Aluminium foil shielding box

In recent 2.4GHz measurement, I suspect the WIFI around here is interfering, to get rid of these annoying WIFI or other 2.4GHz source, a shielding box is made, using aluminium foil, it is found that the top cover need to be in tightly contact with the lower box body, otherwise it's not shielded enough for this bluetooth speaker to shut up. 

Bluetooth speaker as a test sample.

Measuring antenna is insert from the top little hole. Speaker and mobile phone inside.

Active RFID tag spectrum shape and burst time casual measurement

The ADVANTEST R3463 spectrum analyzer is just arrived, it frequency range reached 3GHz and it's a synthesizer type of spectrum analyzer so it get rid of many problem for example frequency drift off the view, ofcause a 3GHz analyzer enabled measuring of abundant 2.4GHz radio systems, including WIFI, bluetooth, zigbee and also this one: 2.4GHz Active RFID Tag.

This little 2.4GHz radio tag is powered by CR2302 battery, firing a short burst of signal in 1 sec time interval. May be it could be used to locate a pet cat.

Because there are many WIFI routers nearby as interference and the tag is very low power (~0dBm), the tag have to put closely to the antenna.

The spectrum view of tag signal, using long time peak hold, because it firing a burst every second, which is quite infrequent. The center frequency is measured 2.457GHz and power -13.5dBm.

As we can see the SPAN setting of this screenshot is 10MHz, and every grid equal 1MHz, the -6dB bandwidth of the spectrum is roughly 1MHz (for compliance to FCC rule for non-frequency hoping system in 2.4GHz ISM band, the -6dB bandwidth should be wider than 500KHz).

One feature of this R3463 spectrum analyzer is Transient analysis, which is a power vs time measurement mode, by using this mode, we can obtain the power envelop of the signal burst, the burst time is 180uS, that's translate to 1/5555 duty cycle.

HP 8590D spectrum analyzer CRT repair attempt

Well, the CRT of HP 8590D spectrum analyzer has been malfunctioned for a few months, a while before i was trying to hook up an external monitor to the machine, hope that i can still make use of this machine for a little while more, however i found this way inconvenient because the context button next to the original screen is far away to the external monitor and I also have to turn on the external monitor and video converter whenever i want to use the spectrum analyzer.
As I know the CRT is actually working quite independently to the spectrum analyzer core, i hope that i can open up the analyzer to check if there are some burnt component to replace or some tuning knob i can adjust so to save back the CRT.
May be it's time to start this exploration journey.

This CRT have been looking like this for a few months, too bad.

Amazing looking, what a sophisticated high tech stuff in 90s, 

as a conventional electronics testing instrument design, it's heavy but, not difficult to take off the cover. As I expected the internal of the machine is highly modularized.

CRT, mixer and various of RF modules are shown, many SMB connectors, RF modules are heavy shielded and connected with semi-rigid cables, the CRT shielding could not be open, may be need to tear apart more module before i can take out the CRT.

The A5 second converter module, and a stuff in the middle of a rigid cable looks like common mode filter.

The bottom side contains an unshielded logic board, may be it's the computer, (90s computer)

Marking on the RF modules shows "LOG AMPL", "BW FILTER", "AMPLITUDE CONTROL" and "THIRD CONVERTER", exciting!

Carefully removed the front cover and power supply module, so the CRT could be extract out.

The extracted cute looking CRT, shielding was removed. I can see some knob showing "Vertical Line", "Horizontal line" and so, may be I could try to adjust these knob.

There we go! after tuning the "Vertical line" knob, the screen is backed to normal. I could also tuning the "Vertical size" knob to make the graphics fully using the whole screen size.

Reassemble the stuff back to the position.

It looks better than before, say goodbye to the external monitor.

Breakthrough of the year 2014 - Long range challenge

Breakthrough of the year! Plover Cove Reservoir main dam long range challenge completed!

It has been a long time goal for me to making a wireless range that could cover the main dam, using 27MHz band and just a monopole antenna. Previous attempts was not successful because the power of transmitter was not enough and sensitivity of receiver is also insufficient. 

It's not easy to get to a good test environment in urban area because the buildings and terrain blocked the signal. The only testing site having straight open channel is the Plover Cove Reservoir main dam.

The simplest form of communication transmitter is to merely feed a carrier to the antenna, no modulation, the key point is big power. Previous attempts was an crystal oscillator stage and one stage of power amplification. In this attempts, one more stage of power amplification is added, coupled by 4:1 step down transformer.

The transmitter with 650mm monopole antenna on ice-cream stick.

27.76 dBm output power, which is more than 500mW

(Note that CRT of this analyzer gone crazy just after boot up for a while)

The transmitter was firstly placed on the beginning of the dam and receiver walks towards the end of the dam. 

The receiver showing the carrier is still detected, at the end of the dam. The sensitivity of this receiver is measured as -90dBm. I guess if need better sensitivity, a double conversion architecture is required.

The receiver signal strength indicator LEDs gradually show lower and lower signal strength until the end, 3 of the total 16 LED still remain shining, what a relief.

Frequency response of 3 types of 455KHz ceramic filter

There are many types of 455KHz ceramics filters out there, what is the different between them? the 455KHz usually comes with suffix A/B/C/D/E, indicating the bandwidth of the filter. what is the filter response actually looks like? In this test we picked 3 ceramics filter, They are -A -D and -E.

The A type filter having an impedance of 600 ohm for both in-out, D and E type having an impedance of 1500 ohm. The filters are loaded with resistor and output RMS voltage are measured using oscilloscope.

The responses are normalized and the result turns out like this. So now I get some concept of what the shape of their frequency response looks like.

2N2222-PIC16F1716 Variable Gain Amplifier - A Quantitative Test

The relationship between control voltage and amplifier gain is an important performance measure of an variable gain amplifier (VGA). In this test, The amplifier is feed by the 8-bit DAC output of 16F1716 microcontroller, the micro is re-programmed each time to generate a different control voltage.

Input frequency=455KHz, Input power=-37dBm

The output level (RMS) is measured by oscilloscope and the relationship of control voltage and output level turns out like that.

On the other hand, let's compare to this, the datasheet figure of Analog Device AD603 Variable Gain Amplifier, the gain is very nicely linearly proportion to the control voltage. I don't know if this linearity is vital in radio IF amplifier application.

2N2222-PIC16F1716 Variable Gain Amplifier brief test

The latest version of 27MHz radio car remote control receiver. In any radio receiver, the IF gain stage must provide variable gain in order to couple with the dynamically changing channel loss. The modification from previous receiver version is minor, the only different is the addition of a buffer amplifier after DAC output, this buffer amplifier is needed because it was found that the DAC output could not provide sufficient current to drive the dual Variable Gain Amplifier (VGA) input. Just soldered the compoents for VGA IF stages, can't wait for doing some test before solder the antenna matching coil.

The latest version of 27MHz RC receiver (DUT)

The previous version of 27MHz RC receiver (VGA not working at all)

455KHz, -41dbm signal is feed into the VGA input port, the VGA can only provide a relatively small dynamic range, so it is easily overloaded.

The PIC16F1716 MCU is programmed to generate this linearly ramp up control signal. From 0V to 5V. DAC resolution is 8-bit.

The output of one of the differential output of VGA. Notice that the VGA is not conducting before control voltage is up to about 0.7V. after 0.7V the gain increase quite linearly as the control voltage increase. Result is quite as expected.

The next test should be the dynamic range. TBC...