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Vintage Test Gear and Workshop Equipment For discussions about vintage test gear and workshop equipment such as coil winders. |
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17th Sep 2018, 9:11 am | #1 |
Tetrode
Join Date: Sep 2018
Location: East Grinstead, West Sussex, UK.
Posts: 54
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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. |
19th Sep 2018, 9:58 pm | #2 |
Nonode
Join Date: Aug 2007
Location: Walsall Wood, Aldridge, Walsall, UK.
Posts: 2,874
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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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It's an enigma, that's what it is! This thing's not fixed because it doesn't want to be fixed! Last edited by Chris55000; 19th Sep 2018 at 10:14 pm. |