Questions about Hittite Attenuators in series, such as HMC624s

[saulbit]

I am quite puzzled about the Hittite attenuators now. I have to make a 60dB range of attenuators. As I have got some HMC624s, which covers 31.5dB each.
So I tried two HMC624s in series directly, expecting a 63dB attenuat range. However, when the input frequency is higher than 1 GHz, the 63dB range won't be reached, with only a 50 dB about. The control word is correct, because each HMC624 works properly individually. So would anyone have the same experience or some advice is welcome.

 

[DDavid]

hi

how u checked this?
did u used with evalutaion boards?
or did u layout it?

maybe u should put amp between them with gain atleast of 15dB...

David

 

[biff44]

One of two things going on:
1) the spacing between the two VVAs is such that there is a resonance
2) there are sneak paths that let the signal go around the VVAs, instead of thru them. Do you have lowpass filters on all the bias/control lines? Do you have some metal shielding stoping waveguide/surface modes from propagating?

 

[tony_lth]

the microstrip radiation is about -50dB, as biff44 said.

 

[saulbit]

The space between the two VVA is a bit small, because I just layout them directly in series. There is no metal shielding between them, as I thought the wavelength of 1 GHz signal is about 300mm/sqr(2.2)=200mm is large enough to neglect the surface propagating modes.
I don't add lowpass filters on the bias/control lines specially and I just add some ceramic capactiors, expecting the filtering efforts.

 

[biff44]

Good, then you have your answer!

 

[saulbit]

Good, then you have your answer!

Thanks to biff44. Then I have got another question. In microwave circuits, the wavelength of the signal is not so little until the frequency is 10 GHz. When the frequency is under 10 GHz, such as 1 GHz, according to the electromagnetic theory, the wavelength is about 100 mm, while the one fourth of the wavelength, about 25 mm, is much larger the much larger than most IC components and other components. In the traditional oppion, the metal shielding won't be needed.
Then it comes to my question, in what frequency we have to take care of the shielding efforts? I mean we have to. Many thanks.

 

[tony_lth]

Hi, saulbit,

1G, wavelength is 300mm, not 100mm. You can consider the microstrip as a antenna, then you can unserstand.