Transformer-less balanced mixer that does not change the phase?

neazoi

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A special application, calls for a:

1. transformer-less balanced mixer
2. that does not cause "serious" phase shifting from it's input to it's output
3. fed by a 3KHz local oscillator. Essentially, shifting the input RF spectrum to 2 output spectrums (DSB), separated by 6KHz, with the initial RF input spectrum to be suppressed.

I have tried this simple mixer, which does the above, but I cannot measure the output signals phase (which are 3KHz away from the initial RF input signal), since they are 2 (sidebands) and the scope shows a distorted waveform.

Any help to satisfy criteria 2. above?
 

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Place this LPF at the IF output and you get rid of distortions.
 

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vfone said:
Place this LPF at the IF output and you get rid of distortions.
Click to expand...
Output is not audio, it is shifted frequency RF by 3KHz (up and down)
 

How do you phase-match two different frequencies, more than once per least-multiple periodically?
 

volker@muehlhaus said:
But isn't this what you expect from the mixer configuration with 3 kHz LO, to create two sidebands at +/- 3 kHz from the RF?
Click to expand...
This is if your LO is in RF. My LO is in AF, so I am using it as a3KHz RF shifter (up and down)
--- Updated ---

dick_freebird said:
How do you phase-match two different frequencies, more than once per least-multiple periodically?
Click to expand...
This is an issue yes. I hope that two HF frequencies (eg 10MHz) spaced by only 3KHz apart, will have a very small phase difference. I am counting on this assumption, but it has to be verified, how many degrees those signals will be off
 

neazoi said:
This is if your LO is in RF. My LO is in AF, so I am using it as a 3KHz RF shifter (up and down)
Click to expand...
No. First order mixer product is RF + LO and RF - LO, two signal frequencies. Instantaneous values of both signals (if you watch them separately) are permantly shifting phase against each other. If you watch the sum, you see a RF signal that switches phase every AF half cycle. To get a steady output signal with fixed phase, you need a SSB mixer.
 
FvM said:
...are permantly shifting phase against each other.
Click to expand...

Hm... please clarify. What do you mean by permanently? They shift at a fixed amount, one in respect to the other? They shift at a variable amount? I would like to know more about it.
Thank you
 

Relative phase of USB and LSB signal is moving according to 6 kHz frequency difference.

Shown for RF frequency of 100 kHz

 
In post #6 you mention the RF as 10MHz, if that is a fixed frequency and you derive the 3KHz from it to maintain constant phase shift between them it might work. It isn't something I have ever tried or attempted to simulate. If I wanted accurate +/- 3KHz shift I would probably use a PLL technique rather than mixing but maybe your application requires both mixer products simultaneously.

Brian.
 

    neazoi

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betwixt said:
and you derive the 3KHz from it to maintain constant phase shift between them it might work.
Click to expand...

That's not the issue.

The mixer is essentially a multiplier, and you will always get the response shown in #9, with an envelope of 3 kHz
 

volker@muehlhaus said:
That's not the issue.

The mixer is essentially a multiplier, and you will always get the response shown in #9, with an envelope of 3 kHz
Click to expand...
Let me put it that way:
Say we have a carrier at 10MHz and then another carrier at 10.003MHz
We mix those with a DSB-SC mixer like the ne612, using a 3KHz local oscillator.
(Ignore the lower output image for simplicity, let's focus on the upper image only.)

What will happen to the phase of the 10MHz signal when it is shifted to the 10.003MHz frequency, IN COMPARISON to the original 10.003MHz carrier that used to be at that frequency (and now shifted upwards too).

Will the original 10.003MHz carrier that used to be at that frequency, have the same (or 180degrees) phase to that of the shifted 10MHz carrier (now occupying the 10.003MHz frequency)?
 

neazoi said:
What will happen to the phase of the 10MHz signal when it is shifted to the 10.003MHz frequency, IN COMPARISON to the original 10.003MHz carrier that used to be at that frequency (and now shifted upwards too).
Click to expand...
If you look at the 10 MHz signal that is sent through the mixer, it is multiplied by +1 or -1 at a 3 kHz rate. So the phase jumps by 180° at a rate of 3 kHz.
 

    neazoi

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neazoi said:
Say we have a carrier at 10MHz and then another carrier at 10.003MHz
We mix those with a DSB-SC mixer
Click to expand...
Means, you are adding both carriers before sending it to the mixer? Then you get 4 output signals, 9.997, 10.000, 10.003, 10.006 MHz, each with different phase.

Or are you sending only 10 MHz carrier to mixer and comparing output with 10.003 MHz carrier? Then both 10.003 signals have ideally constant phase shift, not necessarily 0, depending on initial phase of all involved signals. However, as discussed before, mixer output is actually sum of 9.997 and 10.003 MHz signal.
 

    neazoi

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I present both 10MHz and 10.003MHz in the input of the mixer.
This 10.003MHz, I also present it as a reference signal (eg to a scope), so at to compare it's phase, with the phase of the shifted 10MHz signal. One of the frequencies produced by this shifting of the 10MHz signal on the output of the mixer, will be 10.003MHz.

Then I compare the phases of this mixer output product (10.003 MHz) with the original 10.003MHz applied to the input of the mixer.
How will those phases compare?

I know it is confusing and I apologize, but does it make better sense now?
 

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