Narrowband Walkie Talkie Build

[StudentSA]

Hi All,

I would like to ask what is the best demodulation method to use when trying to design a simple, cost effective yet clear FM receiver operating in the "walkie talke" frequency range (447.00Mhz-455.00Mhz)?

I have been reading up on Phase Locked Loops as well as Super Hetrodyne receivers. Presently I am tending towards PLL however I am unsure if a single package exists that operate in this frequency and if it is possible to split out the functional blocks and source a chip for each?

Any advise or direction is appreciated.

Kind Regards,
StudentSA

 

[MikeMl]

First thing you need to do is determine the legality of building a transmitter on that frequency in your country. It would be illegal here (unless you hold the appropriate license, and then only on 447 to 450Mhz).

 

[JimB]

The conventional arrangement of such a receiver is as follows:

Low noise RF amplifier at the signal frequency.
Mixer, converts the signal frequency to the (first) Intermediate Frequency.
Intermediate Frequency (IF) amplifier and filter. Usually at 10.7 or 21.4 MHz.
Second mixer, converts 1st IF to 2nd IF. Usually 455kHz.
Second IF amplifier and filter.
Demodulator. Recovers the audio from the FM signal at the second IF. There are various ways of doing this, one of which is a phase locked loop.
Audio Amplifier, drives the loudspeaker.

There are also two oscillators, the first local oscillator (LO) usually runs at (signal frequency - 1st IF frequency). The output of this oscillator is fed into the first mixer.
The second local oscillator runs at (1st IF + 2nd IF) or (1st IF - 2nd IF). The selection is made when the radio is configured, so that harmonics of the second LO do not fall near the signal frequency.

JimB

 

[StudentSA]

First thing you need to do is determine the legality of building a transmitter on that frequency in your country. It would be illegal here (unless you hold the appropriate license, and then only on 447 to 450Mhz).

Hi MikeMl, as per my original post I would like to only build a receiver. We have a community type broadcast that is licensed through the 447.00Mhz-455.00Mhz range. The receivers that we have available is of poor quality and I would like to improve it.

JimB, wow, I`ll need to spend some time decoding your post, thanks for all the info. Would you mind assisting me step by step?
1. Low noise RF amplifier at the signal frequency:

Am i correct that the antenna will feed into this stage?
I was thinking of using a standard 1/4 wavelength mono-pole antenna designed at optimal length for 451MHz = 166.3mm
Can you suggest a basic RF amp circuit that has decent characteristics? I will also do some research and post my findings.
​

Thanks for the assistance guys,
StudentSA

 

[JimB]

JimB, wow, I`ll need to spend some time decoding your post, thanks for all the info. Would you mind assisting me step by step?
1. Low noise RF amplifier at the signal frequency:
Am i correct that the antenna will feed into this stage?

Yes the antenna feeds the RF amplifier.

I was thinking of using a standard 1/4 wavelength mono-pole antenna designed at optimal length for 451MHz = 166.3mm

That sounds near enough.

Can you suggest a basic RF amp circuit that has decent characteristics? I will also do some research and post my findings.

Have a look at this old post:
https://www.electro-tech-online.com/threads/rf-amplifier.21123/#post-134906
I am not sure that it is applicable to a portable radio, but it is UHF and does work.

JimB