On Mon, Sep 2, 2024 at 09:53 PM, Jeff Kruth wrote:
Some of the responses did not answer the question you asked. To answer the question below: modern signal generators, like the 8340, have a leveling loop to maintain a preset level. If you investigate to practical result of using this, it basically gives an infinitely low generator VSWR (very high return loss, good match) as no external load can cause significant amplitude ripple due to mismatch, as the loop automatically corrects this. I learned this from an HP APP note many years ago..
In my experience, and this is across many decades of using all kinds of lab sig gens, the signal generators that use a classic levelling loop do not give an infinitely low generator VSWR up at high RF frequencies. This is because a compromise has to be made between the source defining resistor at the output of the levelling loop and the added ESR of all the attenuator switches and other connections/circuitry after the levelling loop. The source VSWR is often quite good at low frequencies but the ESR (and general mismatch) of everything after the levelling loop creeps up with increasing frequency and this can spoil the source match at the output port of the sig gen.
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Usually, the source impedance will be have a VSWR spec of about 1.5:1 at higher generator output levels because of this. For sig gens that operate up to about 20GHz, the source VSWR may climb as high as 2:1 in some cases.
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Once the attenuator sections get used for lower output levels, the source match should improve in most cases.
