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Old 12th Mar 2018, 8:14 pm   #1
jpdesroc
Triode
 
Join Date: Feb 2013
Location: Québec City, Québec, Canada
Posts: 42
Default Leader LSG-245 generator output level issues..

I have a Leader LSG-245 RF frequency generator
that has a weird keyboard response issue..
It's a micro-processor based test equipment.
On the front panel you can set the gen frequency,
modulation percentage and its output level value in dBu.
Well that's where my problem starts..
According to the specs this generator can deliver -10 to +120dBu
(in 1dBu steps) on a 50 ohms load using +/-10dB and +/-1db buttons
on the keyboard. that is 131 steps overall.
My problem is that the display shows a max of +114dBu
and a minimum of -16dBu.
You can see there is a -6dBu offset there on all the readings !!!!
I cannot see what is causing that -6dBu offset..

http://www.arcenson.com/public/Leade...ice_Manual.pdf
http://www.arcenson.com/public/Leade...ion_Manual.pdf

The keyboard buttons are 'scanned matrix' type
read from a micro-processor and seem to do what they should.
There are 131 steps read from -16 to +114dBu.
The 3 x '7 segments output levels digits' are displayed through CD4511
BCD to 7 segments drivers also driven from the micro.
The micro uses 27C64 and 27C128 eeproms program memories.
I'm beginning to suspect the actual program could be
altered and inserts a -6dBu offset on the output scale
keeping the keyboard from going any further than +114dBu.
This generator also accepts external bytes from
a parallel rear external connector. I'm about to try to
'inject' external dBu value bytes and see what the micro
accepts as min/max..

If anybody sees somethnig I did not see
feel free to reply .. I'd be very happy
to see your comments..

Thanks.
J-Pierre
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Old 13th Mar 2018, 10:52 am   #2
Ancient Geek
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Join Date: Aug 2017
Location: Bristol, UK.
Posts: 136
Default Re: Leader LSG-245 generator output level issues..

Does it make any difference what you have the modulation set to? Internal, external etc?
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Old 13th Mar 2018, 12:17 pm   #3
jpdesroc
Triode
 
Join Date: Feb 2013
Location: Québec City, Québec, Canada
Posts: 42
Default Re: Leader LSG-245 generator output level issues..

With or without any modulation, No differences..
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Old 13th Mar 2018, 1:53 pm   #4
PJL
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Join Date: Sep 2005
Location: Seaford, East Sussex, UK.
Posts: 3,690
Default Re: Leader LSG-245 generator output level issues..

My guess is the difference between source dB and output dB into a 50 ohm load, -6dB being the voltage division by 2.
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Old 13th Mar 2018, 2:53 pm   #5
jpdesroc
Triode
 
Join Date: Feb 2013
Location: Québec City, Québec, Canada
Posts: 42
Default Re: Leader LSG-245 generator output level issues..

Quote:
Originally Posted by PJL View Post
My guess is the difference between source dB and output dB into a 50 ohm load, -6dB being the voltage division by 2.
I think I got the answer here from a later post:

... The manual for the Leader signal generator states that the output of the generator is -10dBu to +120dBu emf.

The key to your query is the emf, which means that is the voltage available with no load. The source impedance of the generator is 50 ohms, so when loaded with 50 ohms, the potential difference (PD) across the load resistor will be one half the emf. This corresponds to being 6dB less than the emf. Thus the output range into a 50 ohm load will be -16dBu to +114dBu.

It's easy to get confused by this sort of thing, some signal generators are calibrated in emf, others in PD, so you need to take care when interpretting the levels....
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