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Quadrature mixing in transceiver to select USB or LSB. LOSC phases needed?

neazoi

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Hi, I want to experiment with a bidirectional single balanced subharmonic quadrature mixer I design.
To select just one sideband and cancel the other, I think I have to feed the first LOSC port with 0 degrees and the second with 90 degrees. Then for the other sideband I need to feed these ports with 180 and 270 degrees. Am I right?

Now, one simple way to achieve all four phases required, is with a 74F74, by dividing the input frequency by four within the chip.

1. Will the square wave generated by the chip be ok to drive these kind of single balanced mixers?

2. To select either sideband, can I use just two phases instead of four, and reverse them (with a mechanical switch)? This will allow me to have a driver oscillator of 2x the output frequency and not 4x.

Thanks
 

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betwixt

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I'm not sure that schematic will work at all but the principle of mixing I and Q signals is fine. You don't need 180 degrees and 270 degrees, you can swap the 0 and 90 over to reverse the sideband.

Yes, you can use square waves but additional signals will be generated which you will have to filter out. Ideally you use pure sine and cosine oscillator signals but as long as the diodes are switched in and out of conduction it will work. You might consider using a quad analog switch instead, for example a CD4066.

Brian.
 

    neazoi

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neazoi

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I'm not sure that schematic will work at all but the principle of mixing I and Q signals is fine. You don't need 180 degrees and 270 degrees, you can swap the 0 and 90 over to reverse the sideband.

Yes, you can use square waves but additional signals will be generated which you will have to filter out. Ideally you use pure sine and cosine oscillator signals but as long as the diodes are switched in and out of conduction it will work. You might consider using a quad analog switch instead, for example a CD4066.

Brian.
I would like to avoid 4066 because it needs 4 of them for bidirectional operation or more switches to switch the inputs and the outputs. Also it is not subharmonic.

I have tested the single mixer version of this subharmonic single balanced mixer and it works. I can only detect and generate signals 2x the LO frequency and not 1x. At the same time the Oscillator amplitude is suppressed.

Going quadrature, and because you said only 2 phases are needed, I can use a 74f74 and generate them and only 2x local oscillator would be needed. Because I use sub-harmonic mixers, this means that I can drive the 74f74 with any single phase oscillator AT half frequency. Combining these two, I can drive this mixer with a LO at exactly the frequency I want to receive/transmit, but now using 2 phases.

Another good thing of using the 74f74 is the LO signal level, which will remain constant, as long as the chip can be driven from the external oscillator, they should output a square wave of constant amplitude, I think...

The quadrature schematic above, will need an input splitter to isolate both mixers, I do not think it will work as it is. It will be a resistor wilkinson divider or probably a wheatstone splitter for big isolation of the mixers.

So to switch sideband, just feed the 0 degrees on the bottom mixer and the 90 degrees to the top instead, will it suffice?
--- Updated ---

May I ask also something about the topology. Here I present you different ways to reject the unwanted sideband. The first comes from a QST issue in 90's and the second from a website.
As far as I see it, both ways will work, as the output signals to the adder, are 180 degrees out of phase.

But please let me know, why do I need these multi-shifts?
Can't I just use a local oscillator with just two output phases 0 degrees and 180 degrees and drive the quadrature mixers?
If so, no other shift will be needed on the receiver, not in the audio and not in the RF input stage. And we will still have 0 degrees and 180 degrees at the audio adder.
This one is puzzling me.
 

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BigBoss

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-The best way to obtain 90 degree Phase Shift for LO, is to use 2xF0 the divide this signal by two.But if LO Frequency is relatively high, you cannot use standard dividers for this purpose.You have to use dedicated RF Dividers or build you own Divider with 6 BJT RF/MF Transistor.
-In your second post, first System Block Diagram is most used to get SSB. Using Diodes for Mixer can be a bit troublesome.
-In order to get a right Quadrature Signals, LO has to be equipped with Differential Output.This is Important
-If you intend to use a single LO Frequency, 90 degree can also be obtained by Polyphase Filter.
 

    neazoi

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BigBoss

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How can such a divider be implemented? Any schematics?
The circuit looks like that..
1631724904113.png


If the frequency is not too high, it's possible to implement this circuit with High Speed Logic FFs.
Also, it's possible to implement one single D-Type FF with discrete transistors like that..
1631724994435.png
 

    neazoi

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neazoi

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The circuit looks like that..
View attachment 171906

If the frequency is not too high, it's possible to implement this circuit with High Speed Logic FFs.
Also, it's possible to implement one single D-Type FF with discrete transistors like that..
View attachment 171907
Interesting!
I am not really sure how to connect the inputs and outputs of the discrete circuit to reassemble the dual flip flop divider.
 

BigBoss

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Interesting!
I am not really sure how to connect the inputs and outputs of the discrete circuit to reassemble the dual flip flop divider.
Very simple..
C=CLK
CLOCK_BAR=CLKN
D=ZP
D_BAR=ZN
Q=OUTP
Q_BAR=OUTN

The circuit demonstrates single D-type FF, you have to use identical circuit for second one.So when you use RF/MF Bipolar Transistors for this circuit, you will obtain a very good Divider by Two circuit that can be used up to GHz range.Use SMD Transistors ( there are many very cheap transistors ) and implement them carefully on a FR substrate within a metal can, that's it.
Output will be square wave as wanted for Mixers and rail-to-rail..
 

    neazoi

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RCinFLA

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Instead of dealing with high frequency LO switching you could just swap I and Q baseband signals.
 

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