newbie question HP8657A

[jim]

Hi,
I'm pretty new to electronics,and have just purchased a HP8657A
signal generator. The trouble is,the voltage readings don't seem to
make any sense. When I connect the generator directly into my scope
,through a 1K load,the voltage readings are way off. For example,
generator reads 300mV the scope reads 1.52V (pk/pk),gen reads 600mV
scope reads 3.5V (pk/pk). So (not having a manual ) my first question
is, are the readings of amplitude on these old generators assumed to
be pk to pk or just straight amplitude from baseline., or RMS? The
second question is why the discrepancy in readings. The scope is
reading an increase of approx. 500mV (pk/pk) for every 100mV increase
in generator output.. I'm using a 10X probe but the tek scope
compensates for the readings,I checked that with my power supply.
Any help would be appreciated. Thanks jim

 

[John Larkin]

On Sat, 27 Sep 2003 03:21:21 GMT, jim <road12fg@netscape.net> wrote:

Hi,
I'm pretty new to electronics,and have just purchased a HP8657A
signal generator. The trouble is,the voltage readings don't seem to
make any sense. When I connect the generator directly into my scope
,through a 1K load,the voltage readings are way off. For example,
generator reads 300mV the scope reads 1.52V (pk/pk),gen reads 600mV
scope reads 3.5V (pk/pk). So (not having a manual ) my first question
is, are the readings of amplitude on these old generators assumed to
be pk to pk or just straight amplitude from baseline., or RMS? The
second question is why the discrepancy in readings. The scope is
reading an increase of approx. 500mV (pk/pk) for every 100mV increase
in generator output.. I'm using a 10X probe but the tek scope
compensates for the readings,I checked that with my power supply.
Any help would be appreciated. Thanks jim

The generator indicates RMS, and assumes a 50 ohm load. Without the
load (ie, into a high-Z scope) the voltage will double. Factoring in
the RMS-to-p/p ratio (2.828) then another 2:1 for the load thing, 300
mV RMS becomes 1.70 p/p, close to what you're seeing.

John

 

[jim]

On Fri, 26 Sep 2003 22:01:53 -0700, John Larkin
<jjlarkin@highlandSNIPtechTHISnologyPLEASE.com> wrote:

On Sat, 27 Sep 2003 03:21:21 GMT, jim <road12fg@netscape.net> wrote:

Hi,
I'm pretty new to electronics,and have just purchased a HP8657A
signal generator. The trouble is,the voltage readings don't seem to
make any sense. When I connect the generator directly into my scope
,through a 1K load,the voltage readings are way off. For example,
generator reads 300mV the scope reads 1.52V (pk/pk),gen reads 600mV
scope reads 3.5V (pk/pk). So (not having a manual ) my first question
is, are the readings of amplitude on these old generators assumed to
be pk to pk or just straight amplitude from baseline., or RMS? The
second question is why the discrepancy in readings. The scope is
reading an increase of approx. 500mV (pk/pk) for every 100mV increase
in generator output.. I'm using a 10X probe but the tek scope
compensates for the readings,I checked that with my power supply.
Any help would be appreciated. Thanks jim

The generator indicates RMS, and assumes a 50 ohm load. Without the
load (ie, into a high-Z scope) the voltage will double. Factoring in
the RMS-to-p/p ratio (2.828) then another 2:1 for the load thing, 300
mV RMS becomes 1.70 p/p, close to what you're seeing.

John
Yep, I put a 47 ohm resistor in and switched to rms readings and sure

enough it tracked,about 15mv off due probably to the resistor. As this
generator seems not to be an ideal source,next step is to calc.
Thevinin,I guess. thanks again jim