On Thu, 26 Feb 2004 15:20:06 -0600, "Steve Nosko"
wrote:
OOPS again! You DO get absolute because you know how much low freq
power you remove. Isn't that the principle of the HP437 & 438's? or are
they doing something else...diode ... er...uh crystal if you're across the p
ond? I always thought they were, caloric, as you say.

Hi Steve,

They are Bolos (caloric) using PRD type elements. From your
description (barring the equipment name), it sounded like the typical
detector application. I can see you originally described a self
balancing bridge:

The HP bridge is powered three ways. It has a DC source to offer
temperature compensation and range changes (zero setting). It has an
AC Oscillator (~10 KHz) source in quadrature (topographically
orthogonal, across the bridge instead of over it). Basically, the
bridge temperature components (the Bolometer) act as an oscillator
level stabilizing feedback loop (much like the famous HP 200 which
used a lamp's negative resistance). The third power input is the
unknown RF which unbalances the heat causing the feedback to
re-regulate where the drive level changes (which is monitored by the
meter as the indication of the RF power applied).

The sum of the operation was that the bridge always operates at null
with the same heat burden for any applied RF power (within constraints
of course).

And yes, they are characterized as 200 Ohm mounts.

73's
Richard Clark, KB7QHC