3 preamp prevents loss of sensitivity and overcomes insertion loss of the filters, couplers. The preamp is preceded by a band-pass pre-select filter.

I will have more information soon, but I wanted to introduce the filter and get the discussion started. Thank you to @goldmyne99, @freqseeker, and @ScannerDude244 for helping to test the first few units. A few minor changes were made. The final results are what is shown in the attached S11/S21 calibrated measurements. S-parameter measurements from 30-4000 MHz will be available on-request.

 

[FreqNout]

Nice work on the LTE band 14 rejection!!! A great new in-line filter product for any radio from @btt!

 

[prcguy]

I would like to see a filter like this start up again at 902MHz and continue to 1300MHz to include the 33cm and 23cm amateur bands. This would also include 935 to 937.5MHz LMR and some other stuff. Not sure if there is any cell phone freqs between 902 and 1300MHz that would need to be notched out. I would also prefer a passive version of this filter to use my own preamps.

 

[btt]

I would like to see a filter like this start up again at 902MHz and continue to 1300MHz to include the 33cm and 23cm amateur bands. This would also include 935 to 937.5MHz LMR and some other stuff. Not sure if there is any cell phone freqs between 902 and 1300MHz that would need to be notched out. I would also prefer a passive version of this filter to use my own preamps.

Interesting. It would have to be a different design. If there is enough interest, I could work on it. The preamp is a SOT-89 footprint, so it can be replaced if desired. I know some people around here really like the PGA-103+. I have a reel of those. I could put that on there as on option. It is a drop-in replacement.

 

[prcguy]

In my opinion, if your going to make a filter for scanner enthusiasts make it as useful as possible. Find out what needs to be received, what needs to be removed and give us the widest spectrum possible so we don't miss anything. Looks like you did a good job removing stuff below about 768MHz, which is a huge problem in my area and also the 869 to 896MHz range. But there are things above that which I listen to and I suspect many others might.

I think a great antenna/filter setup for many people would be a typical large Discone like a Diamond D130 or similar feeding a filter than cuts off below 118MHz then removes FM broadcast 88-108 then VHF TV from 176 to 220MHz then cuts off at 512Mhz on the top end. That would remove most offending signals that can cause problems with preamps but also lets through all public service, business and amateur signals. Then use a separate gain type antenna for 700-900MHz like the L-Com HGV-906U into a version of the BTT SELECT-78 then a separate preamp for each range into a diplexer so its all combined onto one feedline. This makes the best use of a D130 type Discone, which works well from about 100MHz to 500MHz but not so good at 800MHz.

Another option could be a tiny Discone to cover 700MHz up to 3GHz with a version of the BTT SELECT-78 that adds the 902MHz to 3GHz range, etc, then amplify and diplex that with the lower frequency Discone and filter/amp. I do something like this now but I don't have a proper filter that cuts off right below 768Mhz and 5G stuff is blitzing my high freq preamp.

Check out post 72 here, which is a later version of my tower top filter/preamp/diplexer fed by a large Discone and an L-Com HGV-906U at the moment. It needs some better filtering for new 5G stuff. prcguy's mast mount filter/preamp/diplexer thingee

Interesting. It would have to be a different design. If there is enough interest, I could work on it. The preamp is a SOT-89 footprint, so it can be replaced if desired. I know some people around here really like the PGA-103+. I have a reel of those. I could put that on there as on option. It is a drop-in replacement.

 

[FreqNout]

@btt sounds like you may be busy designing a product line of filters to meet various needs.

The Select 78 model would be great for me. What size case does it come in? I would be looking to mount it in my truck to reduce band 14.

 

[btt]

@btt sounds like you may be busy designing a product line of filters to meet various needs.

The Select 78 model would be great for me. What size case does it come in? I would be looking to mount it in my truck to reduce band 14.

It is the same enclosure as the P25RX. It looks almost identical except for the extra SMA connector next to the power connector. I'll have images soon. If one of the testers wants to post an image, that would be fine too.

 

[goldmyne99]

It is the same enclosure as the P25RX. It looks almost identical except for the extra SMA connector next to the power connector. I'll have images soon. If one of the testers wants to post an image, that would be fine too.

 

[NoahWL]

I would love to purchase a couple of these! I would also prefer an option without the integrated preamp so I can choose my own or not use one at all.

 

[btt]

I would love to purchase a couple of these! I would also prefer an option without the integrated preamp so I can choose my own or not use one at all.

If there is enough interest in a passive version, I will work on making one available. In this case, you would need to provide you own antenna pre-select filter and preamp. The insertion loss will be high due to the 1/4 wave couplers, combined filters, and dual-pass-band normalization.

 

[prcguy]

You need to keep insertion loss low as that will directly add to the noise figure of the receive system. I don't know what you think is high but any more than about 1dB loss before the preamp is to be avoided. If your filters have say 3dB insertion loss and you make up for that with a low noise preamp with 1dB noise figure, you now have 4dB noise figure due to the filter loss. Its easy to make filters with great skirts and high insertion loss but doing the same with less than 1dB loss is very difficult but done every day by companies like Dayden, Microwave Filter Company, etc. But a small filter from them might cost $1k these days.

Another thing to avoid is using USB or any kind of computer connection to an LNA. These cables usually have lots of RFI riding on them and you are bringing that right to the most sensitive part of a receive system, the LNA. A nice front end filter and LNA belongs at the antenna anyway so they should have a bias Tee with voltage fed up the coax from a very quiet linear power supply.

If there is enough interest in a passive version, I will work on making one available. In this case, you would need to provide you own antenna pre-select filter and preamp. The insertion loss will be high due to the 1/4 wave couplers, combined filters, and dual-pass-band normalization.

 

[FreqNout]

I see this first filter effort by Todd as meant to help those in the hobby that have a simple radio or RTL setup to enjoy their hobby by reducing LTE interefence. Sure it is great to talk about $1000 filters and tower top amps...that is a different product design for a different user. Just my 2 cents.

 

[btt]

You need to keep insertion loss low as that will directly add to the noise figure of the receive system. I don't know what you think is high but any more than about 1dB loss before the preamp is to be avoided. If your filters have say 3dB insertion loss and you make up for that with a low noise preamp with 1dB noise figure, you now have 4dB noise figure due to the filter loss. Its easy to make filters with great skirts and high insertion loss but doing the same with less than 1dB loss is very difficult but done every day by companies like Dayden, Microwave Filter Company, etc. But a small filter from them might cost $1k these days.

Another thing to avoid is using USB or any kind of computer connection to an LNA. These cables usually have lots of RFI riding on them and you are bringing that right to the most sensitive part of a receive system, the LNA. A nice front end filter and LNA belongs at the antenna anyway so they should have a bias Tee with voltage fed up the coax from a very quiet linear power supply.

Yes, this is why I would recommend using the design as-is and not changing the design to a passive version. The BTT SELECT-78 has a UHF bandpass filter with very low insertion loss before the LNA. The high insertion loss of the couplers and dual-pass filters is AFTER the low-loss-UHF-preselect and LNA. This is why I decided to integrate the LNA. I would think most people probably don't want to design their own UHF pre-select and preamp, but two of the responses have mentioned wanting a passive version, so I am listening.

 

[btt]

I see this first filter effort by Todd as meant to help those in the hobby that have a simple radio or RTL setup to enjoy their hobby by reducing LTE interefence. Sure it is great to talk about $1000 filters and tower top amps...that is a different product design for a different user. Just my 2 cents.

Yes sir. I intended this filter as a way for users living in areas with P25 systems that span across the 770 and 850 MHz bands. This filter can be placed between the antenna and an AirSpy, RTL, or other receiver in order to reject everything except these type of P25 systems. Think of it as an improved front-end for low-cost receivers monitoring P25.

 

[prcguy]

The whole purpose of the filters is to protect the LNA and keep it from creating IMD, all filters need to go in front any LNA. I have lots of live on air experience with my tower top filter/LNA box before it went on the tower. Looking at the input spectrum vs output spectrum with a spectrum analyzer you can see what happens when the LNA is blitzed by out of band and in band strong signals, the noise floor goes way up with lots of ghost signals dancing around and reception is degraded. Placing appropriate low loss filters in front of the LNA calms things down nicely where reception is now improved with an LNA and filters ahead of it.

My tower top system was working great until the new 5G stuff went on air recently and now I can see my 768MHz on up spectrum is trashed and the noise floor is raised and reception is much worse due to the incredibly strong signals in the roughly 718 to 768MHz range which my current filters can't deal with. I have relays in the system that will bypass all filters and preamps letting the signals flow direct from the antennas to the feedline and right now reception is much better with it bypassed because my high freq LNA is being driven into IMD land.

When I was first assembling the filter/preamp box I experimented with the LNA alone then the various band pass, high pass, low pass filters in the system and without filters ahead of the LNA it produced tons of IMD and degraded my reception. Placing the filters after the LNA did nothing as once the LNA is driven into IMD range its over and you can't recover with anything used after the LNA, all filters must be placed before the LNA otherwise they are not buying you anything.

Then we get back to high loss filters, whatever loss is in the filters will degrade reception by at least that much and an LNA can't get it back. Its mainly about system noise figure with a little emphasis on making up for feedline loss, etc. If you degrade the signal to noise ratio at the antenna with high loss filters before the LNA they are gone forever.

Yes, this is why I would recommend using the design as-is and not changing the design to a passive version. The BTT SELECT-78 has a UHF bandpass filter with very low insertion loss before the LNA. The high insertion loss of the couplers and dual-pass filters is AFTER the low-loss-UHF-preselect and LNA. This is why I decided to integrate the LNA. I would think most people probably don't want to design their own UHF pre-select and preamp, but two of the responses have mentioned wanting a passive version, so I am listening.

 

[kb5udf]

I’d definitely be interested as BTT proposed for this fillter.

 

[btt]

The whole purpose of the filters is to protect the LNA and keep it from creating IMD, all filters need to go in front any LNA

I would have to disagree with the last part of that statement. There are many filters in a receive chain that come after the LNA. Yes, you are correct, that a low-loss, band-pass pre-select filter before the first amplifier is desired for a high-end, single/dual band receiver. As previously mentioned, there is a low-loss, band-pass, UHF, pre-select filter BEFORE the LNA in this product. Then, for very high selectivity in the 770/850 MHz bands, the LNA is followed by dual interdigital band-pass filters. These are combined using 1/4 wave UHF couplers. It is a unique product designed specifically for these new P25 systems that are spread across the two public safety bands squeezed between LTE on either side. I don't think you will find another design like this (yet).

 

[prcguy]

If your 770/850MHz filters are after the LNA then if I used that here it would have no benefit, the LNA would get blitzed by my local 5G stuff and the receive spectrum would be trashed. Been there, done that many times over. I've also seen that when doing microwave receiver design and testing at Hughes Aircraft many years ago.

I would have to disagree with the last part of that statement. There are many filters in a receive chain that come after the LNA. Yes, you are correct, that a low-loss, band-pass pre-select filter before the first amplifier is desired for a high-end, single/dual band receiver. As previously mentioned, there is a low-loss, band-pass, UHF, pre-select filter BEFORE the LNA in this product. Then, for very high selectivity in the 770/850 MHz bands, the LNA is followed by dual interdigital band-pass filters. These are combined using 1/4 wave UHF couplers. It is a unique product designed specifically for these new P25 systems that are spread across the two public safety bands squeezed between LTE on either side. I don't think you will find another design like this (yet).

 

[btt]

If your 770/850MHz filters are after the LNA then if I used that here it would have no benefit, the LNA would get blitzed by my local 5G stuff and the receive spectrum would be trashed. Been there, done that many times over.

No, there is a bandpass filter BEFORE the LNA in this product. That will block 88-108 FM, VHF, WiFi, 5G before it reaches the LNA.

 

[GTR8000]

It is a unique product designed specifically for these new P25 systems that are spread across the two public safety bands squeezed between LTE on either side.

Mixed 700/800 P25 sites have been around for at least a decade, so I wouldn't call them "new". Perhaps new to you, but not to the industry. 700 and 800 have always been considered a superset band by the industry, going back at least 15-20 years. Infrastructure and subscribers have been capable of operating in either band for quite some time.

Anyway, back on topic. I'm curious why the gentleman from Texas, one of the first to purchase the MicroP25RX and the first to identify this issue with LTE bands adjacent to 769-775 and 851-862, wasn't one of the people chosen to test this new filter. Odd, he was literally your target demographic for this filter.