Nuts and Volts - June 2008

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 QUESTIONS

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 completed converters?

4) What is the practicality of self-conversion, compared to junking our stuff and buying the completed commercial products?

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