[Microwave] Re: [VHF] Building a noise source/head

Thu, 18 Sep 2003 08:55:59 -0400

Preamps usually have a coil to ground, so it drains off the DC.  Also, all
noise sources need a 10 dB pad or greater between the diode and the device
under test which further attenuates the DC.  I've built these noise sources
here and they work great.  W1GHZ definitely knows what he's doing.

jeff, wa1hco

----- Original Message ----- 
From: "Scott Townley" <[email protected]>
To: <[email protected]>; <[email protected]>;
<[email protected]>
Sent: Wednesday, September 17, 2003 9:15 AM
Subject: [VHF] Building a noise source/head

> I have gathered a few articles on building your own noise
> source.  Hopefully someone on the list(s) have done that before!
> 1. The series diode configuration on the W1GHZ web pages looks simple
> enough...but there is no DC block anywhere.  Wouldn't that allow a little
> bit of the diode DC bias to appear at the noise source output?  I would
> think that's NG for GaAsFET preamp testing.
> 2. Bill Sabin wrote some up in QST (May '94) that are shunt
configurations,
> and included a blocking capacitor.  But...noise sources are intended to be
> very broadband devices (decades) and the use of a *single* blocking cap
> seems to me to be quite bandlimiting, especially when you're aiming for
the
> 5-10GHz range on the upper end.  When I was a wee lad I was taught to
> parallel several caps to make a broadband bias-T (e.g., 10pF, 500pF,
> .01uF)...is that technique (or some variant) applicable here to what
> amounts to a broadband DC-block?
> 3. The noise diode manufacturer shows a very simple shunt configuration
> with a blocking capacitor, and the single-pole rolloff equation on the low
> end, but no word/guidance on dealing with blocking caps above
> self-resonance.  So back to the bias-T question in 2.
>
> Any other experiences or pointers would be appreciated.
>
>
>
> Scott Townley NX7U
> Gilbert, AZ  DM43di
> http://members.cox.net/nx7u
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