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Hello ,
While the spectrumanalyzer pro-and contra's are actual on this group,
may be someone can help with this:
My 70904A RF module (HP70100 analyzer) works perfectly from 10 MHz to
2.9 GHz , however below 10 MHz amplitude flatness goes down rapidly ,
-20 dB at 1 MHz .
Manual says problem with 1st mixer ??
I have difficulty to understand that the mixer can be shot and at the
same time performing above 10 MHz without problem ?

Basic layout of the first stages are : 0-70 dB attenuator ,
switchable AC -DC , then limiter 10 mW dc-3GHz , then 1st mixer ...

I would guess a series resistor burned out , becoming a nice series
capacitor that way ??

Any suggestions for repair ? Thanks Erik Belgium

PS . Other people using HP 70000 mms equipment on this group ,I like
to share experience ?

At 22.03 09/01/2007, Erik wrote:

Hello ,
While the spectrumanalyzer pro-and contra's are actual on this group,
may be someone can help with this:
My 70904A RF module (HP70100 analyzer) works perfectly from 10 MHz to
2.9 GHz , however below 10 MHz amplitude flatness goes down rapidly ,
-20 dB at 1 MHz .
Manual says problem with 1st mixer ??
I have difficulty to understand that the mixer can be shot and at the
same time performing above 10 MHz without problem ?

Basic layout of the first stages are : 0-70 dB attenuator ,
switchable AC -DC , then limiter 10 mW dc-3GHz , then 1st mixer ...

I would guess a series resistor burned out , becoming a nice series
capacitor that way ??

Any suggestions for repair ? Thanks Erik Belgium

PS . Other people using HP 70000 mms equipment on this group ,I like
to share experience ?

Erik,

most probably an open capacitor, or a defective connector. Try skipping all the components before the mixer, and inject a -40dBm signal directly to the mixer; you will find if the problem is in the mixer or before it (most suspect: the AC/DC switch or the limiter).

73 - Marco IK1ODO

Hi,

I am not familiar with the exact part in question, but very often,
spectrum analyzer mixers are of the double balanced diode type (DBM).

When working correctly, a DBM provides at least 20dB of isolation between
the LO, RF, and IF ports. So, when you move the LO towards the low end
of the band, like you are doing at 10MHz, the LO, and IF are approaching
each other in frequency, until you hit zero (DC) Hz, where they are exactly
the same frequency. If the DBM is working correctly, it will provide you
with more than a 20DB isolation between the IF, and LO ports, which will
reduce the amount of LO that goes right into the IF port by at least 20dB.

However, if the DBM has a burned out diode, it will no longer give you
much, if any, isolation between the IF and LO ports, and you will see
a drastically larger amount of LO signal being stuffed into the IF port.
This cannot help but wreck the amplitude flatness at the low end of the
band. At the high end of the band, this lack of isolation goes all but
unnoticed because the IF filters are sufficiently steep to keep the LO
signal out of the IF port.

-Chuck Harris

ericapple2004 wrote:

On Tuesday 09 January 2007 21:30, Chuck Harris wrote:
Hello ,

Thank you for the quick and detailed answer , sounds like that is my problem ,
I did inject -30dB signal directly in the input port of the mixer , problem
is te same, so mixer is blown . I suppose home-repair of a 1st mixer HP
style is not feasable ?

rgds ,
Erik

Hi,

I am not familiar with the exact part in question, but very often,
spectrum analyzer mixers are of the double balanced diode type (DBM).

When working correctly, a DBM provides at least 20dB of isolation between
the LO, RF, and IF ports. So, when you move the LO towards the low end
of the band, like you are doing at 10MHz, the LO, and IF are approaching
each other in frequency, until you hit zero (DC) Hz, where they are exactly
the same frequency. If the DBM is working correctly, it will provide you
with more than a 20DB isolation between the IF, and LO ports, which will
reduce the amount of LO that goes right into the IF port by at least 20dB.

However, if the DBM has a burned out diode, it will no longer give you
much, if any, isolation between the IF and LO ports, and you will see
a drastically larger amount of LO signal being stuffed into the IF port.
This cannot help but wreck the amplitude flatness at the low end of the
band. At the high end of the band, this lack of isolation goes all but
unnoticed because the IF filters are sufficiently steep to keep the LO
signal out of the IF port.

-Chuck Harris

ericapple2004 wrote:

Hello ,
While the spectrumanalyzer pro-and contra's are actual on this group,
may be someone can help with this:
My 70904A RF module (HP70100 analyzer) works perfectly from 10 MHz to
2.9 GHz , however below 10 MHz amplitude flatness goes down rapidly ,
-20 dB at 1 MHz .
Manual says problem with 1st mixer ??
I have difficulty to understand that the mixer can be shot and at the
same time performing above 10 MHz without problem ?

Basic layout of the first stages are : 0-70 dB attenuator ,
switchable AC -DC , then limiter 10 mW dc-3GHz , then 1st mixer ...

I would guess a series resistor burned out , becoming a nice series
capacitor that way ??

Any suggestions for repair ? Thanks Erik Belgium

PS . Other people using HP 70000 mms equipment on this group ,I like
to share experience ?

Hi erik,

Whether you can repair HP mixers at home probably has to do
with what you have at home to work with ;-)

I would have to see the mixer in question. It probably requires
a microscope, and a wire bonding machine... but maybe not.

I have friends that have put new transistors in HP hybrids from
that time frame with a microscope, some silver epoxy, and a steady
hand.

-Chuck Harris

erik wrote: