How to use "Link Filters"

Which is the correct format for “link filters” option in doctypes?
I have a doctype: Units_doctype(unit: data, unit_type:data) and another doctype: Product_doctype(name:data, value: float, unit:link) linked to Units_doctype. I want to filter the linked table by unit_type: [unit_type like ‘Voltage’] in order to see only voltage units from product_doctype.
How can I set the “Link Filters” option in the product_doctype? Do i need to do some scripting?
I tried [“unit_type”, “like”, “Voltage”] in the Link Filters field and get:

I found the option:
sol
And :
[[“Units”,“unit_type”,“like”,“%Voltage%”]]

But still not working, it applies the filter to:
name like Voltage
with no results
instead of:
unit_type like Voltage