DIY active / HiZ / high impedance probe design.

I’ve been wanting to have an high-impedance (Hi-Z) probe for some time now. 

Up until now I’ve been searching for something like an HP 85024a which comes up on the secondhand market from time to time. It would be overkill for my needs, but still nice… :)

Recently I came across an old Elektor/Elektuur article from April 2004 titled “Poor man’s 1GHz active probe” by David Jewsbury. Some more research turned up the bachelor thesis of Thomas Hirschbuechler titled “Design of a low-cost 1GHz active probe“.

I decided it would be a good learning experience to try and recreate/adapt the design and make some measurements on it. 

Design

The board measures 60 by 25mm. As I do not intend to use the probe up to 1GHz I did not bother to use Rogers or some other high-performance material and opted for regular FR4. Design is done in Diptrace V4 which I can recommend as an easy-to-learn EDA.

The current state of the design is in my Github repository although I need to update the schematic with the revisions I made during prototyping.

v0.1prototype board with many changes.

The total cost of the current version is well below €50 where the SMA connector is probably the most expensive part.

Measurements

After tweaking and making some changes on the initial design, measurements turned out pretty good (taking in to account thatI do not intend to use the probe much above 200-300MHz). I made a bode plot 0-25MHz with my MSO5074 scope which has a 25MHz signal generator build-in:

Bode plot to 25MHz

For higher frequenties I turned to my 300kHz-1.3GHz HP vector network analyser (csv at the end):

S21 and S11 from 300kHz to 1GHz

Frequency(MHz); S21(Log Mag),dB;  S11(Log Mag),dB

0.300;         -33.06;            -0.38
5.298;         -20.14;             0.00
10.297;        -17.76;            -0.08
15.296;        -17.05;            -0.16
20.294;        -16.81;            -0.33
25.293;        -16.79;            -0.45
30.291;        -16.85;            -0.55
35.289;        -16.90;            -0.59
40.288;        -16.91;            -0.62
45.286;        -16.89;            -0.61
50.285;        -16.79;            -0.59
55.283;        -16.68;            -0.60
60.282;        -16.60;            -0.64
65.281;        -16.58;            -0.73
70.279;        -16.62;            -0.81
75.278;        -16.78;            -0.87
80.276;        -16.96;            -0.87
85.275;        -17.11;            -0.85
90.273;        -17.16;            -0.81
95.272;        -17.11;            -0.75
100.270;       -16.96;            -0.76
105.269;       -16.71;            -0.80
110.267;       -16.48;            -0.94
115.266;       -16.36;            -1.13
120.264;       -16.39;            -1.31
125.263;       -16.58;            -1.40
130.261;       -16.85;            -1.42
135.260;       -17.10;            -1.30
140.258;       -17.27;            -1.14
145.256;       -17.31;            -0.99
150.255;       -17.22;            -0.94
155.254;       -17.01;            -0.95
160.252;       -16.78;            -1.12
165.250;       -16.60;            -1.39
170.249;       -16.48;            -1.70
175.248;       -16.52;            -1.94
180.246;       -16.70;            -2.03
185.244;       -16.89;            -1.92
190.243;       -17.07;            -1.67
195.242;       -17.17;            -1.44
200.240;       -17.18;            -1.24
205.238;       -17.10;            -1.16
210.237;       -16.97;            -1.22
215.236;       -16.84;            -1.44
220.234;       -16.74;            -1.75
225.232;       -16.64;            -2.12
230.231;       -16.58;            -2.42
235.230;       -16.58;            -2.47
240.228;       -16.57;            -2.32
245.226;       -16.58;            -2.01
250.225;       -16.63;            -1.72
255.224;       -16.68;            -1.49
260.222;       -16.76;            -1.40
265.221;       -16.83;            -1.47
270.219;       -16.87;            -1.68
275.217;       -16.83;            -2.03
280.216;       -16.73;            -2.40
285.214;       -16.56;            -2.68
290.213;       -16.40;            -2.77
295.212;       -16.30;            -2.61
300.210;       -16.31;            -2.27
305.209;       -16.43;            -1.91
310.207;       -16.58;            -1.65
315.205;       -16.75;            -1.51
320.204;       -16.82;            -1.54
325.202;       -16.76;            -1.74
330.201;       -16.58;            -2.10
335.200;       -16.31;            -2.58
340.198;       -16.05;            -3.04
345.197;       -15.94;            -3.27
350.195;       -16.00;            -3.15
355.193;       -16.24;            -2.72
360.192;       -16.51;            -2.22
365.190;       -16.74;            -1.83
370.189;       -16.85;            -1.62
375.188;       -16.80;            -1.59
380.186;       -16.59;            -1.79
385.185;       -16.27;            -2.20
390.183;       -15.90;            -2.83
395.182;       -15.63;            -3.51
400.180;       -15.54;            -3.96
405.178;       -15.62;            -3.83
410.177;       -15.85;            -3.21
415.175;       -16.09;            -2.48
420.174;       -16.27;            -1.93
425.173;       -16.35;            -1.65
430.171;       -16.30;            -1.63
435.170;       -16.15;            -1.90
440.168;       -15.93;            -2.45
445.166;       -15.70;            -3.19
450.165;       -15.54;            -3.97
455.163;       -15.46;            -4.34
460.162;       -15.48;            -4.02
465.161;       -15.58;            -3.20
470.159;       -15.69;            -2.36
475.158;       -15.81;            -1.75
480.156;       -15.88;            -1.49
485.154;       -15.85;            -1.53
490.153;       -15.75;            -1.88
495.151;       -15.56;            -2.50
500.150;       -15.31;            -3.34
505.149;       -15.03;            -4.15
510.147;       -14.77;            -4.45
515.145;       -14.62;            -3.93
520.144;       -14.59;            -2.93
525.143;       -14.76;            -2.02
530.141;       -15.01;            -1.48
535.139;       -15.30;            -1.27
540.138;       -15.47;            -1.38
545.136;       -15.52;            -1.77
550.135;       -15.42;            -2.40
555.134;       -15.16;            -3.19
560.132;       -14.85;            -3.91
565.130;       -14.51;            -4.17
570.129;       -14.30;            -3.68
575.128;       -14.22;            -2.80
580.126;       -14.35;            -1.92
585.125;       -14.54;            -1.39
590.123;       -14.75;            -1.17
595.121;       -14.85;            -1.27
600.120;       -14.86;            -1.63
605.119;       -14.73;            -2.22
610.117;       -14.50;            -2.97
615.115;       -14.20;            -3.69
620.114;       -13.94;            -4.06
625.112;       -13.76;            -3.74
630.111;       -13.73;            -2.86
635.110;       -13.86;            -1.96
640.108;       -14.06;            -1.33
645.106;       -14.23;            -1.09
650.105;       -14.34;            -1.11
655.104;       -14.35;            -1.43
660.102;       -14.22;            -1.98
665.101;       -13.98;            -2.80
670.099;       -13.65;            -3.72
675.097;       -13.32;            -4.27
680.096;       -13.06;            -3.99
685.095;       -12.93;            -2.96
690.093;       -12.98;            -1.85
695.091;       -13.18;            -1.06
700.090;       -13.42;            -0.69
705.088;       -13.58;            -0.69
710.087;       -13.61;            -1.04
715.086;       -13.46;            -1.73
720.084;       -13.14;            -2.69
725.082;       -12.71;            -3.77
730.081;       -12.26;            -4.40
735.080;       -11.98;            -3.87
740.078;       -11.91;            -2.54
745.077;       -12.14;            -1.27
750.075;       -12.49;            -0.48
755.073;       -12.85;            -0.17
760.072;       -13.07;            -0.33
765.071;       -13.13;            -0.83
770.069;       -12.98;            -1.68
775.067;       -12.64;            -2.81
780.066;       -12.18;            -3.99
785.064;       -11.75;            -4.41
790.063;       -11.46;            -3.46
795.062;       -11.37;            -1.86
800.060;       -11.48;            -0.62
805.058;       -11.69;             0.02
810.057;       -11.90;             0.07
815.056;       -12.02;            -0.37
820.054;       -12.07;            -1.25
825.053;       -12.08;            -2.47
830.051;       -12.06;            -3.95
835.049;       -12.09;            -5.28
840.048;       -12.13;            -5.44
845.047;       -12.16;            -4.01
850.045;       -12.16;            -2.23
855.043;       -12.16;            -0.88
860.042;       -12.24;            -0.25
865.040;       -12.36;            -0.19
870.039;       -12.50;            -0.62
875.038;       -12.61;            -1.36
880.036;       -12.64;            -2.42
885.034;       -12.55;            -3.57
890.033;       -12.34;            -4.36
895.032;       -12.05;            -4.07
900.030;       -11.78;            -2.84
905.029;       -11.60;            -1.52
910.027;       -11.61;            -0.67
915.025;       -11.80;            -0.45
920.024;       -12.09;            -0.69
925.023;       -12.37;            -1.35
930.021;       -12.55;            -2.27
935.019;       -12.55;            -3.42
940.018;       -12.39;            -4.53
945.016;       -12.13;            -5.09
950.015;       -11.88;            -4.58
955.014;       -11.74;            -3.37
960.012;       -11.77;            -2.24
965.010;       -11.98;            -1.61
970.009;       -12.27;            -1.54
975.008;       -12.49;            -1.99
980.006;       -12.55;            -2.87
985.005;       -12.38;            -4.19
990.003;       -11.99;            -5.91
995.001;       -11.55;            -7.75
1.000.000;     -11.21;            -8.23

The low frequency response (up to, say, 5MHz) isn’t particulary good. Starting around 5MHz, S21 is fairly flat around -17dB up to 400MHz. From there on it starts to increase.

S11 is between 0 and -4 dB up to 800MHz which I think is very good.

Final thoughts

These results are (much) better than I expected from my first attempt, so I am trying to figure out if I am measuring the correct parameters and whether my interpretation is correct…

Things I’d like to figure out:

• First of all: are my assumptions regarding the measurements correct?
• How do I improve the low-frequency response to be more linear?
• What causes the response (both S21 and S11) to ripple rather continuously? Should I suspect my measurement setup?
• Would I be better to lower the FET supply voltage (or input bias) in order to end up with an even -20dB gain, so that converting the voltage magnitude would be easier?

---

I’ll make a v0.2 with the changes and some additional improvements and then a properly shielded enclosure (likely pcb material).

While this is mostly an exercise and learning experience, I’m rather happy with the results so far.

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HP 8640B Signal generator. Part 1: initial state

I recently acquired a HP 8640B signal generator. A quick summary of the specs shows why this is an interesting piece of equipment:

• 500kHz to 512 MHz (option 2 adds a doubler, extending the range to 1.024 GHz)
• Available power range is -145dBm to +19 dBm, withing +0.5dB across the full frequency range
• Extremely low phase noise (less then -130 dBc at 450 MHz, 20kHz from the carrier)
• Fine tuning (when in locked mode): >+/-20ppm
• Integrated counter, switchable between the internal generator and an external input
• FM and AM modulation
• Beautifully engineered (but that’s just an opinion)

The retail price in the mid seventies was around 12.000 USD.

My unit has the frequency doubler (Option 2), and a serial number starting with 1522U, meaning it was produced in 1975 (add the first two digits to 1960), week 22 in England. As far as I know this is probably in a factory in Erskine, Schotland.

And obviously it doesn’t work as it should. At first sight, the apparent issues are:

• The unit has cracked gears (which is a common issue)
• Intermittent power rail faillures
• Counter shows all zeros (seems to work with external signals when the -5.2v rail comes on)
• No output power on the front connector
• “Reduce FM vernier” is lit

At least, the display shows something, and there are some lights. More lights = better.

It needs cosmetic work as well: there is flash corrosion all over the metal parts, and some kind of brown gooey residue. One of the modulation dials has a broken knob. Plastic has yellowed, the vinyl needs work. The list goes on.

First issues to be solved (apart from cleaning while waiting for components):

• Fix -5.2V power rail (A18U1 needs to be replaced)
• Replace burned diode on the A12 Rectifier board, possibly using an external rectifier placed on a custom made board
• Zener diode A20VR3 (+44.6V LED) seems to have been burned, or is at least moving in that direction

After those issues have been solved, the RF path can be checked.

The following sources of information have been of great help so far:

• The people at the great Yahoo HP/Agilent Test Equipment newsgroup
• Various blog posts, I will list them where apropriate
• The US Army TM 9-4935-601-14-7&P “PATRIOT AIR DEFENSE GUIDED MISSILE SYSTEM” Operator, organizational, direct support and general support maintenance manual

HP 8711A: modifications, tweaks, refurbishing

I recently acquired a fairly well preserved HP 8711A network analyser. As sold, this is an economy model, offering only scalar measurements (although vector information can be acquired through the HP-IB bus).

Conversion

What makes this unit interesting, is that the hardware is the same as the never released 8712A, which has vector capabilities. Some changes in the bootrom allow the user to “convert” the unit, making it behave like a full fledged VNA.

Credits for this rom modifcation go to Massimo Porzio (IK1IZA). As far as I know, he is the one who took the time to figure this out. I also found a lot of information on the site of Michal Lewczuk (SP2XDM).

Before doing anything, a backup of the correction constants was made to disk.

Unit boot screen as received:

I used the TL866 USB High Performance Programmer which can be bought online for around €40. Download the software while you wait for the package to arrive, download speed reminded me of the late nineties…

The bootrom with attached label (on the A1 CPU board, removal of the CRT/PSU module is necessary):

Contents of the ROM at address 0x1FFA0 prior to modifications:

After modifications:

New bootscreen (SRL and fault location enabled as well):

Smith chart as proof of vector capabilities:

CRT realignment and brightness setting

Some adjustments on the CRT were done:

• When looking from a normal viewing angle with the unit placed on the desk, the alignment of the options on the right of the screen seems “off” when compared to the location of soft buttons next to them.
• The screen was rather dim.
• Sizing of the display compared to the cutout in the front panel could be enlarged.

I marked the locations of the front panel cutout and the top and bottom buttons on the CRT:

After disassembling the CRT/PSU module and attaching the PSU to the back, followed by some creative cabling, the unit was powered on while leaving access to the alignment potmeters:

The positions of the potmeters before adjustment were marked:

After adjusting the height/width and brightness, the screen looked much better.

Power supply patch

According to a ECN from HP, units with serials between A00000 and 3325A00941 need a 3W, 680Ω resistor between pin 9 and 16 on J5 in the power supply.

Before:

Resistor added and pcb cleaned:

Speaker “modification”

The A models of this network analyser had an issue where noise would get into the audio circuit, causing a high pitched, squeeling noise. When used in a silent environment this was really bothersome, so I first tried replacing the speaker by a version with a slightly different resonant frequency.

This didn’t help enough, so I ended up with this ugly (but fully working) solution:

Cleaning and restoring the unit

A lot of time was spend cleaning and restoring the unit, as to give it as much of its original appearance as possible.

• The inside of the case was cleaned, connectors on the backplane (which are notorious for causing problems with this model) were cleaned using a combination of MEK, IPA, soda, warm water, patience and lots of love.
• Front panel and keypad were cleaned, and the yellowing of the plastic due to age and UV was reversed through the use of high concentration hydrogen peroxide. Dents, holes and cuts were fixed.
• The vinyl was cleaned, restored and treated.
• Internals of the disk drive were cleaned.

Documenting this is worth a separate post.