11 April 2014

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.

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