Revisit AD8307 Power Meter Calibration

This year at our club, annual pig picking BBQ and show-n-tell I had a chance to borrow the kind services of fellow Knightlites member OM John, KU4AF and his brand new and beautiful digital spectrum analyzer.

Connecting the Si570 based oscillator from my Sweeperino   into the Spectrum Analyzer and measuring the power level of the fundamental frequency we measured a -4dB difference in power when compared to measuring the squarewave on the AD8307 based wideband (~500MHz) Power Meter. Using a Low Pass Filter to filter out the odd numbered harmonics I resolved to measure the power in the fundamental on my work bench.

For the experiment I used the well known 10MHz calibration source as developed by K3NHI. This source can be used to establish the -10dBm point on the power meter through the measurement of a DC voltage level at the DC port. This reference is easy to build and is recommended for any homebrew starting out with a basic 50 Ohm bench.

Indeed my result showed a 4.1dB difference in power reading of the fundamental vs the wideband signal.

Herewith below are my Lab Notes of the procedure adopted.

455 KHz IF Transformer Characterization

I found an unmarked IF Transformer in my junkbox which seems to have reasonable characteristics for use in my 455KHz IF stage for my shortwave broadcast band radio project. This one had a yellow inductor adjustment screw and a number of 29501 printed on one side. What attracted me to this item was the fact that it had a reasonably high inductance of 91uH on the primary. Thus I could resonate it with a capacitor of approximately 1200pF on 455KHz.  The secondary winding has a very low inductance of 0.34uH. Mysteriously the measured inductances between pin 1-2 and 2-3 of the primary do not add up to the 91uH.

Refer to the attached Mystery IF Transformer Characterization. Using the method outlined by W7ZOI on his webpage The two faces of Q . A Bandwidth of approximately 4.6KHz was measured which pertains to an Unloaded Q = 100.

I next connected the transformer to a 455KHz source, using a 10pF coupling capacitor (Xc = 35K ohms) IF Transformer loaded with Detector circuit. This lightly loaded tank circuit exhibited an open circuit voltage ratio of 15 which increased to 17 when the secondary was loaded with the detector. The 3dB bandwidth was measured as 5.3KHz with the 6dB points at 10KHz. Loaded Q was 86. Thus the Equivalent Loaded Parallel resistance on the tank circuit is approximately 25K ohms of which 35K is contributed from the signal source load (via the 10pF coupling capacitor. Thus the IF Amp collector load can be expected to be in the 20K ohm range.

The small signal voltage swing of 1.86 Vp-p on the tank coil gave a secondary voltage into the detector of 100mV. This is enough to generate an acceptably loud signal into the connected speaker.

Clearly this is not going to be a highly selective Receiver although the first IF transformer should improve the selectivity further.

Next task is to build the cascode IF Amplifier. Lets see how much gain can be achieved from that stage?

Test Fixture showing the IF transformer connected to the Detector stage.

AM Diode Simple Envelope Detector Measurements

As a part of a project to build a simple shortwave broadcast radio based on the published KD1JV design Steve KD1JDV Shortwave Broadcast Rx the opportunity was taken to perform some measurements of the 'simple' AM envelope detector circuit. Through this exercise I hoped to gain some practical insights into the behavior of diode detectors.

It is clear from my own observations and from reading the literature on the 'simple' diode detector that it is by no means 'simple'! The more one explores and understands the more complex it appeared to me. Especially with respect to the subject of introduced distortion to the input complex modulating waveform.

The detector used was a Schottky SD101A diode that was biased to approximately 60uA. The spec sheet Schottky diode SD101A shows an ON resistance of approximately 200 ohms at this quescient point. It also shows a voltage input swing of approximately 200mVp-p before the diode would turn off or before the 1mA 'knee' would be reached.

The test fixture and measurement results are shown here

A view of the carrier ripple across the diode output is shown here

Some observations as follows:

• With an input 455KHz carrier (this Rx IF frequency) the measured carrier ripple current was 50mVp-p on the diode output. This seems to be insufficient carrier smoothing since once this RF is amplified by the audio amp it appears on the speaker output at an amplitude of approximately 400mV. Will this be a problem in the final receiver. Can the values of these two .001uF capacitors be increased without causing distortion to the modulating audio signal? TBD.
• The carrier was modulated with a 1KHz tone and a 400Hz tone and Music (Santana) for varying levels of AM modulation from about 8% to 90%. No major distortion was detectable by my ear and when listening to the output with a pair of iphone earbuds.
• The carrier frequency was varied from 100KHz through to 50MHz with no real noticeable difference in music quality output. 
• As the carrier frequency was increased of course the carrier ripple became so small as to be un-measurable.
• The audio amp voltage gain = 20. Quiet room volume level needed an output signal of about 4Vp-p into 8 ohms. This equates to a 200mV input signal needed into the audio amp. Tests were conducted up to 380mVp-p input to the detector before distortion was noted on the input signal. Looking at the diode specs suggests that distortion will occur with an input signal greater than 200mVp-p. 
• Varying the bias level by adjusting the 100K potentiometer in the bias supply did not make a noticeable difference to the audio output waveform.

Calculate VSWR for varying Reactance Values X

Great question from Richard N4PBQ as follows:

"So as I'm analyzing my various antennas I understand that my antenna is at resonance when my analyzer reports X=0 for a particular frequency.  But that never seems to be the case outside of my dummy load and even there the reactance is reported as X=7 for 28 MHz.

So is there a rule of thumb for acceptable amount of reactance or some calculation that I can use to determine the loss in DB for a given frequency and the reactance?"

I got to thinking about this. The complex math becomes messy quickly with many possible combinations and permutations if both R and X are allowed to vary for the load impedance ZL. However if one keeps R = 50ohms constant and assume that the source impedance is 50 ohms then the analysis simplifies.

The referenced table shows a range of X values up to 20ohms for a VSWR not exceeding 1.5:1

Simplified math derivation

Calculate VSWR for varying Reactance Values X

ELECRAFT BL2 Balun Measurements

The ELECRAFT BL2 BALUN Return Loss and Balance were measured as shown in the referenced links. The Balance measurements were performed according to W7EL's referenced document. Refer to pg 163 paragraph named 'Current Balun'.

Elecraft BL2 BALUN Specifications & Operating Manual

BALUNS. What they do and how they do it. W7EL

Test Configurations and BALUN Schematic

Return Loss Measurements

Balance Measurements

Test Fixture 1

Test Fixture 2

Test Fixture 3

Measuring Potentiometer Resistance Taper

I had some fun measuring the resistance of a linear 10Kohm potentiometer from my junkbox and designated B10K.

I then measured an ALPS 10Kohm pot that is intended for audio applications. P/N RK0971221Z05. The taper for this pot is designated as type 3B by ALPS.

Refer to the following link: Potentiometer Taper Measurements

Homebrew Pot Taper Meter with 10 degree graduations. Hooked to a DMM

David Clark Headphones amplifier design 2

The simple common emitter circuit was redesigned with an increased target collector current of 10mA. This allowed enough current to supply the 150 ohm load and ensure that the transistor was operating as a constant current source.   

Varying emitter degeneration and/or collector current(by increasing the rail voltage)  demonstrated the effect on amplifier voltage gain and dynamic range. As can be seen from the measurements, collector current increase results in amplifier dynamic range improvements (seen as reduced gain compression in the measurements). Also increasing emitter degeneration improves dynamic range but at the expense of amplifier voltage gain.

So you cannot 'have your cake and eat it'. Better dynamic range comes at the expense of increased battery consumption or reduced gain.  With a 7V supply I found the gain limit to be less than 6.5. Increasing the gain to the 10 - 13 range required a collector current of 18mA. Clearly either option was problematic for this application. 

After increasing the emitter degeneration resistor to 15 ohms I then tried the amplifier connected to my K2 and using a Vcc of 7.5 volts.  Listening to our club evening 80m net with high QRN I was very interested to note slight distortion in the headphones. The received CW note sounded undistorted to my ear at normal listening levels. However the background noise sounded slightly distorted. The measured 3dB points of the amplifier were 200Hz - 1800Hz. When the K2 audio gain was turned down to a low listening level no distortion was detected. So in spite of measuring an undistorted (as far as I could tell) 1kHz output sinewave approaching 600mV (p-p)  on my bench. In practice the noisy band sounded distorted. Could this be due to noise spikes from the QRN exceeding the dynamic range of the amplifier? Using a single signal input to simulate a real radio channel is simplistic. 

Next it would be interesting to try a 2 transistor amplifier using an emitter follower on the output. This would allow an increase in the load resistance of the common emitter input stage which would allow improved dynamic range and gain at a lower collector current. However at least 10mA would be needed to drive the emitter follower. Is this a zero sum game?

20mV input, 128mV output, Vcc=7V Gain (V) = 6.4 'undistorted'

100mV input, 500mV output, Vcc=7V. Gain (V) = 5 'distorted'

250mV input, 1.3V output, Vcc=7V. Gain(V) = 5.2 'distorted'

250mV input, 1.55 V output, Vcc = 12V. Gain(V) = 6.2 'undistorted'