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.

 

Return Loss Bridge & Directivity Measurements

I built this Return Loss Bridge as a part of my continued quest to support an RF workbench. This is one of the instruments recommended for a basic 50 Ohm RF experimenter's workbench by VE7BPO in his RF Workshop series. Refer to the QRP Homebuilder website.

The following results were measured and are also shown below:

3060KHz   31.9dB
7020KHz   35.4 dB
14060KHz 33.0dB
21060KHz  33.0dB

I was unable to obtain measurements at 28MHz due to limitations at this frequency in my Scope. The Scope was used as the detector.

1	Heading	Return Loss Bridge Directivity Measurements
2	Label	Return Loss Bridge
3	Date	12/07/2013
4	Acknowledgements	1.      Author : ZS6RSH. 2.      Reference: EMRFD Section 7
5	Revision	Rev 1.
6	Revision History	None
7	Scope	Measure the directivity of my homebrew Return Loss Bridge for all HF Ham Bands
8	History	Inspired by VE7BPO’s QRP Home Builder website
9	Configuration	Refer to attached schematic
10	Test equipment specifications	1.      K2 Elecraft Transmitter. 2.      Commercial attenuator pad 20dB 3.      GOS 20MHz Scope 4.      3X4ft coax test leads 5.      50ohm commercial terminator
11	DUT specifications	1.    Home brew Return Loss Bridge (RLB)
12	Workbench process	1.      For each hamband. Setup the K2 to generate minimum power which will be close to 100mW. This is the input signal generator. 2.      Connect 30dB attenuation at the K2 Output. 3.      Connect up the RLB according to the schematic. 4.      With the RLB measurement port Open Circuit (at the end of the coax), measure the voltage on the scope. 5.      Terminate the RLB using a 50ohm terminator. 6.      Measure the voltage on the scope 7.      Do the above procedure for all hambands.
13	Expected Results	The directivity should be at least 30db on all bands. The higher the better.
14	Uncertainties	1.      Accuracy of the scope voltage readings. (scope needs calibration). 2.      Harmonics from the RF generator. 7pole filter ‘assumed’ good. 3.      The 50ohm scope terminator. Commercial 4.      The 50ohm bridge terminator. Commercial 5.      Loss on the coax cables. Same cables used in the same positions for all tests. 6.      Performance of the RLB at different power levels may vary. Measure this in future. 7.      Non 50 ohm RF generator. Assumed negligible with 20dB attenuator in place. CONCLUSION: 1 and 6 could be significant and need quantifying. Assume all the other uncertainties are negligible.
15	Preparation	Done
16	Perform validation measurements	Test the output voltage from the K2 is as expected. Ie around 6.3V into 50ohms.  Test the output voltage is as expected from the 20dB pad. Ie around 200mv into a 50ohm load. Test the output voltage from the unterminated RLB is as expected. Should be ‘higher’ since the unterminated RLB is 100ohms with no scope connected. However once the scope is connected the voltage will drop ‘somewhat’.  Adjust the pad accordingly so that the unterminated RLB WITH scope detector attached is around 250mV to 300mV.
17	Perform the full measurement plan	Done
18	Observations	I could not measure the 10m band. At this frequency range I found that the scope was completely ineffective. Changing the Y attenuators at this frequency revealed no change in the scope measurements. Indicating that the built-in scope attenuators do not work at this frequency. Since this is a 20MHz scope this is not surprising. It brings home the need for a sensitive power meter for these measurements  in the future.
19	Change Control	I started out with the idea of using 30dB attenuation. However 20dB was used to get the open circuit voltage into the 200mV – 300mV range.
20	Computation	Directivity is calculated using the formula RL(dB) = 20 Log10 (Open CCT Voltage/50 Ohm terminated voltage)
21	Analysis	The results all showed a better than 30dB directivity which meets the objective of this homebrew RLB.
22	Conclusions	A sensitive power meter such as the EMRFD chapter 7 version using a logarithmic metering chip is desirable as a detector. This design will allow measurements between -80dBm and +13dBm.
23	Documentation	Done