Monday, November 27, 2017

Replace an HP 16047A Test Fixture with a BK TL89F1

A year or two ago I picked up an inexpensive HP 4274A LCR meter at the De Anza swapmeet in Cupertino, California.


Unfortunately, it did not have a test fixture into which one could plug a component to test, so I needed to scrounge one up.

Hmmm...could I make my own?

Maybe, but the text fixture must have four male BNC connectors to mate with the four female BNC jacks on the LCR meter.


I could probably cobble something together, but why not first look for an HP adapter?

HP made a number of different 4-BNC adapters, and the one which shipped with an HP 4274A was the HP 16047A.  (For other adapters, please refer to:  https://www.alliedelec.com/m/d/11ff3fa41669e4bf3da346504df4abc6.pdf)

HP 16047A Test Fixture:


I happened to find an HP 16047D on eBay -- it is similar to the HP16047A, except its frequency rating is to 40 MHz instead of 13 MHz.

But the HP 16047D only came with one set of component-lead clips.  There are actually 3 different types of clips (as seen in the photo for an HP 16047A, above), and it would be nice to not have to bend leads to fit my sole set of clips.

I could hunt for more clips, but they typically came with test fixtures, and these fixtures, with clips, usually cost in the hundreds of dollars on eBay.

Somehow I happened to stumble across a picture of a BK TL89F1 test fixture, and I was immediately struck by how similar it looked to the HP 16047A.  Given its sub-100 dollar price, it looked like it could be an inexpensive "connector-for-connector" compatible replacement for a 16047A, and it included all 3 styles of component clips.

BK TL89F1 Test Fixture:


How do specifications compare?

HP:

(Click on image to enlarge)

(From: Accessories Selection Guide For Impedance Measurements, Agilent Technologies, April, 2005.)


BK:

Model TL89F1
Test fixture for convenient testing of axial and radial leaded type components.
  • Frequency: DC to 10MHz
  • DC Bias:+/- 40V peak max (AC+DC)
  • Operating Temperature: 0 to 40 degrees C
  • Terminal Connection: BNC to 4 terminal insertion slots (radial or axial)
(From https://www.bkprecision.com/products/component-testers/891-300khz-bench-lcr-meter.html)


Specifications were close enough for me (especially given the fact that the HP 4274A is a 100 KHz instruments), so I ordered one.  Here is the test fixture and its various adapters, as received:


And its size, compared to an HP 16047D fixture:


Doing a quick test to compare HP and BK test fixtures...

1.  A 1000 pF dipped-mica capacitor, tested with my HP 16047D test fixture:


2.  And now, the same capacitor, but tested on my BK TL89F1 test fixture:



Not much difference!

There is one physical difference between the two fixtures:  the HP 16047A has a plastic piece to ensure that the bias switch is forced into its lowest positions -- the BK fixture has no such block.  Unless this feature is important to you, I believe the BK TL89F1 would be a satisfactory (and inexpensive) replacement for an HP 16047A.  (But, of course, do your own research to verify!)


Additional Notes:

For testing SMD components, the BK TL89S1 could probably be used in lieu of an HP 16034E:




Standard Caveat:

As always, I might have made a mistake in my equations, assumptions, drawings, or interpretations.  If you see anything you believe to be in error or if anything is confusing, please feel free to contact me or comment below.

And finally, I should add -- this design and any associated information is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

Thursday, November 23, 2017

The Hilbert Transform and SSB Modulation

(Below are some brief notes to myself on the Hilbert Transform's use for the "phasing" version of SSB modulation.)

The "Phasing" method of SSB generation was a popular way of generating SSB in the early days of Amateur Radio SSB operation.

Some transmitters, such as the Heathkit TX-1, were designed to utilize outboard phasing accessories, such as the Heathkit SB-10, shown below.  (The TX-1 and SB-10 were my first SSB station while in high school).


Other early SSB transmitters had their phasing networks built in, such as the Hallicrafters HT-37 and the Central Electronics CE-100V.

The phasing method requires that the audio frequencies in the voice signal be shifted by 90 degrees.  In those early transmitters, this shift was accomplished with an analog phase-shift network.  Typically its setup would involve a nulling process using several knobs.

Now, with digital signal processing, the requisite 90 degree shift of the audio signal can be accomplished much more accurately and without tuning using a Hilbert Transform.

Below are two visual representations of the math underlying SSB generation via the Hilbert Transform in terms of sines and cosines.  For visualization I find it useful to express sines and cosines in their complex-exponential form.  E.g:

cos(2πfot) = (ej2πfot + e−j2πfot)/2

and

sin(2πfot) = j*(e-j2πfot - ej2πfot)/2

Note that j =  ejπ/2.   When multiplying (e-j2πfot - ej2πfot)/2 by j, the "π/2" term in j's exponential results a +90 degree rotation of each of the two exponentials in (e-j2πfot - ej2πfot)/2.  The result is that the negative-frequency exponential (e-j2πfot) is rotated by +90 degrees, and the positive-frequency exponential (- ej2πfothas a total rotation of 270 degrees: 90 degrees due to the multiplication with j, and an additional 180 degrees due to the minus sign in front of it.

Or, in other words, the positive-frequency exponential is rotated by -90 degrees rather than +90 degrees.

Here's a visual representation of LSB generation:

(Click on image to enlarge)

And second, USB generation:

(Click on image to enlarge)

If the input audio were a sawtooth waveform, the image below shows the signals at various stages of the modulation process.

(Click on image to enlarge)
Note, in the image above, that the resulting SSB signal can have amplitudes larger than the peak values of its input audio (due to the Hilbert Transform's phase shifting).

This difference in input versus output signal magnitudes can be seen more easily in the image, below.  The sawtooth input has had its peak magnitude defined to be 1.0.  The peak value of the modulator's output, however, is 1.8.

A significant difference!

(Click on image to enlarge)

Therefore, because the level of the modulator's output IQ signal level can differ dramatically from the input audio's signal level, feed-forward transmitter gain control is ideally accomplished using the magnitude of the modulator's output IQ signal, rather than the magnitude of the input audio signal.

This conclusion is also true for the Weaver method of SSB generation.


Resources:

http://flylib.com/books/en/2.729.1/hilbert_transform_definition.html

http://k6jca.blogspot.com/2017/02/sdr-notes-weaver-modulation-and.html


Standard Caveat:

I might have made a mistake in my designs, equations, schematics, models, etc.  If anything looks confusing or wrong to you, please feel free to comment below or send me an email.

Also, I will note:

This information is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

Saturday, November 18, 2017

Repair Log: Tektronix SC-502 Oscilloscope

[Note:  My "Repair Log" blog posts contain my notes on equipment I've recently repaired.  Posted here in case someone else might find them useful.]

A couple of days ago I was thinking it would be nice to add a small oscilloscope to my FPGA SDR station for monitoring my transmit RF, and I remembered that, several years ago, I had picked up a broken Tek SC502 oscilloscope module at the De Anza swap-meet.


It had been languishing in my "projects for the future" pile (a very large pile!), and I thought that it, coupled with a TM503 chassis, could be a great addition to the station.

So I retrieved the SC502 from the pile, plugged it into the TM503, turned it on...and started to hear periodic snapping.

Oh oh -- something is arcing!

Also, although I could see a glow on the CRT (which was a sign that the CRT was probably good), there was no trace.

Well, I knew it was a project when I bought it.  Time to dig deeper...

After removing it from the TM503 chassis, I noticed that some of the pots were missing their wipers while blowing out the accumulated dust with a can of compressed air.

How could I tell?  Here is a picture of a "good" potentiometer (left) with its wiper "top" and a "bad" potentiometer (right), missing its top.


Below are the four bad boys (with one unattached wiper top) that I replaced, using various trimmer pots from my junk box (none of these replacement pots were of the original (and apparently flakey) style used by Tek in this module):


[After I had reassembled the SC502 I discovered that the wiper-top of R473 was turning freely -- in other words, it was also broken, but I had not noticed this earlier because the top had not fallen off of the pot.  I thought about replacing this one, too, but removing the CRT assembly to get at the PCB pads is a bit of a pain, and I decided that, for my application (monitoring my modulated waveform), R473 isn't that critical (it is used to set the frequency response of the cascode stage driving the vertical deflection plates).]

With the four potentiometers replaced and the SC502 reassemblied, I plugged this module back into the TM503 chassis and powered it up.

No snapping!  A good sign!

And now I could see a trace -- but it was very bright -- much too bright even with the INTENSITY knob turned to minimum intensity.

And another problem -- although there was a trace, I could not get it positioned above the lowest graticule marker on the CRT.  In other words, I could not move the trace up to the vertical center of the screen.

Attacking the latter problem first -- there was clearly a problem with the vertical deflection.

With no input signal, it seemed to me that the voltage on the CRT's vertical plates should be equal if the trace were to be centered vertically on the CRT screen, otherwise the electron beam would be deflected vertically in one direction or the other.

But measuring the collectors of the cascode plate drivers (Q470 and Q475), it was clear no amount of position-pot rotating would make these voltages equal.

So, with the SC502's MODE switch set to CH 1, I first adjusted the CH1 POSITION potentiometer so that the voltages at the collectors of Q320 and Q325 would be equal (in my case, roughly -2.3 volts) -- my thought being that, if these two voltages were equal here (assuming equal collector loads), this equality-of-voltages should apply to the differential signals along the entire vertical amplification chain.  And problems would be revealed if the two complementary voltages were ever unequal.

I've annotated Tek's schematic with my voltage measurements, below.  You can see that the complementary-voltages become unequal in the second schematic, at the bases of the Q460/Q465 pair (in fact, Q460 isn't even forward biased!).

(Click on image to enlarge)

(Click on image to enlarge)

With power off I made a quick in-circuit resistance measurement of R447 and R454 -- being in circuit, each should measure no more than their 698 ohm values (and possible much less, depending upon what is in parallel with them), but their values should be equal.

Instead, I discovered that both values measure significantly greater than 698 ohms (by at least 1K ohms),  and that the two measured values differed significantly, too.

So I removed R447 and R454 from the circuit and remeasured their values.  R447 was 1.78K, and R454 was open!

Per the manual, these are both 1/4 watt resistors, and, seeing how they and the board below them had darkened from heat, it seemed pretty clear that heat from power dissipation had probably affected their values.

Interestingly, using the voltage measurements above, the heat dissipation across each resistor should have been about 0.3 watts with the trace centered vertically on the CRT screen -- in other words, they were operating above their power-rating specification!

This too-high power dissipation raises the possibility -- perhaps the -5 volts I measured at the emitters is out-of-spec, and that this voltage should actually be lower -- if no base current were being drawn by Q450/Q455, then the voltage their bases would be about -8 volts (i.e. R449/R450 voltage divider), putting the emitters at about -7.3 volts.  In this ideal case, though, R447/R450 power dissipation would be about 0.23 watts -- too close to the 0.25 watt resistor specification for my tastes.  And with actual base current (rather than an ideal base current of 0 mA), the base voltage of Q450/Q455 will only become more positive, thus raising the emitter voltages and the power dissipated by the two resistors.

Concerned about their heat dissipation, I replaced R447 and R454 each with a series-connection of three 232 ohm, 1/8 watt resistors (to improve the overall power rating), and mounted them a bit away from the PCB so that they would get some air circulation around them and not discolor the board further.  (Note, these mods could be made without disassembling the SC502).

With this mod, the power rating of R447 and R454 becomes 0.375 watts.  But if more power-dissipation margin is desired, perhaps a better choice would be to replace R447 and R454 each with a series-connection of four 174 ohm, 1/8 watt resistors.

(Click on image to enlarge)

With the SC502 plugged back into the TM503 and powered up, success!  The trace(s) could now be centered vertically on the CRT!

The blindingly bright intensity was fixed by adjust the beam-current potentiometer (R873).  Although the manual says the current should be adjusted so that the test-point measures 0.4 volts, I found 0.2 volts to be a bit better, in my opinion).

Here's the TM503 and SC502 undergoing some bench checkout, just after I finished adjusting the beam current...


One final note -- the SC502 is a 15 MHz scope, and I had planned to used it for monitoring my transmit RF at frequencies up to 30 MHz.  Because I am more interested in seeing how the waveform looks rather than making accurate voltage measurements, it does not bother me if the response rolls off above 15 MHz, but I was curious how much this roll off would be.

Using my HP 3335 signal generator set to +10 dBm and feeding the scope directly via a length of coax (terminated in 50 ohms at the scope input), I measured the following frequency response.  Note, the amplitude measurement is in vertical division of the scope's CRT.


So, at 30 MHz the frequency response is only down about 3 dB from the response at 1 MHz.  Not too bad, and certainly acceptable for my application!

That's it for this post.


Resources:

Instruction Manual PDFs (which include schematics) can be downloaded here:

http://bee.mif.pg.gda.pl/ciasteczkowypotwor/Tek/SC502%20.pdf

http://bama.edebris.com/manuals/tek/sc502/



Standard Caveat:

I might have made a mistake in my designs, equations, schematics, models, etc.  If anything looks confusing or wrong to you, please feel free to comment below or send me an email.

Also, I will note:

This information is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.