Rf signal source. Return Loss tests

Following up from my previous blog I was puzzled as to why the level had dropped from the initial tests of 16dBm to around 7dBm. On further inspection I discovered that the 220 Ohm resistor was still in place across the buffer amplifier broadband transformer!

The lesson learnt here being.....if there is an unexplained test then believe the test and investigate...as to why!

On removing the resistor the Return Loss was re-measured. Refer to lab notes for details. I was very pleased to see that the RL now was measured at 21dB. Definitely a good news story!.

Output power was measured as follows:

at f= 6651KHz = 15.36dBm.

I perceived that this was off the scale of the power meter so a 20dB attenuator was added. This derived a power output of 16.2dBm. Indeed, the above measurement was off the scale of the Power Meter.

Next inserted a 30dB pad and the output was derived at 16.1dBm. This output is 9dB higher than expected. Why? The output amp does run hot. Do calcs for the output amp to check the collector current.

A quick check of linearity. (Using a 30dB pad).

Fmin = -15.6dBm
Fmax = -13.95Bm
Change over the range = 1.65dB. This is good.

Next complete the inclusion of the bandspread caps and check frequency stability of the oscillator.

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?

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.

OBSERVATIONS
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.

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

 

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.