09 April 2014

RF signal source. Buffer stage measurements

Making measurements on the RF signal source buffer stage would be a good learning opportunity. The RF signal source as per EMRFD Fig 7.27 deploys a Common Base Amplifier using a 2N3904 transistor and a 4:1 broadband transformer in the collector. Refer to the schematic below. The output is then terminated in a 3dB pad before connecting to the power amplifier stage. The function of the stage is to create high reverse isolation so that the oscillator frequency will not 'pull' based on changes to the output load.

I analyzed the circuit using equations presented in Chapter 2 of EMRFD. Essentially the stage has unity current gain. The input to the stage is derived as a current source since the emitter input impedance is very low. This being the reason for the high reverse isolation characteristics. Thus it is a challenge to drive power into the stage. Using a theoretical Voltage gain of 200, I was unable to measure that much voltage gain. However using a unity current gain model, the measured and calculated output power correlated.

The open circuit oscillator output voltage was measured using a 10X scope probe. The stage Vin was then measured with the buffer connected. Using a simple small signal model to derive the stage input impedance (222 Ohms), the oscillator source impedance of 25.9 Ohms  was derived. Should this in fact be 50 Ohms (to be investigated). I assume that this is not critical since the oscillator stage is lightly coupled to the buffer stage through a 220 Ohm series resistor.

Some results are presented as follows:

Vcc = 12.17V
Ftest = 2825.09KHz
Ie (emitter quiescent current) calculated = 11mA (I used the long equation 2.11 EMRFD for this calc)
Ve (emitter quiescent voltage) calculated = 2.97V, measured = 2.8V
Ve (base quiescent voltage) calculated = 3.57V, measured = 3.45V
gm (transconductance) calculated = 0.4231, Rin (emitter) = 1/gm = 2.36 Ohms
Rl (collector load resistance) calculated = 200 Ohms.
Vosc open circuit = 960mVp-p
Vin = 860mVp-p
Rin (stage) calculated = 222.4 Ohms.
Rsource oscillator calculated = 25.9 Ohms.
Ib (base current) = 3.86mA
Power output in collector calculated based on voltage gain = 2mW (+3dBm)
Power output in collector calculated using unity current gain = 0.38mW (-4dBm)
Power output in collector measured (v*v/8R) = 0.38mW (-4dBm)
Power output after 3dB pad = -7dBm

All 'in situ' measurements were made using a 10X scope probe. Output power measurments at 50 Ohms were made using the RF power meter or by using the scope connected through a 50 Ohm thru terminator.

Next the stage input and output Return Loss, Power Gain and Reverse Isolation will be measured.

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