Can I apply an AC signal through a bias tee?

[thundrous]

We are trying to apply 1MHz and 1V signals using a signal generator, through one port of bias tee.

I just wanted to check whether this is possible because usually, people use a bias tee with a DC signal.

 

[dick_freebird]

That's the whole point of the bias-T, you have a DC
port and an AC port and a combined port.

Now, the DC port is inductor blocked and the AC,
capacitor blocked. So you can't push DC into the
AC (SMA) port, and any AC that can get through
the DC port (solder posts?) will be limited to very
low frequencies.

You should be OK with 1MHz into the AC port, but
check the specs - economics tends to make products
segmented to frequency ranges, and your port
blocking cap with 50 ohms Zout makes you a high
pass filter, while big caps' ESL would impose an
upper end to useful frequency. So "horses for courses".

The Mini-Circuits T I have, is good for 0.1MHz to 6GHz
so would be fine.

 

[thundrous]

That's the whole point of the bias-T, you have a DC
port and an AC port and a combined port.

Now, the DC port is inductor blocked and the AC,
capacitor blocked. So you can't push DC into the
AC (SMA) port, and any AC that can get through
the DC port (solder posts?) will be limited to very
low frequencies.

You should be OK with 1MHz into the AC port, but
check the specs - economics tends to make products
segmented to frequency ranges, and your port
blocking cap with 50 ohms Zout makes you a high
pass filter, while big caps' ESL would impose an
upper end to useful frequency. So "horses for courses".

The Mini-Circuits T I have, is good for 0.1MHz to 6GHz
so would be fine.

Thank you very much for your response. I want to apply 1MHz and 1V signals using a signal generator with a Minicircuit bias tee built into the eVNA.

Vector Network Analyzer (VNA)

Mini-Circuits is a global leader in the design and manufacturing of RF, IF, and microwave components from DC to 86GHz.

www.minicircuits.com

Do you think it would be ok if I use the eVNA bias tee itself instead of a separated bias tee like ZFBT-6GW+?

 

[FvM]

Which signal do you want to combine with 1 MHz signal? If it's higher frequency RF, you want a diplexer rather than a bias tee.

 

[thundrous]

Which signal do you want to combine with 1 MHz signal? If it's higher frequency RF, you want a diplexer rather than a bias tee.

I want to sweep the frequency from 300kHz to 6GHz by VNA.
Right now I checked the 1v and 1MHz signal connecting to the bias tee of the VNA, and it seems it filters the signal (I checked it with an oscilloscope).
it successfully can transfer 1v and 5kHz but it is not enough for me. Do you think I need a Diplexer?

Any suggestion appreciated.

 

[dick_freebird]

Can you be more clear about -which- bias-T port you are
trying to inject to? As I mentioned, the "DC" port will attenuate
HF (the BW limits rated, are for the "RF" port to "Combined"
port path). The "RF" port I imagine would be the VNA "input"
if there's a bias-T front end.

Yout test instrument may have limitations which a standalone,
wide-BW biast-T "nugget" does not?

 

[FvM]

I want to sweep the frequency from 300kHz to 6GHz by VNA.

In this case, neither bias tee nor diplexer can work to add 1 MHz signal. A resistive "power divider" (6 dB coupler) would be my first approach.

 

[thundrous]

In this case, neither bias tee nor diplexer can work to add 1 MHz signal. A resistive "power divider" (6 dB coupler) would be my first approach.

Thank you for the response, but I need to apply two inputs one from signal generator and one from VNA.
To clarify, here is the video in which we do the signal modulation for one port of the VNA. As you can see we are sweeping the frequency from low to high (300kHz to 6GHz) meanwhile we are applying the 1V and 5kHz from signal generator to the bias tee built inside the VNA.

However, if I increase the signal to 1 MHz, bias tee filter the signal.
I am new in the RF field, but as long as the literature I have read I am kind of confused about using which of these passive devices.

--- Updated Mar 10, 2023 ---

Can you be more clear about -which- bias-T port you are
trying to inject to? As I mentioned, the "DC" port will attenuate
HF (the BW limits rated, are for the "RF" port to "Combined"
port path). The "RF" port I imagine would be the VNA "input"
if there's a bias-T front end.

Yout test instrument may have limitations which a standalone,
wide-BW biast-T "nugget" does not?

Thank you very much for your response. here is the schematic of the bias tee built in the VNA:

we connected the signal of 1V and 1MHz to the bias tee port from the signal generator. and as I mentioned it filters the signal. However, if I change the signal to 5 kHz the signal modulation is successfully applied while the frequency is sweeping from 300kHz to 6GHz by the VNA. You can see the results in the following video:

Do you think if we buy a separate bias tee, we can do this signal modulation with 1MHz successfully?
Any suggestions are appreciated.

 

[FvM]

Now you are talking about modulation, previously it was about "applying a signal", respectively adding it to another signal. The latter problem was precisely answered in my previous post: use a coupler.

A bias tee can add signals with distinct frequency difference but not modulate one signal by another. This can be e.g. done by a balanced mixer.

--- Updated Mar 10, 2023 ---

The video shows added signals, not modulation. So we are probably back to a coupler.

 

[thundrous]

Now you are talking about modulation, previously it was about "applying a signal", respectively adding it to another signal. The latter problem was precisely answered in my previous post: use a coupler.

A bias tee can add signals with distinct frequency difference but not modulate one signal by another. This can be e.g. done by a balanced mixer.

--- Updated Mar 10, 2023 ---

The video shows added signals, not modulation. So we are probably back to a coupler.

Unfortunately, there are no RF mixers in the wide range of frequencies.

 

[FvM]

y = x1 + x2 added signals
y = x1 * (a + x2) amplitude modulation

--- Updated Mar 10, 2023 ---

Unfortunately, there are no RF mixers in the wide range of frequencies.

There are, but apparently you don't want a mixer.

 

[thundrous]

y = x1 + x2 added signals
y = x1 * (a + x2) amplitude modulation

--- Updated Mar 10, 2023 ---

There are, but apparently you don't want a mixer.

What is your suggestion?
Sorry If I have not explained it very clearly, as I am new in this field.
I want to apply a 1V and 1MHz signal while sweeping a frequency from 300kHz to 6GHz. How can I do that?

 

[FvM]

Use a resistive "power divider" (6 dB coupler) to add the signals.

--- Updated Mar 10, 2023 ---

 

[thundrous]

Use a resistive "power divider" (6 dB coupler) to add the signals. View attachment 181701

--- Updated Mar 10, 2023 ---

View attachment 181702

Do you think this model works?

2 Ways Resistive Power Splitter, DC - 12000 MHz, 50Ω | ZFRSC-123-S+ | Mini-Circuits

Mini-Circuits is a global leader in the design and manufacturing of RF, IF, and microwave components from DC to 86GHz.

www.minicircuits.com

--- Updated Mar 10, 2023 ---

Use a resistive "power divider" (6 dB coupler) to add the signals. View attachment 181701

--- Updated Mar 10, 2023 ---

View attachment 181702

It seems that the resistive power combiner sums the signals from each input port and deliver the resulting combined signal to the output port.

So does it mean that if I have a y signal and an x signal, the output of x+y?
if yes it does not work for me because I need the apply of both signals separately into my device:
1v and 1mhz
sweeping of 300khz to 6ghz
any suggestions are appreciated.

 

[FvM]

if yes it does not work for me because I need the apply of both signals separately into my device:
1v and 1mhz

No idea what this means in technical terms.

--- Updated Mar 11, 2023 ---

It should be possible to describe the intended operation mathematically or with circuit.