30 October 2014

LTspice IV learning part 1

In a recent conversation with Gary N3GO he suggested that I might benefit and gain further insights into electronic circuit characteristics through the use of the well known circuit simulator LTspice. Indeed Gary was correct. Thank you! 

 EMRFD Chapter 2.2 is a study in Amplifier Basics. The schematic below is the EMRFD study example of a single transistor audio amplifier. 

I performed the 'manual' calculations using the classic common emitter simplified model with the aid of a spreadsheet. The results are shown in the value(rms) column. I then converted the small signal voltages and currents into pk-pk values as shown in the value (pk-pk) column so that I could compare them with the LTspice DC operating point simulator and the AC linear analysis simulator results. 

The LTspice simulation uses a 2N2222 transistor while the manual calculations assume a generic  transistor with a beta value of 100 and a base-emitter voltage drop of 0.6V. The manual calculations also use the diode equation derived formula to re=26/Ie.  

Comparing the results are interesting. Some observations as follows:
  1. The emitter bias voltages show a difference of 90mV. Why?
  2. The base bias currents show a difference of  10uA. Almost double in the manual calculation.
  3. The small signal analysis shows reasonable correlation between the two models although there is some difference between the collector voltage calculation.

As a next step I plan to build and measure a similar circuit on the bench. 

It will be very interesting to compare those results.

01 October 2014

Checking the calibration of my squarewave & sinewave calibrators and AD8307 rf power meter

At our recent QRP club show-n-tell activity I was fortunate to be able to check the calibration of my home brew instruments. For this exercise an HP Spectrum Analyzer (model unknown) belonging to Gary, N3GO, was used to check my 10MHz, -10dBm squarewave calibrator and my  11MHz -10dBm, sinewave calibrator (see a previous post for details of these two units).

 I have also been lent (on a longterm loan by Chris KD4PBJ) an HP Signal generator HP8657B. Herewith are the results of the calibration check of my homebrew AD8307 rf power meter.

As can be seen below. The results are not consistent.

10MHz, -10dBm squarewave calibrator

Fundamental -12.56dBm  (54.9uW)
Third             -23.05dBm  (5uW)
Fifth              -27.76dBm  (1.68uW)
Second          -39.9dBm
Fourth           -40.13dBm

Total =  61.78uW  This is equivalent to -12.09dBm

Thus this shows a reading that is 2.09dB lower than the nominal 'calibrated' value of -10dBm. The -10dBm level was calibrated using a DC calibration technique.

Next we measured the sinewave generator

11MHz, -10dBm sinewave calibrator

Fundamental -8.77dBm
Second          -39.69dBm
Third             -48.53dBm
Fourth           -58.34dBm

Thus this shows a reading that is 1.23dB higher that the nominal 'calibrated' value of -10dBm.

The tables below show measurement comparisons between the power meter and the scope readings. This result shows good correlation. It is reasonable to assume that the HP8657B signal generator is accurate to tolerances significantly in excess of the power meter or the scope. This shows a maximum deviation of 0.53dB. This is much better than the comparisons above!

Next I will ask Gary if we can run the tests once again. Check the HP8657B against Gary's Spectrum Analyzer.