I have been toying with the idea of making a Si5351A breakout board ever since I saw the chip details on Silicon labs web site. It seems SI5351A is an ideal choice for Variable Frequency Oscillator (VFO) for a homebrewed radio. The SI5351 is an i2c programmable any frequency clock generator. The chip can generates up to 8 non-integer-related frequencies from 8 kHz to 160 MHz depending on the model. There are 3 different variations of the chip currently available. For the home brewer SI 5351A looks promising as it is available in a 10 pin MSOP package.
For the ham radio home brewer, such a chip which can be programmed over many ham bands offer numerous possibilities. The most obvious choice would be as a VFO. Currently, the analog devices DDS chips are ruling the market. Compared to them the Silicon Labs chips are cheaper and less cumbersome to handle. I am yet to see the noise performance of these two chips compared.
In fact, there are a couple of commercial Si5351A breakout boards targeted at the home brewer. They cost around 10$. But the chip itself cost only 1.33$ on the Silicon Labs website and you need only a few additional components to complete the board. The total cost need not be more than 3 $. Along with a cheap Arduino clone, Si5351A gives you lot of flexibility. An added advantages is that the SI5351A has three independent programmable clock generators. You can combine the VFO and BFO into a single chip.
My idea was to build a single side PCB so that any one can build one via toner transfer. Last week, I had a long chat with my friend and colleague Amal Dev about designing a simple board. The board routing was done by Amal Dev on eagle, We have designed an experimental single sided board and it may not be conforming to RF design principles always.
For the interested home brewer a toner transferable image is here. You can buy the SI5351 from RS components or from Mouser. The board is to be powered from a 5v source. The SI5351A needs 3.3v for its operation. The LM1117 is used for generating this. Note that the LM1117 is SMD and is soldered on the copper side. The two MOSFETs ( BS170) are used to convert the I2C levels from 5v to 3.3v. Check the pin outs before you solder. The board can be directly powered from an arduino.
For programming the board you can try the example code from Hansummers. Alternately you can try the excellent si5351 arduino library. Tom Hall AK2B has some sample sketches using this library. Please note that I2C address of the chip may be different from those found in the above code. Please use an I2C scanner to find out your chips address and then modify your code accordingly.
The pin out of the SI5351A is shown below.
A special thanks to Amogh Desai who tested the first version and reported couple of errors. Further comments and error reports are welcome.
I have been trying to program cheap Chinese DDS modules recently. I wanted to add a DDS VFO to one of my bitx rigs. These modules are available at $4.5 from aliexpress. They claim that it can work up to 40Mhz.
I used an arduino Uno along with a rotary encoder for controlling the module. The DDS module pin out is shown below.
The connections to arduino and rotary encoder is shown below.
This note describes a compact Antenna Tuning Unit (ATU) that I assembled recently. It is capable of handling the full 100-watt output of my FT-840 transceiver.
For efficient radiation of the RF power otput of a HF transmitter, its output impedance, the characteristic impedance of the transmission line (usually coaxial cable), and the radiation resistance of the antenna should all be the same. Over the years, this standard impedance has evolved as 50 ohms for communication equipment — applicable to RF transmitters, receivers, coaxial cables and even the Standing Wave Ratio bridge (SWR bridge). The departure of the impedance seen at the transmitter output from this standard value is shown by the SWR bridge. A SWR of 1.0 indicates an impedance of 50 ohms resistive.
An antenna, when cut for the band of operation, is said to be a resonant antenna. At the antenna end of the transmission line, the RF impedance of a resonant antenna is a pure resistance known as its ‘radiation resistance’ whose value, being different for different types of antennas, is not always close to 50 ohms. Moreover, when it is not 50 ohms, the coaxial cable might transform this impedance to some other value at its transceiver end. So, even a resonant antenna might be seen by the transceiver as having a SWR higher than 1.0. Because of the difficulty of putting up an antenna for each band of operation, we are often constrained to operate using a non-resonant antenna, which appears as a complex impedance made up of its ‘radiation resistance’ plus a significant capacitive or inductive reactance. Both these components vary with the frequency of operation.
Present-day solid-state transceivers, which have protective circuits that sense the SWR seen by the rig, would not load antennas that show a high SWR. Moreover, these transceivers make use of a bank of bandpass filters near the antenna terminals that would provide the required bandpass characteristic only when seeing a 50 ohms impedance. Therefore, these rigs need an ATU to work with antennas that show even a moderately high SWR exceeding 1.3.
In many situations, we rely on an ATU to enable us to operate with a ‘short antenna’, i.e. one whose resonant frequency is higher than the frequency of operation. A short antenna appears to the transmitter as a complex impedance in which the resistive component or ‘radiation resistance’ is much lower than that of a resonant antenna, whereas the capacitive reactance is substantial and dominates over the resistive component. It is the job of the ATU to transform the complex antenna impedance to 50 ohms resistive as seen by the transceiver. However, when we operate with a short antenna in this manner, a part of the transmitted power is wasted (1) as ‘line radiation’ from the coaxial line, and (2) as increased ‘resistive losses’ in the coax and the antenna due to the higher RF currents needed to radiate power from the lower ‘radiation resistance’ of the short antenna. The ‘resistive losses’ occur due to real resistance of the conductors at RF (which again is different from the resistance at DC), whereas the ‘radiation resistance’ of the antenna is a virtual resistance which can be calculated theoretically for any antenna (and frequency of operation), and which governs the RF power radiated by the antenna.
Long ago, when I commissioned my ham radio equipment, I was keen on building a good ATU. The general belief then was that a Rotary inductor was an essential component of the ATU. Since I couldn’t locate either a rotary inductor or a 12-position antenna switch for use with a tapped inductor, my ATU project did not take off and, after a while, I lost interest in it. I was QRV only on the ham bands for which I had antennas.
About a year ago, my friend Salim, VU2LID / N8LI, who works in USA and visits India often, suggested that I try operating on 80 metres, and loaned me his SPC Transmatch, which enabled me to tune my 40-metre dipole on 80 metres. Because of the success of the 80-metre operation, my interest in ATU’s got revived. The elusive 25 uH rotary inductor was also finally located. However, the cumbersome size of the rotary inductor and its dial drive put me off, and I started surfing the internet for circuits of compact ATU’s that didn’t need a rotary inductor.
I was able to locate some articles on the Z-match ATU authored by G3VGR, VK5BR and others. Their circuit used a tapped coil (toroidal or air-core) with a link feeding the antenna (Fig.1). I tried this circuit and found that its tuning range was very limited, necessitating tricky adjustment of the total turns and taps of the coil. Also, I didn’t like the link coupling for RF power transfer. So I continued my search, and finally located the article by XS4ALL on the elegant Fri-match ATU, originally developed by PA0FRI. Fig.2 shows the circuit diagram of the Fri-match ATU. This ATU uses a single tapped coil (toroidal or air-core), which couples directly to the antenna. An interesting feature of the Fri-match ATU is that the input and output of the ATU can be interchanged. The conjugate configuration is said to work better in some situations.
A brief discussion on the use of BC-type air-variable tuning condensers for RF power transfer would be appropriate here. In many published articles, the suggested plate spacing for the variable condenser of a 1 kW ATU is around 2 mm. For RF power levels of upto about 200 watts, such wide-spaced condensers are unnecessary, especially when low-impedance antennas are used. I always test BC-type variable condensers before using them in my ham projects, by connecting a 230-volt 10-watt bulb in series with the variable condenser, then applying 230-volt AC to the combination, and turning the condenser knob to and fro to check for arcing. Fig.3 shows the test setup. A good-quality 2-gang 500-pF BC-type air-variable tuning condenser (Polar, Sanyo etc.) would generally pass this test without any arcing. This means the condenser can handle 230 volts RF, which appears to be quite adequate. In fact, it is not the plate spacing of the condenser that appears to be critical here, but rather the RF current-carrying capacity of the wiper of the condenser. At any rate, a good BC-type air-variable condenser, tested before use and having a clean wiper, should be quite adequate for RF power levels of upto 200 watts.
I had with me 2 Nos. of 1.56 in. o.d. toroids of unknown permeability characteristics, which were found to be good for HF. I stacked the 2 toroids, wound teflon tape over them, and then wound the coil using 14-gauge enamelled copper wire. Winding the toroidal coil was a real pain. Silver-plated multi-strand soft copper wire with teflon insulation would have made the job easier, but it is not available here. The number of turns needed for the toroidal coil depends on the core area and permeability of the core. Suggested number of turns is 15, 20 or 25. I used 20 (n) turns with taps at 4 (n/5), 8 (2n/5) and 12 (3n/5) turns.
To make a long story short, my Fri-match ATU was completed in March 2011, nearly half a century after I first thought of building an ATU! Fig. 4 shows a photo of this ATU. It has just 2 controls, and no rotary inductor. It outperforms the conventional Z-match with regard to ease of tuning and tuning range, and is almost as good as the SPC Transmatch. And interestingly, so long as the Fri-match ATU is able to match an antenna within its tuning range, it is able to bring down the SWR to exactly 1.0. This is something that I had not expected from a 2-knob ATU that is free of the burden of a variable inductor!
No reduction drives are used in this ATU. Though the tuning of the condensers is very sharp, it is manageable, even for a person aged 75 years! An analog SWR bridge is needed for tune-up. A point to be kept in mind is that, if one of the condensers is very much off-tune, tuning the other condenser would not produce any dip in the reflected power. Therefore, in the absence of calibrated dials, visual monitoring of the condensers is necessary. The body of one of the condensers has a RF potential but, since it is tied to the transmitter output, there is no hand-capacitance effect.
The Fri-match ATU sits to my right near the front edge of the operating table, not far away from the FT-840 transceiver. From the antenna switch, a 70-ft. length of RG-223 coax feeds a 40-metre dipole antenna, and a 50-ft. length of RG-213 coax feeds a HY-GAIN 12AVQ 3-band ground-plane antenna. The ATU enables me to use the 40-metre dipole on 20, 40 and 80 metres, and the 12AVQ ground-plane on 10, 15, 20 and 40 metres — all with a SWR of 1.0 as seen by the transceiver. So much so, the ATU is useful even when a resonant antenna is used for the band of operation. On 20 metres and the higher bands, I normally use the 12AVQ ground-plane. The only time I operate with a non-resonant antenna is when I use my 40-metre dipole on 80 metres. Signal reports then indicate that I am roughly 1 S-point weaker than similar stations using a 80-metre dipole. That’s not bad, and I am quite happy with the performance of the ATU.
I recommend this ATU to all hams. When an ATU is available, we can fabricate a dipole, ground-plane or any other antenna simply to the dimensions suggested by theory, and dispense with the trimming of the antenna. In many situations, trimming of the antenna to lower the SWR is unscientific, because the problem is not in the antenna, but elsewhere! It is better to rely on the ATU to take care of the fine tuning of a resonant antenna.
( OM Jayaraman VU2JN has kindly permitted me to put this note on this blog. Many thanks to him for sharing his experiences. VU2SWX. )
I have been playing with the ISDR software defined radio from OM VU2DEV , Ramaprabhu ( from Bangalore )for the past one week. ISDR is a low cost SDR for 40m band. The radio is performing wonderfully. I bought 2 assembled kits from DEV and I recommend the kits to any one who wants to have a taste of SDR technology. The fully assembled radio is available with VU2DEV at Rs 750 ( US $15 approximately) and comes packed in a rugged metalic enclosure and with necessary audio cable. ( Email vu2dev at yahoo.com for availability, exact pricing and shipping info. ).
The following pictures show the professional quality ISDR .
OM VU2DEV has also sent me the schematic diagram and component values and permitted me to post the same on my blog.
The block diagram of the radio is shown below. (Click on the picture for a larger view)
At the heart of ISDR is a 28.224Mhz crystal which is used to create stable oscillations. This frequency is divided by 4 using two 74HC74 dual flip flops to produce two 90 degree out of phase local oscillator signals for the radio.
On the RF side signals from the antenna are filtered for the 40m band and some RF amplification is given. The RF signal is then mixed with local oscillator signal using two diode ring mixers. The resultant signals are sent to a simple low pass filter to get rid of unnecessary mixer products and then amplified. The two quadrature signals are fed to the mic input of a PC sound card. ( You need a sound card with stereo input).
The sound card is used as analog to digital converter. The normal sound card found on your PC will have a sampling rate of 44khz ,with 16 bit accuracy. The sampled signals are processed digitally inside the PC using SDR software and you can hear the radio via PC speakers. Your tuning range will be half of the PC sound card’s sampling rate. So with 44Khz card , you can tune from 7034Khz to 7078Khz. If you have a professional quality sound card, your milage with SDR will be better.
The circuit diagram and component values are given below.
If you want to look at the circuit in finer detail, download this pdf file. The info sheet provided by vu2dev is here . The component values for the circuit can be found here. T
I have successfully used Rocky, kgksdr and winrad on my PC running Windows XP ( 🙁 , I had to install it at last ). I am yet to test the SDR on linux and will update as and when I am able to do so.
My Intel G35 mother board seems to supports ( I am suspect ) 96khz sampling via the onboard Realtech card. Hence , I am able to tune almost the entire 40m band. Here is a recording from the radio using Rocky.
OM VU2DEV at his morning QTH M/s Micronova Impex Pvt Limited is making a large number of kits including ISDR for the benefit of the community. You may contact them directly. The contact email id is <email@example.com>.
I have used his JOTA transceiver and reviver . Both are valuable additions to your shack. He has also provided me with PCBs of his new 72 watt linear, I am planning to build them during my next vacation
Here are photographs of my shack. I have built several equipments and circuits over the years. These pictures show the status of my shack as on September 2010. I took them last week on a Nikon CoolPix L21 .