On Wed, Feb 19, 2020 at 7:48 PM Dan G <dgajanovic@...> wrote:
The Prescale Input contains a high-pass filter (R2368/R2466/L2362/C2362)
designed to engage Input-Out-Of-Range at low frequencies that could cause
false triggering of the U2350 prescaler IC. The exact frequency response of
the input depends on this filter, R2422 Threshold Adjust and the 4-stage
UHF amplifier. I suspect that the combined tolerances provide a safety zone
around the 100 MHz - 1.3 GHz range large enough to ensure that an
instrument will meet spec, but 50 MHz is far enough from spec that it might
depend on the particular unit and calibration.
On one of my DC508 units (post-B020000 s/n, whose prescaler board
is fairly similar to the DC508A), the lower cut-off frequency for a
10 mV RMS (28.3 mV p-p) sine wave is 51.3 MHz. At 20 mV RMS,
the cut-off frequency decreases to 37.9 MHz.
On another early, recently restored and calibrated DC508 unit
(pre-B020000 s/n, slightly different prescaler board design), the lower
cut-off frequency for a 20 mV RMS sine wave is 54.6 MHz. It can count
a 1.75V RMS signal down to 39.3 MHz.
These data points are for DC508, but I would expect the DC508A to
behave in a similar fashion. In short, while the Prescale Input of your
unit might be able to count a 50 MHz signal of sufficiently high amplitude,
I would not consider its inability to do so an indication of malfunction.
Just to be clear, all this applies to the Prescaler (Left) Input only.
The Direct (Right) Input has no such high-pass filter, of course,
and can count very low frequencies.
Gary Robert Bosworth@grbosworth