CMD258C3 behavior and controls

RF Solutions RF All Other (switches, controls, attenuators, ..)
1

hello,
Few questions:

  1. Datasheet states the VVA supports DC to 20GHz.
    What is exactly DC? Does it support 50KHz?
  2. I want to confirm the input impedance of Vctrl1 and Vctrl2 is Hi-Z.
    What is the input capacitance on these pins?
  3. Can you state the device attenuation close to Vctrl limits?
    I mean, at Vctrl1 close to 0 and Vctrl2 close to -5V and vice-versa, at Vctrl1 close to -5V and Vctrl2 close to 0.
    How does it change when getting close to these limits?

thank you
Gil

2

I think you mean the CMD285C3.
This is a FET based attenuator, and as such will likely work down very close to DC in terms of small signal s-parameters. The issue is that parameters such as P0.1dB, P1dB and IP3 would be reduced significantly at 50KHz. Qorvo has not tested these parameters at such low frequencies, so hard to comment definitively.
VCTRL 1 and 2 are FET gates, and will be high impedance. The input capacitance will likely be <1pF, this has not been measured. It must be low to give the fast switching times specified in the datasheet.
The attenuation at control voltage limits is plotted in the datasheet on page 6. You need to use the tables on page 3 to reference the state versus the voltage on VCTRL 1 and 2.

3

Hi Chris,

Thank you.
With regard to my 3rd question, the tables on page 3 show attenuation values for 0V and 0.5V.
The curve behaviors (page 6 plots) in the minimum and maximum attenuation do not give accurate information regarding, e.g. how much attenuation I get in Vctrl1=-0.1V/Vctrl2=-4.9V or Vctrl1=-4.9V/Vctrl2=-0.1V.
How tight should I have the Vctrl input reach 0V and -5V?

thank you
Gil

4

You can see from the curves in the datasheet that you only get quite a small change in the 0.5V step closest to the min/max settings. There doesn’t seem to be data available with finer resolution on the control voltages.

5

Hi Chris,

What would happen if I connect both Vc to the same voltage?
I think I would get an attenuation which is somewhere in the middle of its dynamic range, and I would stay there for other “same voltage” values. Am i right?

To your knowledge, if I work in the “complementary” way, would I be able to sweep through all the device dynamic range with high enough resolution?
What are the drawbacks of working in the complementary way?

thanks,
Gil

6

Using Vcc at the same voltage would not give you much attenuation control or optimum performance. If you consider that the part contains both series and shunt FETs, then you can see that these need to be controlled differently. This is a voltage variable attenuator so using complementary control will give you very fine resolution. The datasheet just plots certain ‘states’ for convenience, 0.5V steps in control voltage, but you can use finer control voltage steps to get finer resolution. No particular drawback using complementary control, but if you need better IP3 then the voltages can be adjusted to achieve this as shown in the datasheet.