QRP Direct Conversion SSB Transceiver For 80 meters

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Introduction

Many construction project designs have been published for "home brew" amateur radio equipment over the years and this is to be applauded. However these projects tend to suffer from some of the following difficulties,

The project (in progress) presents a complete, fully functional low power "QRP" transceiver based on commonly available components without undue complexity or design criticality. It is based on a modular approach and can be built "as is" or expanded to, or modified, or built in a cut down form. For example, the design can have an optional 20 W PEP RF Power Amplifier (RFPA) added, or have its receiver section replaced with an alternative design, or be built just as a receiver, or just as a transmitter etc.

This does involve some concessions of course. For example the design is not intended to be "state of the art" or the "world's best rig". Many companies already invest millions of dollars developing such products and it is unrealistic to expect that amateur radio "hobby" enthusiasts will have access to such resources and expertise. It is more appropriate to consider construction projects that provide educational opportunity in the field of radio communication theory without being loaded down with unnecessary construction effort and design complexity.  

Project Objectives

Primary Objectives

This project follows a different path. Its primary objectives are,

The design path adopted, based on these ambitions, is proposed to be,

Further, a low transmit power, i.e. QRP approach is adopted to reduce cost, simplify thermal issues and reduce battery requirements for portable operation.

Secondary Objectives For QRP Transceiver

Having established a Direct Conversion approach, based on a minimum complexity and cost requirement, it remains to determine the trade-offs between Double Sideband (DSB) versus Single Sideband (SSB) voice modulation and demodulation capability. 

Double Sideband (DSB) Modulation Format

Double Sideband (DSB) can be consider to be Amplitude Modulation (AM) with a suppressed carrier. Since the carrier energy doesn't convey information, this energy is effectively a waste of transmitter energy. In a low power QRP transmitter, with limited output power resource, it is best to preserve as much transmit energy as possible for conveying actual (voice) information and eliminate spectral energy contributions that detract from the overall transmit energy budget.. 

Amplitude Modulation "wastes" significant energy in it's carrier energy. For example, AM with an amplitude modulation index "m" as high as m = 100% (sine wave modulation is assumed here) has a carrier signal power that is four times higher (i.e. +6 dB) than either of its sideband components. As an example, consider a QRP transmitter with 1 Watt available output power. Its carrier energy will be (by definition) equal to 1 watt but each sideband will only have 250 mW energy. Further, a modulation index of m = 100% is not feasible, and m = 60% may be selected based on practical considerations (associated with simple, standard modulation architectures). This further reduces the energy in each sideband to P_USB = P_LSB = 250mW × [ m / 100 ]² = 90 mW. 

In Decibel terms, this implies that each information containing sideband is -10.5 dB lower than the transmitters output (carrier) power availability. DSB, in contrast, makes much better use of available transmitter power resources. Assuming complete carrier suppression, each sideband may be considered to contain 500 mW of energy - however this is not completely accurate. DSB transmit power is usually defined in terms of Peak Envelop Power (PEP). When single sinusoidal modulation is applied, each sideband will add in phase at certain times and the peak envelope will be four times the power contained in either sideband.

This phenomenon implies that each sideband will contain 250 mW PEP energy for 1 Watt PEP transmit power. This is superior to the previous case associated with AM. However it should be appreciated that a DSB transmitter draws variable energy from its power supply whilst an AM transmitter has a relatively high (an near constant) power supply energy drain. This attractive feature further motivates the avoidance of AM in QRP situations.

Note: AM is still used by some radio amateurs and is still a useful modulation format for radio communications. If energy efficiency is not a prime factor, other considerations may cause AM to a format of personal choice.   

Single Sideband (SSB) Modulation Format

DSB represents a worthwhile energy improvement but still suffers from the transmission of an unnecessary "mirror image" sideband. Commercial SSB receivers will ignore one of these sidebands and therefore "loose" ½ of the available transmitted power from a DSB source (-3 dB lost energy). A DSB receiver would process both sidebands, but its RF bandwidth would be twice that of an equivalent SSB receiver. A DSB receiver is therefore more susceptable to interference and has an input noise floor that is 3 dB higher than a SSB receiver.

However, how much carrier and sideband suppression is required?

Commercial (high power) SSB transceivers target different technical requirements compared to a QRP approach. Since these products are intended to present the clearest possible signals across large distances they aim at achieving maximum legal output power and present receiver performance that extracts signal information despite high levels of background interference. Similarly, they must transmit clean spectrum so as not to add to overall spectral pollution (i.e. being a "good neighbor" policy)..

The ambitions for QRP are somewhat different. The primary objective is to make best use of finite power supply energy resource, such as that available from battery (or solar or wind) power. This suggests that SSB is the best choice for transmit. The actual amount of relative sideband suppression is less critical than compared with a high power commercial transmitter. For example, a commercial SSB transmitter may provide 100 Watts (+50 dBm) PEP wanted sideband power, and -50 dB suppression to the unwanted sideband. The unwanted sideband energy will therefore be 0 dBm PEP. In contrast, a 1 Watt PEP (+30 dBm) QRP transmitter would only require -30 dB unwanted sideband suppression to create the same level of unwanted (other) sideband interference of 0 dBm PEP.

From an energy utilization perspective, unwanted sideband suppression equal to -30 dB represents a minimal loss of wanted power (i.e. 0.1 %). Even -20 dB suppression corresponds to 99% energy transfer to the wanted sideband and only 1 % lost. A similar argument applies to unwanted carrier power - this is much less contributing to spectral wastage as it is very close to the main sideband signal. 

This project will aim at suppression targets consistent with this reasoning so as not to "over design" a radio communication project that is intended to be fun to build, use and operate within a constrained financial (and time) budget.

On the other hand, how about the receiver approach? DSB and SSB architectures can have similar power supply energy requirements. DSB receivers are simpler but Direct Conversion (DC) approaches are still relatively simple for both formats.

Interoperability Issues Between DSB and SSB

It is constructive to note that DSB and SSB transceivers interoperate fairly well. A DSB receiver will demodulate a SSB signal and only suffer from some potential interference that may be present on the other sideband. A SSB receiver will demodulate a DSB transmission with a minor penalty of loosing ½ the available transmitted signal energy (i.e. information). A 1 Watt DSB transmission will appear to be equivalent to a 500 mW SSB transmission as an example.

However DSB transmission is very hard to demodulate with a DSB receiver (unless some use is made of the residual carrier energy to allow "synchronous" detection - which is an additional design complication). 

From an energy perspective, transmitting SSB is a logical preference. This also allows compatibility with commercial SSB receivers and other QRP DSB or SSB receivers. Transmitting DSB, in contrast, is energy inefficient compared to SSB and prevents reception on other receivers that may only have have DSB demodulation capability.

Further, a QRP receiver may benefit from having optimum sensitivity and interference immunity available from the use of SSB demodulation, but this would only apply for cases when a corresponding QRP transmitting station is being contacted. Contacts with amateur radio operators that use a commercial "rig" will be received with fairly reasonable signal strength due to their higher transmitted power. Given that QRP transceivers are far less common than commercial transceivers, the need for SSB reception is questionable. 

The approach recommended is therefore to use SSB for transmit and DSB for receive. However it is still advantageous to consider SSB for reception providing some additional design complexity is tolerable. In a DC approach this requires a 90 degree phase quadrature LO (as used on Tx), additional receiver mixer and a 90 degree audio signal combiner (can be as simple as a Quad OpAmp, 4 capacitors and 12 resistors!).

80 m QRP Transceiver Target Specification

The design presented will aim to achieve the following design specification targets. These are somewhat relaxed compared to commercial products but are not expected to interfere with normal radio communication capability or operational enjoyment. Improvements can of course be made, but at the expense of complexity or cost. Such modifications are encouraged of course, i.e. the presented project should be best seen as a demonstration product with fairly open-ended future potential depending on the electronic enthusiast's experience or interests.

QRP Transceiver System Design Concept 

The system concept is kept purposefully simple in order to allow its operation to be clearly understood by prospective constructors and experimenters. Its main sub systems are 

Notes;

The proposed system is based on a modular approach. For example the transmit path can be omitted if the project is only required to be a receiver. Further, the RF PA subsystem can be substituted for a higher power version if more output transmit power is required - on an additional power amplifier can be added between the RFPA output and the antenna Tx-Rx switch. Further the receiver can be simplified to DSB as mention beforehand. This also applies to the transmitter. If both are simplified to DSB, the the VCO may also then be replaced with a simpler "on f" design, avoiding the need for frequency division to produce 90 degree (phase quadrature) LO outputs.

80m QRP Schematics
80m QRP Circuit Simulations
80m QRP Component Layouts
80m QRP Enclosure
Component Data For 80m Project

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