An experimental HF 6-band SSB transceiver – Part 7: The Transmitter

This unit basically consists of two parts:

  • SSB-Generator and TX-mixer
  • TX-power amplifier stages

The SSB-Generator and TX-Mixer  Board

After having built this respective board with two NE612 ICs (one for DSB generator, one for the TX mixer) I was not satisfied with carrier suppression of the DSB generator. It turned out as only 40dB. Afterwards I constructed a new board with an old SIEMENS Mixer IC (S 042 P) that is still available NOS from various sources. With this one I gained carrier suppression rates of around 55dB. I think this is OK for a homemade transceiver.

The board looks as follows, set up on a 6x4cm 0.1″ veroboard:

DK7IH 6 band QRP SSB TRX 2019 - SSB-Generator and TX-Mixer board
DK7IH 6 band QRP SSB TRX 2019 – SSB-Generator and TX-Mixer board

The circuit starts with an AF amplifier equipped with a bipolar transistor where also a power supply for Electret microphones has been added. The radio now can handle dynamic and Electret microphones adequately.

DK7IH 6 band QRP SSB TRX 2019 - SSB Generator and TX mixer
DK7IH 6 band QRP SSB TRX 2019 – SSB Generator and TX mixer (Full size schematic)

Afterwards we see the S042P mixer IC where I have changed the circuit slighty to the one used in my 40-meter-QRO TRX. Audio input signal is now to PIN8 of the IC, Lo input on the rf side of the IC to PIN11 and PIN13. To reduce carrier level and enhance carrier suppression a 5.6pF cap is in series because the relatively high level of signal coming from the LO amp would deteriorate the performance of the DSB generator without countermeasures.

Output from this DSB generator is also symmetric and fairly high. Thus a low valued capacitor has been inserted prior to the SSB filter, sited on the RX board.

After that we see an amplifier with limited gain due to high emitter degeneration and the NE612 as TX mixer. The latter one also with an symmetric output to get more gain from it by using the two inherent output transistors.

TX-power amplifier stages

As I have described in the article of my “Give me 5“-Transceiver some years ago, building a broadband power amplifier is challenging due to one special problem related with the wide range of frequencies that this amplifier must be able to cope with. an extra gain of 5 to 6 dB is commen, when the frequency is divided by the factor of 2. Usually the necessary compensation is done by adding adequate capacitors and inductances using their frequency depending reactance.

With this radio I tried something new. I added an amplifier that is gain controlled by an adjustable voltage. Here a dual-gate MOSFET with gain control to gate 2 sets up the initial stage of the whole amplifier strip. The stage’s gain is set by a simple bipolar driver transistor controlled by a digital-analog-converter (DAC). A numeric value for each individual band is stored with in the EEPROM of the MUC. This numeric value is calculated during adjustment, then stored in the MUC and recalled whenever the radio is switched to a certain band. The DAC is an MCP4725 breakout board, containing a 12-bit device.

DK7IH 6 band QRP SSB TRX 2019 - Power transmitter
DK7IH 6 band QRP SSB TRX 2019 – Power transmitter (full size picture)

After that we see an amplifier that is common solid state technology. Preamp stage and predriver stage are set to A mode which requires a heat sink for the predriver stage. Here a 2N3866 is used as amplifying element.

Driver stage is single ended, operates in AB-mode and also is protected by a heat sink.

After that a somehow uncommon technique has been applied. Instead of using a broadband transformer to reduce the stages output impedance to the some ohms input impedance of the final stage, a set of 6 switchable low-pass-filters is used.

DK7IH 6 band QRP SSB TRX 2019 - Intermediate LPF section
DK7IH 6 band QRP SSB TRX 2019 – Intermediate LPF section

This filter section has been optimized to an output impedance of 50 ohms for each band thus enabling me to test and optimize the transmitter to a maximum with a defined output impedance (remember, this is an experimental radio! 😉 ).

After this filter section the final amplifier stage follows which is able to drive the output power up to 15 to 20 watts on all bands but depending on the DC voltage used for transmitting. The max. power gained during tests was 22 watts pep at 15V DC with two NTE236 transistors. Unfortunately the turned out not to be so rugged and blew in the tests. The eleflow 2SC1969 inserted later showed no problems at all. Thank God! When running on 12.0 V DC the amplifier puts out 12 watts at all bands.

The final part of the transmitter section is the last low-pass filter that is positioned next to antenna relay in the same compartment:

DK7IH 6 band QRP SSB TRX 2019 - Low Pas  Filter Unit for TX
DK7IH 6 band QRP SSB TRX 2019 – Low Pas Filter Unit for TX

The whole transmitter looks like this:

DK7IH 6 band QRP SSB TRX 2019 - Practical setup of the transmitter board
DK7IH 6 band QRP SSB TRX 2019 – Practical setup of the transmitter board

The various units are:

  • 1: DSB-Generator and TX mixer
  • 2: Amplifier stages 1 to 4
  • 3: MCP4725 transmitter gain controller
  • 4: Intermediate LPF board
  • 5: Power amplifier
  • 6: Final LPF section
  • 7: TX/RX switch board

Here a little bit of analysis to end with the article. First is the output of the SSB-Generator/TX-mixer board with maximum output (Around 500mV pp) set to the 40m band.

DK7IH 6 band QRP SSB TRX 2019 - TX-Mixer's output signal
DK7IH 6 band QRP SSB TRX 2019 – TX-Mixer’s output signal

Nest we see the carrier suppression when dual tone audio in has been suspended. Carrier is about 55db under the signal peak.

DK7IH 6 band QRP SSB TRX 2019 - TX-Mixer's output signal, suppressed carrier only
DK7IH 6 band QRP SSB TRX 2019 – TX-Mixer’s output signal, suppressed carrier only

And here an output signal with max. power at 3.5 and 7 MHz:

DK7IH 6 band QRP SSB TRX 2019 - TX output at 80m band
DK7IH 6 band QRP SSB TRX 2019 – TX output at 80m band
DK7IH 6 band QRP SSB TRX 2019 - TX output at 20m band - Pout = 12 W PEP
DK7IH 6 band QRP SSB TRX 2019 – TX output at 20m band – Pout = 12 W PEP

So, that’s all for today, thanks for watching and 73!

Peter (DK7IH)

An experimental HF 6-band SSB transceiver – Part 5: Analog Affairs – Getting Measurement Data

This short article will describe the adapter board that is connected to analog data sources and that is converting the respective voltage data into suitable voltage levels for the ADC inputs PA0:PA4 at the microcontroller:

DK7IH 6 band QRP SSB TRX 2019 - Analog Adaptor Board
DK7IH 6 band QRP SSB TRX 2019 – Analog Adapter Board

The following data will be converted and later shown on the display:

  • User keys (Key1:Key3)
  • TX power measurement
  • PA temperature (Sensor: KTY81-210 switched against GND)
  • Battery/Supply voltage
  • AGC output (DC) from receiver => S-Meter

This article covers the remaining digital (or “analog to digital”) stuff, next on the agenda will be the receiver.

Vy 73 de Peter

An experimental HF 6-band SSB transceiver – Part 1: Basic outline

Abstract

An SSB radio for the HF bands will be presented. Featuring 12 to 20 Watts of output power (depending on DC supply), full DDS frequency generation, covering 6 major frequency bands (1.8, 3.5, 7, 14, 21 and 28 MHz) within the short wave amateur radio spectrum. The rig also features colored LCD and front panel backlight.

Content

Project description

In this upcoming series of articles a relatively complex project will be discussed. It is some sort of „remake“ of my last multi-band QRP SSB transceiver that has been entitled the „Gimme Five“-Transceiver and that was finished in 2015. „5“ in that case stands for the 5 major (i. e. „classical“) RF bands: 80m, 40, 20m, 15m and 10m the radio covered. This new project (called the „Midi6“, because it is not a “Micro” or a “Mini” transceiver 😉 ) covers one band more, the range has been extended to 160m.

The basic features of this construction are:

  • Dual DDS frequency generation (AD9951 as VFO, AD9834 as LO),
  • Colored LCD (CP11003) with resolution 240×320 pixels,
  • Microcontroller ATMega128,
  • Single conversion superhet receiver, interfrequency 9 MHz,
  • 5 stage high quality transmitter, Pout=20W (max. at 15V DC) , featuring a microcontroller driven regulated gain stage to ensure absolute constant output on all bands,
  • Integrated 2-tone oscillator for testing and tuning,
  • Front panel full backlight.

“Experimental radio” means that there is enough space inside the cabinet to change boards and test new ideas in the same space. Also certain components like the SSB-filter have been  made as “plug-in” components to enable quick change of the part. Also the connector between the various transmitter and receiver stages have been done by “jumpers” and header strips so that resistors and capacitors can be changed quickly to experiment with other values.

The radio has been realized with standard veroboards (0.1″ pitch), SMD components and been put into a homemade aluminum cabinet using 2 layer sandwich construction inside the cabinet.

Here a snapshot of the operational transceiver. Cabinet size, by the way, is 7.5 x 16.5 x  19.5 centimeters (2.95 x 6.5 x 7.68 inches). These dimensions are in the range of other multiband QRP transceivers like the Elecraft K2 (larger) or the Icom IC703 (a bit smaller).

The
The “Midi6” – An experimental HF radio for 6 amateur bands and SSB modulation. By DK7IH (Peter)

Stay tuned for the next article(s)!

 

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