09 January 2015

Homebrew Attenuator Measurements contd

It was interesting to measure the insertion loss and maximum attenuator accuracy over a wider frequency range.

The BIG attenuator has a maximum attenuation of 68dB.
The Copper attenuator has a maximum attenuation of 71dB.

As for the previous measurements I used the HP 8657B as the 'standard' against which to compare.

I also used a 0dBm input level for the 71dB (copper attenuator) and 68dB (Big attenuator) measurements in order to move the measurements comfortably up from the lower end of the Power Meter measuring capability. The Power Meter in fact showed a consistent slope ranging from 20.5mV/dB for 1.6MHz to 19.69 mV/dB at 450 MHz. Quite cool for a homebrew power meter!

Reference the following links:

68dB slope calculation

Insertion Loss Measurements

Maximum Attenuation Measurements

In conclusion both attenuators show significant insertion loss and maximum attenuator errors for the 148MHz, 224MHz and 450MHz measurements.

'BIG' Attenuator

'Copper' Attenuator

Homebrew Attenuator Measurements contd/

Today I characterized another attenuator recently acquired and known as the 'BIG' attenuator on this blog. The performance of this attenuator over the HF frequency band is not as good as the 'copper' attenuator (results shown in the previous blog). The worst case measurement was on one of the 20dB pads at +1.06dB error. Although this unit is solidly constructed, I assume that the fact that it does not have shielded compartments between each attenuator must degrade performance over the frequency range. Perhaps also the switches used are not as good as slide switches for this application.

The shown measurements in fact are optimistic since they do not include insertion loss. I performed separate insertion loss measurements which are shown in the next blog.

As a part of these measurements I also characterized the slope of the Power Meter.

In conclusion, the attenuator certainly is useful for many functions at HF. Also the use of 1/4 Watt resistors will allow use in higher power applications.

The following links

Measured Attenuation

Pi network resistor values and calculations

Slope Calculations

07 January 2015

Homebrew Attenuator Measurements

I recently acquired a homebrew attenuator (called the copper attenuator in this blog). The construction is modeled in line with the QST article of September 1982 titled "A Step Attenuator You Can Build" by Bob Shriner WA0UZO and Paul Pagel N1FB although the named attenuator pad values were different.

On opening up the unit I was mystified by the values used for four of the PADS. The parallel and series values appeared to have been transposed in error. I reworked these values using standard nearest value resistors.

The switches are in excellent condition with no signs of wear. As can be seen from the linked spreadsheets below, I tested the attenuator over the HF frequency range only. A maximum of 0.1dB insertion loss was observed at 28MHz.

The worst case error observed was +0.4dB for the 16dB PAD.

With all PADS switched in to yield a maximum attenuation of 71dB the maximum error was 0.8dB at 10MHz.

Click on the link below for details of the measurements made. To return to this page please use the back button.
Attenuator Measurements

Click on the link below for details of the resistor Pi Network Values. To return to this page please use the back button.
Pi Network Resistor Values, dB(calculated), Return Loss (calculated)

28 December 2014

ears to our world, HumanaLIGHT and the Joule Thief

I was inspired by the recent excellent You Tube video made by W2AEW showing the HumanaLIGHT.

Reference: https://www.youtube.com/watch?v=qfgX93o8HzY

REF: W2AEW  You Tube #187. " A single cell LED light supporting EARS TO OUR WORLD"

Since the money for this kit goes to a noble cause, I decided to buy one. Check out www.etow.org for more information. In the process I also learnt something about the relaxation oscillator and the idea of the Joule Thief. A fascinating journey for me.

Since all is said on the video I thought I would simply record the results of my measurements here on the blog.

First I tried a dead battery that had a voltage of 0.47 volts open circuit. This did not light the LED.

Attaching a variable voltage power supply to the circuit I found that the LED just turns on at 0.8 volts and 'well on' at 1.02 volts.  Full on was measured as 1.74 volts.

Rail voltage was 1.3 VDC for the first three measurements below.

The circuit oscillates at 10kHz. Symmetrical wave form once fully on.

Here below are some views of the scope waveforms.

Output wave form to the LED. 3 volts p-p. Vcc was 1.3 v dc.
Base of final transistor that is connected to the LED. 

Capacitor feedback waveform 

Just turning on with a rail voltage of 0.79 volts

'Well on' rail voltage is 1.02 volts. 

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.