by Walter H. Schwartz
----- The Sencore SC 60MHz Oscilloscope.
What do you do with a 60MHz scope? Obviously you can see frequencies well into the VHF range--so what? Well, first with the increasing clock frequencies and faster pulses in microprocessor and other digital circuitry you want to be able to see rather fast square waves and other fast rise time pulses. To properly view a high frequency square wave an oscilloscope must have a bandwidth several times the fundamental frequency of the waveforms displayed--to five or seven times it for good waveform accuracy.
With clock frequencies going to 50MHz or higher a wideband-scope is becoming quite necessary. Many of the pulses you will find in digital circuitry are equivalent to fundamental frequencies in the 12-15MHz region requiring a scope with response to 75 or 90MHz. Sencore specifies the SC 60 vertical amplifiers to be 3dB down at 60MHz. 6dB down at 80MHz and 12dB down at 100MHz, with a rise time of 6 nanoseconds. Tests with a calibrated output signal generator confirmed these specifications.
What does this mean in TV servicing? In most cases not a great deal--but it will allow you to observe signals, assuming sufficient amplitude, in the 44MHz video IF. It is very nice to be able to look at both the input and output of the video detector--and you might not realize it but the signal levels in most of the IF's are of adequate amplitude for observation.
The alternative is, of course, a demodulator probe, which because of cable and scope input capacitance, really shows nothing but sync pulses--all video is lost. The 60MHz response will also allow excellent, and easy, modulation checking of CB and ham, AM and SSB transmitters thru 54 MHz.
The SC 60 offers most of the features you now probably expect in a general purpose oscilloscope. Besides its 60MHz frequency response it is dual trace (and a vector scope with well matched X and Y channels), has sweep speeds commensurate with its frequency response -and has a X10 expansion of the sweep. The maximum vertical sensitivity is 5mv per division. The input attenuators are calibrated beginning at 0.05v, 50mv, to be direct reading with the X10 probes in use. Trigger can be from either channel A or channel B or from the line or an external source. The SC 60 also has a video preset position on the timebase control and when this is used either vertical or horizontal can be selected by pushbutton switches.
In spite of Sencore's statements about the SC 60's trigger circuits. I found it no easier or harder to trigger than most other good quality scopes. When viewing waveforms in video IF amplifiers I found it most satisfactory to trigger externally from a horizontal sweep source. (External trigger can simplify triggering in many cases.) A feature that finds particular favor with me is the probes, rated at 2kv. These have more than 3 times the safety factor and input capability of most other scope probes. It is quite useful to be able to look at a horizontal output transistor collector waveform, which may be nearly 1kv p-p.
Are there any disadvantages to such a wideband scope? Not many - a wideband vertical amplifier may generate a little more internal noise resulting in a trace that doesn't appear to be quite as sharp as it could be. It does not seem to really make much difference in operation. You may also pick up your local FM station or some other source of RF and if you do not realize what you have it can be confusing--a minimum length ground on the probe helps.
The panel layout of the SC 60 is very convenient--wide spaced knobs--full sized push buttons, etc. -- and there is no problem with clumsy fingers. Sencor provides good manuals with all its instruments; the SC 60 manual is typical. The SC 60 comes complete with two probes in a very nice storage compartment for $1895.
(source: Electronic Technician/Dealer)