On Jan 31, 1:51 pm, Owen Duffy wrote:
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
Yes, there are several linear-in-dB RF detectors out there. Linear
Technology also have them. I really like that idea; they're typically
much more temperature stable than a diode detector. But Suzy wanted
to avoid SMT.
I've used a Harris chopper-stabilized op amp with HSMS-2850 zero-bias
detector diodes, and it works well, but I did learn something about
the need to be really careful around the chopper capacitor pins on
that op amp before getting it right... But it's also not difficult to
find a digital voltmeter that will go down to pretty low voltage at
high impedance. A 4.5 digit meter on a 200mV scale does ten
microvolts, and the simulation I ran last night suggests you could see
down to about -50dBm power level with that. When you get down to
10uV, you have to get serious about avoiding thermal emfs.
I suppose it makes sense to just drive the detector hard and run it
right into an analog meter movement, and then calibrate the meter.
Actually, at that level, the detector should be pretty linear in
voltage. That actually makes it easier to detect down closer to 1:1
SWR anyway.
I posted not too long ago about a load I made with four 200 ohm 2 watt
metal oxide resistors that shows what to me is remarkably good return
loss out to well beyond 450MHz. It was very cheap to make. But
there's no guarantee that some other brand of resistor would give such
good results. It may have just been a fluke that the one I made
turned out so good. (But I'm not about to toss it out!)
Cheers,
Tom