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help building a cheap fm vhf receiver

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mrfamilia

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hey people i want to be able to build a basic fm receiver to receive 152.275 mhz.... ive heard about the mc3362p being the best option but i have no idea where to start? anyone with help :)
 
mrfamilia said:
hey people i want to be able to build a basic fm receiver to receive 152.275 mhz.... ive heard about the mc3362p being the best option but i have no idea where to start? anyone with help :)

Your first design decision is to find out what type of FM transmission you are wanting to receive? - NBFM (Narrow Band FM) and WBFM (Wide Band FM - normal broadcast radio) generally require different receivers.

I would presume you are looking at NBFM?, based on the frequency, so you will require a dual-conversion receiver. As such there have been many such receivers over the years intended to receive the radio amateur 2M band (144MHz to 146MHz) - it wouldn't require much work to alter one to 152MHz. The MC3362P is presumably a suitable IF IC for such a receiver?, including most of it, except the front end parts.
 
ok ive got this circuit that runs at 173.750mhz and i want to put the frequency down to 152.275, what parts do i remove/replace. the crystals on the picture are 32.6100 mhz and 10.245 mhz and theres this funny looking black plastic box part i have not seen before with the numbers/letters LTW455E
 

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The LTW455E is a ceramic filter, which gives the radio its selectivity, the filter is in the second IF stage at 455khz.

The input frequency 173.75 Mhz is converted to the first IF of 10.7Mhz by mixing with the 5th harmonic of the 32.61Mhz crystal.
The first IF 10.7 Mhz is converted to the second IF 455khz by mixing with the 10.245Mhz crystal.

To recieve 152.275 Mhz you will need to change the 32.61 Mhz crystal and retune the RF circuits, the two "cans" side by side at the top of the photo.
Re-tuning will be VERY difficult without a signal generator of some sort.

As for the new crystal frequency, there are two possibilities.
Keep with the existing "low side injection" where

F_crystal = (F_recieve -10.7)/5

giving a new crystal frequency of 28.315 Mhz.

Or, change to "high side injection" where

F_crystal = (F_recieve +10.7)/5

giving a new crystal frequency of 32.595 Mhz.

In this case high side injection has the advantage that the oscillator circuits are running at nearly the same frequency and will not need any adjustment.

JimB
 
the three items that can modify a radio's frequency is a crystal (which I dont use), inductor (or coil), and a capacitor.
 
so people what would you recomend because im trying to keep this as simple as possible so i can build a few of these receivers...

should i change the crystal? if so are there 28.315mhz crystals and 32.595 mhz crystals?

or should i do something else? whats the best option?
 
mstechca said:
the three items that can modify a radio's frequency is a crystal (which I dont use), inductor (or coil), and a capacitor.

Dont forget the divider ratio in the local oscillator synthesiser (used quite frequently).

JimB
 
mrfamilia said:
so people what would you recomend because im trying to keep this as simple as possible so i can build a few of these receivers...

should i change the crystal? if so are there 28.315mhz crystals and 32.595 mhz crystals?

or should i do something else? whats the best option?

What are you trying to achieve?
What is the application for these receivers?

Do you have a source of the original receiver which you are going to modify, or copy the circuit and build from scratch?

Crystals for "odd" frequencies like this will have to be ordered and can be expensive in small quantities, typically £8 for one in the UK.
It all depends what you are trying to achieve.

JimB
 
im trying to build receivers for my friends community radio station, just a little add on circuit to put onto normal 88-108 receivers so that people can listen, ive looked at two similar private radio station receivers running at higher frequency, thinking i might be able to duplicate the circuit and modify it for my friends radio station, but i want to be able to build something simple and cheap so it doesnt cost the community to much for the radios.. so help me guys.... if you can, thanks
 
A "community radio station" on 152Mhz, now I am confused. In which country is this?

JimB
 
in australia we have private frequencys that community's can purchase from the government, i think they run from 140- to 180 mhz...
if you have a scanner you can pick up all sorts of community and language radio stations on this frequency...

anyway so theres the story, any idea's........
??? about building or altering the receiver
 
yes ive realised that they are narrow band and not normal wide band, i think wide band is only used for 88-108 mhz... because every jump i think either between 25khz or 50khz theres a station, if it were wide band it would take the entire 152mhz, however there are several stations using that frequency....
so what do you people recommend, i should alter the board i have from 173 mhz to 152 or should i build a entire new one... help appreciated :)
 
mrfamilia said:
so what do you people recommend, i should alter the board i have from 173 mhz to 152 or should i build a entire new one... help appreciated :)

Alter the board you've got!, it's almost exactly what you need - it just needs a crystal change, and perhaps a slight adjustment - the change is small enough that the existing tuning circuits will probably have sufficient adjustment range (otherwise you could add small capacitors across the tuning inductors.
 
ok can you tell me how to do that, like what exactually should i do... where should i put the capacitor over the tuning bit or should i get new crystals, and if so wat crystal should i change and what frequency should it be? thanks
 
mrfamilia said:
ok can you tell me how to do that, like what exactually should i do... where should i put the capacitor over the tuning bit or should i get new crystals, and if so wat crystal should i change and what frequency should it be? thanks

You HAVE to change the main tuning crystal, it's the component that sets what frequency it's tuned to. A previous poster gave the required calculations and suggested the frequency crystal you would need.

There should be two crystals in the receiver, the other is the second conversion crystal which is a lower frequency, again as the previous poster specified.

You might not have to add any capacitors, simply change the crystal and try and align the front end, if it won't tune low enough, then try adding a little extra capacitance across the tuning coils - or for air cored free standing coils, push the windings closer together.
 
Several posts ago I told you the crystal frequency to use.
Whether you need to put some small capacitors in parallel with the RF tuned circuits is impossible to say from 8000+ miles away.

At the risk of appearing to be Mr Negative, I think that this is not a project for someone who has no (apparent) experience of this type of radio.

JimB
 
no no i have experience with radios, manly transmitters, however i've never gone this high in frequency before, and i normally dont use crystals to lock the frequency. so thats why im stuck...

i useto do alot of work with am radios... cb radios where my thing... however they now are also on uhf.. which doesnt help me.
 
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