10 April 2014

RF Signal source. Buffer stage measurements 2

Following on from my last blog I next worked to try to measure the Return Loss, RL, of the buffer stage. This was not so easy. Some notes as follows.

Refer to the below schematic of the test configuration using the MFJ259B as the signal source into the Return Loss Bridge with a 6dB PAD. The objective was to measure the output Return Loss of the buffer amplifier.

With the oscillator connected to the buffer stage and the oscillator tank circuit short circuited (to disable the oscillator), the Return Loss (RL) was measured at 5.48dB which is very low. The target being to achieve better than at least 18dB. This being an SWR of 1.2:1.

Next the input to the buffer was terminated in 50 Ohms with the oscillator disconnected. The RL measured remained unchanged. Turning the stage power on/off had no effect. Shorting the input to the buffer had no effect.

The 3dB Pad was disconnected from the broadband transformer and terminated in 50 Ohms. This gave a better RL of 26dB. Indicating that the 3dB pad is working.

The 4:1 broadband transformer was reconnected to the Pad but disconnected from the JFET collector. RL= 5dB. The transformer was then terminated in a 200 Ohm resistor. RL = 26dB, Good! This showed that the broadband transformer was working at the test frequency of 7020KHz. The RL remained at 26dB when the transformer was connected back in the collector circuit with the 200 Ohm resistor in place. RL remained unaffected by the stage being turned on. The output however dropped from -5.7dBm with no 200 ohm resistor to -11.45dBm with the resistor in place.

The output RL was completely unaffected by any change to the terminated input impedance. Perhaps this comes as no surprise because the input impedance into the emitter is very low at a calculated value of 2.3 Ohms. Thus to get an input match would be very difficult.

In order to get an acceptable stage output RL the broadband 4:1 transformer must be terminated in a 200 Ohm resistor on the primary side, however at the expense of a loss of output signal level. This since half the power developed in the collector is being dissipated in the 200 Ohm resistor.

At the end of the day perhaps it does not matter if the RL is high from the buffer since the objective is to ensure high Reverse Isolation and not amplification. But what about harmonic distortion introduced in the stage? Is that of concern? The stage appears to be working correctly in spite of the high return loss.

In the lab notes below the RL is shown as dBm. This is incorrect. RL is expressed in dB.


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