Do you need a tuning circuit for a direct conversion receiver?

[obrien135]

Is a tuning circuit connected to the antenna required for a direct conversion receiver as shown in the link:

**broken link removed**

Or is the tuning of the oscillator / mixer sometimes sufficient?

 

[betwixt]

The 10K resistor drops the signal level greatly and it isn't really a direct converson receiver anyway. In answer to your question, theoretically no, it will work without the input tuned circuit but in the real world it will not work properly without it. The reason is that if you do not pre-select the receive frequency you feed the whole radio spectrum into the mixer (oscillator in your case). Suppose you are trying to receive a microvolt signal at 7MHz and a nearby MF transmitter is pumping out 100KW, the big signal swamps the little one. Of course, there are actually thousands of radio stations and other sources of RF in the ether so your tiny signal doesn't stand a chance. If you can reject the ones you don't want to receive, it gives it some hope of getting through.

Brian.

 

[obrien135]

Is a direct conversion receiver the kind that has the image and quadrature frequencies? I thought the ones like this one were refferred to on the web bas direct conversion also.

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Is a direct conversion receiver the kind that has the image and quadrature frequencies? I thought the ones like this one were refferred to on the web bas direct conversion also.

 

[betwixt]

No, they are different things. What you have is a circuit where the oscillator runs at reception frequency so the mixer product is f(in) - f(osc) = 0. You are shifting the RF directly down to audio frequencies and amplifying them to drive your loudspeaker. Your bandwidth is controlled by the frequency response of the amplifier. In some circumstances this technique is very useful, for example in GPS receivers where a closely spaced group of signals can be shifted down to much lower frequency band so the individual signals can be picked out by software.

What you are thinking of with Iand Q signals is slightly different although direct conversion can be used as part of the receiving process. The I and Q signals are the outputs of TWO mixers, one fed directly from the oscillator and the other fed through a 90 degrees phase shift circuit (90 degrees = 1 quarter = Quadrature). The resulting signals can be digitally processed to recover a data stream.

Your circuit would work far better if you used a different oscillator circuit and a seperate mixer stage. The Colpitts oscillator you show is fine if you can adjust both the tuning capacitors at the same time or you have a variable inductor, otherwise they are difficult to tune accurately. You could try a center tapped coil and single variable capacitor as in a Hartley oscillator but the problem they have is that both sides of the tuning capacitor are 'live' and prone to frequency shift if you touch them. A better solution would be a variation on Colpitts design with two fixed capacitors across the tuned circuit with the feedback inserted between them. This gives you the advantage of a single tuning control with one side of it grounded.

Brian.