10 April 2014

RF Signal Source. Amplifier stage 1

I now build the amplifier stage for my homebrew RF signal source as per EMRFD Fig 7.27. The schematic is shown in the lab notes below. This stage uses a 2N3866 transistor as the amplifier with a standard 4:1 broadband transformer in the collector. A 3dB pad is inserted in the output to improve Return Loss. Some signal is fed off to a separate port for connection to a frequency counter. The build came together easily after about 45 minutes construction time UGLY style! No issues with getting the stage to work.

Initial quick tests showed an output of 12.3dBm. However subsequent more careful tests showed the output as follows which are more in line with expectations.

Fmax at 1381KHz output = 7.4 dBm
Fmin at 2783KHz output = 5.67 dBm

Why is the output higher for the higher frequencies than the lower frequencies? Investigate further.. Is 2 dB difference over the frequency range in line with expectations?

The oscillator tank circuit was shorted out to test the total system RL which = 18dB. This is a satisfactory result although not stellar.

The RL was then measured with the tank circuit running at an offset frequency (3243KHz) as opposed to the test frequency of 7022KHz. This did not work indicating a negative RL.

The amplifier stage was then disconnected from the buffer and terminated in 50 Ohms across the base 330 Ohm resistor. This did not work as the RL was low.

The 50 Ohm termination was then moved to the input of the 3dB pad. This worked and resulted in a RL of 18dB.

Referring to the previous blog on buffer stage testing, this result was revealing. The buffer RL tests showed a poor 5dB RL. However when the buffer stage is connected to the amplifier the RL output is an acceptable 18dB. This RL figure is exactly the same as that obtained when the amplifier stage was tested in isolation with it's input terminated in 50 Ohms. Why is this? Perhaps there is an error in the procedure used to measure the RL of the buffer stage in isolation?

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