dsp10-1.pdf

By Bob Larkin, W7PUA

The DSP-10: An All-Mode 2-Meter

Transceiver Using a DSP IF and

PC-Controlled Front Panel

Part 1—What’s neat about this 2-meter transceiver is that most of it is in

software! Your PC is its front panel. You can operate it as a stand-alone

QRP rig, with an amplifier or with UHF and microwave transverters!

ALL PHOTOS BY JOE BOTTIGLIERI, AA1GW

T he complexity of amateur all-

mode transceivers has grown to

the point that their construction

is generally left to commercial

manufacturers. For many of us, merely envi-

sioning the total number of components re-

quired to build such a project is overwhelm-

ing! Duplicating the mechanical structure of

a modern front panel seems to require a ma-

chine shop and the talents of an artist. So, like

most hams, I have usually gone shopping and

come back with a factory-made transceiver.

This time, I built the transceiver myself.

Over the years, several things have

changed that make such a homebuilt radio a

possibility once more. Digital signal proces-

sors (DSP) are available at low cost. These

devices allow considerable simplification in

transceiver construction by using software to

replace much of the hardware. Once the soft-

ware is written, it is possible to get the hard-

ware functions working with very little ef-

fort. As a bonus, with DSP we can perform

some types of filtering and signal processing

that would be impractical to implement in

hardware.

PC availability nowadays is such that one

can be dedicated to controlling a transceiver.

This means we can have a front panel, smart

controls and all the “bells and whistles” that

we want without drilling a single hole!

The benefits of moving portions of a

radio’s circuit action into software are affect-

ing the designs of commercial radios. 1 A

number of products now available use a PC

and appropriate software to create the front

panels. Final IF and audio-stage implementa-

tions in DSP are becoming more common. As

the performance of digital/analog conversion

hardware improves, expect to see the percent-

age of radios operating in DSP to increase

further. 2 This project gives you an opportu-

nity to see how such a radio is designed—and

the chance to build your own!

The DSP-10 Transceiver

The DSP-10 is a low-power, all-mode

2-meter transceiver using DSP at the last IF

and audio stages. You control the radio via a

PC acting as the virtual front panel. A built-

in audio spectrum analyzer allows you to see

what is happening at the audio level. A num-

ber of features make this rig particularly well

suited for use as an IF radio for UHF and

microwave transverters.

An inside view of the transceiver with the

DSP assembly removed.

Notes appear on page 41 .

September 1999

Figure 1—This transceiver block diagram shows the receive path and the hardware portions of the transmit path. Most of the circuits

are bidirectional, being used for transmit and receive. The dashed line at the output of the 150-MHz low-pass filter indicates the signal

path to the TR switch during transmit. Two frequency conversions shift the signal from 146 MHz down to the 15-kHz IF. All IF and

audio processing is done using DSP. One detector is used for SSB or CW and a second detector for FM. Fine tuning for the SSB/CW

modes comes from the 12.5- to 17.5-kHz software BFO.

Three basic components are involved. A

minimal amount of RF hardware (on a single

PC board) translates the signal frequency up

and down using the DSP of an Analog De-

vices demonstration board. 3 A DSP program

processes the IF and audio portions of the

radio signals. Finally, software running in the

PC controls the DSP and presents a front-

panel interface to you, the user.

PC requirements are minimal. Almost any

PC running DOS equipped with a 640 × 480

VGA display can be used. No extended or

expanded memory is needed, nor is a math

coprocessor required. The program can oper-

ate with very slow processors. Most of this

transceiver’s testing was done using a

20-MHz ’386 laptop computer. Communica-

tion between the DSP and the PC is at 9600

baud.

Constructing a piece of electronic hard-

ware requires a description through schemat-

ics, PC layouts and the like. To help you un-

derstand the inner workings of this radio—as

a starting point for customizing your radio, or

for building a new project altogether—this

project’s source code is available. (More on

this in Part 2 .)

Figure 1 is a block diagram of the trans-

Figure 2—(See facing page) The transmit and receive RF signal paths. Extensive RF

filtering ensures a clean transmit signal and freedom from spurious signals during

reception. All resistors on the main board are 5%-tolerance 1206 Xicon chips. These

are available in small quantities from Mouser Electronics. Unless otherwise noted, all

capacitors are 1206 or 0805 chips. Capacitor values less than 470 pF are NP0; values

of 470 pF and greater are any general-purpose ceramic, such as X7R or Z5U.

Component sources and abbreviations are listed in the sidebar “ Parts Sources . ” Those

identified generally are only one of several that manufacture or distribute equivalent

parts. Equivalent parts can be substituted.

C24—0.04 pF “gimmick” capacitor

constructed from a 0.2-inch length of

#24 tinned wire spaced 0.05 inch above

the adjacent PC-board pad.

D1, D2—HSMP-3804 dual PIN diode

(HP HSMP-3804)

FIL1, FIL2, FIL5—470-pF pi filter,

Panasonic EXC-EMT471BT

(DK P9806CT)

L1, L2, L8, L9, L10, L11—100-nH variable

inductor, 10 mm, Toko BTKENS-T1044Z

(DK TK1402)

L3, L19—39-nH chip inductor

(DK TKS1008CT.) This and all other

chip inductors are from the Toko 32CS

series.

L4, L5, L6, L7, L14, L15—0.33-

L18—0.10-

H chip inductor

(DK TKS1013CT)

L23, L24, L25—0.36

H; 17 turns #26

enameled wire on a T-25-17 toroid core.

P1, P2, P3—2-pin 0.1-inch in-line header.

This and the other multi-pin headers are

all from the Molex 22-23-20x1 series,

where x is the number of pins.

(DK WM4200)

Q2—2N5109 NPN RF transistor

(ME 511-2N5109)

U1, U4—MSA0686 with leads bent

(HP MSA0686), or MAR-6 with leads

trimmed and bent (MC MAR-6)

U2—MSA0386 with leads bent

(HP MSA0386), or MAR-3 with leads

trimmed and bent (MC MAR-3)

U3—TUF-1 mixer (MC TUF-1)

U5—MSA0486 with leads bent

(HP MSA0486), or MAR-4 with leads

trimmed and bent (MC MAR-4)

H chip

inductor (DK TKS1019CT)

L16, L17—Ferrite SMT bead, 1206, 600

Ω

at 100 MHz, Stewart HZ1206B601R

(DK 240-1019-1)

September 1999

r 9

ceiver, showing the receive path. This is a

conventional double-conversion design. An

RF amplifier builds up the signal sufficiently

to overcome the first mixer noise. Two RF

filters ensure that the image frequency, which

is in the FM-broadcast band, is adequately

rejected. The first-conversion synthesizer in

the 125-MHz region is programmable in

5-kHz steps. The first mixer produces a first

IF at 19.665 MHz, which is equipped with a

crystal filter. This filter’s bandwidth is about

12 kHz and provides image rejection for a

second IF at only 15 kHz. This low-frequency

second IF allows use of a low-cost audio

analog-to-digital converter (ADC) to prepare

the signal for the DSP.

All 15-kHz IF and audio-signal process-

ing is done in DSP. The software BFO for

SSB and CW can be programmed in steps

smaller than 1 Hz; this is image-reject mixed

with the IF signal to produce audio. At au-

dio, you can select band-pass filtering 4 or a

least-mean-square (LMS) denoise algo-

rithm. 5 Following the audio processing, a

DAC readies the signal for the audio-power

amplifiers. At audio, a fast Fourier trans-

form (FFT) spectrum analyzer is always

operating, sending the resulting data to the

PC through a serial port.

The FM detector also operates at the

15-kHz IF. No fine-tuning control is avail-

able for this mode, so it is tunable in 5-kHz

steps, adequate for most applications. The

spectrum-analyzer continues to operate on

the detected audio for FM. The FM squelch

is derived from the spectrum analyzer out-

put by examining the level of the high-

frequency noise.

The transmit path is essentially the receive

path in reverse. The CW, SSB or FM signal is

generated by the DSP at about 15 kHz. This

signal is then double-converted to 2 meters

using the same mixers and filters that are used

in the receive path. A three-stage amplifier

raises the power output to more than 20 mW.

Provision is made to use external amplifiers

to raise the power level further.

Figure 3—Measured response of the four-pole interstage filter. The greatest attenuation

is on the same side as the first-conversion oscillator and the related image, yielding

excellent spurious rejection during receive and transmit.

dB. This high gain level is needed to over-

come the first-mixer noise. It does, however,

make the front end more prone to overload.

Following the RF amplifier is a second

dual-PIN-diode switch, D1. This, too, serves

dual roles as a TR switch and as a variable

attenuator for the receive path. Here is an-

other 18 dB of RF gain control, again under

control of the PC.

The four-pole bandpass filter built

around L8, L9, L10 and L11 provides most

RF-signal filtering. A conventional top-

coupled, or Cohn, filter, it has its greatest

rejection on the low-frequency side. C24 is

added to produce a notch at about 126 MHz.

A “gimmick” capacitor, C24’s value is very

small (about 0.04 pF) and consists simply of

a piece of tinned wire placed near a PC-board

pad. The filter response, plotted in Figure 3 ,

shows this notch with an attenuation of about

97 dB. Rejection exceeds 85 dB for all fre-

quencies below 128 MHz, which includes the

conversion-oscillator and image frequencies.

Filter-insertion loss is about 10 dB and is

compensated for by the RF-amplifier gain.

A Mini-Circuits TUF-1 double-balanced

mixer (U3) converts the 2-meter input signal

to a 19.665-MHz IF. When transmitting, U3’s

signal passes through the RF filter, goes

through TR switch D1 and arrives at the first

transmit amplifier (U4) at a level of about

–27 dBm. Two MSA amplifiers (U4 and U5)

and Q2, a 2N5109 operating class A, provide

a gain of about 40 dB to raise this level to

+13 dBm (20 mW). The measured 1-dB com-

pression point of this amplifier is +18 dBm,

making it very linear at the operating point. A

low-pass filter consisting of L19 and three ca-

pacitors (C51 through C53) reduces the trans-

mitter harmonic levels. The transmitter out-

put does not go directly to the PIN-diode TR

switch D2. Instead, the lines go to a pair of

connectors identified as P2 and P3 that attach

to rear-panel jacks. Such routing allows the

transceiver to be connected to a transverter or

a power amplifier without the need for an-

other TR switch. P2 and P3 can be connected

together for stand-alone QRP operation.

First and Second IF

As shown in Figure 4 , the receive path

accepts a 19.665-Hz signal from the first

mixer, U3. A four-pole crystal filter using

low-cost standard crystals (X1 through X4)

provides selectivity. The series-crystal con-

figuration used has a rejection notch at a fre-

quency above the passband. 7 This rejects the

image before the second mixer. L12 and L13

along with C25 and C29 form L networks that

step up the 50-Ω impedance to the 1.5 kΩ

required by the filter. Figure 5 is the mea-

sured response of this crystal filter. The plot

does not extend far enough to show the out-

of-band response, but at the image frequen-

cies above 19.690 MHz, the rejection is

greater than 70 dB; passband insertion loss is

just over 1 dB.

A second TUF-1 double-balanced mixer,

U15, converts the received signal to the next

IF at 15 kHz. A three-pole, elliptical, low-

pass filter, built around L32, restricts the band

of signals passed on to the IF amplifier. The

cutoff frequency of this filter is about 28 kHz.

Next, the received signal is amplified by

a 50-dB low-noise amplifier using Q1 and

U10A. This circuit is essentially the same as

that used by KK7B in his R2 receiver, 8 but

the roots of the grounded-base IF appear to

go back to Roy Lewallen, W7EL. 9 No active

power-supply decoupling is needed because

the lowest frequency amplified (set by C32)

is a few kilohertz. D4 and R15 disable Q1

during transmit. This circuit provides flat re-

sponse to frequencies well beyond 20 kHz

and provides the gain needed to drive the DSP

board ADC. CMOS switches U12A and

U12B determine whether the ADC is con-

nected to the IF amplifier for receive or to the

microphone for transmit.

Received signals are converted to digital

Transceiver Hardware

Figure 2 shows the received-signal path.

Signals from an antenna (or a transverter) go

to P1 on the main circuit board. A dual PIN

diode (D2) is used for TR switching. The

current through this diode is under PC con-

trol (through the DSP) and is used as an ad-

justable RF attenuator. This is a very simple

way to achieve an attenuation range of about

18 dB. It is, however, a compromise method

because the impedance seen by the following

filter varies with the attenuation level. This,

in turn, causes some distortion in the RF pass-

band response. However, because the attenu-

ation is set for minimum except when han-

dling a strong local signal, this approach does

not cause problems.

The signal passes through a two-pole fil-

ter consisting of L1, L2 and associated ca-

pacitors. This filter derives from a design by

Rick Campbell, KK7B, 6 and has a 20-dB re-

jection 25 MHz out of band. The filter’s in-

sertion loss is about 2 dB. Two RF amplifier

stages, U1 and U2, provide a gain of about 32

September 1999

Figure 4—First- and second-IF circuitry. The crystal filter and second mixer, U15, are used for transmit and receive. The IF amplifier is

bypassed by CMOS switches for transmit. Some component designators differ from QST style.

C35, C39—2.2- µ F, 50-V surface-mount

electrolytic (DK PCE3046CT.) This and

the other surface-mount electrolytic

capacitors are from the Panasonic HB

series.

C37—47-

F, 16-V surface-mount

electrolytic (DK PCE3033CT)

D4, D5, D6—BAR74 diode (DK

BAR74ZXCT)

L12, L13—2.2-

H variable inductor,

10 mm, Toko BTKANS-9447HM

(DK TK1413)

L27, L31—Ferrite SMT bead 1206

(DK 240-1019-1)

L32—330 µ H; 52 turns #32 enameled wire

on an Amidon F-22-43 toroid core.

Q1, Q5—FMMT3904 NPN transistor,

SOT-23 (DK FMMT3904CT)

Q6—FMMT3906 PNP transistor, SOT-23

(DK FMMT3906CT)

U10—LM833M low-noise op amp

(DK LM833M)

U11, U12—CD4066BCM

(DK CD4066BCM)

U15—TUF-1 mixer (MC TUF-1)

X1, X2, X3, X4—19.6608-MHz crystal,

Epson CA-301 type (DK SE3437)

Figure 5—Measured response of the four-pole crystal filter. The 6-dB bandwidth is about

12 kHz to allow use on FM. For this IF, the conversion oscillator is on the high-frequency

side at 19.680 MHz. This provides the best spurious rejection because the filter drops

off fastest on the high-frequency side.

September 1999

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