High Dynamic Range RF Amplifier for Noise Measurement

This simple amplifier is handy in situations where a low-level signal in the 5 - 1500 MHz range needs to be amplified to levels around +15 to +17 dBm without adding a lot of noise. Such an application might arise when using a double-balanced mixer in a quadrature PLL to measure the phase noise of a high performance crystal oscillator. Because these oscillators often include little or no internal buffering, they're vulnerable to injection locking unless the test setup includes external amplifiers and/or attenuators to provide the necessary reverse isolation.

This was the purpose of the K22 option for the HP 3048A phase noise measurement system. The K22 amplifiers are relatively uncommon in the surplus market, but they're easy enough to homebrew. This version offers comparable performance, with slightly lower additive noise and the ability to run from a +12 to +15 volt supply. It can be built in an afternoon for around $50 in parts....

... or at least, it could before NXP decided to EOL all of their good RF bipolars. For the time being, the remaining BFG591s on the market can be found here. Check eBay as well. (Update: as of March 2019, the 2014-vintage BFU590G is still in production. It should be an excellent substitute, at least going by the data sheet.)

Click on photos below to enlarge

The plot below shows the residual phase noise at VHF of this version compared to the original K22 amplifier under identical conditions. Spurs in both cases are artifacts of the test setup.

S21 and S12 are reasonably flat to 1.5 GHz:

Both are comparable to the original HP K22 amplifier:

S11 is significantly better in this version, at least below 1 GHz:

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