Improvement for Front-end RF Receiver Paths for SMR

[turbovectorz]

Hi ALL,

I am planning to build a hybrid digital/analog ESMR scanner front-end receivers that will work between 806-870 MHz.
What combination of antennas, preselectors, diplexers, and/or LNA that I could use to improve the signal and reduce interference?

I would like the capability to receive Astro, iDEN, LTR, etc.

Anyone here has experience in this type of RF engineering?
Hope I make sense here.

Sincerely,

TVZ

[tvsjr]

So you're going to build (presumably) a "commercial" single-band scanner that will decode P25, iDEN (both codecs?), LTR, and more... and you don't have a basic understanding of what RF conditioning components you'd need to get started? I'd say you have a lot more to worry about than a few cans...

[turbovectorz]

So you're going to build (presumably) a "commercial" single-band scanner that will decode P25, iDEN (both codecs?), LTR, and more... and you don't have a basic understanding of what RF conditioning components you'd need to get started? I'd say you have a lot more to worry about than a few cans...

Thank you for your quick response.
Let me explain further.

I am expert in the digital hardware domain and cryptanalysis, but I am not an analog expert like most HAM radio enthusiasts are. I am very open-minded to possible solutions that anyone can provide. I already figured most of everything out -- "codecs" (i.e. vocoders), OSI layers, encryption, trunking protocols, etc. Honestly, most of everything already works.

There are times I receive a interference in the analog domain causing frame error rates, and I just want to reduce that so everything else I can do the decoding in software.

Furthermore, this is not a commercial product, I build homebrew scanners as a hobby and to further my education.

Sincerely,

TVZ

[turbovectorz]

I suppose to further explain my request, here's my current and proposed setup:

(For example, let's take iDEN 800MHz Band 2 from my current setup)

Receive Omni Antenna (806-870, 2.5dBi)---> Diplexer (806-928MHz) ----> Two Receive Paths:
(A) Forward/Outbound Base Station Frequency 851-866MHz
(B) Reverse/Inbound Mobile Subscriber Frequency 806-821MHz

For receive path A, what I measured is around 22.5 Watts Max Top-of-Rack to the receive antenna.
For receive path B, what I measured is around 0.8 Watts Max to the receive antenna.
Therefore, in order to improve the signal for receive path B, I had to put a Low Noise Amplifier (LNA) before the Analog/Digital Converter (ADC).

My proposed setup is to separate the receive paths somehow as follows:

For receive path A:

Antenna --> Programmable Attenuator --> Preselector (tuned to 851-866 using RF Service Monitor and Tracking Generator) ---> ADC

For receive path B:

Antenna --> Preselector (806-821) --> LNA --> ADC

I have used Telewave and Sinclair preselectors/diplexers (comb and cavity), but just wondering if there's better RF components out there.
Also, in the receive paths A or B prior to the ADC, could I use low-pass and high-pass filters without introducing spurious noise assuming the proper grounding and low insertion loss?

I also realized that different modulation schemes require different preferred RF front-ends, M-16QAM versus C4FM/CQPSK versus DPQSK. If I am correct, the adjustments are minor to the RF front-end and it should not matter once converted to the digital domain.

I know there are really talented HAM radio enthusiasts out there that are experienced in this type of RF tuning and the associated techniques (at least in principle) that should apply to this to the get BEST RF front-end signal conditioning possible.

Any suggestions and recommendations?

Thank you all in advance!

Sincerely,

TVZ