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DC508A Frequency Range


Gary Robert Bosworth
 

The DC508A is a 1.3GHz frequency counter. The left-hand side is for high frequencies at 50 Ohms. There is an LED labeled "Input Out Of Range". It lights if the input is not of high enough frequency or of proper voltage. The question is - What is the low end frequency of this input? I believe it requires a minimum of 100MHz at greater than 100mV p-p. Does anyone know for sure if this input will work down below 100MHz and how far down will it work???


Bruce Atwood
 


Gary Robert Bosworth
 

Thank you Bruce. I already had the Operator's and Service Manuals. The
specifications state that the 508A will work from 100MHz to 1.3GHz but does
not state exactly how low it will work. It appears that there is a
high-pass filter that passes frequencies above 100MHz, but I was hoping
someone knows if the counter will readout at 50MHz without the error LED
coming on. It is not made crystal clear. I have been coping with
specsmanship since 1964.

Gary

On Sat, Feb 15, 2020 at 2:47 PM Bruce Atwood <atwood.1@...> wrote:

See http://w140.com/smb/DC508A_SM.pdf page 1-1.



--
Gary Robert Bosworth
@grbosworth
Tel: 310-317-2247


 

Hi Gary,
The DC508A and every frequency counter works down to almost DC limited by the duration of the period between measurements. A 1sec measurement period can measure to 1Hz. A 10 sec period can measure down to 0.1Hz. And so on. There is no lower limit. Without a long enough period you won't get any resolution unless you have a reciprocating counter.
For HF you will almost always use 50 ohm impedance. For low frequency work 1Mohm is more typical.

The manual clearly states the resolution enhancement circuitry only works between a certain audio frequency range. I don't recall the range off the top of my head. The answer is DEFINITELY in the service manual.

Dennis Tillman W7pF

-----Original Message-----
From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of Gary Robert Bosworth
Sent: Saturday, February 15, 2020 6:02 PM
To: TekScopes@groups io <TekScopes@groups.io>
Subject: Re: [TekScopes] DC508A Frequency Range

Thank you Bruce. I already had the Operator's and Service Manuals. The specifications state that the 508A will work from 100MHz to 1.3GHz but does not state exactly how low it will work. It appears that there is a high-pass filter that passes frequencies above 100MHz, but I was hoping someone knows if the counter will readout at 50MHz without the error LED coming on. It is not made crystal clear. I have been coping with specsmanship since 1964.

Gary

On Sat, Feb 15, 2020 at 2:47 PM Bruce Atwood <atwood.1@...> wrote:

See http://w140.com/smb/DC508A_SM.pdf page 1-1.



--
Gary Robert Bosworth
@grbosworth
Tel: 310-317-2247





--
Dennis Tillman W7PF
TekScopes Moderator


 

Hmm...would like to know as well.
I have a DC505a counter, but went with an HP5386A for bigger range afterward. Most of my older HF gear to 3Ghz is combined Tektronix & HP/Agilent. RiGOL for the newer 'daily use' stuff, a bit more user friendly and portable.


Dan G
 

Hi Gary,

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.

dan


Gary Robert Bosworth
 

On Wed, Feb 19, 2020 at 7:48 PM Dan G <dgajanovic@...> wrote:

Hi Gary,

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.

dan



--
Gary Robert Bosworth
@grbosworth
Tel: 310-317-2247


Gary Robert Bosworth
 

Thanx Dan. Your response is exactly what I wanted to see. I very much
appreciate your time in answering my question.

Gary

On Thu, Feb 20, 2020 at 5:17 AM Gary Robert Bosworth <@grbosworth>
wrote:



On Wed, Feb 19, 2020 at 7:48 PM Dan G <dgajanovic@...> wrote:

Hi Gary,

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.

dan



--
Gary Robert Bosworth
@grbosworth
Tel: 310-317-2247

--
Gary Robert Bosworth
@grbosworth
Tel: 310-317-2247