# Computer controlled varactor am/fm receiver

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#### bowen

##### New Member
Greetings,

I'm trying to build a computer controlled AM/FM receiver. I know a enough about electronics to get in the ball park of knowing what I need to do but now I'm a little flummoxed so I turn to you.

I need to control 12 radios simultaneously from a computer, to achieve this I'll use a 12 output channel audio interface. I'll step up the voltage and have this voltage controlling a varactor cicuit. I can write the code for the computer but It's finding the correct circuit I'm having trouble with.

The circuit needs ideally to have a linear tuning characteristics, if this is impossible then at least a curve I can bias for in the computer code

I don't need anything too fancy with regards to locking into stations, this is for an art project and not to build a superb radio receiver.

Is there anything I'll need to be careful with in regards to stepping up the voltage?

Many many thanks,

O

#### audioguru

##### Well-Known Member
Modern radios use a frequency synthesizer circuit, not a varactor for tuning.
Each varactor diode will have a different capacitance/voltage.

#### bowen

##### New Member
Modern radios use a frequency synthesizer circuit, not a varactor for tuning.
Each varactor diode will have a different capacitance/voltage.
Thanks Audioguru,

It doesn't need to be a modern radio, in fact the the art installation is based on a piece from the 50s!

The main issue is that I need to be tuning 12 radios simultaneously and I figured that a 12 channel audio interface hooked up to 12 varacator tuners was the simplest way of achieving that.

#### JimB

##### Super Moderator
At the risk of appearing rather negative, there are a LOT of potential problems here.

12 Channel Audio Interface
What is this? A souce of tuning voltage for the varactor diodes?
You need a DC voltage for a varactor, not an audio signal.

If they are VHF FM radios, I can see that you would get away with connecting the varactor tuning circuit to the local oscillator in the radio and it would work (sort of).
But if they are AM radios, you are looking at a large capacitance change to tune from station to station, and you will have to also tune the antenna circuit at the same time using a second varactor circuit, otherwise you are likely to be limited to one station you can hear and a few others which you can hardly hear.

The tuning characteristics of the various radios will not be the same, unless they are identical and built in the same batch.

From an electronic point of view, this project would be much easier using modern radios with a synthesiser (as AG suggested).
If you want "old radios", hide the new radios inside the old ones, or just run a pair of wires from the new radio to the speaker in the old radio.

JimB

#### bowen

##### New Member
At the risk of appearing rather negative, there are a LOT of potential problems here.

12 Channel Audio Interface
What is this? A souce of tuning voltage for the varactor diodes?
You need a DC voltage for a varactor, not an audio signal.

If they are VHF FM radios, I can see that you would get away with connecting the varactor tuning circuit to the local oscillator in the radio and it would work (sort of).
But if they are AM radios, you are looking at a large capacitance change to tune from station to station, and you will have to also tune the antenna circuit at the same time using a second varactor circuit, otherwise you are likely to be limited to one station you can hear and a few others which you can hardly hear.

The tuning characteristics of the various radios will not be the same, unless they are identical and built in the same batch.

From an electronic point of view, this project would be much easier using modern radios with a synthesiser (as AG suggested).
If you want "old radios", hide the new radios inside the old ones, or just run a pair of wires from the new radio to the speaker in the old radio.

JimB
Thanks JimB. Negative is good though. I'm who Alexander Pope was referring to when he said "A little learning is a dangerous thing"

Yes the audio output would be used as the tuning voltage, though I guess I'd need to boost the actual voltage.

I can output a DC voltage from the interface though. I won't be outputting a waveform just a constant positive signal, which I could vary the amplitude of, it's just a cheap and easy way of getting 12 DACs and I wouldn't have to worry about programming them or building them. I can just vary the amplitude of the positive only audio signal in the software.

The piece I'm trying to automate used AM radios, I'd like the ability if possible to be able to switch to FM.

If as I've suggested I can get DC output would varactor tuning be an option?

these are the circuits I've been looking at but I don't know if I'm looking at the right sort of thing

Practical electronics for inventors - Google Books

#### mneary

##### New Member
If you are willing to use FM-only and don't care about quality, then you could take twelve cheap scan-type FM radios ($1 to$4). These radios are tuned by a varactor. Their chip is the TDA7088 or 9088. It should be easy to identify where to disconnect the internal scan voltage and insert your own 0-3V tuning control signal.

It's too bad you are NO WHERE. Some of us could suggest local sources for the radios. If you decide to be somewhere, you can put that location into your UserCP like the rest of us.

A warning - since you insist that accuracy isn't required, I have to assume that your 'art' has nothing to do with any signals they might receive. The radios at their best could receive very few of the normal stations. In this arrangement they'll also interfere with each other.

Now you need a 12-channel D/A.

#### bowen

##### New Member
If you are willing to use FM-only and don't care about quality, then you could take twelve cheap scan-type FM radios ($1 to$4). These radios are tuned by a varactor. Their chip is the TDA7088 or 9088. It should be easy to identify where to disconnect the internal scan voltage and insert your own 0-3V tuning control signal.

It's too bad you are NO WHERE. Some of us could suggest local sources for the radios. If you decide to be somewhere, you can put that location into your UserCP like the rest of us.

A warning - since you insist that accuracy isn't required, I have to assume that your 'art' has nothing to do with any signals they might receive. The radios at their best could receive very few of the normal stations. In this arrangement they'll also interfere with each other.

Now you need a 12-channel D/A.
Thanks Mneary, I'm now somewhere

Thanks for the tip, this will be good place to start. I'd still ideally like them to be switchable but I've got a one button scan type radio in the cupboard I can play with.

I can use two 6 channel D/A interfaces as an aggregate device in osx so don't have to fork out for a 12 channel interface.

With regards to interference is this due to proximity or the way I'm trying to go about this?

#### mneary

##### New Member
Two six channel D/As is fine, I was ambiguous.

The interference among 'receivers' would be due to their poor design. With an Intermediate Frequency of 75 kHz, image rejection is nonexistent. And there's no tuning at all in RF stage.

#### bowen

##### New Member
Two six channel D/As is fine, I was ambiguous.

The interference among 'receivers' would be due to their poor design. With an Intermediate Frequency of 75 kHz, image rejection is nonexistent. And there's no tuning at all in RF stage.
Thanks Mneary. I'll build a mockup using your suggested method.

Next questions based on the replies

Could I use variable voltage to drive a frequency synthesiser?

or perhaps use a microcontroller such as an Arduino?

#### mneary

##### New Member
Could I use variable voltage to drive a frequency synthesiser?
A synthesizer would normally use a regulated power source. Arduino driving the power? I wouldn't normally think so; Arduino's primary purpose is as a controller, but I suppose if your board has extra power...

Perhaps you want to learn more about synthesizers. They usually need a stable power source. Their operation is controlled via a digital command input.

#### bowen

##### New Member
A synthesizer would normally use a regulated power source. Arduino driving the power? I wouldn't normally think so; Arduino's primary purpose is as a controller, but I suppose if your board has extra power...

Perhaps you want to learn more about synthesizers. They usually need a stable power source. Their operation is controlled via a digital command input.
sorry poor explanation on my part again, what I meant to ask is whether I could use an arduino to control a digital command input for the synthesiser, which you've answered anyway.

Is there a chip or a certain circuit I should be looking at to start researching? what kind of logic input do these synth circuits need?

again many many thanks.

#### mneary

##### New Member
Most synthesizer chips use I2C or SPI digital data to control them. The Arduino is an open platform that comes in many forms. The ATMega328 (one of the available options) has both interfaces but you should research to find what's accessible.

Lots of synthesizer chips. I found the app notes for the PLLatinum series to be readable, others may have info on more modern parts and their app notes.

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