01 November 2014

LTspice learning part 2

Following on from LTspice Learning part 1, I now proceeded to measure the behavior of the audio common emitter circuit. The objective being to compare the simple model, LTspice model and actual measurements.

I built a 1.59KHz phase shift oscillator based on EMRFD Figure 7.25. This was an interesting exercise and will form the topic of a separate writeup. This generator gave me a clean signal with less than 2mV of noise as seen on the scope.

For an amplifier signal output voltage of 1volt and assuming a voltage gain of 24 for the amplifier, I calculated that I needed about 70mV input to the amplifier. After measuring the open circuit voltage of the generator and the output resistance of the generator, and knowing the calculated input resistance of the amplifier, I calculated the need for a 2.82k voltage dropping resistor between the generator and the amplifier. I inserted a 2.7k resistor which gave me a measured amplifier input voltage of 63mV. This was in the ballpark.

I then measured the DC bias characteristics and the signal input and output voltages of the amplifier. I also tried to measure the signal voltage on the base of the 2N2222, however this signal was really too small to measure accurately on my scope.

Next I measured all the resistors in the circuit using a Keithley 179 DMM. These results were compared to the FLUKE 8024B results and seen to be within a few percent of each other. I also measured the transistor beta which varied between 160 and 175 depending on the DMM used. I carried out these measurements twice. Once on friday evening and again on this saturday morning. This morning the temperature was 47deg F in my attic where I was working. Certainly colder than last night although I did not measure the temperature then. All measurements were consistent except the beta measurement. I used a value of 168 for the calculations. I did not measure the ESR values of the capacitors however.

I then used these measured component values in the simple model calculations and the LTspice simulations. The results are summarized in the table directly below.

The results are extremely interesting and insightful to me. I found that adding an ESR (equivalent series resistance) of 2 ohms into the emitter 100uF bypass capacitor, I was able to obtain close correlation between the measured result and the LTspice simulation. Especially when comparing the signal load voltage results. The voltage gain measurements shown below have less credibility due to the difficulty in measuring the base input voltage (used to calculate the voltage gain). It is also interesting to compare the difference between the simulation with and without the 2 ohm resistor. There is a difference of 806mV-724mV = 82mV. Meaning a difference of 33mW power output.

The shown results for the simple model also include a 2 ohm regeneration resistor. Certainly this brought this result closer to the simulated and measured result but there is still a significant variance. Why?

Perhaps a useful conclusion from this exercise is that manual calculations are great to predict a ballpark result, however the LTspice IV simulator can get one much closer to the measured result. This conclusion cannot be validated, of course, without the ability to do many more precise measurements. Certainly beyond the scope of my simple 1970's era equipment workbench. Or is it?
 

Selected Parameters that serve to illustrate the differences in model results compared to the measured results

Bench test configuration


The LTspice generated schematic showing actual measured component values


A detailed spreadsheet showing all calculated and measured results


The phase shift test oscillator is powered by a 9V battery. Shown on the right and coupled to the audio amp on the left  with 1:1 test leads connected to the scope. No impact on the measurements, were seen from the test leads.

Scope display showing the amplifier input and output waveforms. The waves 'look' like 'clean' sinewaves.





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.


23 August 2014

Motor starter capacitor components

Recently I determined that the failure of my powered garage door opener was as a result of a failed motor starter capacitor. This was easy to troubleshoot since smoke was coming out the bottom of the capacitor.

I was able to buy a replacement capacitor on eBay for $9.50 landed at my home. The garage door opener now works perfectly.

In an idle moment I disassembled the failed capacitor to see 'what made it tick'. As the pictures below show,  it is a regular waxed paper dielectric capacitor. I identified the failed area as an area where the paper was completely dry. Exactly why that section was dry remains an open question.

The capacitor is made up of 2 plates of dimensions 22 inches long by 2 inches wide and made of a thin conducting material with a rough surface. I understand that the roughness increases the surface area and thus the capacitance. We all understand how this works as a capacitor...

What is a mystery to me is the inclusion of a 3rd conductor made of the same material as the two plates. This piece is 48 inches long by 2 inches wide and appears to be overlapped by a few inches with one of the two plates. In other words in electrical contact with one of the plates.

Thus one plate has a  much larger surface area (I believe) than the other.

A Google search did not reveal the answer to me.

What purpose would that 3rd conductor fill?

Does anyone know the answer?

Outer protective sheath with specifications.

Removed inner components still rolled up

The two plates showing wire connecting points. The mystery 3rd element and the waxed paper dielectric.

Installed replacement starter capacitor bought on eBay




29 July 2014

Adventure Radio Society Flight of the Bumblebees 2014

I operated in this fun event which is sponsored by the Adventure Radio Society and known as the 'Flight of the Bumblebees'. This year I was Bumblebee #94.

http://arsqrp.blogspot.com/

This is a 4 hour event where QRP stations operate from the field for a period of 4 hours. The operating time this year being Sunday July 27th from 1pm - 5pm eastern.

I set up in a lovely spot overlooking Lake Jordan which is about 20 minutes from our home. Using my trusty Norcal 40A and an End-Fed Halfwave. The weather was hot and humid but with a slight breeze.

Band conditions were terrible (to use an understatement). There were a number of stations that I copied that could not copy me. There was a period of an hour in the middle of the contest where I failed to make a contact hi! I know that Paul AA4XX who was operating from Topsail Island NC experienced the same poor band conditions.

Always an enjoyable event and a nice chance to get out into the woods.

Lets hope the Skeeter Contest in August will see better condx.




26 July 2014

Excalibur 2 Project. Final Takedown.

Today was a good day at Excalibur 2. The weather was hot and steamy!





  1. Roof Shingles removed to the dump site.
  2. Rood trusses dismantled.
  3. Wall Framing taken down.
  4. Flooring removed.



Next Steps:
  1. Lift the Floor Frame.
  2. Remove the foundation concrete blocks.
  3. Install the black plastic sheeting.



Framing 


Floor Frame. Not much of the insulation has survived. Why? Was it carted off by critters?



20 July 2014

Excalibur 2 Project. Shack Rebuild Procedure

PROCEDURE FOR REBUILD OF THE EXCALIBUR SHACK (Known as EXCALIBUR 2)

I thought that this information would add huge value to the Knightlites archives and body of knowledge. After all, Amateur Radio covers not only radio, but a huge range of allied disciplines including the all important construction of the radio shack.

This information was compiled by Sir Marty W4MY. Marty has great experience in construction. The Knightlites have a fantastic range of experiences to draw on. Here is one very practical example. Thanks Sir Marty!

At this stage the shack has been disassembled down to the floor frame. This frame currently sits on concrete blocks a few inches above the ground. The timber used for the floor frame is pressure treated timber.

The original Excalibur 1 shack was a kit purchased from one of the hardware stores in the area and really intended as a garden shed. The aim now is to rebuild the shack and to make it more comfortable for the anticipated many amateur radio operations. Also to ensure that the shack will withstand the test of time.

Herewith the procedure from Sir Marty (format editions and small clarifications by N4HAY).

The rise of EXCALIBUR 2. Shack Rebuild Procedure

1.      Lift the floor off its present location and add an additional course of blocks to make the floor clear the ground by 6 inches.

2.      Put a single piece of 6 mil plastic sheet directly on the ground under the block stack.  If a single sheet is not possible, overlap the seams by12 inches.  

3.      Place rolled batten insulation and staple between the floor joists with the paper side facing the ground.  

4.      Install steel flashing between the top course of blocks and lowest part of the wood floor. This is for termite protection.  It comes in 6 inch wide strips about 30 gauge thick (1/16" approx).  It can be nailed to the bottom of the floor in the area where it will rest on the top course of concrete block.  

5.      Level is very important. Rig a simple "hydraulic" level to do this.  You need a bucket of water and about 20 feet of clear fish tank tubing.  

6.      Install the sub floor. This must be 5/8" C-D plywood with the "C" side facing up.  

7.      Place roofing tar paper sheet on top, overlapping about 6" and sealing with general purpose "roofing tar" commonly dispensed in a caulk tube. 

8.      Frame the walls, door, and windows on top of the sub floor.  

9.      Place OSB board on the outside of the wall framework.  

10.   Frame roof and ceiling.  Use 1/2" plywood on roof, not OSB.  

11.   Do not sheet ceiling or soffits at this time.  

12.   Install ridge vent, tar paper, drip edge and shingles on roof.  

13.   Structure is now "dried in"

14.   Install Owens Corning 1/2" foam board on top of OSB walls,

15.   Wrap entire four walls with Tyvek vapor barrier.  

16.   Cut out windows and door openings.  

17.   Seal edges of Tyvek with construction adhesive. 

18.   Install finish floor on top of tar paper with 1/2" "Pressboard". Not OSB

19.   Sheet facia and soffits and install soffit vents. 

20.   Install electrical wiring.   

21.   Install batt insulation on walls between the studs, paper side facing inside.  

22.   Install batt insulation between ceiling rafters with paper side facing down. 

23.   Sheet interior walls and ceiling with 3/8" drywall.  

24.   Finish interior drywall and seal.  

25.   Install siding to exterior walls,

26.   Paint/finish as required.  

27.   Finish floor (Linoleum suggested) 

28.   Hang windows and doors

29.   Finish interior trim.  

30.   Paint inside. 

31.   Move/Build furniture 

32.   Knightlights enjoy!