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Phase shifter shows constant amplitude on simulation but not in real test.

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neazoi

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This HF Phase shifter shows constant amplitude on simulation but not in real test.
What can be wrong?
LTspice file.
 

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  • test1.zip
    1.1 KB · Views: 93

How much deviation in amplitude does the real test show?
For example on 7MHz, I see 3vpp to 600mvpp as I tune the phase potentiometer, in the circuit I have prototyped. Whereas on simulation the amplitude is almost constant.
 

You should include non-standard symbols and libraries when posting LTspice files. pot symbol missing:

1641733638878.png


What's the intended operation frequency? Is see phase shift between 23° and 168° with 10 MHz and 0 - 9V control voltage.

1641733796615.png
 

    neazoi

    Points: 2
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You should include non-standard symbols and libraries when posting LTspice files. pot symbol missing:

View attachment 173783

What's the intended operation frequency? Is see phase shift between 23° and 168° with 10 MHz and 0 - 9V control voltage.

View attachment 173784
What you did is ok. You removed the potentiometers (used as potential dividers) and directly varied the control and input AC voltages.
The frequency of operation is anywhere between 1.8-30MHz.
You will find that the shift is minimal on lower frequencies with just the varicap. To add more range, I will add parallel capacitors switched in parallel to the varicap on lower frequencies. I have simulated that and it seems it works.
For for the shake of simplicity let's consider one frequency to evaluate it and whereas you see in your simulation very small amplitude variations, in practice my prototype shows huge (example on 7MHz, I see 3vpp to 600mvpp as I tune the phase potentiometer).
It is to be used for phasing the noise out of a noise antenna and then canceling it from the main antenna. I opt for a 360 degrees phaser, so more than one stages need to be cascaded. But the performance of the single stage shown here, as loaded to the 1M probe scope, is not what is expected from the simulation... What could cause this?
 

in practice my prototype shows huge (example on 7MHz, I see 3vpp to 600mvpp as I tune the phase potentiometer).
What's the input voltage in the test? The cap diode can't work above about 1 Vpp with 0V control voltage.

1:1 probe has about 70 pF input capacitance, considerably reducing the phase control range. You should at least use 10:1 probe.
 

    neazoi

    Points: 2
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What's the input voltage in the test? The cap diode can't work above about 1 Vpp with 0V control voltage.

1:1 probe has about 70 pF input capacitance, considerably reducing the phase control range. You should at least use 10:1 probe.
I use a 10:1 probe at the output of the phaser and a 1:1 probe (a coaxial cable) to ptobe the 50R output of the generator. The output voltage of the generator is 300mv to 380mv depended on how I set the phase potentiometer in the phaser. I do not know why there is this interaction.

Another weird interaction is that, when I set the gain potentiometer near maximum, the phase does change, where it should not!
 

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