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Old 17th Sep 2018, 9:11 am   #1
DoubleWound
Tetrode
 
Join Date: Sep 2018
Location: East Grinstead, West Sussex, UK.
Posts: 54
Default Interfacing Function Generator to Control Grids

I have recently acquired a Siglent SDG1020 Function Generator which has a minimum output of 4mVpp and variable load impedance settings of up to 10,000kOhm (plus "Hi Z").

Service sheets normally just say connect to control grid via a 0.1uF capacitor.

My questions to knowledgeable members are:

1. What is the best way to attenuate output voltage?
2. What is the best load impedance setting?

A resistor in series with the capacitor and Hi Z seems to work but I'm wondering if there is a more appropriate method.

My questions apply to both RF and audio control grids.

I have searched forums for a definitive answer and I apologise if this has been covered before.
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Old 19th Sep 2018, 9:58 pm   #2
Chris55000
Nonode
 
Join Date: Aug 2007
Location: Walsall Wood, Aldridge, Walsall, UK.
Posts: 2,853
Default Re: Interfacing Function Generator to Control Grids

Hi!

A simple resistive potential divider is all that's needed up to the maximum o/p frequency of most F.G.'s – for this 20 MHz generator, if you use a 9k1 upper resistor and 910R lower resistor, you will get typically 0–400uV p–p output, whilst a 91k and 910R divider would give you typically 0–40uV p–p output – a lot less than many inexpensive meters, both standard DMM's and purpose–made audio electronic meters can read reliably – and you'd not be able to see a 0–40uV attenuated F.G.'s output with the average oscilloscope's amplifier with a 10mV/div max. gain setting – the "grass" present in the scope's amplifier would mask it!

(There are specialist oscilloscope Differential Amplifiers that will read down to 20/50uV/div but their bandwidth is extremely limited and they're notoriously difficult to use!)

The 0u1 (1000p in R.F./I.F. stages) mentioned in Service Sheets is simply a d.c. blocker to prevent any d.c. in the signal generator's output upsetting transistor or valve bias, and MUST be included with function or DDS synthesised generators – it is purely a safety component and serves no direct attenuation function.

Chris Williams
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Last edited by Chris55000; 19th Sep 2018 at 10:14 pm.
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