After having fun with the VFO4351A 140MHz~4.4GHz RF signal generator on my Rigol and Hantek oscilloscopes, I decided to do the same for the three analog scopes I use the most, namely the Hitachi VC-6025, GW-INSTEK GOS-6103 and Kenwood CS-5275.
The first thing to note is that, for whatever reasons, the VC-6025 will power on in 10x probe mode. To set the probe mode back to 1x, push the SELECTOR button down and adjust the VARIABLES knob.
As a start, I fed a 100MHz (Vpp=1V) into the VC-6025, rated for 50MHz. The scope was able to trigger on this signal, with Vpp measured at 337mV:
Despite the heavy signal attenuation, manual frequency measurement using cursor features, with the help of the x10 MAG button which amplifies the horizontal scale by 10 times, is fairly accurate, showing the expected value of 100MHz:
Next, I fed a 200MHz (Vpp=1V) signal into the VC-6025, and once again, the frequency measurement was accurate:
This is surprising, as the peak to peak voltage is now only a tiny 2.20mV, an amount that could easily be disregarded as noises in a modern digital storage oscilloscope:
Although I have to say that it takes a bit of efforts (using the x10 MAG button, adjusting the trigger level carefully, tweaking the vertical division knob and also adjusting the trace intensity) to get the oscilloscope to trigger on such a high frequency signal, my results only mean that the VC-6025 is well-built and can be used for frequency well above 50MHz so long as you are aware of the limitations (e.g. amplitude attenuation).
I have no such luck with the CS-5275 100MHz analog oscilloscope, an “overlapping” sine wave is the best I can get despite tweaking various settings:
With the GW-INSTEK GOS-6103 100MHz analog oscilloscope, I was able to get a stable sine wave, however the readout text is overlapping, which seems to only happen with frequency above 160MHz. I suspect the overlapping text was caused by high frequency components leaking into the readout circuitry, which did not have the necessary filters, since the oscilloscope was designed for 100MHz.
Reducing the frequency to 150MHz and the Kenwood has no issues showing a stable sine wave in x10 mode (there are no readout displays on the Kenwood):
The Instek also has no issues with 150MHz:
Vpp is now measured to be 0.5V on the Instek, half the expected value of 1V:
Of course the Hitachi has no issues with 150MHz too:
Vpp is shown as 56mV on the Hitachi, still heavily attenuated:
In case you are wondering, my VC-6025 was not able to trigger on frequencies higher than approximately 210MHz. Everything has its limits, I guess.
My conclusion from the tests is that analog oscilloscopes can still be useful well above their rated bandwidth, provided you know what you are doing. Also, it seems that the Hitachi VC-6025, despite its lower bandwidth of 50MHz, is more well-built and can handle high frequency much better then the other two scopes, which are rated for 100MHz. I am not sure if this is simply due to marginal components (these scopes are at least 30 years old) or if the two 100MHz would also perform well at 200MHz after a proper calibration. A similar model, Hitachi VC-6045, is rated for 100MHz so it is certainly possible that the 6025 also shares several circuitry designed to support the higher bandwidth, thus explaining its better performance.
The Hitachi has a simple “auto-set” feature – pressing the AUTO button will cause the scope to configure what it thinks is the correct settings for the horizontal timebase. It has primitive storage features (1K points) which can be activated by pressing the STORAGE button near the right corner of the CRT. In storage mode, the scope can stably display a 1Vpp sine wave up to around 85-90MHz. Various digital settings (interpolation, equivalent time sampling, etc.) can be configured by pressing the MENU button, and then by using the VARIABLE knob. The HOLD button will freeze the waveform at the instance. Press SAVE (after pressing HOLD) will save the waveform into memory, which can be retrieved later by the RECALL button.
This is a 85MHz signal on the VC-6025 in digital mode:
Vpp is 468mV (expected around 1V):
Most analog oscilloscopes have a delayed time base setting, which allows the user to zoom into part of the primary signal in order to locate things like distortions or glitches. Triggering for the delayed timebase can be set to start a short time after a main trigger, and is usually marked as B on the front panel. To display both the full signal and the “zoomed-in” section, display mode should be set to ALT. On the Kenwood CS-5275, the zoomed-in part is highlighted (with the proper trace intensity settings):
Use the TRACE SEP knob to adjust the separation between the two traces. If you can’t see the highlighted section, make sure that trace intensity is not set to too low or too high. To configure the length of the highlighted signal, use the knob to set the horizontal division for timebase B, e.g. delayed timebase.
On the VC-6025, the trace is not highlighted. Instead, a vertical cursor appears to indicate the zoomed-in part. Normal cursor behaviour (to measure voltage, time or frequency) can’t be used in delayed timebase mode:
The same behavior can be seen with the GOS-6103:
The GOS-6103 also has SAVE/RECALL buttons on the front panel, but those buttons are for saving/retrieving the horizontal/vertical/trigger/etc. settings only and do not save waveform data as the 6103 is a pure analog oscilloscope.
In short, I love my analog oscilloscopes. These devices were built to last and can still be very useful for modern day engineering work. The x10 MAG button makes them even more useful when looking at high frequency signals, since the lowest 5ns/division horizontal scale setting is not adequate. With an analog oscilloscope, what you see is what you get. The same can’t be said for a digital oscilloscope, which can trick your eyes (and your mind) at times, due to bandwidth limitations, aliasing, software bugs, and whatnot.