[time-nuts] DIY Frequency extension for HP Agilent 53181A, 53131A or 53181A

[J. L. Trantham]

Rick,

I have been 'reading the mail' on this discussion and have a somewhat off
topic question.  I acquired a 5334B that has a 'noisy' channel C.  It
originally did not read when fed a signal.  I then adjusted R328, the peak
detector, and then it correctly read the signal.  However, when I disconnect
the signal, it continues to 'gate' and read some numbers.

Any thoughts?

Also, is it possible to add a 'channel C' to a unit that does not have it.
I have noticed that the unit I have without a channel C has most (if not
all) of the parts populated on the board except for the 8 pin IC that plugs
into the socket, U302, which appears to be an MB506.  Is all that is
required is to plug in an MB506 and add a connector to the front panel, or
is there a firmware issue as well?  If a firmware issue, is it possible to
'rewrite' an EPROM or some such to deal with this?

Thanks,

Joe

-----Original Message-----
From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On
Behalf Of Rick Karlquist
Sent: Sunday, February 15, 2009 1:54 PM
To: mccorkle at ptialaska.net; Discussion of precise time and frequency
measurement
Subject: Re: [time-nuts] DIY Frequency extension for HP Agilent 53181A,
53131A or 53181A

Richard H McCorkle wrote:
> Hi Samuel,
>
> I suggest starting by downloading the component level information for
> the 53131A and 53132A from tha Agilent site at the following links.
>
> http://cp.literature.agilent.com/litweb/pdf/5989-6308EN.pdf
> http://cp.literature.agilent.com/litweb/pdf/5989-6307EN.pdf
>
> The 53132A document includes a schematic of the channel C board with
> parts list and may assist in getting you started.

I see that they have a factory select resistor that unbalances
the input slightly to keep the prescaler from oscillating with
no input.  This a simple, obvious "solution" to this problem.
It is also non-optimum, for three reasons.  It desensitizes the
prescaler, it uses a factory select part that may not work
consistently over temperature, and it still results in a range
of input power where the counter displays garbage.

In the 5334B C channel, I used a diode detector that turned on
the counter at a repeatable input power.  Thus you never got garbage
on the display due to low input power.  The detector used the
well known circuit where you bias the diode on with a few 100
microamps of DC and have a compensating diode that is not excited
by RF.  A difference amp subtracts the compensating diode voltage.
Simple, but effective.

It is also interesting to note that in 1987 I used the MB506 prescaler.
Here they have simply upgraded to the MB510.  Not likely to be
any better on noisy sources.

Rick Karlquist N6RK


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