Nuts & Volts is the first place I turn
to when a new copy arrives. I enjoy
your ideas and insight and have used
and adapted many of your circuits.
Thanks for any assistance you
might be able to give me.
— Gary Casper
ASince the output meter
does not vary with load, it
must be calibrated for 600
ohms load. The open circuit
output (Eg) is two times (+ 6 dB) the
meter reading. The output (Eo) for
any other load (Rl) can be calculated:
Eo=Eg*Rl/(Rl+600) where Eo and Eg
are in volts, not dB.
When the op-amp is connected
as a non-inverting, unity gain buffer,
the output impedance is reduced
by the open loop gain. With a gain
of 90 dB and 500 ohms output
impedance, the closed loop output is
.015 ohm and can be neglected. Just
add the desired resistance at the
op-amp output to match the filter
(see Figure 3). As long as the load is
matched, the meter will read correctly.
The OPA544 op-amp would be
a good choice for the buffer. It can
supply full output to 100 kHz, has a
max DC offset of 5 mV, and can
drive a one ohm load at the levels you
are using. A power supply of ± 12 volts
is needed, with output current of 30
mA minimum at a 150 ohm load.
The diodes and 10K resistor at the
op-amp input in Figure 3 are for static
protection. The input impedance is very
high and sensitive to static damage.
DIGITAL RADIO AND TV
QVery soon, the digital world
will affect television, and now
affects radio entertainment.
I have questions on converters for presently-owned RF devices.
1) Does this affect AM, FM, or TV only?
2) Can we build them?
3) Can we find a source for kits, or
4) What is the practicality of self-conversion, compared to junking
our stuff and buying the completed
5) What are the band spectrums
now, compared to the conversion?
6) Is there a block diagram that
simply explains the application to
radio or TV, or telephone?
— Bob Stoddard
A1) Only TV requires a
converter. The hybrid digital
system works with the
analog receivers. I expect
this system will remain in place for
20 years or so, until most analog
receivers have been replaced.
2) There is no law to prevent building
your own; you just have to have a
design and the expertise.
3) I don’t believe there are any kits
available. You can get two coupons
worth about $40 from the government toward converter boxes which
is about half the cost. Your local
electronics store will be able to tell
you how to make the application.
4) Converting an analog TV to digital
is not practical because the entire
design should be focused on the
digital parameters. The converter
box is a complete receiver, lacking
■ FIGURE 4
28 June 2008