ungblood, AC5OG
We conclude this series with a description of a dc-60 MHz
transceiver that will allow open-software experimentation
with software defined radios.
I
t has been a pleasure to receive
feedback from so many QEX read-
ers that they have been inspired
to experiment with software-defined
radios (SDRs) through this article se-
ries. SDRs truly offer opportunities to
reinvigorate experimentation in the
service and attract new blood from the
ranks of future generations of com-
puter-literate young people.
1
It is en-
couraging to learn that many readers
see the opportunity to return to a love
of experimentation left behind because
of the complexity of modern hardware.
With SDRs, the opportunity again ex-
ists for the experimenter to achieve
results that exceed the performance
of existing commercial equipment.
Most respondents indicated an in-
terest in gaining access to a complete
SDR hardware solution on which they
can experiment in software. Based on
this feedback, I have decided to offer
the SDR-1000 transceiver described in
this article as a semi-assembled,
three-board set. The SDR-1000 soft-
ware will also be made available in
open-source form along with support
for the GNU Radio project on Linux.
2
Table 1 outlines preliminary specifi-
cations for the SDR-1000 transceiver.
I expect to have the hardware avail-
able by the time this article is in print.
The ARRL SDR Working Group in-
cludes in its mission the encourage-
ment of SDR experimentation through
educational articles and the availabil-
ity of SDR hardware on which to ex-
periment. A significant advance to-
ward this end has been seen in the
pages of QEX over the last year, and
it continues into 2003.
This series began in Part 1 with a
general description of digital signal
processing (DSP) in SDRs.
3
Part 2 de-
scribed Visual Basic source code to
implement a full-duplex, quadrature
interface on a PC sound card.
4
Part 3
described the use of DSP to make the
PC sound-card interface into a func-
tional software-defined radio.
5
It also
explored the filtering technique called
FFT fast-convolution filtering. In this
final article, I will describe the SDR-
1000 transceiver hardware including
an analysis of gain distribution, noise
figure and dynamic range. There is
also a discussion of frequency control
using the AD9854 quadrature DDS.
1
Notes appear on page 28. Mar/Apr 2003 21
To further support the interest
generated by this series, I have est-
ablished a Web site at home.
earthlink.net/~g_youngblood. As
you experiment in this interesting
technology, please e-mail suggested en-
hancements to the site.
Is the Tayloe Detector
Really New?
In Part 1, I described what I knew
at the time about a potentially new ap-
proach to detection that was dubbed
the Tayloe Detector. In the same is-
sue, Rod Green described the use of
the same circuit in a multiple conver-
sion scheme he called the Dirodyne.
6
The question has been raised: Is this
new technology or rediscovery of prior
art? After significant research, I have
concluded that both the Tayloe De-
tector and the Dirodyne are simply
rediscovery of prior art; albeit little
known or understood. In the Septem-
ber 1990 issue of QEX, D. H. van
Graas, PAØDEN, describes The
Fourth Method: Generating and De-
tecting SSB Signals.
7
The three pre-
vious methods are commonly called
the phasing method, the filter method
and the Weaver method. The Tayloe
Detector uses exactly the same con-
cept as that described by van Grass
with the exception that van Grass uses
a double-balanced version of the cir-
cuit that is actually superior to the sin-
gly-balanced detector described by
Dan Tayloe
8
in 2001.
In his article, van Graas describes
how he was inspired by old frequency-
converter systems that used ac motor-
generators called selsyn motors. The
selsyn was one part of an electric axle
formerly used in radar systems. His
circuit used the CMOS 4052 dual 1-4
multiplexer (an early version of the
more modern 3253 multiplexers ref-
erenced in Part 1 of this series) to pro-
vide the four-phase switching. The
article describes circuits for both
transmit and receive operation.
Phil Rice, VK3BKR, published a
nearly identical version of the van
Graas transmitter circuit in Amateur
Radio (Australia) in February 1998,
which may be found on the Web.
9
While he only describes the transmit
circuitry, he also states, . . . the switch-
ing modulator should be capable of
acting as a demodulator.
Its the Capacitor, Stupid!
So why is all this so interesting?
First, it appears that this truly is a
fourth method that dates back to at
least 1990. In the early 1990s, there was
a saying in the political realm: Its the
economy, stupid! Well, in this case, its
the capacitor, stupid! Traditional com-
mutating mixers do not have capacitors
(or integrators) on their output. The
capacitor converts the commutating
switch from a mixer into a sampling
detector (more accurately a track-and-
hold) as discussed on page 8 of Part 1
(see Note 3). Because the detector op-
erates according to sampling theory, the
mixing products sum aliases back to the
same frequency as the difference prod-
uct, thereby limiting conversion loss. In
reality, a switching detector is simply a
modified version of a digital commutat-
ing filter as described in previous QEX
articles.
10, 11, 12
Instead of summing the four or
more phases of the commutating fil-
ter into a single output, the sampling
detector sums the 0
°
and 180
°
phases
into the in-phase (I) channel and the
90
°
and 270
°
phases into the quadra-
ture (Q) channel. In fact, the math-
ematical analysis described in Mike
Kossors article (see Note 10) applies
equally well to the sampling detector.
Is the Dirodyne Really New?
The Dirodyne is in reality the sam-
pling detector driving the sampling
generator as described by van Graas,
forming the architecture first de-
scribed by Weaver in 1956.
13
The
Weaver method was covered in a se-
ries of QEX articles
14, 15, 16
that are
worth reading. Other interesting read-
ing on the subject may be found on the
Web in a Phillips Semiconductors ap-
plication note
17
and an article in
Microwaves & RF.
18
Peter Anderson in his Jul/Aug 1999
letter to the QEX editor specifically
describes the use of back-to-back com-
mutating filters to perform frequency
shifting for SSB generation or recep-
tion.
19
He states that if, on the output
of a commutating filter, we can, add
a second commutator connected to the
same set of capacitors, and take the
output from the second commutator.
Run the two commutators at different
frequencies and find that the input
passband is centered at a frequency
set by the input commutator; the out-
put passband is centered at a fre-
quency set by the output commutator.
Thus, we have a device that shifts the
signal frequency, an SSB generator or
receiver. This is exactly what the
Dirodyne does. He goes on to state,
The frequency-shifting commutating
filter is a generalization of the Weaver
method of SSB generation.
So What Shall We Call It?
Although Dan Tayloe popularized
the sampling detector, it is probably
not appropriate to call it the Tayloe
detector, since its origin was at least
10 years earlier, with van Graas.
Should we call it the van Graas De-
tector or just the Fourth Method?
Maybe we should, but since I dont
know if van Graas originally invented
it, I will simply call it the quadrature-
sampling detector (QSD) or quadra-
ture-sampling exciter (QSE).
Dynamic Range
How Much is Enough?
The QSD is capable of exceptional
dynamic range. It is possible to design
a QSD with virtually no loss and 1-dB
compression of at least 18 dBm
(5 V
P-P
). I have seen postings on e-mail
Table 2Acceptable Noise Figure
for Terrestrial Communications
Frequency Acceptable
(MHz)
NF (dB)
1.8
45
3.5
37
4.0
27
14.0
24
21.0
20
28.0
15
50.0
9
144.0
2
Table 1SDR-1000 Preliminary Hardware Specifications
Frequency Range
0-60 MHz
Minimum Tuning Step
1 µHz
DDS Clock
200 MHz, <1 ps RMS jitter
1dB Compression
+6 dBm
Max. Receive Bandwidth
44 kHz-192 kHz (depends on PC sound card)
Transmit Power
1 W PEP
PC Control Interface
PC parallel port (DB-25 connector)
Rear Panel Control Outputs
7 open-collector Darlington outputs
Input Controls
PTT, Code Key, 2 Spare TTL Inputs
Sound Card Interface
Line in, Line out, Microphone in
Power
13.8 V dc 22 Mar/Apr 2003
Fig 1SDR-1000 receiver/exciter schematic. Mar/Apr 2003 23
reflectors claiming measured IP3 in the
+40 dBm range for QSD detectors us-
ing 5-V parts. With ultra-low-noise au-
dio op amps, it is possible to achieve an
analog noise figure on the order of 1 dB
without an RF preamplifier. With ap-
propriately designed analog AGC and
careful gain distribution, it is theoreti-
cally possible to achieve over 150 dB of
total dynamic range. The question is
whether that much range is needed for
typical HF applications. In reality, the
answer is no. So how much is enough?
Several QEX writers have done an
excellent job of addressing the sub-
ject.
20, 21, 22
Table 2 was originally pub-
lished in an October 1975 ham radio
article.
23
It provides a straightforward
summary of the acceptable receiver
noise figure for terrestrial communi-
cation for each band from 160 m to
2 m. Table 3 from the same article il-
lustrates the acceptable noise figures
for satellite communications on bands