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Radio Wrangler · 27th Mar 2021, 7:03 pm

The 51R resistor will keep the mixer happy, broadband, just so long as the following transistor Q19 doesn't load it much.

This means that the majority of the power from the mixer winds up in the resistor, and that the power gain demanded of the two transistors is rather large. This is not a good move in terms of local noise figure, but OK so long as the gain in earlier stages makes it relatively unimportant.

Using a high-Z probe may show that there isn't much voltage gain going on in Q19, Q20, but the input impedance arrangement may mean that there is plenty of power gain just to keep the voltage similar while the impedance level changes significantly.

David

27th Mar 2021, 7:03 pm	#156
Radio Wrangler Moderator Join Date: Mar 2012 Location: Fife, Scotland, UK. Posts: 22,901	Re: Spectrum Analyser TR4172 The 51R resistor will keep the mixer happy, broadband, just so long as the following transistor Q19 doesn't load it much. This means that the majority of the power from the mixer winds up in the resistor, and that the power gain demanded of the two transistors is rather large. This is not a good move in terms of local noise figure, but OK so long as the gain in earlier stages makes it relatively unimportant. Using a high-Z probe may show that there isn't much voltage gain going on in Q19, Q20, but the input impedance arrangement may mean that there is plenty of power gain just to keep the voltage similar while the impedance level changes significantly. David __________________ Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done