CMOS Switch Mixer too simple?

[potcore]

Hi,

I wondered if readers could offer any validation for this CMOS Switch Mixer for an AM receiver that i just designed? I have built the receiver and now seeking the 455KHZ IF filter. However, i am having second thoughts about my mixer and wondered if suggestions for improvement or opinions could be offered?

The Switch Mixer...

Switch 3 switches signal to signal_ground (5V rail)
Switch 2 switches off signal path while switch 3 switchs
Switch 1 ensures previous stage sees same Z(in) during switching.
(5V rail needed as CMOS 4066 switch only switchs signals "inside" its own power rails. Also, this configuration prevents the transistor going into saturation during switching.)

The receiver (so far) into which this mixer is built:-

Documents used in the mixer's design (last three mostly)

It seems a little unbelievable that a mixer operating so simplistically and even derived from another mixer operating "erroneously" could work. Any thoughts greatly appreciated.

 

[Comcokid]

As VE1BLL pointed out in thread240-181369 where you were asking about 455 kHz IF filters - . . .Diodes are common. Anything non-linear will probably get the job done.

Mixers are typically made from a bridge of diodes. But, I have seen receiver circuits where the input RF and the LO were both coupled capacitively to the base of a transistor - the transistor both adding some gain and mixing at the same time. I've even see where XOR gates were used as a mixer.

If you look at the data sheet for the 4066, you will find it is made of an array of MOS transistors. In your schematic, it's hard to say exactly where the mixing is really occuring - in the switch arrangement, in the MOS transistors that make up the switches, or at the base of the BF199, or a combination of all three.

 

[jimkirk]

I suggest you redesign your topology so you aren't adding a 2.5 volt square wave to your signal, that will likely swamp out everything else after it in your chain.

Also, your local oscillator is 216 kHz? So with a 1026 kHz RF signal your IF is either 810 or 1242 kHz, not 455 kHz. Or is that a typo?

And some of the components and time constants in the following stage just don't add up for me either.

And while bypassing the emitter resistors with electrolytics at these frequencies might work... I'd recommend using ceramic caps.

 

[potcore]

Hi, sorry for the confusion on the local oscillator. At the moment it's 216KHz, but since i've recently managed to source a steep-sided 455khz IF filter, i will change my oscillator frequency to 571khz to give me the 455 khz difference frequency and change the circuit etc.

Regards the square wave, you're quite right it does swamp everything else out -in the above circuit (first thread) there is a 2-65pF decoupling capacitor feeding the stage following the mixer, i adjust this while watching the signal on the scope and thus high-pass filter it till its well down.

The above circuit does work (listenable but not pleasant) though i need to redesign it so that its not just detecting the 1026khz original station frequency. -And hope that this mixer does actually work.

 

[Skogsgurra]

I am a little confused. Is this to prove that a "fun" design also works? Or is there an advantage over standard mixers? Like better gain (usually not a problem), less noise (I think au contraire), better economy (doesn't look like), less PCB space (not likely). Or is it just to have some fun at work - in that case, I can understand.

Gunnar Englund

http://www.gke.org

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