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Improving selectivity on a reciver

 

Hello Everyone,
?
Let me first say this is an experimental improvement to see it will work.? ? I have a hamtronics reciver board that is running in the 220MHZ space.? ? ?It's one the last board produced before the owner had a fatal bicycle accident.? ??
?
I wanted to see if I could improvement the selectivity of the receiver by narrowing filters or am I look at more of a total redo of the circuit.? Like I said, I'm doing this as a experiment.?
?
I wanted to narrow it to the current 222-225 Mhz from the old 220 to 225 Mhz.? ?
?
Any suggestions would be helpful.?
?
Thanks
Steven H.
N8RLW?
 

Your helicals should already be much narrower than that.?? What problem are we trying to solve?

Andy Zorca
WJ9J

On Tue, Apr 8, 2025 at 1:28?PM steven harvey via <sharvey=[email protected]> wrote:
Hello Everyone,
?
Let me first say this is an experimental improvement to see it will work.? ? I have a hamtronics reciver board that is running in the 220MHZ space.? ? ?It's one the last board produced before the owner had a fatal bicycle accident.? ??
?
I wanted to see if I could improvement the selectivity of the receiver by narrowing filters or am I look at more of a total redo of the circuit.? Like I said, I'm doing this as a experiment.?
?
I wanted to narrow it to the current 222-225 Mhz from the old 220 to 225 Mhz.? ?
?
Any suggestions would be helpful.?
?
Thanks
Steven H.
N8RLW?
 

Can you post the current IF filter mfr and model #??
 

Never mind, I looked it up and its a custom 10.7MHz crystal filter using four discreet crystals. If you have the equipment you could sweep it for response then shop for a packaged filter with better specs. I don't see any adjustment or matching on the filter output to the receiver IC so its hard to say how well a drop in filter will perform but it may not be too bad.
 

On 4/8/2025 2:46 PM, Mike via groups.io wrote:
Never mind, I looked it up and its a custom 10.7MHz crystal filter using four discreet crystals. If you have the equipment you could sweep it for response then shop for a packaged filter with better specs. I don't see any adjustment or matching on the filter output to the receiver IC so its hard to say how well a drop in filter will perform but it may not be too bad.
The OP has inquired how to improve input selectivity - not I-F selectivity.? Go back and re-read the inquiry.

Kevin W3KKC
 

Steven,

Receivers provide "selectivity" at various stages - and each have different purposes.

The front-end is selective, and operates at the fundamental operating frequency.? The front-end filtering is generally narrower than the whole band - depending on the type of filtering that's used.? Hamtronics receiver used helical resonators and L/C tuned circuits depending on the model.? Therefore - a receiver generally doesn't pass the entire 220-225 MHz range even if the receiver was built to cover this entire range.? Front-end selectivity is necessary to provide rejection to nearby signals - a few to several MHz away from the operating frequency.? Without front-end filtering - a receiver can be driven into overload easily from other signal sources.

The receiver's first I-F frequency is 10.7 MHz and provides a basic width of the intermediate frequency for two-way radio.? The receivers second I-F is 455 kHz and provides the final selectivity.? These filters determine how well a receiver will tolerate a signal on an adjacent channel that's 15 - 20kHz away or more.? Neither of these filters have anything to do with how wide the front-end is.

My question to you is - what are you wanting to accomplish?? Or, what is the problem that you feel narrowing something is going to help?

Kevin W3KKC

On 4/8/2025 2:28 PM, steven harvey wrote:
Hello Everyone,
Let me first say this is an experimental improvement to see it will work.? ? I have a hamtronics reciver board that is running in the 220MHZ space.? ? ?It's one the last board produced before the owner had a fatal bicycle accident.
I wanted to see if I could improvement the selectivity of the receiver by narrowing filters or am I look at more of a total redo of the circuit.? Like I said, I'm doing this as a experiment.
I wanted to narrow it to the current 222-225 Mhz from the old 220 to 225 Mhz.
Any suggestions would be helpful.
Thanks
Steven H.
N8RLW
 

The schematic for the receiver in question shows a single broad band pass filter before the FET 1st amplifier then a three section helical resonator after that. Not a very good design which would rely on the duplexer band pass to protect the FET from overload, generating IMD, etc. Depending on the duplexer used a single 1/4 wave cavity filter ahead of the receiver module would go a long way to reduce the repeater transmitter or out of band signals from degrading receive.?

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