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...

ADF7020 ISM BAND TRANSCEIVER module

Found out the ADF7020 radio module is quite popular in market, so just brought some back to get a hands on it.

The ADF7020 chip
This chip, once frustrated me, because it's such highly integrated and it make ones feels that most of the study in discrete RF circuit design is worthless, LNA, quadrature mixer, IF filter, IF amplifier, modulator, demodulator, ADC, power amp, PLL... all-in-one single chip. What a respectable piece of art. It looks like there are no space left for RF designer. But I have to overcome this frustration and test test this little modules.

The module

The module is supposed to be inserted between two UART device as a cable replacement device. It consists of ADF7020 ISM band transceiver, a MCU, which responds for initialization and translate the external UART signal to the data format required for ADF7020. A 11.0592MHz crystal , what an odd frequency, some matching circuit, a SMA connector, a power regulator, and 7 pin interface terminal.
The manufacturer provide with an PC tool to config the module, the tool controls the carrier frequency, RF output power, baud rate etc. I set the carrier to around 443MHz and trying to see if any RF comes out from the SMA connector.

The output

It just fire up RF output once power is turn on, clearly seeing the fundamental 441MHz output, with power level of 15dbm (adjusted figure due to -3db attenuator) and some harmonics, about be 30dbc lower than the fundamental. This is a good start, some real field test have to be done to see how this power level is translating to the actually usable distance.

HP8590D Spectrum analyzer death CRT

The CRT display of 8590D spectrum analyzer is dead. There are a BNC composite video output on the back. Looks like it can output to a TV or something.

Brought back a composite video to VGA converter, it turns out like this.

Mixer compare - NE602 vs BFR93 Discrete Glibert cell

The discrete transistor Gilbert cell mixer which is briefly tested a month before, is that mixer really having a decent performance? Let's compare it with a classic, popular NE602 mixer. Because both mixer do not have 50 ohm output impedance so this time a 4:1 transformer is added to make it more match with the 50 ohm spectrum analyzer.

The board for mixer comparison is consist of upper section discrete Gilbert cell and lower section of NE602 mixer. Left side is RF and LO input connector and the output connector is on the right side.

The 1.6 mm board is a mistake, 1.2 mm would be better.


Transistor Gilbert cell under test, with LO input 27MHz -20dbm and RF input 32MHz -20dbm,
beside the wanted 5MHz mixing product, many other frequency component is observed.


With same input signal. The NE602A mixer give way less spurious component, but the mixing product also lower in power. This may caused by the bias current of NE602 is smaller than the discrete mixer. It is observed that the LO isolation in NE602 is better.

This comparison attempt did not draw any conclusion, further analyst and more test is required to be done.