06 September 2021

Laptop WiFi antenna coax

Laptop computers usually have the WiFi antenna in the upper border of the screen/cover. When you dismantle one down to the last bit, you end up with about 60 cm of coax, a super small coax plug connector and a J-pole looking antenna for 2.4 GHz (I haven't torn down a laptop with 5 GHz yet). There might/should actually be two antennas for reception diversity (improves signal quality).

Laptop wifi antenna.
Laptop computer WiFi antenna, coax and connector.

Since these antennas work at 2.4 GHz, I wanted to know if that coax is any better than our HAM radio cables. Not an easy task without markings on the cable, but starting from the connector name, probably U.FL, I found a ready-made cable with detailed specifications.

Well, the manufacturer declares 3.2 dB/metre at 2.4 GHz. As a comparison, the "tiny" RG-174 we use in HAM radio, looses "only" ~2.4 dB/metre at 2.4 GHz.

Conclusion: unless you need a WiFi antenna for some homemade module (like ESP8266/ESP32) or a very tiny coax, these things are not a keeper.

15 August 2021

Topward TFG-8104: the multimeter challenge

SPOILER ALERT. This post contains my take to the question asked in the previous post: "how to determine if it works with just a multimeter." If you want to play the same game please skip one post backwards and read my challenge.

Here we go.

The Topward TFG-8104 Function Generator powers up and generates enough RF that can be picked up by a nearby AM receiver. What else can be said with just a multimeter?

Since the output frequency can be reduced down to 0.1 Hz, all variations can be slowed down enough to be seen and appreciated even on a slow digital multimeter.

According to the manual it should produce a 20 Vpp signal. So I set:

  • lowest possible frequency, 0.1 Hz or so
  • square wave output
  • maximum amplitude
  • no DC offset
  • no modulation
  • no attenuation

The output alternates between -11.5V and +8V while it should be -10/+10V. When the amplitude is reduced readings collapse towards 0V and the asymmetry reduces, which is good. I can't think of further checks that are possible with a multimeter at the output connector. Well, there is output impedance.

So, even before checking the output at the oscilloscope, the asymmetry should mean something had happened to the output stage. A visual inspection of the circuit reveals signs of human intervention or fault on the output attenuator: some signal has tried to enter the function generator?!

While the box is open, the multimeter can check voltages. Both +15V and -15V are present. The circuit diagram mentions +/-20V but they are not marked on the board, nor there are accessible test points.

10 August 2021

Topward TFG-8104: does it work?

Does the Topward TFG-8104 Function Generator (from the previous post) work? Well, considering the source, the correct question is: "What is broken in the Topward TFG-8104 that I now own?"

A Topward Function Generator TFG-8104 in mint aesthetic conditions.

While it looks good aesthetically - if you ignore the heavy uniform yellowing of the plastic case -, some tools are needed to determine how the inner electronics are doing.

First test: power-up. It is quite likely that someone else has plugged it in before selling the device, so a power-on harm has probably already been done. There's always a bit of adrenaline shock when pushing the ON button, and the red LED lights up. Nothing else can be tested using eyes and nose.

Next check: does it generate something? This step is easily done with the recently acquired Hantek oscilloscope, that is now resting in its box out of sight and immediate use.

First of all I tried tuning the generated signal on an AM radio. It is easier to slowly sweep the generator over the tuned frequency than doing it in reverse. Got it. With and without modulation. Good sign, it's worth going further. Oscilloscope? Not yet.

I opted for a more fun approach: understand as much as possible with a simple multimeter.

How would you do that? What can you check?

You can contribute in the comments, and read how it I did in the next post.

05 August 2021

Topward multimeter and function generator - for repair

I got two Topward instruments at a fundraising event: they give away truly untested and almost certainly broken electronic devices that might have a second chance of life in a different form. Most of the hardware available is composed of old computers and parts from 10-20 years ago, with the occasional electric/electronic lab novelty.

Topward TFG-8104 and 1302
Topward TFG-8104 and 1302.

I brought home a Topward Function Generator TFG-8104 and a Topward Digital Multimeter 1302. Built in mid-1980's in Taiwan by a company that is still around in 2021, and has released scans of manuals and diagrams.

Both boxes are very light, even though they contain a real transformer. I initially thought they were empty! I would say that even back in 1980's these guys were entry level devices and according to the front panel wear, they haven't been used much.

The multimeter seems OK. Voltage readings are all a bit low. Resistance is in the right ballpark and I haven't measured a current, which is the faulty part in other multimeters around my lab.

The function generator, while quite obsolete today, it provides uncalibrated 0.1 Hz to 2 MHz sine/square/triangle and AM/FM modulation, which makes it suitable for driving optical transmitters or "transmitting" a signal to a nearby AM radio. I had a look at the schematic diagram before buying it and its use of standard components makes it very suitable for repairs, which is the fun part I am looking for.

30 July 2021

Winding QCX toroids as left-handed

The other day I resumed building the QCX transceiver kit I bought a couple of years ago in 2018. I had left it to the point of winding toroids.

All seemed fine for L4 (the red one), a bit loose but can be moved to match the PCB silkscreen.

Then comes the middle one from the output filter (L2) and ... I realised that winding toroids is another thing that left-handed people do differently! Loose ends of the winding end up on the opposite side vs. what is expected on the PCB and the inductor sits diagonally.

QCX toroids by a left-handed
QCX toroids by a left-handed builder.

When winding L1 I paid attention to the position of the wire and it matches better the silkscreen. Since I've had to remove L3 I might fix L2 as well so that these coils are perpendicular to each other as designed by Hans Summers.

Regardless the toroid is done with the right-hand or with the left-hand in the direction expected by the PCB, it really goes against the left-handed nature!

05 July 2021

From Tortona 2021, Nixie and CRT

The picture shows part of the spoils from Tortona June 2021 flea market.

On top is the original box containing a DP7-32 CRT. The letter "P" in the name means "long persistence, dual color". It has been opened to check the content presence and integrity, and that's all.

On the lower part of the picture is the display board from R&S receiver, showing a nice algorithm of hand routing PCB traces. Something had burned in this board because a resistor had been replaced but another one has a different [lower] impedance which causes lots of current to pass into a Nixie and the associated 74141 driver, which, in turn, is not functional any more. So the board gives 7 sockets, probably 6x 74141 and 5 good ZM1182 Nixies.

01 July 2021

Testing the equipment by testing other things

I found myself stuck in a testing loop while learning on-the-field how to use the Hantek oscilloscope: I was creating test cases for the oscilloscope that dubbed into test cases for accessories.

See this. The scope has a -3dB 150 MHz bandwidth, so I picked up my 14 MHz Marker Generator and tried to visualise the output terminated on a 50 ohm dummy load. The marker generator creates a short impulse in the time domain, which results in many peaks in the frequency domain. The short square impulse should be 10 ns long inside a period T of 71 ns at 14 MHz.

Grab all the probes in the lab and see which one renders better the impulse train. Don't forget to set the probe to 10x.

Probes were tested in this order:

  1. P6100, 100 MHz
  2. P6100, 100 MHz
  3. PK-8150, 150 MHz
  4. PP150B 150 MHz, Hantek
  5. 88025, 250 MHz, Greenpar

The first stitch below shows 4 periods of the test signal, while the second image is a zoom-in on one impulse.

According to the screenshots (wow!), all five probes behave almost in the same way! Even the input square wave show ringing.

Well, I do want to see an impulse train so I will either change the XTAL in the marker generator or use the arbitrary function generator embedded into the scope. Or both.

When time allows I will repeat the experiment with a 200 MHz Siglent oscilloscope (not mine).