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Superhetrodyne receiver

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Armadillo

New Member
Hi

I am doing a small project and one of the things I need is a radio receiver as I am trying to pick up the signal from a remote control. It would actually be ideal if I could just buy a receiver off the net somewhere, but unfortunately it isn't quite an ordinary receiver I need.

I have no interest in the information carried in the signal but only in how the carrier wave "looks". Ideally I would like to lead the signal directly into an AD converter and sample the "raw" signal. Unfortunately the signal is at 434.5 MHz which would mean an AD converter well outside my price range (not to mention the circuit design needed).

So I need some kind of radio receiver. After looking around on the net I figured that the superhetrodyne receiver would work, but instead of actually going all the way and extracting the data I would simply chose the IF to be in the low Mhz and sample that signal (I have a 25MHz ADC).

I have attached my receiver design as it is right now. It does work somewhat, but it isn't really that sensitive. If the remote control is more that 10m away the signal is very bad. Another thing is that if I do a FFT of the IF the overtones (IFx2,3,4,5,6.... etc) are quite strong in the signal and generally there's a lot of noise.

A few hard fact about the system:
Overview:
Antenna-->Bandpass(center frequency 434.75MHz)-->LNA-->LNA-->Mixer(431MHz LO)-->amp->Bandpass(center frequency 3.75MHz)-->amp-->ADC

I am using using ICs where possible to try and avoid having to design RF amps myself.

Mixer is a SA612A
LNA (amplifier) is a BSG2011 (I have used this one for all my amps).
Received signal is 434.75 MHz.


Finally, the question.
How could I improve it or what design would be better? By better I guess I mean more sensitive and with less noise/overtones in the IF.

I hope it was explained well enough and that you can help me.

EDIT: Ups... just noticed C36 on the diagram. I used it to bypass the amplifier to see how much of a difference the amplifier did, so it's not on the actual circuit board.
 

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Nigel Goodwin

Super Moderator
Most Helpful Member
It's still pretty unclear really what you're trying to do?.

This is the licence free band, used for simple RF remotes etc. and receivers are often just super-regen's rather than superhet, there's no need for the complexity and expense of a superhet.

Transmitters are usually AM (simple carrier ON/OFF) or FM (FSK - carrier frequency shift), both of which normally send Manchester coded data packets. So it seems a bit 'over kill' to want to A2D it?.

For the receiver, why not just buy one?, licence free modules are cheap, and you know the exact frequency you require.
 

Armadillo

New Member
I am not interested in the data that is being sent.

I will try to explain what I need in a bit more detail.

Ideally I need a periodic signal with no data in at all and then AD convert it so I could recreate it if needed. So if I could I would just sample the carrier signal and be done with it. As the data rate of the remote is around 1.25k and (from what I can see on a scope) it is using PSK the signal is periodic between the bitshifts.
I want to do this because I am using an antenna that can change its radiation pattern and I want to see how the signal is affected by each one. So a fast scope would be ideal but I need a more compact solution.

I don't have the AD converter to be able to sample a 434.75MHz signal fast enough though so get around that I try to get the signal to a lower frequency via a mixer and sample that signal. The signal should still be significantly faster than the 1.25k that is the data rate though so it still appears periodic is sampled in a short enough time period.


Not sure if that explanation makes it any clearer... but looking into super-regens at the moment though, so thanks for that tip :)

PS. Now that I think of it just checking the RSSI might work... will have to test that too.
 

JimB

Super Moderator
Most Helpful Member
Now that I think of it just checking the RSSI might work... will have to test that too.
For the last 100 years, the way to assess the radiation pattern of an antenna was to use a receiver with a signal strength meter.
So yes, use the RSSI it will be much easier and simpler, and you will get sensible answers which I dont think you will get by analysing the recieved data.

JimB
 

Tesla23

Member
It's still pretty unclear really what you're trying to do?.

This is the licence free band, used for simple RF remotes etc. and receivers are often just super-regen's rather than superhet, there's no need for the complexity and expense of a superhet.

Transmitters are usually AM (simple carrier ON/OFF) or FM (FSK - carrier frequency shift), both of which normally send Manchester coded data packets. So it seems a bit 'over kill' to want to A2D it?.

For the receiver, why not just buy one?, licence free modules are cheap, and you know the exact frequency you require.
THis is off topic for the OP.

Nigel, have you seen what's been happening in these bands in the last few years? The regs have been refined as folks tried to send more and more data. For example, looking at the ADI site, for less than $3 you can get a single chip transceiver that does GFSK (and others) at up to 25kbps ADF7021, TI and others have similar devices. Direct conversion and low IF superhets are becoming commonplace.
 

Nigel Goodwin

Super Moderator
Most Helpful Member
THis is off topic for the OP.

Nigel, have you seen what's been happening in these bands in the last few years? The regs have been refined as folks tried to send more and more data. For example, looking at the ADI site, for less than $3 you can get a single chip transceiver that does GFSK (and others) at up to 25kbps ADF7021, TI and others have similar devices. Direct conversion and low IF superhets are becoming commonplace.
Try reading the original post! - he referred to a 'remote control' - which only need a slow data rate and use the simplest (and cheapest) radio systems.
 
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