K7ITM wrote in news:9e844e58-a673-4ec0-9a0b-ec15f8cc8f30
@c4g2000hsg.googlegroups.com:

On Jan 31, 12:31 pm, Cecil Moore wrote:
K7ITM wrote:
To me, having a
linear power scale is a big advantage, because then you can
reasonably
accurately figure SWR without having to worry about temperature
compensation of the detectors.

Can you define what you mean by linear? Straight line?
Since we can only measure voltage and current, in order
to obtain a linear power scale from a linear meter, it
is necessary to supply some pre-display computing
ability (microcomputer).
--
73, Cecil http://www.w5dxp.com

See earlier posting in this thread. See various Avago ap notes, such
as AN 969. A diode detector run at low input provides an output DC
voltage that's a constant times the square of the input RF voltage.
If the input voltage is, or is assumed to be, at some constant
resistive load impedance, the DC output is linear with RF power
input. The proportionality is temperature dependent, but if two
detectors are constructed the same and run at the same temperature,
and run in the signal level region where that relationship holds, then
the ratio of the output DC voltages is a very good approximation of
the ratio of the input RF power levels, and thus is useful for finding
the SWR if the detectors are attached to the forward and reverse ports
of a good directional coupler. Top end of the useful "linear power"
range using an HSMS-2850 single diode detector is about 10mV DC
output. If you can measure the DC accurately down to 1uV (a bit
tough, given thermal emfs, but possible), that gives you about a
10000:1 power range, or 100:1 RF input voltage range -- or about
1.02:1 SWR. Chances are very good that a home-built coupler won't be
accurately enough matched to 50+j0 ohms to worry about anything that
low anyway, even if you had a reason to care about it.

Cheers,
Tom


Tom,

This is further from Suzy's needs, but...

Operation of a diode detector in the square law region isn't out of the
question, but it takes some serious gain to drive a meter. There are some
good chopper stabilised op amps out there that have uV offset levels and
single supply rail and input to below the negative rail eg LTC1050.

Another alternative is the AD8307AN log amps for a linear dBW scale. You
could even use one on FWD and REF detectors and difference the outputs in
an op amp for a direct indicating VSWR or RL scale. I have thought of
getting one of these chips and seeing whether its response is fast enough
to drive a PEP amplifier for SSB telephony.

Back to Suzy's problem...

The instrument downstream of the sampler is not so much the issue as
building and calibrating a sampler when you have no test gear.

Suzy, if you see a Revex W560 going on VKHAM for $100 or so, it is a good
buy. It has HF to 70cm (two independent couplers, ie four coax
connectors), and works pretty well.

For a dummy load, the market was flooded with terminations from 25W to
about 60W that had been scrapped from AMPS base station equipment, and
they were sold at hamfests for $20 or so, you may find them if you look
around.

Owen