Interfacing colored LCD ILI9341 with 8-bit Microcontroller

For my current project, a compact sized multiband transceiver, I wanted to have a colored LCD module as display. On the web I found the ILI9341 LCD. This display has got up to >200000 different colors (depending on the respective mode you chose) and a resolution of 320×240 pixels. It can be driven in various parallel and serial (SPI) modes, therefore it is very versatile. Price for the LCD is about 10 US$ (11€).

First I developed code for a 4-line SPI interface. The display worked, but I found that it was much too slow. A lot of data has to be transferred because due to the higher resolution of the LCD I chose a 12×16 pixel font. That is very much for a small microcontroller (I am using an old ATMega128) clocked to 16 MHz via serial transmission

But I loved the colors and the luminance of the LCD. After a brief research I found that there is also a PCB available for parallel driving. This is sold as a “CP11003” display. The ATMega128 has plenty of ports and that made me think of driving it in parallel mode.

This display has 16 data connectors (DB0:DB15) of which 8 can be used for driving it in parallel mode. As common for parallel bus type LCDs these are the higher 8 bits of the data bus, thus DB8:DB15. DB0:DB7 are not used and therefore not connected.

As control lines there are “RS” (Data or command indicator), “WR” (write operation indicator),”RD” (read operation indicator) and “RES” (reset) are used for control.

CS (chip select) can be connected to GND when the LCD is the only device connected to the 8-bit bus.

ILI9341 LCD TFT display - 8bit parallel bus mode
ILI9341 LCD TFT display – 8bit parallel bus mode

Software development was easy using the GNU C compiler vor AVRs. (I still don’t use Arduino libraries! ;-)) The code can be found after this article.

Final hint: The module also has a touchscreen integrated but that is not in use here!

73 de Peter (DK7IH)

Code for ili9341_par_8bit

Curing transmitter instabilities with the”High Performance TRX” for 14MHz

I have used a simple dipole for 14MHz temporarily. This antenna was not much as quiet as the Deltaloop but much simpler to install and fix, compared to the loop that has been my standard antenna for 14MHz for years. The noise in the receiver when using the dipole was beyond words.

Thus I changed the dipole for a quad loop again the last weekend. When connecting the transceiver I found that in spite of good SWR (1:1.2) the transmitter was self-oscillating. With the spectrum analyzer I could recognize that there were oscillations in the medium wave range as well as around 14 MHz. This made me examining the transmitter more closely.

After estimated a dozen of checks and failed attempts I changed to improve the performance I decided to alter the input transformer of the final stage. This item,  which by origin had a winding ratio of 4 to 2+2, then was changed by a transformer having 3 to 3+3. This even deteriorated the situation.

Next I concluded that the second winding might have too high impedance. So I went to a 4 to 1+1 transformer (pig nose core BN43-202).

This finally cured the problem to a 100% and gave a very stable transmitter. Output power increased to 28 watts when fully modulated. This might be the direct outcome of better impedance matching.

Here is the current transmitter circuit with all the improvements made so far:

DK7IH - High performance Transceiver - Transmitter section
DK7IH – High performance Transceiver – Transmitter section (Full size image)

73 de Peter and “Thanks for watching!” 😉

More output power for the “High Performance Transceiver” for 14MHz

The first power amplifier for this transceiver project initially was capable to produce 20 watts of SSB pep rf power on the 20 meter band. The transistors in use are the 2SC1969 bipolar types by eleflow.com. A single device is rated to max. output power of 16 watts according to data sheet. In push-pull mode this nearly doubles because each of the semiconductors only has to amplify half of the duty cycle. Hence I started trying to get a little bit more power out of the PA assembly.

First I modified T3. The former data was: 1+1 turn primary center tapped and 4 turns secondary on a homemade pig nose core of 2×3 stacked FT50-43 toroids.

The new transformer has got 1 secondary turn more. Wire is 0.8mm enam.wire both for primary and secondary.

DK7IH - High performance Transceiver - Transmitter section - Modified PA output transformer
DK7IH – High performance Transceiver – Transmitter section – Modified PA output transformer

First I tried out 0.5mm enam. wire for the secondary which resulted in about 23 watts output pep.  Next I used 0.8 mm diameter enam. wire which reduces the consequences of skin effect significantly . Penetration depth of a 14MHz ac current is only some 50 µm, thus every increase in surface area reduces resistance.

This set of measurements greatly improves two figures:

  • Output power increases from 20 to 25 watts pep,
  • carrier suppression improves by 5 dB.

The enhanced signal on the RIGOL DS1054 scope:

DK7IH - High performance Transceiver - Transmitter section - Output with modified PA
DK7IH – High performance Transceiver – Transmitter section – Output with modified PA

About 100 volts pp. (i. e. 35.5 volts rms)  at 50Ω equals to about 25 watts pep. Nice improvement for a minor change that was done within 20 minutes!

vy 73 de Peter

 

Constructing a high performance transceiver for voice communication on 14MHz

 

Abstract

A general and good practice in engineering is a steady process of improvement. This  article describes the construction of a high performance transmitter/receiver for SSB (voice) communication covering the 14MHz (20 meters) high frequency amateur radio band.

Various modules that have proven high performance, liability and ruggedness in recent constructions will be combined to form a radio with outstanding receiver performance, an ultra linear transmitter with output range 15 to 20 watts and a top audio sound quality both on transmit and receive.

DK7IH - High performance SSB 14MHz 20meters Transceiver - Front view
DK7IH – High performance SSB 14MHz 20 meters Transceiver – Front view

Key features are:

  • Dual DDS frequency generation with AD9834 (Local oscillator) and AD9951 (VFO),
  • Microcontroller (MCU): ATMega644P by ATMEL,
  • Single conversion superhet receiver with 9MHz interfrequency (IF) and preamplifier, mixer and IF  amplifier equipped with Dual-Gate-MOSFETs,
  • Audio-derived automatic gain control (AGC),
  • Transmitter with MC1496 as double sideband (DSB) modulator and NE602 as transmit mixer,
  • power transimitter with 4 stages, final stage in push-pull mode.

Another version of this radio has been built before. But this one was equipped with a variable frequency oscillator (VFO) because of nostalgia reasons. Unfortunately a VFO lacks certain features (frequency stability above all) which can be overcome by using digital frequency synthesis without losing performance. Usage of a high performance DDS systems is a prerequisite to achievement and a possible solution.

Most building blocks of that respective radio have been redesigned except the VFO section that turned out as not being able to deliver the projected frequency stability to a 100% degree. Frequency instability occurred because of the flatness of the former cabinet that brought the aluminum case too close to the VFO tuned LC circuit. Aluminum  has a huge tendency to expand under the influence of heat so the rig was very temperature sensitive. That undeniable fault lead to a complete reconstruction using the old RX and TX modules and building a new set of frequency generators.

Parts of the old cabinet were reused but because of the fact that the whole rig got increased vertical expansion, the cabinet was “stretched” with two lateral strips of Aluminum.

Also a full electronic transmit/receive switch with p-channel power MOSFETs has been designed to avoid usage of a DC switch relay and get a “smooth” switching.

Another objective of this radio was to get out the absolute best performing circuits of the recent projects and to build a real high-performance radio. Hence this transceiver is also some sort of an improvement of the “Old school SSB TRX” as well. The circuit empirically turned out to be very good for communication in the 14MHz band. Because of this frequent readers of this website might detect certain similarities. 😉

The Receiver section

The design objectives were:

  • Low noise (achieved by using Dual-Gate-MOSFETs with the receiver to a large extent)
  • High dynamic range (achieved by using a Dual-Gate-MOSFET as receive mixer)
  • High AGC range (achieved by taking RF preamp and IF amp into the AGC chain)
  • Good audio quality (achieved by using a TBA820M as integrated AF amplifier circuit and a 5 cm loudspeaker)
DK7IH - High performance Transceiver 14MHz- Receiver Frontend & Mixer
DK7IH – High performance Transceiver 14MHz- Receiver Frontend & Mixer (Full size picture)

RF preamplifier and receive mixer

The radio frequency preamplifier has been designed primarily to improve the receiver’s noise figure. Delivering additional gain only is relevant in second order.

Preselection is performed with only one tuned circuit int G1 line. The center frequency of this circuit is 14.180MHz. In the output section of the stage an another identical LC circuit has been installed. This turned out to be sufficient because there is no immediate need of higher preselection. The subsequently placed mixer, that is also equipped with a Dual-Gate-MOSFET has very good high level processing qualities. No interfrequency feedthrough could be observed with various antennas. No IMD occured even when signals were very strong. Testing out in the field with partable antenna very far from man-made noise sources the receiver was very quiet and even very weak stations could be received and read with Q5.

To get most of gain swing from AGC the preamplifier is controlled by a DC voltage between 0 and 12V supplied by the AGC control stage to be described later. This voltage is halved by a 1:1 resistor voltage divider  because maximum gain of the Dual-Gate-MOSFET occurs with about 6V DC applied to G2.

Clipping diodes that are sometimes used to prevent high voltage entering the 1st stage have not been installed because they are prone to produce unwanted IMD products if signal levels from the antenna are too high and undesired mixing takes place there.

To prevent self-oscillation in the preamplifier, the tuned circuit LC1 and LC2 are connected together in a special way. G1 is connected to the tuned section of LC1. This section has high impedance, thus it should be connected to a load which also has high impedance. The coupling section of the coil with low impedance is connected to the 50Ω antenna. There are not two tuned parts of the LC circuits together in one stage.

The output of the Dual-Gate-MOSFET (low impedance) is connected to the coupling winding, the high impedance tuned part is going the high impedance of G1 of the mixer. The impedance ratio between the two coils is 16:4 due to the winding ratio of 4:1 of the coil set.

The sensitivity and noise figure of the whole receiver is determined by these two stages. Measurements showed that the minimum discernable signal is about 0.1µV which is very good for a short wave receiver.

SSB-Filter, IF amplifier, Demodulator, AF amp and AGC

The following stages are some sort of best practice combination of circuits that have proven to perform very well in the recent projects.

DK7IH - High performance Transceiver 14MHz- Receiver - SSB filter, IF amp, ddemodulator, AF preamp, AF final amp, AGC.
DK7IH – High performance Transceiver 14MHz- Receiver – SSB filter, IF amp, ddemodulator, AF preamp, AF final amp, AGC. (Full size picture)

SSB-Filter and relay

The SSB filter is switched with a special rf relay by Teledyne® ensuring excellent isolation of relay ports with very low capacities in the unswitched signal path. Here the usage of shielded cable is mandatory for connecting the relay/filter section to the transmitter (see later text!). A clamp diode has been installed to eliminate high voltage peaks due to self-induction when the relay is switched. This will prevent the MOSFETs in the switching unit from excessive voltage and possible destruction.

IF amp

A proven and reliable circuit can be found here as well. One stage delivers IF gain of about 12dB which is sufficient because the mixer following as a demodulator (NE612) also propduces some dB of gain. Too much gain in this section only contributes to high noise in the speaker later and is not desirable.

The Dual-Gate-MOSFET in this stage is also integrated to the AGC chain. Together with the RF preamp installed in the front and also being part of AGC control end we will get some 20 to 25 dB of gain swing when AGC is fully driven. This turned out to be enough, only in some rare cases I found that the manual gain control (also included in this recevier) needs to be used in addition when AGC is not able to cope with excessive signal levels.

Compared to a MC1350 IC equipped IF amplifier I found that gain control is much smoother because the V->dB function is very much less precipitously with the Dual-Gate-MOSFET than it is with the MC1350.

Demodulator

NE612 is built-in here. The main advantage of this IC is that it requires only a few components and it has got an additional gain of about 12dB or more.

In VDD line you will find a 5.6V Zener to bring 12..14V supply voltage down to about 6V. There are also two capacitors. The 0.1uF is for bleeding off rf energy from or to the supply rail, the same is the purpose of the 10uF cap for audio frequencies or low frequency noise present on VDD line. This noise sometimes originates from the digital components in the radio and should be eliminated at all reasonable points in the circuit. Also it will help to prevent the high gain amplifier chain from self-oscillating in the audio frequency range.

Audio frequency amplifier section

Two ICs are used here. The first is an operational amplifier (uA741) with a 150kΩ resistor as part of negative feedback circuit. This value is comparatively low. If (in rare cases) higher gain should be needed it can be replaced by e. g. 330kΩ or even more.

The main audio amp is the TBA820M, an integrated audio amplifier in 8 pin DIL case. It is an interesting alternative for LM386 because tendency for self-ocillation is much lower within the TBA820M. But it requires some more components. TBA820M can be switched with the load (speaker) to VVD or GND. I use a headphone jack in the radio, that is grounded, hence I prefer the latter version.

A “good” loudspeaker with 5cm of diameter was found by ordering a larger series of different speakers from Chinese vendors via ebay. The differences in sound quality are breath-taking. So it is worthwhile spending some money and order a larger variety of speakers and install the very best one.

AGC

This is a circuit I have used many times and it has proven to work very reliable. If you wish to have different settings concerning attack and decay time then another cap can be added via a switch to GND in parallel to the 47uF cap. Another 100uF for example will give a few extra fractions of a second in attack/decay time.

A 20kΩ variable resistor is used for manual gain setting. The AGC voltage that is near to VDD (12V or more) is divided and so AGC and manual gain control can be combined. At least until the point where noch AGCing will take place because the resulting voltage is <3V.

The “AGC thres.” variable resistor shown in the schematic will determine the point where AGC becomes active. I usually set it that way that solely band noise does not affect the AGC. Stronger stations (coming with S5 or 6 with a commercial transceiver) should give first minor influence on the AGC voltage. That is the point where amp gain should start dropping gradually. Strong stations must set AGC voltage to nearly 0 V.

DK7IH - High performance SSB 14MHz 20meters Transceiver - Receiver and switchboard modules
DK7IH – High performance SSB 14MHz 20 meters Transceiver – Receiver and switchboard modules (Left: Switchboard with 2 P-channel MOSFETs. Center: receiver front end and SSB filter. Right: Demodulator and AF section plus AGC)

The Transmitter section

The transmitter generally consists of two parts:

  • The SSB generator and the TX mixer, and
  • the Power Amplifier.

The full schematic of the two parts together:

DK7IH - High performance Transceiver - Transmitter section
DK7IH – High performance Transceiver – Transmitter section (Click here for updated schematic of this assembly)

Microphone amplifier

Starting from the left we see the microphone amp. A nostalgic but still available operational amplifier integrated circuit (741) is used here. The amp has high gain (about 30dB) to make a dynamic microphone connectable. There is no DC feeding for an electret microphone. If you should wish to use one then the negative feedback resistors should be lowered to about 47kΩ and the audio level should be carefully observed to avoid excessive driving. DC must also be applied for htis type of microphone!

Double sideband generator

The MC1496 (still available as NOS in 14 pin DIL case or fresh from the market in SMD package by ON Semiconductors) offers high carrier suppression of about 50 to 60 dB. Therefore a network of 2 x 10kΩ and a 50kΩ variable resistor has been installed. The crucial point: To make full usage of this network the carrier offset must be set properly. If you should notice that there is no point within the full swing of the 50kΩ variable resistor then the carrier frequency should be readjusted.

A balanced output transformer has been installed to improve carrier suppression and to enhance output voltage.

SSB filter coupling out

The usage of shielded cable is mandatory here to avoid transfer of rf stray energy into the DSB and SSB line!

Transmit mixer

This stage also is equipped with an NE612 doubly balanced mixer due to reasons of circuit simplicity.

14MHz Band pass filter

This filter also needs observation. I use the TOKO style coil formers familiar from other projects. The winding data can be found in the schematic. The coil formers must have the ferrite caps and metal shield cans on to avoid incoupling of rf energy from the subsequent power stages. The filter should be placed away from the higher power stages to avoid self-oscillation inside the transmitter section.

RF amplifier power stages

The amplifier presented here has been tested in 2 different radios so far and has proven to be very stable, very linear and very rugged against antenna mismatch. The power levels are about 10 db gain per stage. From the second stage on the output impedance is 50Ω. This makes it easier to measure power levels with a 50Ω standard dummy load.

The 2 watt driver stage uses a PI-filter instead of a broadband transformer. This is because I intended to save some space on the veroboard and for a monoband transmitter this is a practical solution. If you should find out that there is a mismatch that results in losing gain, then the capacitors can slightly be modified because the L-network has impedance transforming capabilities. By knowing input versus output impedance and calculating a “Q”-factor subsequently L and C can be computed to get a defined step-down impedance (Link for further information). This is a useful method and, in case of low pass filter like applied here, there is also a filter for harmonics.

Driver and PA power amp are biased for AB-mode, all other stages operate in A-mode to ensure best linearity. Strategies using emitter degeneration and negative feedback are inherent in preamp and predriver stage.

All transistors apart from preamp stage require usage of heat sinks.

Impedance matching is either not done (stage 1 to 2), by transformer (stage 2 to 3) or by L-network (stage 3 to 4). Whereas from stage 3 to 4 also there is a transformer applied to split the signal symmetrically to the two bases of the final transistors.

If there should be a tendency for self-oscillation within this stage the input transformer secondary winding can be center tapped and put to GND via a 0.1 capacitor.

Power out depends on DC power voltage and is about 20 watts when run on 13.5 V DC power supply an the amplifier terminated to a 50Ω load.

Here

DK7IH - High performance Transceiver - Transmitter section - Spectrum output signal at 20 watts
DK7IH – High performance Transceiver – Transmitter section – Spectrum output signal at 20 watts
DK7IH - High performance Transceiver - Transmitter section - Spectrum carrier
DK7IH – High performance Transceiver – Transmitter section – Spectrum carrier

This is a spectroscopical analysis of the fully driven transmitter (f=14.200kHz, Pout = 20.1 watts, VDD=13.0V) and the remaining carrier.

Harmonics are filtered very effectively . This is achieved by using a push-pull final stage driven in AB mode. Some authors say this is useful to eliminate odd number harmonics. On the other hand there are two sections of low pass filtering (one between driver and PA, one following PA). The figure of the output spectrum between and 50 MHz below:

DK7IH - High performance Transceiver - Transmitter section -Showing harmonic suppression
DK7IH – High performance Transceiver – Transmitter section -Showing harmonic suppression
DK7IH - High performance SSB 14MHz 20meters Transceiver - TX modules
DK7IH – High performance SSB 14MHz 20 meters Transceiver – TX modules: Left the SSB generator, center 3 driver stages, right: final amp and antenna relay.

The Dual DDS Oscillator System

The DDS has got the following features:

  • VFO: AD9951 + amplifier,
  • LO: AD9834 + amplifier,
  • MCU: ATMega644P,
  • LCD: NOKIA 5110,
  • Tuning: Optical rotary encoder by Bourns,
  • User interface: 4 keys to control the digital settings,
  • Analog inputs: User keys (ADC0), VDD (ADC1), S-Value (ADC2), TX PWR (ADC3), PA Temp. (ADC4).

The schematic:

DK7IH - High performance Transceiver - Dual DDS AD9951 and AD9834
DK7IH – High performance Transceiver – Dual DDS AD9951 and AD9834 (Full size image)

The control lines for DDS1 (AD9951) and DDS2 (Ad9834) are as follows:

//DDS1
//IO_UD: PB0 (1) (green) 
//SDIO (DATA): PB1 (2) (white)
//SCLK PB2 (4) (blue)
//RESET PB3 (violet)

//DDS2
//FSYNC: PC0 (1) (green) 
//SDIO (DATA): PC1 (2) (white)
//SCLK PC2 (4) (blue)
//RESET PC3 (pink)

The colors are the colors used for the cables in my radio.

The LCD is connected likewise:

//LCD
//RES: PD4
//DC: PD5
//DIN: PD6
//SCLK: PD7

The NOKIA5110 LCD has been designed for VDD=3.3V. Please use 10kΩ resistors in the control lines which are not in the schematic! 3.3V are derived more or less closely by switching 2 Si-Diodes in series which results in a voltage drop of about 1.4V. Hence the LCD gets 3.6V DC from the 5V supply chain which is no problem for the module. One big advantage of the Nokia LCD should not be forgotten: It is very quiet and does not produce any discernable digital noise. Thus it is my favourite meanwhile for receivers on the RF bands.

For both DDS modules coupling out the rf is done with symmetrical circuits using trifilar broadband transformers. 10 turns on a FT37-43 core are a good choice. This will enhance gain and reduce spurs.

DDS2 is clocked to 110MHz, but keep in mind, that AD9834 is specified for 75 MHz max. clock rate only. I found out that modules from the “grey market” sometimes fail and produce lousy signals when overclocked. You can see that on a scope when extra peaks appear or with the spectrum analyzer when spurious signal are frequent. I recommend buying with Mouser or anther trusty vendor for example or reduce clock rate in case of problems in signal quality.

Power consumption is not excessive because both DDS modules are for low power application, unlike the AD9850 or AD9835, that draw much higher current. Power rate is 300mA when in receive mode with LCD backlight on.

The C-code for the software has about 2600 lines source code and can be downloaded here.

DK7IH - High performance SSB 14MHz 20meters Transceiver - Dual DDS modules
DK7IH – High performance SSB 14MHz 20 meters Transceiver – Dual DDS modules (Left: AD9951, center ATMega644P, right: AD9834)

Rear view:

A standard CB DC supply cable is used here. Unfortunatel the plugs equipped with a cable and fuse holder are widely availabe but the sockets have to be stripped from old CB trasnceivers.

DK7IH_14MHz_20DK7IH - High performance SSB 14MHz 20meters Transceiver - Rear view
DK7IH_14MHz_20DK7IH – High performance SSB 14MHz 20 meters Transceiver – Rear view

On the air the transceiver performs great. Audio is clear and powerful what the QSO partners often tell me. The receiver is fun to listen to, sounding soft AND precise. Maybe I will do a YouTube video the next weeks to prove it! 😉

General construction

All my rigs are for portable, hiking, bicycle trips and travel to foreign countries. I use Aluminum as a basis for the hardware to keep the radio lightweight. With this radio a ground plane made of 0.8mm Aluminum sheet metal has been used that one has been enforced with a lateral additional ground plane carrying the DDS system (see pictures in this article, please). Thus the base frame is pretty rigid and not prone for bending.

Front an rear panel are made from 0,.8mm Al sheets (rear) and 0.5 mm Al sheet (front).

The various subassemblies (DDS, receiver, transmitter) are split into different modules and are seperatelay fixed with bolts and washers mounted to special spacer bolts for screws of 2mm diameter. This ensures better grounding instead of using larger veroboards. Connections are made from flexible stranded hook-up wire and shielded cable for rf and audio signals.

On the undersides of the single boards copper foil is used for lines with GND portential.

Vy 73 and thanks for watching! Peter (DK7IH)

 

HF Propagation can not be so bad! ;-)

I am currently testing a remake of the “Old School SSB TRX” modified by adding a Dual-DDS-VFO to enhance frequency accuracy and stability.  The rig is completed and a description will follow within the next weeks. But another finding is notable.

Currently I am using WEBSDR sites to monitor my own signal from time to time. On sdr.hu you can find a large number of WEBSDR sites from all over the world. Most of them are somehow insensitive because they seem to use suboptimal antennas. M0RZF appears to be different.

I first had an initial test monitoring the two-tone signal for some seconds on 14.200 MHz with the WEBSDR and the transceiver connected to a half wave dipole antenna.

After some seconds I switched the transmitter off, disconnected the antenna and connected a BNC cable terminated by a 50Ω dummy load. I watched the signal on the scope and the spectrum analyzer and did some measurements. Output power was about 20 watts PEP.

When adjusting the transmitter I, by accident, got the PC monitor from the corner of the eye with the SDR still on it. I was pretty surprised when I noticed this figure:

SSB signal to dummyload received on UK WEBSDR (PWR 20 watts PEP)
SSB signal to dummyload received on UK WEBSDR (PWR 20 watts PEP)

Well, the antenna cable is on the desk about half a meter distant from the dummy load and it seems that there is enough stray energy coupled into the 50Ω-antenna-cable to produce a discernable signal over 1000 kilometers away. But nonetheless, pretty surprising.

Conclusion: Always be careful when testing your transmitters with a dummy load what you are about to  talk into the microphone! Use a two-tone test generator instead! 😉

Vy 73 de Peter (DK7IH)

DDS and spurious signals – The role of the post-DDS-amplifier (Part I)

Abstract

When building a direct-digital-synthesis (DDS)  frequency generator, the engineer has to take into account one inherit shortcoming of this state-of-the-art technology: Spurious signals that are an unwanted product of generating a radio frequency signal with a synthesizer. These signals occur in a wide range of frequencies and are a limiting factor for receiver performance particularly sensitivity. A method of examining and evaluating these unwanted signal products will be described and some guidelines for amplifier design will be presented.

Introduction

Spurious signals in a DDS system originate from various signal sources in

  1. the microcontrollers (MCU) driving the DDS. The responsible parts inside the MCU are clock oscillators, dividers, pulse-width-modulation (PWM) timers etc.
  2. the DDS chip itself mainly from clock dividers and the digital-analog-converters (DACs) used.

Various factors contribute to the problem. First the topology of the synthesizer itself. These systems contain a DAC to form a sine wave signal out of a computer calculated synthesizing data model. DACs have a wide variety of bitwidth. A rule of the thumb is: The more bits the DAC has, the lower the number and the weaker the spurious signals will be. 14-bit DACs by ANALOG DEVICES e. g. perform sufficiently for low noise receivers.

Another topic is clock rate. The occurrence of unwanted signals out of the synthesis process is a reciprocal function of clock rate. So it is highly recommended to use the highest possible clock rate the respective chip is designed for. When experimenting with the AD9951 the following findings occured: From 200MHz primary clock the number of spurious signals significantly decreases. The usage of an internal clock multiplier (if available) is not recommended since it will deteriorate phase noise because another oscillator is added to the signal generating chain..

So far the theory that is common today. Another aspect should now be brought into discussion: The role of the stages that are the successors behind the mere synthesizer. First stage usually is an amplifier that is used to lift the signal level of the synthesizer (usually about 1Vpp.) to a level that it is needed for a certain type of mixer.

The DDS circuit

To eliminate any weakness in the basic generator underlying this research a high-level performance synthesizer has been constructed. This ensures a pure sine wave output which is essential because we want to examine the potentially negative outcomes of the various small signal amplifiers succeeding the synthesizer.

The DDS chip used in this circuit is the AD9951 by Analog Devices that incorporates a 14-bit digital-analog-converter (DAC). Clock rate for the chip here is 200MHz (400MHz max. according to data sheet), clock output is 1.8Vpp. which is the maximum signal level that is suitable for this 1.8V-technology based DDS.

The AD9951 DDS integrated circuit needs 2 supply voltages: 1.8V for the digital and analog circuits and 3.3V for DVDD_I/O, the output driver voltage.

Controls lines are 5V applicable which makes the DDS suitable for being controlled by a 5V microcontroller as well as a 3.3V system.

AD9951 DDS synthesizer (DK7IH 2018)
AD9951 DDS synthesizer (by DK7IH 2018)

The signal outlet in this case is made from a symmetrical transformer (3 parallel windings 10 turns each on a FT43-37 core) using the IOUT1 and its corresponding paraphase outlet IOUT2. To use the balanced output is another effective possibility to reduce spurious signals as well as to enhance signal voltage by about 3 to 5 dB.

As first step the unamplified signal shall be inspected. In a wider spectroscopic range (f0=8.65MHz, fgen.=16MHz, f1=250.0MHz) the signal performs as shown below where f0 and f1 are the edge frequencies of the spectrum analyzer and fgen. is the output frequency of the DDS):

AD9951 wide spektrum analysis
AD9951 wide spektrum analysis

A first spurious signal can be detected with a signal level of more than 40dB below the generated signal. The signals at around 100MHz supposedly are strong FM stations in the VHF radio band whose energy from a nearby radio tower is coupled into the laboratory via the short wave antenna cable ending on top of the workbench. The peaks around 200 MHz are likely generated by the DDS clock oscillator.

Switching to a more narrow spectrum, we get this reading:

AD9951 narrow spectroscopic analysis
AD9951 narrow spectroscopic analysis

Remarkable that there is no even first harmonic that should be expected around the 32 MHz region. It is highly probable that this elimination of the 1st harmonic is caused by the symmetrical decoupling of the signal from the DDS. Hence we know that push-pull operated amplifiers reduce distortion and therefore tend to minimize even harmonics production.

Remarkable on the right falling edge of the main peak there another signal occurs hidden by the main curve which requires  further examination.

Examining amplifiers for a DDS system

1.) Bipolar RF preamplifier circuit (adapted from DeMaw et. al., Solid state design for the Radio Amateur)

The first amplifier under test is a simple circuit containing a bipolar transistor. To reduce distortion emitter degeneration and negative feedback (from collector to base) have been installed. This is an amplifier that is often used in rf amplifiers as first stage of the power strip. Therefore it should contribute less to the overall distortion of the circuit.

Overall voltage gain with a 2SC829 RF transistor (fT=230MHz) is 13dB with 1 MHz, decreasing about 3 db per octave, power gain has not been evaluated.

DDS amplfier, DUT 1
DDS amplfier, DUT 1

With the settings of the spectrum analyzer unchanged it turns out that this amplifier obviously produces new signals that are prone to disturb the receiver of a radio where this amplifier is installed:

AD9951_narrow_spektrum_DK7IH_2018_with_amp_1

Signal level is about 2V pp.

One countermeasure is to carefully check the input level of the amplifier. Excessive input voltage will bring the amplifier into the clipping area thus generating IMD products and harmonics. We usually do not only observe the output signal with an oscilloscope but use the spectrum analyzer in parallel. This ensures optimized signal quality.

Proper biassing is essential for this type of amplifier, aside from the linearization described before. The operating point (also referred to as “Q-point) must be set in the middle of the linear part of the IBE->IC function.

bipolar-transistor-ibe-ic-function

Usually this is achieved by applying a positive voltage (for NPN transistor) so that a given “quiescent” current flow through the base-emitter line. In the most simple case a voltage divider with the base connected to the joint of the two resistors works satisfactory.

Any AC voltage applied now will alter the voltage sum of DC (quiescent) and AC around the Q-point.

Vres.= VDC+VAC

whereas VAC=V0*sin(ωt)

(To be continued)

by Peter, DK7IH

French proposal to assign 2m-Band from Amateur Radio Service to Air Traffic Control

The 2m-Band is in danger! French authorities have proposed to take the frequencies from 144MHz to 146MHz from amateur radio service to aeronautical radio service.

What you can do:

  1. Get informed and download the CEPT papers here!
  2. Ask your national amateur association for assistance, provided you are a member!
  3. Sign the petition here!
  4. Donate! (Just done as well!)
  5. USE THE BAND!

Thanks de Peter (DK7IH)