Characterization of a 14.5MHz low pass filter

 I built a Chebyshev n=5 filter and then tried to characterize it. Since I am using an 11MHz sine wave generator for calibration I started out with the aim of realizing a 13.2MHz filter. Refer to EMRFD Chapter 3 for filter design. Since I didn't have any 330pf caps on hand I ended up with a 14.5MHz (peak cutoff) filter using 300pF capacitors.

I was unable to detect the ripple in the filter. Instead it showed a flat response until about 9MHz with an insertion loss and ripple that I could not measure with my power meter.

The following results were obtained.

RL = 22dB
-3dB cutoff point = 15.61MHz
-5dB Passband = 16.4MHz.

JFET biasing experiments

Today I continued with the biasing experiments. I redid the tests for Sample 1 and MPF102 and the results were the same as previous measurements.

I then ran the same tests on Sample 2 MPF102 and Sample 3 J310

The results are shown below for sample 2 and 3.

Using the FET equation to try to achieve a curve fit I estimate the Vp and Idss values as follows:

Vdd = 10.18V

SAMPLE        Vp        Idss
1 MPF102      -2.27      9mA
2 MPF102      -3.6        9.7mA
3 J310             -2.98     33.8mA

For the J310 the specification states that Idss can fall between 12mA and 30mA. Clearly this sample is at the upper end.

According to this analysis sample 1 would have the lowest power output and the J310 the highest. Next step will be to build a Hartley oscillator and measure the power outputs.

JFET test fixture

Sample 2 MPF102

Sample 2 MPF102

Sample 3 J310

Sample 3 J310

Power Meter calibration using the Weinschel step & fixed attenuators

Now that I had established a few calibration points I wanted to establish further reference points. These would assist in determining the variances in linearity over the range -10dBm through -70dBm.

On hand I had a step attenuator with the following markings:

MANUFACTURER: Weinschel Engineering, Gaithersburg, MD.
MODEL: 9332
SERIAL: 441 0955-0085.
STEPS: 0dB, 10dB, 20dB, 30dB, 40dB, 50dB

I compared the Weinschel with my fixed attenuators using the power meter as the comparator.

The following results were obtained @11MHz.

dB(marked)         Weinschel DVM (mV)         Fixed Attenuator (mV)        Difference (mV)
0                             1565                                      1566                                  1
10                           1361                                      1368                                  7
20                           1167                                      1166                                  1
30                           962                                         959                                   3

The above shows excellent fit between the fixed and step attenuators with a maximum difference between the 10dB pads of 0.5%.

I now derived a result for the 40dB & 50dB attenuators based on a nominal slope of 20mV/dB.

dB          Weinschel DVM (mV)     Slope     dB derived              
0                1565                                -             -
40              761                              20            40.2
50              565                              20            50

This gave me confidence that the Weinschel was indeed an accurate attenuator .

By combining my fixed attenuators with the Weinschel settings I was able to measure reference points from -10dBm through -70dBm in steps of 0dB, 3dB, 6dB, 10dB. Refer to the table as shown below.

Referring to the linear equations on W7ZOI's essential notes. http://w7zoi.net/Power%20meter%20updates.pdf 

For two reference points P1(dBm) & P2(dBm)  having respective DVM voltage measurements V1(mV) and V2(mV)

The linear equation has the following form  P(dBm) = A + B*V(DVM)

Where B = (P1-P2)/(V1-V2), A= P1-((P1-P2)/(V1-V2))*V1

Using an excel spreadsheet I developed an equation for each decade of the slope. This can be used to obtain the most accurate possible power meter reading should it be so desired. It is interesting to examine the table below. The dB/mV slopes are indeed 'slightly' different for each decade.

The equations below are tedious to plug into a calculator, however they do yield the most accurate result. Alternatively the last column slope numbers can be used.

   

Calibration of the AD8307 Power Meter using fixed attenuators

I was now in a position to calibrate some additional points on the Power meter having completed the calibration of the 11MHz sine wave generator to the -10dBm reference point.

On hand I have the following attenuators which were purchased new from a commercial source. These attenuators do not have a specified tolerance, however they are marked to be used upto 1GHz. For this purpose I assume they are accurate and will form the baseline for calibrating the system. The attenuator values are:
3dB, 6dB, 10dB, 20dB,

The attenuators now allowed me to derive the slope in mV/dB at each measurement point. This was an encouraging result shown in the following table.

Pad(dB)           dBm        DVM(mV)      mV/dB
0                      -10            1565               ----
3                      -13            1507              19.33
6                      -16            1449              19.33
10                    -20            1367              19.80
20                    -30            1166              19.95
30                    -40              958              20.23
39                    -49              775              20.26

This shows a slope variation over the measured range of 20.26-19.33 = 0.93mV  which equates to 0.93/20 = 0.05dB. Not sure this is a meaningful calculation?

Looking at it a different way.  Midpoint over the range -10dBm to -49dBm
= (20.26-19.33)/2 + 19.33 = 19.80

Thus at the bottom of the range @ -49dBm and using 19.8 we derive (1565-775)/19.8 = 39.9dB
ie. at the reference point of -49.9dBm the power meter shows an error of 0.9dB. Acceptable?

Herewith Lab notes showing the calculations.

AD8307 Power Meter. The Analog Meter Calibration

In this procedure the aim was to adjust the current to voltage converter that drives the analog meter so that the meter would read about 75% of full scale deflection (FSD) at the maximum rated input of the power meter which is +15dBm. 

Since the meter has a FSD of 1mA the 75% point would require a drive current of 750uA. I started out using 2 X 2.2K resistors which resulted in a reading of 517uA for +14dBm. 

Subsequent calculations and tests (shown in the lab notes below) resulted in the use of 2 X 1.5K resistors which gave a reading of 835uA at +14dBm. The input voltage to convert to 835uA with this resistor of 3K was 2.51volts. This was deemed to be satisfactory and would give enough safety to ensure that the meter would not be damaged if excess input is applied in error. 

For these measurements I used my K2 Elecraft as the RF source since that is what I had. I had previously calibrated my 1N4148 based RF power meter using DC voltages. Thus this was the starting point. Using this meter I set the K2 power output to 1 watt (+30dBm). I then inserted fixed attenuators (16dB) to bring the level down to +14dBm. This level was then used to set the meter deflection.

Calibrating the slope of the AD8307 Power meter

The first task after building the Power meter was to adjust the Calibration output (Port B) pot to as close as possible to the specified 20mV/dB.

The iterative procedure followed is shown in the lab notes below. At this stage in the calibration the slope was set to 19.95mv/dB.

SARL Field Day Contest February 2014

We travelled to a 'dog friendly' guest farm in the Magaliesberg for the weekend of February 8th and 9th. This is a really nice farm called Stonehill, not far from the town of Magaliesburg. We did some fantastic walks around the property. All good from an exercise point of view.

The weather on Saturday was perfect but on Sunday it became overcast and colder.

Purely coincidentally was the fact that it was the SARL Field Day Contest weekend. The cottage we were staying in was about 2/3rd's the way up a gentle slope with a nice take-off angle to the south. Upon surveying the trees, I elected to deploy a doublet up about 6 meters and between 2 thorn trees. The horizontal section was 23 meters long. This is a field deployable antenna consisting of two lengths of wire about 40 meters long. Once the horizontal section was deployed I then turned the rest of the wire into a ladder line by fixing portable spreaders between the wires. These spreaders being spaced about a meter apart. This is not a quick antenna to deploy but I was able to get it all assembled and in the air in about 45 minutes.  The original design was shown for years on the Adventure Radio Society Website which does not appear to be functioning any more.

After draping the feeder over the braai area and along some flowers I was able to connect it to my trusty balanced tuner, my ZM2. This tuner has been on hundreds of field trips and still provides wonderful service.

The first thing I noticed was the extreme quiet of the bands. This had to be a combination of band conditions, location and the balanced feeder system. What a pleasure.

I pre-arranged a QSO with Barrie ZS6AJY and Dave ZS6AZP on Saturday morning at 6:30am on 80m. Condx were a pleasure and I received solid reports from both stations which was very pleasing. Thanks Barrie and Dave for coming on frequency.

I did not operate for more that maybe 5 hours in the contest. I operated under Class 1C which is QRP field portable. Condx were nice and I was able to work all stations that I could hear. Check out the results, pics and details below.

Definitely a good location for radio and a fun weekend!